A versatile aqueous reduction of bio-based carboxylic acids using syngas as a hydrogen source

Article abstract of DOI:10.1002/cssc.201200674

Syngas as a versatile hydrogen source: Using readily available and economically favorable syngas as a convenient hydrogen source, an efficient and sustainable aqueous reduction of bio-based carboxylic acids has been achieved over a highly robust catalyst system consisting of gold nanoparticles supported on acid-tolerant single-phase monoclinic zirconia (Au/m-ZrO₂). A range of bio-based multifunctional carboxylic acids have been selectively converted into their corresponding lactones or diols in high to excellent yields. Copyright

Full text of DOI:10.1002/cssc.201200674

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DOI: 10.1002/cssc.201200674
A Versatile Aqueous Reduction of Bio-Based Carboxylic
Acids using Syngas as a Hydrogen Source
Lei Yu,[a] Xian-Long Du,[a, b] Jing Yuan,[a] Yong-Mei Liu,[a] Yong Cao,*[a] He-Yong He,[a] and
Kang-Nian Fan[a]
Faced with the rapid depletion of natural resources and serious
environmental issues, there is an ever-increasing demand for
clean fuels and chemicals.[1] This reinforces a greater utilization
of renewable resources to replace the conventional fossil-
based feedstocks for chemicals and synthetic fuels.[2] Lignocel-
lulose constitutes a major fraction of terrestrial biomass, which
is readily available and does not compete with the food
supply.[3] The challenge for the efficient utilization of these nat-
ural resources is to develop cost-effective processing methods
to transform highly functionalized carbohydrates into value-
added chemical structures, which is believed to be essential to
build novel sustainable supply chains for the production of in-
dustrial chemicals.[4] In this respect, a potentially viable ap-
proach is the conversion of the carbohydrate fraction of ligno-
cellulose into a set of defined platform molecules, which can
serve as key intermediates or building blocks for the synthesis
of a plethora of commodity and fine chemicals.[4b,5] The imple-
mentation of such advanced biorefinery concepts requires the
development of novel and versatile catalytic processes for the
selective transformation of biomass-derived feedstocks to
a broad range of valuable chemicals and materials.
carboxylic acids have been reported,[6a–e] these reduction pro-
cedures rely mostly on a large-scale use of pure H2. The H2 pro-
duced today mainly comes from non-renewable fossil sources
by very energy-intensive processes. From both economic and
sustainable points of view, the development of new catalytic
reduction processes that could substantially reduce the need
for expensive H2 gas would be of high interest.
Syngas, also known as synthesis gas, is a standard industry
feedstock that comprises mainly a mixture of hydrogen and
carbon monoxide (H2/CO). As a key intermediate in the chemi-
cal industry, syngas is heavily used in a number of highly selec-
tive syntheses of a wide range of chemicals and fuels and also
playing an increasing role in energy conversion.[7] Syngas can
be produced from almost any carbonaceous sources ranging
from natural gas and oil products to coal and biomass by oxi-
dation with steam and oxygen.[7a–c] As a result, it represents
a key for creating flexibility for the chemical industry and for
the production of synthetic fuels (synfuels). The present use of
syngas is primarily for the manufacture of ammonia and meth-
anol, followed by the use of pure hydrogen for hydrotreating
in refineries.[7a] Recent trends in the use of syngas are dominat-
ed by the conversion of inexpensive remote natural gas into
liquid fuels (GTL) and by a possible role in a future “hydrogen
economy” mainly associated with the use of fuel cells.[7a,d,e]
Notwithstanding these versatile and broad applications, the
direct use of syngas as a convenient and truly viable hydrogen
source for valorization of bio-resources via catalytic reduction
has not been reported.
