Influence of the carrier and composition of active phase on physicochemical and catalytic properties of CuAg/ oxide catalysts for selective hydrogenolysis of glycerol

Article abstract of DOI:10.1007/s11164-015-1999-0

The aim of the present study is to investigate the influence of the support and composition of the active bimetallic phase on both the physicochemical and catalytic properties of catalysts for use in glycerol hydrogenolysis reaction. Two series of catalysts with different amounts of copper oxide and/or silver supported on Al₂O₃ or TiO₂ oxides were prepared. To determine the physicochemical properties of the catalysts, the following techniques were used: Brunauer-Emmett-Teller, reactive N₂O adsorption, X-ray diffraction, and temperature-programmed reduction TPR-H₂. Physicochemical characterization revealed that addition of silver modifies the redox properties of the catalysts containing copper oxide and influences their specific surface area. It was found that the type of carrier determines the catalytic activity and selectivities for desired products, strongly influencing their distribution. The Al₂O₃-supported catalysts were much more selective for 1,2-propanediol, whereas 1-propanol was the main reaction product for the titania-supported catalysts. The best catalysts (6Cu/Al and 2Cu/Ti) achieved 38 % glycerol conversion with 71 % selectivity for 1,2-propanediol and 44 % conversion with 62 % selectivity for 1-propanol, respectively.

Full text of DOI:10.1007/s11164-015-1999-0

Res Chem Intermed
DOI 10.1007/s11164-015-1999-0
Influence of the carrier and composition of active phase
on physicochemical and catalytic properties of CuAg/
oxide catalysts for selective hydrogenolysis of glycerol
1
K. Samson1 A. Zelazny R. Grabowski1
_
•
•
•
M. Ruggiero-Mikołajczyk1 M. Sliwa
1
´
•
•
K. Pamin1 A. Kornas1 M. Lachowska2
•
•
Received: 27 October 2014 / Accepted: 3 March 2015
ꢀ The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract The aim of the present study is to investigate the influence of the support
and composition of the active bimetallic phase on both the physicochemical and
catalytic properties of catalysts for use in glycerol hydrogenolysis reaction. Two
series of catalysts with different amounts of copper oxide and/or silver supported on
Al2O3 or TiO2 oxides were prepared. To determine the physicochemical properties
of the catalysts, the following techniques were used: Brunauer–Emmett–Teller,
reactive N2O adsorption, X-ray diffraction, and temperature-programmed reduction
TPR-H2. Physicochemical characterization revealed that addition of silver modifies
the redox properties of the catalysts containing copper oxide and influences their
specific surface area. It was found that the type of carrier determines the catalytic
activity and selectivities for desired products, strongly influencing their distribution.
The Al2O3-supported catalysts were much more selective for 1,2-propanediol,
whereas 1-propanol was the main reaction product for the titania-supported cata-
lysts. The best catalysts (6Cu/Al and 2Cu/Ti) achieved 38 % glycerol conversion
with 71 % selectivity for 1,2-propanediol and 44 % conversion with 62 % selec-
tivity for 1-propanol, respectively.
Keywords Glycerol Á Hydrogenolysis of glycerol Á CuAg/Al2O3 Á CuAg/TiO2
&
K. Samson
ncsamson@cyf-kr.edu.pl
1
2
Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul.
Niezapominajek 8, 30-239 Krakow, Poland
Institute of Chemical Engineering, Polish Academy of Sciences, ul. Bałtycka 5, 44-100 Gliwice,
Poland
123

