Insights into the reaction pathways of glycerol hydrogenolysis over Cu-Cr catalysts

Article abstract of DOI:10.1016/j.molcata.2012.08.004

In order to well understand reaction pathways of glycerol hydrogenolysis over Cu-Cr catalysts, hydrogenolysis of glycerol was investigated as a function of the molar ratios of Cu to Cr, reaction time, reaction temperature, hydrogen pressure, and glycerol concentration. The intermediates in glycerol hydrogenolysis were identified under Ar atmosphere or relatively mild condition. Hydrogenolysis of propanediols was also investigated for understanding the formation of propanols as by-products. The structure of Cu-Cr catalysts, prepared by an epoxide-assisted route, was determined by X-ray diffraction and scanning transmission electron microscopy. The high conversion of 85.9% and high selectivity toward 1,2-propylene glycol of 98.5% was achieved over the CuCr(4) catalyst in the hydrogenolysis of glycerol. As expected, extending reaction time, or elevating temperature and hydrogen pressure favored the conversion of glycerol. Interestingly, the conversion of glycerol and the selectivity to 1,2-propanediol increased with increasing the glycerol concentration at the same ratio of catalyst to glycerol. It was found that the hydrogenolysis of glycerol not only involved glycerol directly dehydrated and hydrogenated to 1,2-propanediol (DH route), but also involved glycerol dehydrogenation to glyceraldehyde, which was subsequently dehydrated and hydrogenated to 1,2-propanediol (DDH route), while 1,2-propanediol was further converted to propanol through H⁺ transfer from alcohol compounds.

Full text of DOI:10.1016/j.molcata.2012.08.004

Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Insights into the reaction pathways of glycerol hydrogenolysis over Cu–Cr
catalysts
Zihui Xiaoa, Chuang Lia, Jinghai Xiua, Xinkui Wanga, Christopher T. Williamsb, Changhai Lianga,∗
a Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China
b Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, SC 29208, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 May 2012
Received in revised form 3 August 2012
Accepted 5 August 2012
Available online 14 August 2012
In order to well understand reaction pathways of glycerol hydrogenolysis over Cu–Cr catalysts,
hydrogenolysis of glycerol was investigated as a function of the molar ratios of Cu to Cr, reaction time,
reaction temperature, hydrogen pressure, and glycerol concentration. The intermediates in glycerol
hydrogenolysis were identified under Ar atmosphere or relatively mild condition. Hydrogenolysis of
propanediols was also investigated for understanding the formation of propanols as by-products. The
structure of Cu–Cr catalysts, prepared by an epoxide-assisted route, was determined by X-ray diffrac-
tion and scanning transmission electron microscopy. The high conversion of 85.9% and high selectivity
toward 1,2-propylene glycol of 98.5% was achieved over the CuCr(4) catalyst in the hydrogenolysis of
glycerol. As expected, extending reaction time, or elevating temperature and hydrogen pressure favored
the conversion of glycerol. Interestingly, the conversion of glycerol and the selectivity to 1,2-propanediol
increased with increasing the glycerol concentration at the same ratio of catalyst to glycerol. It was found
that the hydrogenolysis of glycerol not only involved glycerol directly dehydrated and hydrogenated to
1,2-propanediol (DH route), but also involved glycerol dehydrogenation to glyceraldehyde, which was
subsequently dehydrated and hydrogenated to 1,2-propanediol (DDH route), while 1,2-propanediol was
further converted to propanol through H+ transfer from alcohol compounds.