Carboxylic acids are key platform chemicals for use as bio-
based alternatives for fossil-fuel-based applications:[2b,c,6] eight
out of twelve platform chemicals identified by the US Depart-
ment of Energy as top value-added chemical targets to pro-
duce from biomass are organic acids.[5a] These are used in the
production of polymers, pharmaceuticals, solvents, and food
additives, projecting an important market volume of carboxylic
acids.[2b,c] The most common and straightforward approach for
controlled and selective conversion of these renewable build-
ing blocks is through catalytic hydrogenations, which are often
performed under highly agreessive reaction conditions due to
the low intrinsic reactivity of the carboxylic acid group with
molecular hydrogen (H2).[6f] Although a plethora of transition-
metal-based systems capable of selective hydrogenation of
The use of catalysts based on supported gold nanoparticles
(NPs) or clusters for the processing and transformation of bio-
mass into valuable chemicals is an area of growing interest.[8]
Recently, from our continuing studies on heterogeneous gold
catalysis,[9] we have discovered that small gold NPs (ca. 1.8 nm)
deposited on a monoclinic/tetragonal biphasic zirconia (Au/b-
ZrO2) can deliver a facile hydrogen-independent conversion of
aqueous levulinic acid (LA) and formic acid (FA) into g-valero-
lactone (GVL),[10] a highly versatile intermediate that has at-
tracted tremendous interest for preparing a range of valuable
chemicals and liquid fuels from lignocellulosic biomass.[11] The
essential role of supported gold is to facilitate rapid and selec-
tive decomposition of FA to produce a CO-free H2 stream,
thereby enabling a highly efficient reduction of LA without re-
quiring an external source of H2. Herein, we report that by
using a single-phase monoclinic ZrO2 supported gold catalyst,
it is possible to realize a versatile aqueous reduction of a range
of functionalized carboxylic acids to their corresponding lac-
tones or diols utilizing syngas as a direct and economically
more favored hydrogen source.
[a] L. Yu, Dr. X.-L. Du, J. Yuan, Dr. Y.-M. Liu, Prof. Dr. Y. Cao, H.-Y. He,
Prof. K.-N. Fan
Department of Chemistry
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials
Fudan University
Handan Road 220, Shanghai 200433 (P.R. China)
Fax: (+86)21-65643774
E-mail: yongcao@fudan.edu.cn
[b] Dr. X.-L. Du
Shanghai Institute of Applied Physics
Chinese Academy of Sciences
Jialuo Road 2019, Shanghai 201800 (P.R. China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200674.
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In the first stage of our work, the conversion of LA using
a H2-rich syngas with H2/CO ratio of 2, which is typical for
methanol or Fischer–Tropsch synthesis,[7a,e,f] was chosen as
a model to explore the feasibility of the proposed reaction.
We initially examined the reduction of a diluted aqueous LA
solution (0.45m) at 1208C in the presence of a series of zirco-
nia polymorphs supported gold catalysts having a similar aver-
age gold particle size (ca. 2 nm). When using Au/b-ZrO2, which
has been reported as a very active and selective catalyst for
FA-mediated LA conversion into GVL,[10] high conversion and
selectivity were obtained with an initial turnover frequency
(TOF) of 737 hÀ1 (see the Supporting Information) calculated
from the initial reaction rate, as the number of GVL molecules
formed per hour per gold atom. However, with the catalyst
comprising gold NPs deposited on single-phase monoclinic zir-
conia (Au/m-ZrO2), a much higher TOF (up to 958 hÀ1) can be
attained under identical reaction conditions. Importantly, the
aqueous LA can be quantitatively converted to GVL over Au/
m-ZrO2 within 4 h (Table 1, entry 2). This result is remarkable,
and becomes more relevant as the catalyst can be reused at
least five times, while maintaining a 98% conversion and
a yield up to 98% (Table 1, entry 3). Furthermore, it was con-
firmed by inductively coupled plasma atomic emission spectral
(ICP-AES) analysis that there was no leaching of gold during
the reaction, verifying the inherent stability of the Au/m-ZrO2
catalyst.
syngas-mediated LA reduction was examined (Figure S5). The
reaction proceeded smoothly and was completed within 4 h.
