Hydrogenation of Lactic Acid to 1,2-Propanediol over Ru-Based Catalysts

Article abstract of DOI:10.1002/cctc.201701329

The catalytic hydrogenation of lactic acid to 1,2-propanediol with supported Ru catalysts in water was investigated. The influence of catalyst support (activated carbon, γ-Al₂O₃, SiO₂, TiO₂, and CeO₂) and promoters (Pd, Au, Mo, Re, Sn) on the catalytic performance was evaluated. Catalytic tests revealed that TiO₂ yields the best Ru catalysts. With a monometallic Ru/TiO₂ catalyst, a 1,2-propanediol yield of 70 % at 79 % lactic acid conversion was achieved at 130 °C after 20 h reaction. Minor byproducts of the hydrogenation reaction were propionic acid, ethanol, 1-propanol, and 2-propanol. For the bimetallic catalysts, the addition of Pd and Au slightly enhanced the performance of Ru/TiO₂, whereas the addition of common hydrogenation promoters such as Re, Mo, and Sn impaired the activity.

Full text of DOI:10.1002/cctc.201701329

Accepted Article
Title: Hydrogenation of lactic acid to 1,2-propanediol over Ru-based
catalysts
Authors: Evgeny Alexandrovich Pidko, Kaituo Liu, Xiaoming Huang,
and Emiel Hensen
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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ChemCatChem
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Hydrogenation of lactic acid to 1,2-propanediol over Ru-based
catalysts
[a]
[a]
[b,c],#
Emiel J.M. Hensen*[a]
Kaituo Liu, Xiaoming Huang, Evgeny. A. Pidko,*
Abstract: The catalytic hydrogenation of lactic acid to 1,2-propanediol
The reductive transformation of LA to 1,2-propanediol (1,2-
with supported Ru catalysts in water was investigated. The influence
PDO) via catalytic hydrogenation is regarded as an attractive
green alternative to the current process in which 1,2-PDO is
obtained via the hydration of propylene oxide.[11] The main
challenge in establishing a catalytic process for the hydrogenation
of LA to 1,2-PDO is to reduce the carboxylic acid group while
preserving the α-hydroxy moiety. Given the intrinsic difficulty to
of catalyst support (activated carbon, γ-Al
and promotors (Pd, Au, Mo, Re, Sn) on the catalytic performance was
evaluated. Catalytic tests revealed that TiO yields the best Ru
catalysts. With a monometallic Ru/TiO catalyst, a 1,2-propanediol
2 3 2 2 2
O , SiO , TiO , and CeO )
2
2
yield of 70% at 79% lactic acid conversion was achieved at 130°C
after 20 h reaction. Minor by-products of the hydrogenation reaction
were propionic acid, ethanol, 1-propanol, and 2-propanol. For the
bimetallic catalysts, the addition of Pd and Au slightly enhanced the
reduce carboxylic acids with H
2
, harsh conditions are typically
required to achieve high conversion,[ which usually has the
drawback of also promoting undesired hydrogenolysis side-
reactions. The development of an active catalyst for selective
hydrogenation of LA to 1,2-PDO under mild reaction conditions is
highly desired.[
12]
2
performance of Ru/TiO , while the addition of common hydrogenation
promotors such as Re, Mo and Sn impaired the activity.
13]
Table 1. Overview of LA hydrogenation over Ru (5wt%) based catalyst in
aqueous solution. C
S(1,2-PDO) – selectivity to 1,2-PDO in %.
0
(LA) – initial LA concentration, X(LA) – conversion in %,
Introduction
P
/
H2
S(1,2-
PDO)
/ %
Lignocellulosic biomass is a potential low-cost and renewable
carbon source for the chemical industry.[1] A common approach to
valorize biomass is its conversion to platform chemicals, which
then serve as the starting chemicals for further applications. Lactic
acid (LA) is a recognized biobased platform molecule,[2] which is
commonly produced by fermentation. Given sustainability issues
with this enzymatic route, there is also significant interest in
obtaining lactic acid by chemocatalytic approaches.[3]
Polymerization of a homochiral LA via the intermediate formation
of the cyclic lactide molecules results in ploylactic acid (PLA), a
biobased and biodegradable plastic.[ Dehydration of LA at the α-
hydroxyl position yields acrylic acid, which is the primary
monomer for the synthesis of acrylate polymers.[5] The oxidative
dehydrogenation of LA gives pyruvic acid, which finds diverse
applications in food, cosmetics, and pharmaceutical
industries.[6],[7] Other important LA conversion routes include
decarboxylation to acetaldehyde,[8] condensation to 2,3-
C
/ M
0
(LA)
T
/ °C
X(LA)
/ %
Catalyst
t / h
Ref.