Keywords:
Cu–Cr catalysts
Glycerol hydrogenolysis
Glyceraldehyde
1,2-Propanediol
H+ transfer
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
and high efficiency for C O bond hydrogenation and dehydro-
genation. For the first time, Suppes et al. [5] reported that copper
Glycerol is the major byproduct of vegetable oil transesterifica-
tion and accounts for 10 wt.% of the biodiesel product [1]. Therefore,
the utilization of glycerol to produce high value-added chemicals
may partially compensate for the production costs of biodiesel. The
global projection in glycerol production estimated by Procter &
Gamble in 2010 show that the markets are reacting strongly to
the increased availability of this feedstock [2]. The resulting sharp
decrease of glycerol prices from increasing production of biodiesel
is expected to elevate glycerol as a major platform chemical. Indeed,
glycerol has been recently identified by the US DOE as an impor-
tant building block for future biorefineries [3,4]. Recently, there has
been growing interest regarding catalytic glycerol conversion. The
hydrogenolysis of glycerol would be one attractive pathway for the
production of renewable high value-added chemicals.
chromite catalyzes glycerol hydrogenolysis with good conversion
(54.8%) and propanediol selectivity (85.0%) under relatively mild
reaction conditions (i.e. 200 ◦C, 1.4 MPa). Liang and co-workers
showed that Cu–Cr catalysts prepared by sol–gel route and car-
bon templating method resulted in different performance [6–8].
More recently, the high glycerol conversion (80.3%) and selec-
tivity to 1,2-propanediol (83.9%) obtained by using Cr promoted
Cu catalysts was reported by Yi et al. [9], while the kinetic
mechanism, as a point of debate for a long time, was not dis-
cussed in these reports. There are two mechanisms that have
been proposed for glycerol hydrogenolysis to 1,2-propylene gly-
col in the literature. The most commonly proposed involves a
dehydration–hydrogenation (DH) mechanism [10–19], although a
dehydrogenation–dehydration–hydrogenation (DDH) mechanism
has some support [20,21]. Clearly, these two routes require dif-
ferent catalytic functions, and a systematic study of the activity
and selectivity of this reaction over Cu–Cr catalysts is required.
Obviously, besides the catalyst structure, the pH of solution has a
significant influence on the hydrogenolysis mechanism of glycerol
as well [22]. Under alkaline conditions, 1,2-propanediol is formed
via glyceraldehyde, through an initial dehydrogenation followed by
water elimination and finally two reduction steps [23]. While under
In previous studies, copper chromite catalysts have also been
reported to have a superior performance in glycerol hydrogenoly-
sis [5–9]. This is due to their poor activity for C C bond cleavage
∗
Corresponding author. Tel.: +86 411 84986353; fax: +86 411 84986353.
E-mail address: changhai@dlut.edu.cn (C. Liang).
URL: http://finechem.dlut.edu.cn/liangchanghai (C. Liang).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.004

Z. Xiao et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
25
acidic conditions, 1,2-propanediol is formed by direct dehydration
and subsequent hydrogenation [24].
CuCr2O4
CuO
Cu
Herein, in order to systematic understand the catalytic per-
formance of the Cu–Cr catalysts, the hydrogenolysis of glycerol
was investigated as a function of the molar ratios of Cu to
Cr, reaction time, reaction temperature, hydrogen pressure, and
glycerol concentration. For further exploring the reaction path-
ways of glycerol hydrogenolysis and determine the intermediates,
the hydrogenolysis of glycerol was performed under Ar atmo-
sphere or relatively mild condition. The formation of by-products
(propanols) was also investigated by employing propanediols as
reaction substrates. It was found that hydrogenolysis of glycerol
not only involved glycerol directly dehydrated and hydrogenated
to 1,2-propanediol, but also involved glycerol dehydrogenation to
glyceraldehyde, which was subsequently dehydrated and hydro-
genated to 1,2-propanediol, while 1,2-propanediol was further
converted to propanol through H+ transfer reaction.
CuCr(4), Used
CuCr(4), Reduction
CuCr(6)
CuCr(4)
CuCr(2)
CuCr(0.5)
CuCr(0)
15
30
45
60
75
2. Experimental
2 Theta (degree)
2.1. Catalyst preparation
Fig. 1. XRD patterns of the CuCr(X) catalysts with various Cu/Cr molar ratios.