K. Samson et al.
Introduction
In recent years, biodiesel production has increased very sharply. This has been
caused by many factors, including diminishing sources of fossil fuels. This
increase in biodiesel production is accompanied by an increase in production of
glycerol, which is a main byproduct of biodiesel production [1]. Biodiesel is
produced by trans-esterification reaction of fats, which co-generates 1 mol of
glycerol per 3 mol of synthesized esters, approximately 10 wt % of the total
product. This results in a large quantity of glycerol available for conversion. All of
this has resulted in an oversupply of glycerol and a drastic drop in its market price
[2]. As a consequence, glycerol is considered one of the most important
compounds in conversion of biomass-derived feedstock to value-added chemicals
[3, 4]. Due to decreasing fossil fuel resources and increasing air pollution, effective
utilization of glycerol as a renewable resource by conversion into value-added
chemicals is essential [4].
Among value-added chemicals, propylene glycol (PG) is a major commodity that
is widely used as a biodegradable functional fluid, e.g., as a de-icing reagent,
antifreeze, coolant, and precursor in syntheses of unsaturated polyester resins [5–7].
The conventional method for PG production from petroleum derivatives is
expensive and environmentally unfriendly, so its production from renewable
resources is highly desired [8].
Glycerol hydrogenolysis is one of the reactions which can be applied to obtain
value-added chemicals. This reaction (Scheme 1) is considered as a two- or three-
step reaction, depending on the acido-basic conditions: (1) dehydration of glycerol
to acetol in the first step, followed by hydrogenation of intermediate in the second
step to 1,2-propanediol [10], or (2) dehydrogenation to glyceraldehydes, followed
by dehydration to 2-hydroxyacrolein and hydrogenation to desired propanediol [9,
10].
Scheme 1 Glycerol reaction pathways in acidic (1) and basic (2) reaction conditions
123

Influence of the carrier and composition of active phase on…
For this reason, use of bifunctional catalyst, possessing both acidic and redox
centers, is required. Acidic centers are responsible for glycerol dehydration, while
redox centers take part in the second stage, i.e., hydrogenation of acetol to PG [11].
So far, two kinds of catalysts for glycerol hydrogenolysis have been reported in
the literature: supported noble-metal catalysts and catalysts containing transition-
metal oxides [4]. Noble-metal-based catalysts exhibit high activity in glycerol
hydrogenolysis reaction, but their selectivity for 1,2-propanediol is low [10].
Another disadvantage of noble-metal catalysts is their high cost. On the contrary,
transition-metal catalysts are cheap and offer much greater selectivity for 1,2-
propanediol (even close to 100 %), but the glycerol conversion is limited and
requires severe reaction conditions [2]. Most transition-metal catalysts contain
copper, as it is known to be an effective catalyst in glycerol hydrogenolysis to 1,2-
propanediol by selective cleavage of C–O bond instead of C–C bond [12]. Zhou
et al. [4] found that silver addition increases the activity of Cu/Al2O3. They also
studied the activity of Ag/Al2O3 catalysts which did not contain copper, allowing
the achievement of 46 % glycerol conversion and 96 % selectivity for 1,2-
propanediol [13]. The present work focuses on investigation of the activity of
catalysts containing copper, silver or both, deposited on titania and acidic alumina,
in glycerol hydrogenolysis reaction. The influence of the support and the
composition of the active phase was also investigated.
Experimental
Preparation of catalysts
Mixed CuAg catalysts with varying Cu and Ag contents (see Tables 1, 2 for details)
were prepared by a wet impregnation method using aqueous solutions of copper and
silver nitrate (Sigma Aldrich and POCh., p.p.a., respectively). Commercial Al2O3
(Fluka, acidic, SSA = 172 m2/g) and TiO2 (anatase tioxide, batch NP 93/205, with
traces of SO4 by XPS analysis, SSA = 94 m2/g) were used as supports (Al and Ti,
respectively, hereinafter). The synthesis of the catalysts included evaporation of the
Table 1 CuAg/Al series and their physicochemical characterization
Sample,
mmolCummolAg/
1 g Al2O3
SaSA (m2/g)
Porositya
Cub metal
area (m2/g)
Copper particle
sizeb (nm)
Total pore
volume (mL/g)
Pore
size (A)
˚
Al
172.3
111.9
138.6
134.0
123.5
124.6
0.27
0.18
0.21
0.21
0.19
0.18
38.5
38.7
38.6
38.5
38.5
38.4
–
–
225.8
81.7
26.5
15.4
–
6Cu/Al
1.0
0.9
2.2
2.5
–
2Cu/Al
1.5Cu0.5Ag/Al
0.5Cu1.5Ag/Al
2Ag/Al
123