bar
80
50
60
35
80
80
80
[19]
Ru/SiO2
Ru/C
1
130
130
190
120
120
120
120
7
30
80
[
20]
1
2
70
84
[21]
RuSn/C
Ru/C
1.66
0.55
0.55
0.55
0.55
4
>95
19
>95
>95
95
[
[
22]
23]
2.5
18
2
RuMoO
Ru/C
x
/C
>95
13
4]
[23]
93
[
23]
Ru/SiO
2
2
4.7
92
Hydrogenation of LA using heterogeneous catalysts has been
discussed since the 1950s. Neat lactic acid could be successfully
reduced in the presence of a Re black at 150°C and 258 bars of
pentanedione,[ and esterification to LA esters.
9]
[10]
[14]
2
H , yielding up to 80% of 1,2-PDO. The hydrogenation of LA in
the gas phase has been reported using a Cu-based catalyst at
temperatures between 160°C and 220°C, resulting in high 1,2-
PDO yields.[12, 15] Supported Ru has been recently identified as a
promising catalyst for the aqueous-phase hydrogenation of
carbonyl-containing biomass-derived oxygenates including
carboxylic acids.[16] The activity of Ru can be promoted by Sn or
Mo to enable the selective hydrogenation of carboxylic acids and
their esters. Earlier studies on the hydrogenation of aqueous
lactic acid identified carbon-supported Ru catalysts as active and
highly selective catalysts.[18] However, the optimal performance of
[
a]
b]
K. Liu, Dr. X. Huang, Prof. Dr. E. A. Pidko, Prof. Dr. E. J. M. Hensen
Laboratory of Inorganic Materials Chemistry
Schuit Institute of Catalysis
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven, the Netherlands
E-mail: e.j.m.hensen@tue.nl (E.J.M.H.)
Prof. Dr. E. A. Pidko
[
[17]
ITMO University
Lomonosova str. 9, St. Petersburg 191002, Russia
#
Current address: Prof. Dr. E.A. Pidko
Inorganic Systems Engineering group,
Department of Chemical Engineering, Delft University of Technology
Van der Maasweg 9, 2629 HZ Delft, the Netherlands
E-mail: e.a.pidko@tudelft.nl (E.A.P.)
2
such catalysts could only be established at H pressure as high
as 140 bar. Subsequent studies mainly focused on reducing the
temperature and pressure requirements for the hydrogenation
process and the most representative results are summarized in
Table 1.[13,21-24]
Previous studies indicate that the highest 1,2-PDO selectivity
can be obtained in the 110°C-200°C temperature range. At higher
Supporting information for this article is given via a link at the end of
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temperatures hydrogenolysis results in 1-propanol and 2-
propanol. Furthermore, the solvent has a pronounced effect on
and in the 400°C-600°C range were observed. These peaks are
most likely related to the reduction of oxygen-containing
-supported catalysts, functional groups of the activated carbon surface. [ For the
28]
the performance of the Ru catalysts. For TiO
2
water has been identified as the preferred solvent.[ Competitive
adsorption of water with 1,2-PDO for surface sites and high
solubility of the product in water shorten the residence time of the
product on the catalyst surface, thereby limiting hydrogenolysis
reactions.
24]
Ru/CeO
sample, the broad peak at high temperature at 750°C is
2
due to the bulk reduction of the support.[ The smaller feature
below 300°C is due to ceria surface reduction facilitated by
hydrogen spilling over from the reduced Ru particles.
29]
In this work we carried out a systematic study of LA
hydrogenation by supported Ru catalysts. Particular attention was
paid to the influence of the support and common hydrogenation
promotors. For the best performing catalyst, the influence of the
process conditions on 1,2-PDO selectivity was evaluated.
Results and Discussion
Catalyst identification. A series of 2 wt% Ru catalysts supported
on TiO (P25), CeO , SiO , C, and γ-Al O were prepared by
2 2 2 2 3
incipient wetness impregnation. XRD patterns of the reduced
catalysts are given in Figure 1a. The XRD patterns only contain
features of the supports, implying the absence of large Ru and
RuO
that TiO
structure, γ-Al
this semi-crystalline oxide, while C and SiO
The textural properties of the catalysts are listed in Table 2. The
highest surface area is obtained for the amorphous carbon
2
particles. With regard to the support features, XRD shows
is a mixture of anatase and rutile, CeO has the fluorite
gives rise to the well-known weak reflections of
are X-ray amorphous.