The Cu–Cr catalysts were synthesized by sol–gel route [7,8]. In a
typical synthesis, 2.9 g Cu(NO3)2·3H2O and 9.8 g of Cr(NO3)3·9H2O
at a desired molar ratio (Cu/Cr = 0.5) were dissolved in 47 mL of
ethanol at 60 ◦C to give a clear dark blue solution. After adding 18 mL
of 1,2-propylene oxide to the solution, a dark green transparent
gel formed within a few minutes. The obtained wet gel was aged
under air atmosphere and subsequently dried at 75 ◦C for 15 h, and
the resulting xerogel was calcinated at 500 ◦C for 120 min. The as-
prepared oxides were reduced by 10% H2 in Ar at 300 ◦C for 2 h
in a fluidized bed reactor to obtain the final Cu–Cr catalysts. The
prepared catalysts were designated as CuCr(X), where X denotes as
the molar ratio of copper to chromium.
internal standard. The conversion of the glycerol was calculated as
follows:
sum of moles of all products
sum of moles of reactant
Conversion (%) =
× 100
The selectivity to each product was defined based on carbon as
follows:
moles of carbon in specific product
moles of carbon in all products
Selectivity (%) =
× 100
3. Results and discussion
2.2. Catalyst characterization
3.1. Characterization of Cu–Cr catalysts
X-ray diffraction (XRD) patterns of the samples were measured
The crystalline structures of the CuCr(X) catalysts were investi-
gated by XRD measurements. Fig. 1 shows XRD patterns of calcined
CuCr(X) catalysts with various molar ratios. The CuCr(0) catalyst
showed a single phase of Cr2O3. As the copper ratio was increased,
the characteristic peaks corresponding to Cr2O3 phase decreased
and new characteristic peaks of the copper chromite (CuCr2O4)
spinel phase appeared at ca. 18.6◦, 29.6◦, 31.1◦, 37.7◦, 42.3◦, 61.4◦
and 64.8◦ for the CuCr(0.5) catalyst, corresponding to the (1 1 1),
(2 2 0), (0 2 2), (1 1 3), (4 0 0), (4 4 0) and (4 0 4) planes, respectively
[25]. This indicates that the formation of the CuCr2O4 spinel phase is
energetically favorable in mixtures of Cu and Cr. Further increases
in the copper ratio resulted in a decrease in the CuCr2O4-related
coupled with the appearance of the CuO phase. On the basis of
these observations, it is apparent that the phase composition is
significantly affected by increasing Cu/Cr molar ratios. Apart from
the CuCr(0), the higher the loading of copper was, the sharper the
diffraction peaks were, indicating a greater degree of crystallinity
and crystallite growth.
in a D/MAX-2400 diffractometer with a Cu K␣ monochromatized
˚
radiation source (ꢀ = 1.5418 A), operated at 40 kV and 100 mA.
Composition analysis was performed in the scanning transmis-
sion electron microscopy (STEM) mode in combination with energy
dispersive X-ray spectroscopy using an analyzer system (EDAX) in
the same microscope. High-resolution TEM (HRTEM) and energy
dispersive X-ray spectroscopy (EDX) were performed by using a
Philips CM200 FEG transmission electron microscope operated at
200 kV.
2.3. Catalytic reaction
Glycerol catalytic conversion was performed in a 100 mL
stainless steel autoclave with mechanical stirrer and an electric
temperature controller, operated under H2 pressure of 4.1 MPa at
210 ◦C for 600 min. About 60 mL aqueous solution of 60 wt.% glyc-
erol and 5 wt.% (based on glycerol) of the catalysts was charged
into the autoclave. The reactor was sealed and pressurized to the
required hydrogen pressure, and then heated to the desired tem-
perature. After the reaction, the autoclave was cooled to ambient
temperature, then brought to atmospheric pressure and opened
to allow the reaction mass to be discharged and centrifuged for
removing the catalyst. The products were analyzed with a Bruker
450-GC gas chromatograph equipped with a flame ionization
detector. ChemStation software was used to collect and analyze
the data. A FFAP GC column (30 m × 0.32 mm × 0.5 ␮m) was used
for separation. The analysis was carried out using n-butanol as the
Typically, TEM, HRTEM, STEM, elemental maps, and EDX anal-
ysis were used to determine the catalyst structural properties, e.g.