K. Samson et al.
Table 2 CuAg/Ti series and their physicochemical characterization
Sample,
mmolCummolAg/1 g
TiO2
SaSA
(m2/g)
Porositya
Cub metal area
(m2/g)
Copper particle
sizeb (nm)
Total pore
volume (mL/g)
Pore size
˚
(A)
Ti
94.0
59.2
82.0
73.3
44.6
38.6
0.45
0.24
0.27
0.30
0.29
0.26
65.9
66.0
66.1
66.2
67.4
33.2
–
–
24.5
9.4
6Cu/Ti
9.3
8.0
0.8
0.2
–
2Cu/Ti
1.5Cu0.5Ag/Ti
0.5Cu1.5Ag/Ti
2Ag/Ti
68.6
165.2
–
a
Measured by BET/Barrett–Joyner–Halenda (BJH) method
Measured by dissociative N2O adsorption
b
solvent, followed by drying for 10 h at 120 ꢁC and calcination under air flow for 5 h
at 350 ꢁC.
Physicochemical characterization of prepared catalysts
The prepared catalysts were characterized by determining their surface area and
porosity (BET/BJH method), the active surface of Cu using the method of reactive
adsorption of N2O, the phase composition by XRD, and reducibility by the
temperature-programmed reduction (TPR-H2) method.
Specific surface area and porosity by BET/BJH method
The specific surface area (SSA) and porosity of the oxide supports and synthesized
catalysts were determined by the BET method using an Autosorb-1 Quantachrome
apparatus, with nitrogen as adsorbate at 77 K. Prior to the measurements, the
samples were preheated and degassed under vacuum at 393 K for 18 h. The
micropore area was obtained by t-micropore analysis, while the pore size
distribution was calculated by the BJH method.
Active surface of copper
The active surface of copper of the fresh catalysts was determined via reactive
adsorption of N2O at 363 K according to the method described in Ref. [14]. The
measurements were carried out in a quartz flow microreactor. Approximately 0.25 g
of catalyst was reduced at 523 K during 2 h and cooled to 363 K. Then, 100-ll N2O
pulses were injected until the reaction was complete. The amount of reacted N2O
was determined using a mass spectrometer (VG/Fison Quartz 200D). It was
assumed in the calculations that reoxidation of the surface copper follows the
equation 2Cu(s) ? N2O(g) = Cu2O(s) ? N2(g) and that 1 m2 of elemental copper
corresponds to 6.1 lmol O2.
123