2
2
2
O
3
2
Figure 1. (a) XRD patterns and (b) TPR profiles of different supported 2 wt. %
Ru catalysts (Peak indications: A = anatase, R = rutile, * = γ-Al O ).
2
3
2
support, the lowest for CeO .
Figure 2 shows representative TEM images of the reduced Ru
catalysts along with the particle size distribution and the average
Ru particle size. Except for the low-surface area ceria support (d
Table 2. Physical-chemical properties of supported 2 wt. % Ru catalysts.
Catalyst
S
BET
2
/
Pore volume /
cm³ g
Average pore size /
nm
=
4.15 nm), all catalysts contained on average smaller than 1 nm
Ru particles.
The activity of the supported Ru catalysts was evaluated in
–1
–1
m
58
3
g
Ru/TiO
2
0.09
0.005
1.25
0.37
0.54
5.3
5.3
12.1
2.9
7.6
the batch hydrogenation of an aqueous 0.1 M LA at 130°C. The
LA conversion and product yield obtained after 4 h are displayed
Ru/CeO
2
in Figure 3. Ru/TiO
LA conversion of 39%. The Ru/SiO
were less active with respective 1,2-PDO yields of 11%, 21%, and
3%, respectively. Ru/CeO was the least active and selective
2
gave the highest 1,2-PDO yield (27%) at a
Ru/SiO
2
293
2
, Ru/C, and Ru/Al catalysts
2 3
O
Ru/C
1470
192
1
2
2 3
Ru/Al O
catalyst with only 3% yield of 1,2-PDO obtained at a LA
conversion of 23%. We also tested Ru supported on pure-phase
anatase and rutile, as well as the high surface area CeO
m /g). They were all found to be nearly inactive compared to
mixed-phase P25-supported catalyst (Figure S3). The origin of
such a different activity behaviour is not clear and will be
The reducibility of the Ru supported catalysts was
investigated by H -TPR. The TPR traces of the samples are
shown in Figure 1b. The trace for Ru/TiO shows a sharp
reduction feature at 120 °C due to the reduction of RuCl to
metallic Ru (Figure 1b). The position of this peak is shifted to the
lower temperature compared to the unsupported catalyst, for
which the reduction peak is observed at ca. 155 °C [
2
(150
2
2
2
3
investigated in our further studies. The low activity of Ru/CeO
likely due to the large Ru particle size. The superior activity of the
P25 TiO -supported catalysts is tentatively attributed to their high
reducibility. TPR data reveals that Ru nanoparticles supported on
P25 TiO are reduced already at 120 °C allowing to maintain their
reduced state under the conditions of the catalytic reaction
130°C). Previous studies also highlight the importance of specific
Lewis acidity of the TiO support that additionally polarizes the
2
is
25] [26]
,
’
suggesting that the TiO
For the other catalysts, the main reduction feature is located at
2
3
support facilitates the reduction of RuCl .
2
around 150 °C. The higher reducibility of Ru/TiO
explained by the acid-base characteristics of the TiO
which can catalyse the dissociation of H
studies indicated the possibility of a heterolytic cleavage of H
interfacial sites between Ru nanoparticles and basic bridging
hydroxyl groups of the TiO
surface.[27] For Ru/C, besides the
main peak at 150°C, two additional reduction features at 200°C
2
may be
2
2
surface
(
2
. Previous computational
at
2
2
carbonyl group of LA making it more susceptible to reduction over
the Ru nanoparticles. [17b, 30]
2
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ChemCatChem
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50
40
30
20
10
0
50
1
,2-PDO
EtOH
2-PrOH
1-PrOH
PrA
40
30
20
10
0
Ru/TiO2 Ru/CeO2 Ru/SiO2 Ru/C Ru/Al O
2
3
Figure 3. LA conversion (left) and product yield (right) for the 2wt. % Ru
catalysts (conditions: 0.1M LA, 130°C, 4h, 40 bars H ).