the CuCr(4) sample was shown in Figs. 2 and 3. The HRTEM image
in Fig. 2c revealed that the CuCr2O4 and CuO phases were formed,
which was coincide with XRD result. Meanwhile the structure of
CuO supported CuCr2O4 was observed. The d-spacing of 0.476,
0.256 and 0.301 nm for CuCr2O4 corresponding to the (1 0 1), (2 1 1)
and (2 0 0) planes, and lattice spacing of d = 0.251 nm was identified
¯
as CuO (1 1 1). EDX, STEM and elemental maps results indicated
that the dispersion of Cu and Cr species was homogeneous. It was

26
Z. Xiao et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
Fig. 2. TEM (a), STEM (b), HRTEM (c) and elemental maps (d–f) analysis of the CuCr(4) catalyst.
noteworthy, however, that content ratio (Cu/Cr) of about 3.6 in
the EDX results was lower than the theoretical value of 4, which
likely resulted from experimental errors, or the special structure of
CuCr2O4/CuO led to the increased of surface chromium.
of active copper in the catalyst. Further increasing the Cu/Cr ratio
to 6 resulted in no obvious difference in glycerol conversion and
selectivity, which may be a result of increase in particle size (the
same as XRD results), even though the increase of active copper.
From that on, the CuCr(4) catalyst gave the maximum activ-
ity with the best selectivity of 1,2-PD. Thus, the CuCr(4) catalyst
was further explored to determine the influence of reaction param-
eters on glycerol hydrogenolysis. Fig. 5a shows the results of
glycerol hydrogenolysis with increasing reaction time. About 44%
of conversion and 82% selectivity toward 1,2-PD were achieved
within 3 h. As expected, the glycerol conversion increased with
increasing the reaction time, reaching a value of about 56%
in 17 h. At the same time, the selectivity to 1,2-PD and ace-
tol decreased with time. While the selectivity to the byproduct
1-propanol (1-PO) increased with time, due to the gradual for-
mation from 1,2-PD, which would be discussed in the following.
There was no obvious change observed for 2-propanol (2-PO)
selectivity.
3.2. Hydrogenolysis of glycerol over Cu–Cr catalysts
Given those interesting structural changes, glycerol hydrogenol-
ysis over these CuCr(X) catalysts were investigated as shown
in Fig. 4. Not surprisingly, there was no activity for glycerol
hydrogenolysis over CuCr(0) catalyst. However, as loading of cop-
per increased, there was a uniform increase in the glycerol conver-
sion from 32% over CuCr(0.5) catalyst to 49% over CuCr(4) catalyst.
The similar tendency for 1,2-propanediol (1,2-PD) selectivity was
observed, it successive increased from 55% to 68%. Glycerol can
also be dehydrated into acetol, and then hydrogenated to propy-
lene glycol over copper metal catalysts [6,26,27]. Thus, we believe
that the increase of glycerol conversion is likely due to the increase

Z. Xiao et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
27
Fig. 3. EDX analysis of the CuCr(4) catalyst.
The effect of reaction temperature on hydrogenolysis of glyc-
erol was also investigated, and the results are presented in Fig. 5b.
Glycerol conversion greatly improved from 9% to 64% in the range
of 180–230 ◦C, while the selectivity to 1,2-PD decreased from 94%
to 45%. As an intermediate in the first dehydration step, the selec-
tivity of acetol increased with temperature since the formation
of acetol is thermodynamically favorable, while the acetol hydro-
genation is limited by thermodynamic equilibrium [28]. Due to the
enhancement of dehydration at elevating temperature, the com-
bined selectivity to 1-PO and 2-PO was increased, e.g. from 7%
to 54%. These results are probably due to hydrogenation of ace-
tol being favored at low reaction temperature and high hydrogen
partial pressure from the viewpoint of chemical equilibrium [28],
while the dehydration of glycerol favors high temperatures [17,28].