Influence of the carrier and composition of active phase on…
X-ray diffraction (XRD) measurements
Diffraction patterns of the supports and prepared catalysts were collected using an
X’PERT PRO MPD diffractometer working in Bragg–Brentano geometry. Cu Ka
˚
radiation (k = 1.54178 A), a graphite monochromator of the secondary beam
(PW3122/00), and an X’CELERATOR detector were used. Measurements (at
40 kV and 30 mA) were performed in the 2h range from 5ꢁ to 90ꢁ with interpolated
step size of 0.02ꢁ. The patterns were recorded at room temperature.
Reducibility by TPR-H2 method
TPR-H2 profiles of the catalysts were obtained using a U-shaped quartz flow reactor
(diameter ca. 5 mm) in the temperature range of 20–600 ꢁC. About 0.025 g of
sample was used for TPR measurements. A mixture of 5 % H2 in Ar was used as
reducing agent. Before TPR analysis, all samples were kept in a helium stream at
373 K for 1.5 h to remove physically adsorbed water. Subsequently, each sample
was cooled down to ambient temperature, and TPR analysis was performed with a
temperature ramp of 10 ꢁC/min and a flow rate of H2 ? Ar reducing mixture of
30 cm3/min. The TPR profiles were recorded using a thermal conductivity detector
(TCD).
Catalytic activity measurements
Glycerol hydrogenolysis reaction was carried out in a 100-mL PARR batch
stainless-steel reactor with mechanical stirring and electronic temperature con-
troller. For a conventional run, 30 mL of 50 v/v % aqueous solution of glycerol
(Sigma-Aldrich, p.p.a.) and 2 g of catalyst were used. The typical conditions for a
hydrogenolysis test were: pre-reduction of each catalyst (T = 200 ꢁC, pH2 = 2 atm,
t = 2 h, 100 rpm) and proper hydrogenolysis reaction (T = 200 ꢁC, pH2 = 40 atm,
t = 24 h, 400 rpm). The liquid and gas products of the reaction were analyzed by
HP gas chromatograph (GC) using a flame ionization detector (FID). Only traces
(below 5 %) of methane and COx—the only gaseous products—were detected.
Analysis of liquid products was carried out by the n-butanol internal standard
method.
Glycerol conversion was calculated according to the following equation:
conversion of glycerol [%] = [moles of glycerol consumed/moles of glycerol
initially charged] 9 100.
Product selectivities were calculated as carbon selectivity according to the
following equation: selectivity [%] = [moles of carbon in specific product/moles of
carbon in all detected products] 9 100.
123

K. Samson et al.
Results and discussion
Physicochemical characterization of the catalysts
Acidity of supports, their specific surface area and porosity determined by BET/BJH
method, and active surface of copper
Quantitative pyridine sorption experiments for Al2O3 and TiO2 oxides were
performed using the procedure described in Ref. [15]. No Brønsted acidity was
detected, but the concentration and strength of Lewis acid sites (LAS) were
different, depending on the support used. The Al2O3 support was characterized by
possessing a major amount of LAS (0.58 lmol/m2) with considerably (nearly twice,
0.9) higher strength than for TiO2 (0.32 and 0.5, respectively). Tables 1 and 2
present lists of prepared catalysts supported on Al2O3 and TiO2, respectively, their
specific surface area and porosity obtained by the BET/BJH method, and cupric
metal area calculated from dissociative N2O adsorption.
For both the CuAg/Al and CuAg/Ti series, the specific surface area of the
catalysts, SSA, was lower than that of the pure Al2O3 and TiO2 supports. The
presence of the copper active phase, as well as the silver addition, led to a decrease
of SSA; this effect is particularly visible for the CuAg/Ti series. A decrease in the
specific surface area of the support after deposition of active copper or silver
phases has frequently been observed in supported metal catalysts [16] and may be
due to sintering of the support in the presence of deposited phase or blocking of
the support pores, especially by addition of silver. The texture of the CuAg/Al
catalysts was similar, with both specific surface area and porosity having
comparable values. The surface areas of the CuAg/Ti catalysts were much lower
2
˚
(39–94 m /g) and the pore sizes were larger (*65 A) than for the CuAg/Al series
2
˚
(112–172 m /g and *38 A, respectively). The active copper surface, as deter-
mined by reactive adsorption of N2O, depended on the Cu loading and the support
used (Tables 1, 2). The copper surface area for the catalysts supported on Al2O3
varied in the range from 0.9 to 2.5 m2/g, while for the catalysts supported on TiO2
it varied across a broader range, i.e., from 0.2 to 9.3 m2/g. For both series there
was no clear correlation between copper content and active copper surface area.
However, as can be seen from Tables 1 and 2, for the studied catalysts, the smaller
copper particle size resulted in higher values of copper surface area for the
measured samples. In both series, for the catalysts containing only copper, the Cu
metal area was almost the same, irrespective of the copper content (2 or 6 mmol).
It should be noted that, in Cu metal surface area measurements, the catalysts were
reduced at 250 ꢁC, therefore only easily reducible copper is reduced, as emerges
from the TPR profiles (see ‘‘Reducibility by TPR-H2 method’’ section for details).
In view of the reaction temperature (200 ꢁC) at which glycerol hydrogenolysis is
conducted, only this type of copper is responsible for glycerol conversion or
catalytic activity.
123