2
For all catalysts, by-products obtained in relatively small
amounts were propionic acid (PrA), ethanol (EtOH), 1-propanol
(
1-PrOH), and 2-propanol (2-PrOH). For Ru/TiO
of these by-products is about 3%. Gas-phase analysis of the gas
cap of the experiment with Ru/TiO (Table S1) revealed the
formation of small quantities of alkanes such as CH (0.3%,
carbon yield), C (0.1%), and C (1.0%). No CO and only
negligible amounts of CO were detected. The overall yield of
2
, the overall yield
2
4
2
H
6
3 8
H
2
these gaseous products was 1.4%. Similar amounts of gaseous
products were formed with the other catalysts. It is important to
note that the carbon balance for LA conversion could not be
2 2 2 3
closed for the experiments with Ru/CeO , Ru/SiO , and Ru/Al O .
The conversion of LA was substantially higher than the total yield
of analysed liquid products. The difference cannot be accounted
for by the small amount of gaseous products. We were not able
to identify the products responsible for this loss of carbon neither
1
by the chromatography nor by H NMR spectroscopy, where all
major peaks were identified and quantified.
2
Catalyst optimisation. Having identified TiO -supported Ru as
the most promising catalyst for the selective hydrogenation of LA
to 1,2-PDO, we aimed to optimize performance towards Ru
loading and by using promoters.
2
Firstly, we optimized the Ru loading of the Ru/TiO catalyst.
Figure 4 shows representative TEM images of the reduced
catalysts. The catalysts containing 5 wt% and 8 wt% Ru contain
on average 1.9 ± 0.7 nm and 1.8 ± 0.8 nm particles. Increasing
the loading to 10 wt% Ru results in nanoparticles with an average
size of 2.2 ± 1.2 nm. Thus, increasing the loading leads to a
modest increase of the particle size from about 1 to 2 nm with a
concomitant broadening of the particle size distribution.
2 2 2
Figure 2. TEM images of reduced (a) Ru/TiO , (b) Ru/CeO , (c) Ru/SiO , (d)
Ru/C and (e) Ru/γ-Al O catalysts.(Ru loading: 2 wt%)
2 3
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1,2-PDO
EtOH
2-PrOH
1
00
1-PrOH
PrA
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
2
wt%
5 wt%
8 wt%
10 wt%
Ru loading
Figure 5. LA conversion (left) and product yield (right) for Ru/TiO
Ru loadings (conditions: 0.1 M LA, 130 °C, 4h, 40 bars H ).
2
with different
2
Figure 5 shows the LA hydrogenation data for these catalysts.
The LA conversion and 1,2-PDO yield show a maximum at a Ru
content of 8 wt%. Notably, the carbon balance for the experiments
for the 5 wt% and 8 wt% catalysts was closed. Although the 8 wt%
2
Ru/TiO resulted in the highest 1,2-PDO yield of 70%, it also
catalysed formation of ethanol, 1-propanol and 2-propanol in
larger amounts than the 2 wt% catalyst. By normalizing the 1,2-
PDO yields to the total Ru contents, turnover number (TON)
values can be estimated to be 34, 28, 25 and 13 for Ru/TiO
2
containing 2, 5 ,8 and 10 wt. % Ru, respectively. These data
suggests that the intrinsic activity of the catalyst remains almost
same up until 8 wt. % Ru loading. The increase of the particle size
observed at higher Ru loading is accompanied by a substantial
drop in the intrinsic activity, suggesting a pronounce structure-
sensitivity of the hydrogenation reaction with optimum activity
towards 1,2-PDO formation obtained for the intermediately sized
Ru nanoparticles.
Although high-loading Ru/TiO
PDO yield, this catalyst also leads to more by-products. With the
goal of further improving the activity and selectivity of the Ru/TiO
2
catalyst gave a higher 1,2-
2
catalyst, we explored the influence of metal promotors. Previous
studies showed that the formation of alloyed nanoparticles can
enhance the catalyst activity by modifying the electronic or
structural properties of the active phase.[31] For instance, Sn has
2 2
Figure 4. TEM images of (a) 2 wt% Ru/TiO , (b) 5 wt% Ru/TiO , (c) 8 wt% Ru
2 2
/TiO and (d) 10 wt% Ru/TiO , and the corresponding particle size distribution
histograms.
been widely studied as an additive to hydrogenation catalysts and
it might also play a role in activating the carboxyl group in
carboxylic acid reactants.[21, 30a] The formation of bimetallic RuPd
nanoparticles resulted in a superior hydrogenation activity (99%
selectivity towards γ-valerolactone in the hydrogenation of
levulinic acid) due to the dilution and isolation of the active Ru
sites by Pd.[32] Bimetallic RuAu/C improved the activity and
stability in LA hydrogenation.[33]
Inspired by these earlier studies, we prepared several
2
bimetallic RuM/TiO with M being Mo, Sn, Re. The Ru loading was
kept at 2 wt%, while the atomic amount of promoter was the same
as that of Ru.