As shown in Fig. 5c, reaction pressure had a significant posi-
tive effect on glycerol conversion and a negative effect on 1,2-PD
selectivity. As the pressure increased from 2 to 5 MPa, glycerol
conversion increased from 32% to 58%, and subsequent 1,2-PD
selectivity decreased from 84% to 44%. In contrast, the combined
selectivity to 1-PO and 2-PO increased with pressure. This further

28
Z. Xiao et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
100
80
60
40
20
0
100
80
60
40
20
0
increasing glycerol concentration [5]. In contrast, increasing the
glycerol concentration from 20% to 100% led to an increase in con-
version from 32% to 92%. The reason is that, in the previous report,
the amount of catalysts was not changed regardless of glycerol
concentration, while the ratio of both was identical in our stud-
ies. The similar results had been reported by Zhou et al. [30], in
which the glycerol concentration had a positive effect on conver-
sion, when the substrates with identical molar ratio Ag to glycerol
were injected. As a solvent, water has a diluting effect, which is
detrimental to the conversion of glycerol. Considering that water is
a byproduct of the glycerol hydrogenolysis, vast amounts of water
in solution can impede the equilibrium in the forward direction.
Interestingly, the selectivity to 1,2-PD exhibited the same ten-
dency as the conversion of glycerol, increasing from 10% to 85%.
While about 7% of others were detected for solvent-free glycerol
hydrogenolysis, which were not identified due to polymerization
[5]. According to previous report [31], glycerol had a higher affin-
ity for adsorption sites than 1,2-PD. The catalysts had sufficient
reaction sites for glycerol and 1,2-PD conversion at low glycerol
concentration. As the concentration increased, the reaction sites
for 1,2-PD converting to 1-PO were occupied by glycerol, resulting
in the increased of 1,2-PD selectivity.
1,2-PD
1-PO
2-PO
Acetol
Cr2O3
CuCr(4)
CuCr(0.5)
CuCr(2)
CuCr(6)
Cu/Cr molar ratio
Fig. 4. Effect of molar ratios on hydrogenolysis of glycerol over the CuCr(X) cata-
lyst (reference conditions: 60 mL 60 wt.% aqueous glycerol solution, 5 wt.% catalyst
(based on glycerol), 210 ◦C, 4.1 MPa H2, 10 h, 150 r/min).
suggested that more byproducts were formed at higher pressure.
Vannice has also proposed that active hydrogen species on metal
sites increase with increasing hydrogen pressure [29], with a resul-
tant increase in the rate of acetol hydrogenation. This is the reason
of a decrease in the acetol selectivity with increasing pressure.
The effect of glycerol concentration on the performance of glyc-
erol hydrogenolysis was examined as shown in Fig. 5d. It was
previously reported that the conversion of glycerol decreased with
3.3. Reaction pathways of the glycerol hydrogenolysis
In the previous reports, several mechanisms have been
suggested for glycerol hydrogenolysis. Since acetol can be hydro-
genated to 1,2-PD over metal catalysts, it is regarded as a key
intermediate for 1,2-PD. The formation of acetol generally involves
100
100
b
Conversion
1,2-PD
1-PO
Conversion
1,2-PD
1-PO
a
80
60
40
20
0
80
60
40
20
0
Acetol
2-PO
Acetol
2-PO
230
210
195
17
7
10
180
3
5
Reaction temperature (oC)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
Conversion
1,2-PD
1-PO
d
Conversion
1,2-PD
1-PO
c
Acetol
2-PO
Acetol
2-PO
80
40
Glycerol concentration (%)
100
4.1
3
Reaction pressure (MPa)
5
20
2
60
Fig. 5. Effect of reaction time (a), temperatures (b), hydrogen pressure (c), and glycerol concentration (d) on hydrogenolysis of glycerol over the CuCr(4) catalyst (reference
conditions: 60 mL 60 wt.% aqueous glycerol solution, 5 wt.% catalyst (based on glycerol), 210 ◦C, 4.1 MPa H2, 5 h, 150 r/min).

Z. Xiao et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
29
Table 1
Hydrogenolysis of 1,2-PD and 1,3-PD over the CuCr(0.5) and CuCr(4) catalysts.