Influence of the carrier and composition of active phase on…
X-ray diffraction (XRD) measurements
X-ray diffraction (XRD) analysis was used to determine the phase composition of
the obtained samples; Figs. 1 and 2 show the XRD patterns for the CuAg/Al and
CuAg/Ti systems, respectively.
For both series of catalysts, it was found that, at high copper content (C1.5 mmol
of Cu), copper was present in the form of copper oxide CuO. For the catalysts
containing small amounts of copper (0.5Cu1.5Ag/Al and 0.5Cu1.5Ag/Ti), no
patterns related to presence of copper were recorded. A similar effect was observed
by Zhou et al. [4]. This can be explained based on the fact that silver addition
promotes formation of well-dispersed CuO crystallites, too small to be detected by
the XRD technique, or that the concentration of copper was too low. For the
catalysts deposited on alumina, silver is present in both metallic and silver oxide
form (Ag2O), while in the case of the catalysts supported on titania there is no
metallic silver. For the catalysts supported on titania, silver is present as two
different silver oxides—Ag2O and Ag2O3. These different forms of silver result
from the different influences of Al2O3 and TiO2. Use of the alumina support
promotes formation of reduced silver (Fig. 1), while use of titania allows one to
obtain catalysts containing unreduced forms of silver (Fig. 2). In the case of the
XRD patterns of the 1.5Cu0.5Ag/Al and 1.5Cu0.5Ag/Ti catalysts, there are no
signals from Ag crystallites, probably due to the too low silver content, below the
limit of detection.
CuO
Ag
Ag2O
6Cu/Al
2Cu/Al
1.5Cu0.5Ag/Al
1.5Ag0.5Cu/Al
2Ag/Al
Al2O3
0
10
20
30
40
50
60
70
80
90
2 Theta [deg]
Fig. 1 XRD patterns of CuAg/Al catalysts
123

K. Samson et al.
CuO
Ag2O3
Ag2O
6Cu/Ti
2Cu/Ti
1.5Cu0.5Ag/Ti
1.5Ag0.5Cu/Ti
2Ag/Ti
TiO2 (anatase)
90
0
10
20
30
40
50
60
70
80
2 Theta [deg]
Fig. 2 XRD patterns of CuAg/Ti catalysts
6Cu/Al
60
50
40
30
20
10
0
2Cu/Al
1.5Cu0.5Ag/Al
0.5Cu1.5Ag/Al
2Ag/Al
Al2O3
0
100
200
300
400
500
600
temperature [°C]
Fig. 3 TPR profiles of CuAg/Al catalysts
123

Influence of the carrier and composition of active phase on…
Reducibility by TPR-H2 method
Figures 3 and 4 show the TPR-H2 profiles of the CuAg/Al and CuAg/Ti catalysts,
respectively.
For the samples without silver (6Cu/Al, 6Cu/Ti, 2Cu/Al, and 2Cu/Ti) two reduction
peaks, related to reduction of different kinds of copper oxides, are exhibited. The low-
temperature peak, with Tmax at 250 ꢁC for the Al2O3 series and 205 ꢁC for the TiO2
series, corresponds to reduction of smaller copper oxide crystallites or reduction of
copper oxide crystallites weakly bonded with the support [17]. Curiously enough, the
intensity of this low-temperature peak is almost the same for catalysts containing 2 and
6 mmol of copper, irrespective of the support type. Increasing the copper content in the
catalystleads to an increase of the high-temperature peak (Tmax at420 ꢁC for the Al2O3
series and 320 ꢁC for the TiO2 series) intensity, which corresponds to reduction of
larger copper oxide crystallites or crystallites that are strongly bonded with the
support, being more difficult to reduce. For catalysts containing copper and silver
(1.5Cu0.5Ag/Al and 0.5Cu1.5Ag/Al) the first peak is distinctly shifted toward lower
temperature, from 250 to 218 ꢁC. This fact suggests that the presence of silver particles
facilitates dissociation of a hydrogen molecule, considered the rate-determining step
(r.d.s.) of the reduction, as observed previously by Luo et al. [18] for catalysts
containing Ag supported on oxides. In the case of the catalyst with low copper content
(0.5Cu1.5Ag/Al) there is no high-temperature peak. This means that this sample
contains only easily reducible copper oxide. For the catalysts containing only silver
phase (2Ag/Al and 2Ag/Ti), the shape of the TPR profiles indicates that there are two
kinds of silver oxide, reducible at lower temperature than copper oxide. In the TiO2
80
6Cu/Ti
2Cu/Ti
70
60
50
40
30
20
10
0
1.5Cu0.5Ag/Ti
0.5Cu1.5Ag/Ti
2Ag/Ti
TiO2
-10
0
100
200
300
400
500
600
temperature [°C]
Fig. 4 TPR profiles of CuAg/Ti catalysts
123