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ChemCatChem
10.1002/cctc.201701329
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of the RuPd catalyst. The increased LA conversion was
accompanied by a decreased 1,2-PDO selectivity. The decreased
1,2-PDO selectivity was due to enhanced hydrogenolysis
reactions as evident from the increased selectivity to 1-propanol
and 2-propanol. This conclusion is also supported by the higher
yields of gaseous products (Table S1). The addition of Sn and Mo
had a strong negative effect on the catalytic performance,
especially with respect to LA conversion. Based on these data,
we conclude that promoters do not bring a real advantage.
Therefore, we further investigated the influence of the reaction
parameters such as reaction time and temperature for the 2 wt%
2
Ru/TiO catalyst. We attribute the observed lack of activity
enhancement in the current bimetallic catalyst formulations to the
current choices of the synthesis methodology and loading of the
promotors as well as other parameters of the synthesis. In this
work, we selected the co-impregnation method for the preparation
of the bimetallic catalysts to achieve a high loading of the
promotor. Previously this method was successfully employed by
us for preparation of a wide range of Re-promoted Ni[34] and Pd
hydrogenation catalysts. Potentially, other synthesis approaches
could have led to more active systems. For example Jong-Min
Lee and co-authors successfully employed
a sequential
coprecipitation-deposition method to prepare RuSn/ZnO catalysts
showing an exceptional activity in the hydrogenation of butyric
acid to n-butanol [35].Takeda et al. employed a sequential
Figure 6. XRD patterns of the Ru-based bimetallic catalysts supported on TiO
2
(Loadings: Ru 2 wt%, Mo 1.9 wt%, Sn 2.4 wt%, Re 3.7 wt%, Pd 2.1wt%, Au 3.9
wt%).
impregnation method to a highly active Ru-MoO
capable of hydrogenating LA at 120 °C and 80 bar H
x
/SiO
2
material
.
. [23]
2
60
50
40
30
20
10
0
1,2-PDO
EtOH
2-PrOH
PrA
60
50
40
30
20
10
0
1
-PrOH
Process conditions optimization. Figure 8 presents the results of
the catalytic tests at varied reaction time at 130 °C with 2 wt%
Ru/TiO
conversion with the reaction time led to a pronounced increase in
,2-PDO selectivity and an improved carbon balance. After 20 h
2
catalyst. Similar to the above findings, the increase of LA
1
reaction time at 130 °C, a 70% 1,2-PDD yield at 79% LA
conversion was obtained along with a nearly complete carbon
balance.
1
00
Carbon balance
100
80
60
40
20
0
Ru
RuAu
RuPd
RuRe
RuSn
RuMo
80
6
4
2
0
0
0
0
Figure 7. LA conversion (left) and product yield (right) for RuM/TiO
2
catalysts
Conv.
1,2-PDO
EtOH
(
conditions: 0.1 M LA, 130°C, 4h, 40 bars H ). (Loadings: Ru 2 wt%, Mo 1.9
2
wt%, Sn 2.4 wt%, Re 3.7 wt%, Pd 2.1wt%, Au 3.9 wt%).
2
1
-PrOH
-PrOH
PrA
The XRD patterns of the bimetallic catalysts are shown in
Figure 6. Only the diffraction peaks of rutile and anatase phases
of the support were observed, indicative of the high dispersion of
the metallic phase. Only for the RuAu catalyst small features due
to the presence of gold particles with an estimated size in the 5-
0
4
8
12
16
20
Reaction time (h)
1
0 nm range were observed.
Figure 7 summarizes results of the catalytic tests with the
Figure 8. LA conversion (left) and product yield (right) for 2 wt% Ru/TiO
function of reaction time (conditions: 0.1 M LA, 130°C, 40 bars H ).