100
1,2-PD
80
Reactant
Catalysts
Selectivity (%)
Conversion (%)
1-PO Acetol 2-PO
1,2-PDa
CuCr(0.5) 89.0
CuCr(0.5) 99.5
6.5
–
6.0
–
3.3
6.3
4.4
4.5
0.5
13.5
0.1
2.1
2.4
0.3
10.5
0.3
50.7
34.5
5.1
Acetol
1,3-PDa
1,2-PDa
CuCr(4)
CuCr(4)
CuCr(4)
CuCr(4)
CuCr(4)
80.5
99.9
94.6
91.3
95.1
1,3-PDa
20
10
0
1,2-PD + ethylene glycolb
1,2-PD + n-butanolb
1,2-PD + cyclohexaneb
0.5
0.5
Conversion
a
Reaction conditions: 60 mL 60 wt.% aqueous reactant solution, 5 wt.% catalyst,
4.1 MPa H2, 210 ◦C, 5 h, 150 r/min.
b
Reaction conditions: 60 mL 60 wt.% aqueous reactant solution (40 wt.% of 1,2-
PD), 5 wt.% catalyst, 4.1 MPa H2, 210 ◦C, 5 h, 150 r/min.
2-PO
40
1-PO
30
10
50
20
two routes. One is direct dehydration of glycerol [10–13,32–35].
In the other route, glycerol dehydrogenates to glyceraldehyde fol-
lowed by dehydration and subsequent hydrogenation into acetol
[20,36,37]. Therefore, it is very significant to determine the forma-
tion route of acetol for understanding the mechanism of glycerol
hydrogenolysis. On the basis of the current understanding mecha-
nism, the glycerol hydrogenolysis product 1,2-PD was also detected
on CuCr(4) catalyst under Ar (other conditions are the same), e.g.
about 67% selectivity to 1,2-PD and 15% selectivity to acetol at 9%
conversion of glycerol, indicating that acetol may be formed not
solely from the direct dehydration of glycerol. H2 is required for ace-
tol to be hydrogenated to propylene glycol under Ar, which should
be derived from glycerol through its dehydrogenation on the metal-
lic Cu surfaces. However, no dehydrogenated intermediate (e.g.
glyceraldehyde) was detected, which was likely due to its known
high reactivity and instability relative to acetol and propanediol
under the given conditions [36]. To verify the mechanism, the
solvent-free glycerol hydrogenolysis was also performed under
mild conditions (2 MPa, 130 ◦C), interestingly, a small amount of
glyceraldehyde was detected.
Tomishige et al. proposed that 1-PO and 2-PO originated from
1,3-PD over Ru/C with an ion-exchange resin catalysts [13,38].
However, 1-PO and 2-PO were observed over Pt/ASA catalysts
via dehydration of 1,2-PD and subsequent hydrogenation [39]. In
agreement with the previous results, 1-PO and 2-PO were also
detected in this study. Therefore, we attempted to elucidate the
pathways of glycerol converted to 1-PO and 2-PO over Cu–Cr cata-
lysts. The performance of CuCr(0.5) and CuCr(4) catalysts for 1,2-PD
and 1,3-PD catalytic conversion has been investigated as shown
in Table 1. Surprisingly, 1,2-PD conversion was much lower under
the given conditions, irrespective of the employed catalysts. How-
ever, there was no obvious changed of glycerol conversion after 3 h,
the selectivity of 1,2-PD decreased with increasing reaction time as
shown in Fig. 5a, while the selectivity of 1-PO increased. As known,
the selectivity is proportional to the concentration at identical con-
version, namely the corresponding concentration exhibited the
same variation trend as well. Thus, it was reasonable that 1,2-PD
could convert to 1-PO. This is contrary with the results in Table 1.
It was likely that the conversion of 1,2-PD promoted by present-
ing some molecules. In the case of 1,3-PD as reactant, the CuCr(4)
catalyst gave much superior 1,3-PD conversion compared with the
CuCr(0.5). Nevertheless, most of 1,3-PD had converted to 1-PO and
a trace amount of 2-PO was detected, reflecting that 2-PO may be
originated from 1,2-PD. Based on these results, the formation of
1-PO is likely favored from the conversion of 1,3-PD and/or 1,2-
PD, while the formation of 2-PO is favored from the conversion of
1,2-PD.