During the whole reaction process, GVL was the only product
without detecting any other intermediates. Furthermore, in
a series of additional studies examining the effect of pressure,
it was revealed that the reaction time was greatly shortened
from 6 to 3 h as Psyngas was increased from 2 to 5 MPa (Table 1,
entries 2, 7–9). Subsequent experiments focused on the effect
of the reaction temperatures revealed that under otherwise
identical reaction conditions the Au/m-ZrO2 catalyst exhibited
rather low activity at 908C (Table 1, entry 5). Notably, at an ele-
vated temperature of 1508C, quantitative formation of GVL
can be attained within 2 h (Table 1, entry 6). It should be
stressed that, in this particular case the specific activity based
on LA conversion is up to 2.54 molgAuÀ1 hÀ1, which is over
three times more active than the previously established FA-
mediated Au/b-ZrO2 catalytic systems.[10]
At this juncture, it is important to note that gold deposited
on single-phase tetragonal ZrO2 (Au/t-ZrO2) can only afford
a very moderate GVL yield (Table 1, entry 4), in line with the
broad literature documenting the profound effect of poly-
morphic structure on the activity of ZrO2-based catalysts.[12]
The superior activity as found for Au/m-ZrO2 with respect to
Au/b-ZrO2 and Au/t-ZrO2 in this reaction could be due to
a higher CO adsorption capacity as well as a higher abundance
of hydroxyl groups of the Au/m-ZrO2 catalyst.[12b] The high effi-
ciency of Au/m-ZrO2 for the direct reduction of LA using a H2-
rich syngas relative to other supported gold catalysts was
shown clearly (Table 1). Au/TiO2 and Au/SiO2 resulted in poor
yields, whereas Au/C did not promote the reduction at all
(Table 1, entries 10–12). These results show that the combina-
tion of gold NPs with suitable polymorphs of ZrO2 is essential
for achieving a high catalytic activity for the selective reduction
of LA into GVL using syngas under mild conditions. Moreover,
we found that gold is uniquely active for LA reduction with
syngas compared with other noble metals. The attempts to
reduce LA to GVL with zirconia- or carbon-supported rutheni-
um, palladium, platinum, or rhodium NPs under the same reac-
tion conditions were unsuccessful (Table 1, entries 13–17).
These observations can be attributed to the intrinsically high
affinity of ruthenium, palladium, platinum, and rhodium for CO
adsorption,[13] which blocks their activity relative to gold.
To gain more detailed insight into the reaction performed
with Au/m-ZrO2, the time-course plot for the above mentioned
Table 1. Reductive transformation of aqueous LA into GVL with syngas in
the presence of various catalysts.[a]
Entry
Catalyst
T
[8C]
t
[h]
Psyngas
[MPa]
Conv.[b]
[%]
Sel.[b]
[%]
1
2
3[c]
4
5
6
7
8
Au/b-ZrO2
Au/m-ZrO2
Au/m-ZrO2
Au/t-ZrO2
Au/m-ZrO2
Au/m-ZrO2
Au/m-ZrO2
Au/m-ZrO2
Au/m-ZrO2
Au/TiO2
120
120
120
120
90
150
120
120
120
120
120
120
120
120
120
120
120
4
4
4
4
4
2
3
5
6
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
3
2
4
4
4
4
4
4
4
4
73
100
98
22
24
100
100
100
100
15
100
100
100
100
100
100
100
100
100
100
100
–
Given the fact that the composition of syngas can vary quite
significantly depending on the carbon source from which it is
produced,[7a,b] we have explored the possibility to produce GVL
via aqueous LA reduction using simulated syngas with varying
H2/CO feed ratio. To this end, three different volumetric com-
positions of syngas were examined in the Au/m-ZrO2-mediated
conversion of aqueous LA. As shown in entries 1–3 of Table 2,
in all cases, LA can be quantitatively converted into GVL, albeit
different reaction time is required. One interesting aspect that
deserves special mention is that the reaction involving the H2-
rich syngas proceeds much more rapidly than with the CO-rich
syngas. Of further interest is that inexpensive, renewable, and
easily accessible CO2-rich bio-syngas can also be successfully
applied to the LA reduction (Table 2, entry 6). These results are
extremely welcome in view of the fact that flexible and versa-
9
10
11
12
13
14
15
16
17
Au/SiO2
Au/C
3
n.r.[d]
n.r.[d]
n.r.[d]
n.r.[d]
n.r.[d]
1
Pd/m-ZrO2
Pt/m-ZrO2
Ru/m-ZrO2
Rh/m-ZrO2
Pd/C
–
–
–
–
–
[a] Reaction conditions: LA (4.53 mmol), metal (0.1 mol%), simulated
syngas (H2/CO=2:1), water (10 mL). [b] The conversion and selectivity
were determined by means of GC using bis(2-methoxyethyl) ether as the
internal standard. [c] Results for the fifth run with a recycled cata-
lyst.[d] n.r.=no reaction.