2
as a
2
RuM/TiO
effect on the catalytic performance.
conversion (45%) was obtained with the RuAu/TiO
the monometallic counterpart (39%), while the 1,2-PDO yield
29%) was the same. Similarly, the addition of Pd and Re did not
affect substantially the 1,2-PDO yield, despite the higher activity
2
catalysts. The addition of Au did not have a significant
slightly higher LA
compared to
A
2
(
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ChemCatChem
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FULL PAPER
conditions. Hydrogenation of carboxylic acid group of LA yields
1,2-PDO as the major product. One side-reaction is the non-
selective hydrogenolysis of the α-OH group of the LA to yield
propionic acid. Another side-reaction is the consecutive
hydrogenolysis of the 1,2-PDO, yielding 1-propanol and 2-
propanol.[36] Further hydrogenolysis of these alcohols results in
1,2-PDO
EtOH
2-PrOH
1-PrOH
100
100
80
60
40
20
0
PrA
80
60
40
20
0
the formation of C
decarboxylation of LA. However, only trace amount of CO
3
alkanes. Ethanol can be obtained via
was
2
detected during reactions, suggesting that decarboxylation might
not be the path for ethanol formation. Given the high yield of
methane, we believe that ethanol is likely formed by C-C bond
cleavage of 1,2-PDO. In this way, ethanol was produced together
with methanol followed by a fast conversion of methanol to
methane.
110
130
150
180
200
Reaction temperature (°C)
O
OH
OH
-H
2
O
HO
OH
+
+H
2
O
CH
4
Figure 9. LA conversion (left) and product yield (right) for 2 wt% Ru/TiO
2
as a
O
function of reaction temperature (conditions: 0.1 M LA, 4 h, 40 bars H ).
2
Lactyl lactate
OH
-H O
2
OH
-H O
2
C
2
H
6
+
2H2
OH +H2
+H2
C H
3
8
-H O +H O
-H O
2
2
2
-
+
H
2O
2
O
-H
H
2
+H 2
OH
OH
Next, we evaluated the influence of the reaction temperature
on the outcome of the catalytic reaction with 2 wt% Ru/TiO
catalyst. The results of the catalytic experiments are given in
Figure 9. Expectedly, the reaction at 110°C gave only a low LA
conversion of ca. 27% with 1,2-PDO yield of 14%. The increase
+H
2
2
O
OH
-H
OH
2
O
O
Figure 10. Possible reaction pathways for LA hydrogenation in water.
of the reaction temperature up to 180°C resulted in
a
simultaneous increase of both the LA conversion and 1,2-PDO
yield. A maximum 1,2-PDO yield of 64% at a 84% LA conversion
was obtained in 4 h reaction at 180°C. The high conversion
resulted in a higher overall yield (13%) of the mono-alcohol
hydrogenolysis products as well as the short-chain alkanes in the
gas phase (8%) (Table S1). Further increase of the reaction
temperature to 200 °C led to a pronounced decrease of the yield
of the target 1,2-PDO product suggesting the range between
Conclusion
Supported monometallic and bimetallic Ru-based catalysts were
synthesized and tested in the aqueous phase hydrogenation of
LA to 1,2-PDO. The XRD and TEM characterizations showed that
2 2
the metal particles are highly dispersed on TiO surface. Ru/TiO
gave the lowest reduction temperature compared to other
supports, which makes it an excellent support for the
hydrogenation of LA. The addition of different metal promotors to
the Ru catalyst did not result in substantial activity enhancement
and, accordingly, the optimization of the reaction conditions and
130°C and 180°C to be optimal for the catalytic LA hdyrogenation
with Ru/TiO
2
.
We performed additional LA hydrogenation reactions by
varying the pressure from 10 to 40 bar at 180 °C for 4h with the
2
2wt% Ru/TiO catalyst. It was found that the yield of 1,2-PDO
catalyst formulation was further carried out with the Ru/TiO
2
linearly increases with increasing reaction pressure (Figure S4).
This is in line with earlier studies demonstrating that the formation
of 1,2-PDO is generally favored at a higher pressure[15a], at least
system. The catalyst shows a pronounced hydrogenation activity
at the temperature as low as 130°C. The optimal performance
was established at this temperature allowing to reach a 1,2-PDO
yield of 70% and LA conversion of 79%. Further increase of the
temperature led to pronounced selectivity decline due to several
side-reactions.
in part because of the strong pressure-dependency of H
2
solubility
in water[19]
.
The increase of the LA feed concentration five-fold to 0.5 M
while keeping the LA/Ru ratio similar to that of the other tests did
not affect substantially the reaction outcome. A similar 1,2-PDO
yield of 68% at 84% LA conversion could be obtained after 4h
reaction at 180 °C. An appropriate amount of spent catalyst was
weighed and utilized in a next catalytic test following the standard
procedure (20mg catalyst, 0.1M LA). In this recycling experiment,
an 1,2-PDO yield of 64% was obtained similar to that in the
catalytic tests employing fresh catalyst.