Reaction time (min)
Fig. 6. Glycerol hydrogenolysis over the CuCr(4) catalyst (60 mL 60 wt.% aqueous
glycerol solution, 5 wt.% catalyst (based on glycerol), 190 ◦C, 3 MPa H2, 150 r/min).
investigated under hydrogen pressure of 3 MPa and 190 ◦C as
shown in Fig. 6. Unexpectedly, apart from the final products, no
intermediates were observed, such as 3-hydroxypropanal, acrolein
and allyl alcohol (intermediates in producing 1,3-PD), which were
detected in the literature [40]. In addition, a more mild condition
(3 MPa and 160 ◦C) was also employed for glycerol hydrogenoly-
sis, and again there were no intermediates. Therefore, this strongly
supports that the origin of 1-PO is via dehydration of 1,2-PD and
subsequent hydrogenation, rather than from conversion of 1,3-PD.
A promotion mechanism was proposed by Schüth et al. [41],
where allyl alcohol could be obtained from acrolein through a
hydrogen transfer reaction with an alcohol as a hydrogen donor.
Whether the similar mechanism presents in the conversion of 1,2-
PD, which needs to be further investigated. Thus, we had conducted
the conversion of 1,2-PD over CuCr(4) catalyst under the existence
of other compounds (ethylene glycol, n-butanol and cyclohexane)
as shown in Table 1. Surprisingly, 1,2-PD conversion was greatly
improved upon injection of ethylene glycol with ca. 35% of 1,2-PD
conversion, whereas it increased only to 5% after adding n-butanol.
However, a promotion had been observed. Besides, 1,2-PD con-
version was not obvious changed as injecting cyclohexane. These
results indicated that 1,2-PD was further converted to propanols
through H+ transfer of an alcohol as reported in the literature [42],
in which 2-propanol could function as a hydrogen donor molecule
in the transfer hydrogenation process to selectively convert glyc-
erol into 1,2-propanediol under N2 pressure. On the basis of the
above discussion, we propose that glycerol hydrogenolysis to 1,2-
PD proceeds as shown in Scheme 1 by its dehydrogenation to
glyceraldehyde, and the subsequently dehydration and hydrogena-
tion to acetol, or direct dehydration and hydrogenation to acetol,
which is then hydrogenated to 1,2-PD, while 1,2-PD was further
converted to propanols through H+ transfer of an alcohol.
3.4. Effect of mass transfer on the hydrogenolysis of glycerol
It easy to find the above results were not showed superiority
comparing to previous study [9]. The difference is likely due to
mass transfer limitations in liquid-phase catalytic hydrogenations.
Therefore, a thorough study should be made of the influence of
mass transfer on the catalyst performance. The influence of the
stirring rate on glycerol hydrogenolysis is presented in Table 2.
At stirring rate of below 300 rpm, the glycerol conversion signifi-
cantly increased with increasing the stirring rate, and subsequently
increased gradually with the stirring rate increase up to 900 rpm.
For a detailed view of the origin of 1-PO, a shortened reaction
time and less forcing reaction conditions are required for detect-
ing the intermediates. Therefore, glycerol hydrogenolysis was

30
Z. Xiao et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
Scheme 1. Reaction pathways of glycerol hydrogenolysis over the Cu–Cr catalysts.
7
6
5
4
3
2
1
0
7
a
b
600 rpm
150 rpm
6
5
4
3
2
1
0
1,2-PD
Glycerol
1,2-PD
Glycerol
1-PO
Acetol
2-PO
Acetol
8
1-PO
2-PO
0
2
4
6
8
10
0
2
4
6
10
Reaction time (h)
Reaction time (h)
Fig. 7. Effect of stirring rate on hydrogenolysis of glycerol over the CuCr(4) catalyst (reference conditions: 60 mL 60 wt.% aqueous glycerol solution, 5 wt.% catalyst (based on
glycerol), 210 ◦C, 4.1 MPa H2).