K. Samson et al.
series, the reduction peak with Tmax at 410 ꢁC is assigned to reduction of the support.
As can be seen, the TPR profiles of the catalysts were significantly influenced by the
composition of the active phase deposited on the different oxide supports. Both the
shifts and intensities indicate that silver addition modifies the redox properties of
copper oxide in the studied catalytic systems.
Results of catalytic activity measurements in hydrogenolysis of glycerol
The catalytic behavior of the studied samples in the tested reaction was found to
depend on the type of oxide support and the composition of the active CuAg phase.
Tables 3 and 4 present the values of glycerol conversion, turnover frequencies
(TOF), and selectivities for major products for the CuAg/Al and CuAg/Ti catalysts,
respectively. The reaction conditions were the same in all experiments as described
in detail in ‘‘Catalytic activity measurements’’ section.
The CuAg/Ti catalysts were found to be more active in the studied reaction than
the CuAg/Al ones. The obtained conversions of glycerol for the CuAg/Ti system were
between 24 and 47 %, whereas for CuAg/Al they were lower, reaching 38 % at most.
Depending on the support used, the main reaction products (with potential industrial
application) were also different. In the case of alumina-supported catalysts, the
highest selectivities for 1,2-propanediol (1,2-PDO) were obtained for the samples
containing only copper phase. For the CuAg/Al samples, 1-propanol and acetol were
the predominant products. On the other hand, the CuAg catalysts supported on titania
were mainly selective for 1-PO (30–64 %), whereas the selectivities for 1,2-
propanediol were low (below 16 %). The highest selectivities for acetol in both series
were obtained for the 2Cu and 1.5Cu0.5Ag catalysts. As can be seen, there were
distinct differences in the selectivities for 1,2-propanediol between the catalysts
containing only active copper phase but supported on the different oxides. Therefore,
in this case, the differences in the selectivities are probably caused by differences in
the acidity of the studied supports. The 2Ag/Al and 2Ag/Ti catalysts, containing only
silver phase, showed the lowest catalytic activity with glycerol conversion of 15 and
24 %, respectively. The main reaction product for both samples was 1-propanol (for
Table 3 Results of catalysis of glycerol hydrogenolysis for CuAg/Al catalysts
Catalyst
Glycerol
TOFa
conversion (Gly/gMe9 for 1,2-PDO for 1-PO (%) for acetol
Selectivity
Selectivity
Selectivity Other
liquid
(%)
h) 9 1021
(%)
(%)
productsb (%)
6Cu/Al
2Cu/Al
38
36
1.73
4.91
1.92
1.93
1.31
71
54
0
16
15
39
43
93
4
31
61
12
2
9
0
1.5Cu0.5Ag/Al 16
0.5Cu1.5Ag/Al 20
0
11
2
34
3
2Ag/Al
15
Gly number of glycerol molecules; 1,2-PDO 1,2-propanediol; 1-PO 1-propanol
a
TOF calculated on gram of metal (Me wt%), i.e., copper and/or silver
b
2-Propanol, ethanol
123