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Table 2. Effect of the syngas composition on aqueous LA reduction.[a]
Table 3. Aqueous reductive transformation of various bio-based carboxyl-
ic acids with syngas in the presence of Au/m-ZrO2.[a]
Entry
H2/CO
P
t
[h]
Conv.[b]
[%]
Sel.[b]
[%]
[MPa]
Entry
Carboxylic acids
T
[8C]
t
[h]
Conv.[b]
[%]
Product
Sel.[b]
[%]
1
2
3
4
5
6
2:1
1:1
1:2
pure H2
pure CO
simulated bio-syngas[c]
4
4
4
4
4
4.4
4
5
6
2.5
10
4
100
100
100
100
100
100
100
100
100
100
100
100
1[c]
2[c]
3
180
180
150
6
8
5
100
100
100
>99
>99
92
[a] Reaction conditions: LA (4.53 mmol), Au/m-ZrO2 (Au 0.1 mol%), water
(10 mL), 1208C. [b] The conversion and selectivity was determined by
means of GC using bis(2-methoxyethyl) ether as the internal standard.
[c] H2/CO=2:1, CO/CO2 =5:1, CH4/H2 =0.045:1; a typical simulated bio-
syngas prepared in accordance with Reference [7f].
4[d]
240
4
100
87
[a] Reaction conditions: 5 wt% bio-based carboxylic acid (10 mL), Au/m-
ZrO2 (Au 0.1 mol%), syngas (4 MPa, H2/CO=2:1). [b] The conversion and
selectivity was determined by means of GC using bis(2-methoxyethyl)
ether as the internal standard. [c] Au (0.2 mol%). [d] Au (1 mol%).
tile syngas can be used as a direct and viable hydrogen source
for selective biomass conversion, which may open the possibil-
ity to develop novel cost-effective technologies for the produc-
tion of industrial chemicals based on platform molecules.
To better understand the essential role of H2 and CO in the
present syngas-mediated LA reduction, two independent ex-
periments using pure H2 and CO/H2O as the hydrogen source
were carried out. Under identical conditions the reaction can
proceed to completion within 2.5 h when using H2 as the sole
reductant (Table 2, entry 4), in sharp contrast to the case with
pure CO as the reductant (Table 2, entry 5). A careful analysis
of the kinetic data allows us to estimate that the reduction
rate of LA using H2 (1.86 mmolgAuÀ1 sÀ1) is around four times
faster than that using CO/H2O (0.45 mmolgAuÀ1 sÀ1). This ac-
counts for the significantly slower reduction rate associated
with the CO-rich syngas atmosphere.[14] To gain deeper insight
into the reaction pathways involved in the CO/H2O-mediated
LA reduction, we performed an additional parallel experiment
to monitor the reaction kinetics of liquid phase water gas shift
reaction (LWGS, CO+H2OQH2 +CO2) in the absence of LA (see
Supporting Information). The rate of H2 formation via LWGS
(0.58 mmolgAuÀ1 sÀ1) in the presence of Au/m-ZrO2 is compara-
ble to that of the pure CO-mediated GVL generation
(0.45 mmolgAuÀ1 sÀ1).[15] This indicates that crucial H2 generation
by LWGS is the kinetically relevant step in the overall reaction
when using sole CO/H2O as the hydrogen source.
under H2 atmosphere, whereby a 1.2:1 mixture of the two re-
gioisomers of methyl-g-butyrolactones (3-MGBL and 2-MGBL)
was obtained in a combined yield of about 80%.[6b] Remarka-
bly, the hydrogenation of IA over Au/m-ZrO2 is highly regiose-
lective, producing 3-MGBL in excellent yield of up to 92%
(Table 3, entry 3).
Furthermore, the catalyst system also shows promise for
highly selective reduction of lactic acid, the most important hy-
droxycarboxylic acid that can be derived from biological fer-
mentation.[2b,c,6a,18] The more challenging reduction of the car-
boxylic-acid function in this molecule as compared to LA was
reflected in a much higher reaction temperature required for
the conversion of lactic acid. In contrast to the situation for
the selective transformation of the C4- and C5-dicarboxylic
acids described above, wherein the formation of correspond-
ing lactones was frequently found to proceed through a dual-
step hydrogenation/dehydration sequence, the present Au/m-
ZrO2-syngas system can deliver a selective reduction of the car-
boxylic acid function in lactic acid, thus offering a renewable-
based pathway to 1,2-propanediol,[19] a high demand commod-
ity chemical currently manufactured by the oxidation of petro-
leum-derived propylene.[18] Again, it is important to note that
neither the Pt, Pd, Rh nor Ru catalysts were active for the
above reactions (see Table S1), which further confirms the in-
dispensable role of Au in mediating the relevant reduction pro-
cess using syngas as a direct hydrogen source.