Experimental Section
Materials and methods. Lactic acid (LA, 90 wt% aqueous solution), 1,2-
propandiol (1,2-PDO, 98%), 1-propanol (99%), 2-propanol (99%), ethanol
(99%), and propionic acid (99%) were purchased from Sigma-Aldrich.
Ruthenium (III) chloride hexahydrate (38 wt% Ru) was purchased from
2
2
VWR. Amorphous SiO
(P25, 50 m /g) and commercial 5wt% Ru/C were purchased from Sigma-
Aldrich. H (99.999%) and N (99.999%) were purchased from Linde Gas.
2 2 3 2 2
(481 m /g), γ-Al O (231 m /g), CeO , and TiO
Mechanistic proposal. Based on the product distributions
observed in the catalytic tests, we put forward a proposal for the
2
2
2
mechanism of LA conversion over Ru/TiO
The commercial 90 wt% LA aqueous solution contains some lactyl
lactate and this dimer will hydrolyse to form LA under the reaction
2
catalyst (Figure 10).
All the reagents were used as received without further purification. MilliQ
water was used in the preparation of catalysts, reactor feeds, calibration
standards, and HPLC mobile phases.
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10.1002/cctc.201701329
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Quantitative analysis of LA solutions with H and H-13C nuclear magnetic
resonance measurements revealed that the starting 0.1 M solution
contains 72.1 wt% LA, 27.4 wt% dimer (lactyl lactate) and 0.5 wt%
oligomers (see the supporting information). No cyclic lactide was observed
in the solution (Figure S1). The dimer is completely hydrolyzed to form LA
monomer after heating at 130°C for 4 h in water without a catalyst (Figure
S2). Therefore, to simplify the data presentation we assume that the 0.1 M
LA solution feed only contains LA monomer. Given the fact that the dimer
totally disappeared after the reaction, the concentration of the remaining
LA monomer as determined by HPLC was used for calculating the reaction
conversion. The quantification of LA and propionic acid were made based
on the calibration curves generated with authentic compound solutions in
HPLC.
1
1
Catalyst preparation. Catalyst comprised of Ru supported on TiO
2
(P25),
CeO
2
,
SiO
2
,
C, and γ-Al
O
2 3
were prepared by incipient wetness
impregnation using RuCl
3
2
.xH O (38 wt% Ru) as the metal precursor. The
loading of ruthenium was fixed at 2 wt% on these supports. All the supports
were dried at 110°C for 24 h before impregnation. For the preparation of
monometallic Ru-based catalysts, milliQ water (3 mL/g catalyst) was
added into the amount of Ru precursor needed. For the bimetallic catalysts
2
(RuM/TiO ), the metal precursor was added to the Ru precursor solution.
The loading of ruthenium was fixed at 2 wt%. The second metal was added
with the molar ratio to Ru of 1:1. The specific loadings of the second metals
are summarized in Table S2. The precursor solution was stirred at 300 rpm
for 10 min before adding it to the support. Then, the slurries were stirred
for 4 h at 250 rpm and pre-dried in a sand bath at 80°C followed by drying
in an oven overnight at 110°C. Finally, the catalysts were reduced in a
H
2
/N
2
(v/v 10/90, 100 mL/min) at 350°C for 2 h after heating at a rate of
1H NMR was used for quantification of the other liquid products including
2°C/min.
1,2-PDO, ethanol,1-propanol, and 2-propanol. For the preparation of NMR
samples, 0.25 ml of the liquid solution was added into a 5 mm NMR sample
tube together with 0.25 ml water and 0.05 ml deuterated dimethylsulfoxide-
Catalyst characterization. Crystal phase analysis of the reduced
catalysts was carried out on a Bruker Phaser 2 X-ray powder diffraction
d
6
(DMSO-d
6
) solvent, which contains 2 mg/ml 1,4-dioxane as the internal
1
(XRD) apparatus using Cu Kα radiation source (2θ range from 5° to 80°).
standard. For quantitative H NMR analysis 32 scans were average using
Textural analysis was performed by N physisorption carried out on a
2
a
relaxation delay of
5 s. All spectra were integrated using the
Tristar 3000 automated gas adsorption system. Prior to analysis, samples
were degassed at 300°C for 6 h. Temperature programmed reduction
MestReNova software.