Influence of the carrier and composition of active phase on…
Table 4 Results of catalysis of glycerol hydrogenolysis for CuAg/Ti catalysts
Catalyst
Glycerol
TOFa (Gly/
Selectivity Selectivity Selectivity Other liquid
b
conversion gMe9h) 9 1021 for 1,2-
for 1-PO
(%)
for acetol products (%)
(%)
(%)
PDO (%)
6Cu/Ti
2Cu/Ti
25
44
1.14
6.00
4.31
4.53
2.09
13
3
44
62
64
57
31
10
19
13
14
2
33
16
19
22
51
1.5Cu0.5Ag/Ti 36
0.5Cu1.5Ag/Ti 47
4
7
2Ag/Ti
24
16
Gly number of glycerol molecules; 1,2-PDO 1,2-propanediol; 1-PO 1-propanol
a
TOF calculated on gram of metal (Me wt %), i.e., copper and/or silver
b
2-propanol, ethanol, 2-hydroxyacrolein, glyceraldehydes
2Ag/Al even about 90 %), whereas the selectivities for 1,2-PDO were insignificant.
This indicates that dispersed copper is more efficient for glycerol hydrogenolysis
reaction than silver, particularly for obtaining desired product such as 1,2-
propanediol. Catalysts containing only copper phase are also more active than
bimetallic samples; for both supports, the samples denoted 2Cu/Ti and 2Cu/Al were
characterized by the highest values of glycerol conversions and TOFs. Depending on
the kind of support used, a different distribution of products was observed. In the case
of CuAg/Al catalysts, 2-propanol and ethanol were produced in small amounts. These
compounds are secondary C–C hydrogenolysis (degradation) products [3] formed via
the mechanism described in the ‘‘Introduction’’ [9, 10]. However, 2-hydroxyacrolein
and glyceraldehyde were present among the products detected in the catalytic tests
carried out with the CuAg/Ti catalysts (Table 4). This indicates that the type of
support and its acidity influence the mechanism of the glycerol hydrogenolysis
reaction. Another mechanism of glycerol hydrogenolysis, proposed in literature by
Montassier et al. [19], suggests glycerol dehydrogenation to glyceraldehyde followed
by dehydration to 2-hydroxyacrolein. In this mechanism, 2-hydroxyacrolein is
hydrogenated to 1,2-propanediol. Feng et al. [20] found that, for the Ru/TiO2 system,
the pH of the reaction mixture influences the glycerol hydrogenolysis mechanism.
They found that a pH increase leads to a change of the hydrogenolysis reaction path to
the route via glyceraldehyde. Our results showed that glyceraldehyde and
2-hydroxyacrolein were only obtained for titania-supported catalysts, which is less
acidic than alumina, which seems to be in agreement with this mechanism. Further
investigation of the acidity of the catalysts will be carried out to determine the
correlation between the composition of the active phase supported on the used oxides
and the glycerol hydrogenolysis mechanism.
Conclusions
The influence of the support type and the active phase composition was investigated
in glycerol hydrogenolysis reaction. It was found that silver addition modifies the
redox properties of the catalysts and decreases their specific surface area. The
123

K. Samson et al.
product distribution and reaction route, in agreement with Scheme 1, depend on the
type of support used. Catalysts with high copper content deposited on more acidic
alumina allowed achievement of the highest selectivity for 1,2-propanediol
(54–71 %) with 36–38 % glycerol conversion. On the other hand, the catalysts
supported on less acidic titania were more selective for 1-propanol, which is also an
industrially important commodity.
Acknowledgments The authors gratefully acknowledge financial support from the Foundation for
´
Polish Science through project POMOST/2011-3/7 and the Marian Smoluchowski Krakow Research
Consortium—a Leading National Research Centre KNOW supported by the Ministry of Science and
Higher Education.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, dis-
tribution, and reproduction in any medium, provided you give appropriate credit to the original author(s)
and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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