This effect may be due to the diffusion retardation of the reac-
tion, namely that the mass transfer resistances of hydrogen at the
gas/liquid interface and of glycerol at the liquid/solid interface was
minimized [43]. Significantly, however, the selectivity to 1,2-PD
increased monotonically from 60% to 98%. These observations can
be explained by the effect of mass transfer of the organic reactants
on the reaction rates [44]. Whereas the hydrogen concentration
on the catalyst surface is in equilibrium with the bulk concentra-
tion at high conversion. The influence of gas–liquid mass transfer
can be neglected. Under low stirring rate, insufficiently fast diffu-
sion of glycerol to the catalyst surface from the bulk and of the
desorbed 1,2-PD to the bulk leads to lower Cglyerol/C1,2-PD in the
liquid–solid diffusion layer compared to the kinetic regime and,
thus, to a lower catalyst surface coverage with glycerol and higher
with 1,2-PD. Therefore, in this case of mass transfer limitations, the
over hydrogenation of 1,2-PD to 1-PO will be occurred. Otherwise,
the mass transfer limitation is eliminated, leads to the increased of
1,2-PD selectivity. The curve of concentration as a function of reac-
tion time at different stirring rate was shown as in Fig. 7, which
suggested that higher stirring rate gave superior hydrogenation
rate.
Catalyst recycle use is important for liquid-phase reactions in
batch reactor. Therefore, the recycling experiments of the CuCr(4)
catalyst were conducted under the optimal conditions (4.1 MPa H2,
210 ◦C, 900 rpm, 10 h). After three recycling, the glycerol conversion
was slightly decreased from 85.9% to 81.1%, while the selectivity
of 1,2-PD maintained at about 98%. This result indicated that the
Cu–Cr catalyst had good stability, which was also proved by the
XRD results as shown in Fig. 1. After reaction, there was no obvious
change in XRD pattern compared to the reduced catalyst, apart from
an increase in the intension of diffraction peaks, indicating that the
particle size of Cu–Cr catalyst increased after reaction.
4. Conclusions
Table 2
The copper species are mainly responsible for the glyc-
erol conversion, which is confirmed by the results of glycerol
hydrogenolysis over Cu–Cr catalysts with various Cu/Cr molar
ratios. The CuCr(4) catalyst exhibits the best activity due to
an increased number of active sites. The glycerol conversion is
increased monotonically with increasing the reaction temperature
and hydrogen pressure. An antiparallel correlation is also observed
for the selectivity toward 1,2-PD. These results indicate that the
dehydration or formation of acetol favors high temperatures, while
hydrogenation of acetol favors low reaction temperature and high
Effect of the stirring rate on catalytic performance for glycerol hydrogenolysis.a
Stirring rates (rpm)
Selectivity (%)
Conversion (%)
1,2-PD
1-PO
Acetol
2-PO
150
300
600
900
59.7
84.6
90.6
98.5
34.9
12.7
7.5
4.7
2.5
1.7
0.4
0.7
0.2
0.2
0
49.2
73.4
78.7
85.9
1.1
a
Reaction conditions: 60 mL 60 wt.% aqueous glycerol solution, 5 wt.% CuCr(4)
catalyst (based on glycerol), 4.1 MPa H2, 210 ◦C, 10 h.

Z. Xiao et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 24–31
31
hydrogen partial pressure. The same tendency is shown for glycerol
conversion and 1,2-PD selectivity under different glycerol concen-
tration, which is due to the reaction equilibrium and competition
adsorption. In addition, the hydrogenolysis mechanism apparently
involves glycerol dehydrogenation to glyceraldehyde, which is sub-
sequently dehydrated and hydrogenated to 1,2-PD (DDH route),
or glycerol is directly dehydrated to acetol and hydrogenated to
1,2-PD (DH route). The 1,2-PD can then be further dehydrated and
hydrogenated to 1-PO and 2-PO, which is promoted by presenting
alcohols through H+ transfer of hydroxyl.
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We gratefully acknowledge the financial support provided by
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