Building upon these results, we started to extend this new
gold-catalyzed syngas-mediated aqueous hydrogenation pro-
tocol to a wider range of bio-derived carboxylic acids. The re-
sults are summarized in Table 3. The reaction proceeds success-
fully with a number of fermentation-derived C4- and C5-dicar-
boxylic acids. For example, succinic and fumaric acids can be
selectively hydrogenated to g-butyrolactone (GBL),[16] a valuable
chemical intermediate widely used as solvent as well as in the
production of plastics and polymers,[2b,c,17] in high yields
(Table 3, entries 1 and 2). In addition, the catalyst system can
be readily adapted to itaconic acid (IA), an unsaturated dicar-
boxylic acid with interesting commercial possibilities.[2b,c] In the
only related precedent to this work a bifunctional ruthenium
complex bearing specifically designed phosphine ligands was
used as a hydrogenation catalyst for the conversion of IA
In summary, the results of the present study demonstrate
that zirconia-supported gold catalysts and in particular the Au/
m-ZrO2 system show a very promising potential for the selec-
tive reduction of bio-derived platform chemicals using readily
available syngas as a convenient and cost-competitive hydro-
gen source. To the best of our knowledge, this new Au-based
catalytic protocol constitutes the first biomass valorization
system directly using versatile and flexible syngas as a viable
hydrogen source and furnishes a new application of syngas
apart from conventional fuel production and chemical synthe-
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sis. As the controlled hydrogenation or reductive transforma-
tion is a very general issue in the conversion of bio-derived
substrates, the procedure described herein is expected to be
of broad applicability in the utilization of biomass.
Experimental Section
Preparation of Au/m-ZrO2 catalysts: Monoclinic zirconia (m-ZrO2)
was synthesized according to the literature procedure.[12a] Au/m-
ZrO2 catalysts were prepared by a modified deposition–precipita-
tion (DP) method by mixing as-synthesized m-ZrO2 powders (2 g)
with suitable amounts of aqueous solutions of chloroauric acid
(100 mL, 1mm), the pH was adjusted to 9.0 by dropwise addition
of 0.25m NH4OH. After 6 h continuous stirring at room tempera-
ture the catalyst was washed five times with deionized water and
separated by filtration. The samples were dried at 1108C in
a forced air oven for 1 h, followed by reduction with a stream of
5 vol% H2/Ar at 3008C for 2 h. Elemental analysis results revealed
that the real gold loading was 0.8 wt% in Au/m-ZrO2.
Safety note: The mixing of gold chloride and ammonia solutions
may lead to the formation of explosive gold compounds (fulminat-
ing gold).[20] Based on our experimental observations, mixing of
ammonia solution (not concentrated, 0.25m) with gold chloride so-
lution with very low concentration (1mm) at room temperature
does not result in explosive and friction-sensitive gold compounds.
General procedure for the reduction of LA: A mixture of LA
(4.53 mmol), supported metal catalysts (metal 0.1 mol%), 2-me-
thoxyethyl ether (2 mmol, internal standard), water (10 mL) were
charged into a 50 mL Hastelloy-C high pressure Parr reactor and
stirred at a rate of 800 rpm under 4 MPa syngas for a given reac-
tion time. The mixture of substrates and catalyst was heated to the
desired temperature in less than 15 min. The liquid products were
analyzed by using
a Shimadzu GC-17A gas chromatograph
equipped with a capillary column HP-FFAP (30 mꢁ0.25 mm) and
flame ionization detector. The identification of the products was
performed by using a GC–MS spectrometer.
Acknowledgements
This work was support by the National Natural Science Founda-
tion of China (20873026, 21073042 and 21273044), New Century
Excellent Talents in the University of China (NCET-09-0305), the
National Basic Research Program of China (2009 CB623506), and
the Science & Technology Commission of Shanghai Municipality
(08DZ2270500 and 12ZR1401500).
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Keywords: bio-based carboxylic acids
· biomass · gold
catalysis · hydrogenation · sustainable chemistry · syngas
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Received: September 10, 2012
Published online on November 9, 2012
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