(TPR) experiments were performed in a flow apparatus equipped with a
fixed-bed reactor, a computer-controlled oven and a thermal conductivity
detector. Typically, 50 mg catalyst was loaded into a tubular quartz reactor.
Before TPR, samples were pretreated at 150°C for 2 h. The sample was
The conversion of LA (X) was calculated as follows:
ꢁꢂꢃ,0 − ꢁꢂꢃ
ꢀ
(%) =
× 100%
reduced in 4 vol% H
room temperature up to 900°C. The H
CuO/SiO reference catalyst. Transmission electron microscopy (TEM)
2
in N
2
at a flow rate of 8 mL/min, whilst heating from
ꢁꢂꢃ,0
2
signal was calibrated using a
2
i
The yield of the liquid component ꢄ (Y) was calculated as follows:
was used to determine the average Ru particle size and the particle size
distribution for the reduced catalysts. To this end, a catalyst sample was
suspended in excess ethanol via sonication, and aliquots were deposited
on 300 mesh carbon film Cu grids (EMS) and dried overnight under
ambient conditions. Images were taken using a JEOL 2010F equipped with
a Schottky field emission gun operated at 200 kV and a Gatan CCD
camera. Particle size distributions were extracted from the TEM images
using image processing software (ImageJ).
ꢁꢇꢈꢉꢊꢋꢌꢍ ꢆ
ꢅ
ꢆ
(
% =
)
× 100%
ꢁꢂꢃ,0
2
The gaseous product (methane, ethane, propane, CO , CO) were
analyzed by an Interscience Compact GC system, equipped with Molsieve
5 Å and Porabond Q columns employing a thermal conductivity detector
2 3
(TCD) and an Al O /KCl column with a flame ionization detector (FID).
Catalytic activity testing. Aqueous-phase catalytic hydrogenation of LA
to 1,2-PDO was performed in a 10 mL autoclave (HOKE Swagelok) at
various reaction temperatures (110°C-180°C) and a (cold) hydrogen
pressure of 40 bar. Typically, 5 mL of 0.1M LA aqueous solution and 20
mg of reduced catalyst were charged into the autoclave in a nitrogen-
Acknowledgements
This work was performed in the framework of the European Union
FP7 NMP project NOVACAM ("Novel Cheap and Abundant
Materials for Catalytic Biomass Conversion", FP7-NMP-2013-EU-
Japan-604319). E.A.P. thanks the Government of the Russian
Federation (Grant 074-U01) and the Ministry of Education and
Science of the Russian Federation (Project 11.1706.2017/4.6) for
supporting his research.
flushed glove box. The sealed autoclave was then purged 4 times with H
before being pressurized at 40 bar with H at room temperature. Typically,
2
2
the autoclave was heated at 130°C and stirred at 1100 rpm (magnetic
stirring). After the reaction, the heater was removed and the autoclave was
rapidly cooled in an ice bath. The gaseous products (methane, ethane,
2
propane, CO , CO) were analyzed by an off-line Interscience Compact GC.
After the remaining gaseous products were released, the liquid product
was separated from the catalyst by filtration (syringe filter 0.45 µm, VWR
International) and transferred to a glass vial.
Keywords: lactic acid • 1,2-propanediol • hydrogenation •
Product analysis. Quantitative analysis of the liquid products was carried
out by a combination of HPLC and H NMR. The concentrations of LA and
Ruthenium • TiO
2
1
propionic acid were quantified using a Shimadzu HPLC equipped with a
Prevail Organic Acid column and UV detector. Phosphate buffer (25 mM,
pH = 2) was used as mobile phase to ensure that all acidic groups of the
compounds were protonated. All the liquid samples were directly subjected
to analysis without dilution. Analysis showed that the commercially
obtained aqueous solution of 90 wt% LA contains LA monomer, lactyl
lactate dimers, and oligomers. The composition depends on the LA
concentration. In this work, a 0.1 M aqueous LA solution was used as the
starting material.
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Titania-supported
nanoparticles are efficient catalyst for
the conversion of aqueous lactic acid
ruthenium
K. Liu, X. Huang, E.A. Pidko*, E.J.M.
Hensen*
to
conditions. Under the optimized
conditions (130 °C, 40 bar H ), 1,2-
propanediol yield of up to 70% at lactic
acid conversion of 79% was obtained.
1,2-propanediol
under
mild
Page No. – Page No.
Hydrogenation of lactic acid to 1,2-
propanediol over Ru-based catalysts
2
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