Effect of redox properties of LaCoO3 perovskite catalyst on production of lactic acid from cellulosic biomass

Article abstract of DOI:10.1016/j.cattod.2015.12.003

Cost-effective conversion of cellulosic biomass to value-added lactic acid with heterogeneous catalysts has attracted much attention recent years. While both solid Lewis acids and bases have been extensively studied, the role of redox catalysts for the production of lactic acid is barely understood. Herein, the LaCoO₃ perovskite metal oxides with strong redox properties and a good stability in hydrothermal media were used as the catalysts for the conversion of a variety of cellulosic biomass to lactic acid. The effects of reaction conditions such as reaction temperature, catalyst loading, and gas atmosphere were investigated. At the optimum conditions, the yields of approximately 40%, 38%, and 24% lactic acid were obtained from glucose, xylose and cellulose, respectively. The key intermediates such as pyruvic acid were used as the probe reactants to explore the reaction mechanism. Unlike Lewis acid or base catalysed sugar conversion reactions, the redox pathway might start from the oxidative decarboxylation of aldose sugars and the lattice oxygen atoms in the LaCoO₃ perovskite structure participate the redox cycles in the conversion of sugars to lactic acid. Lastly, the stability of the LaCoO₃ catalyst in hydrothermal reaction media was discussed.

Full text of DOI:10.1016/j.cattod.2015.12.003

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Effect of redox properties of LaCoO3 perovskite catalyst on production
of lactic acid from cellulosic biomass
Xiaokun Yanga, Lisha Yanga, Wei Fanb, Hongfei Lina,∗
a Department of Chemical and Materials Engineering, University of Nevada, 1664 N. Virginia St., Reno, NV 89557, USA
b Department of Chemical Engineering, University of Massachusetts Amherst, 686 N. Pleasant Street, Amherst, MA 01002, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Cost-effective conversion of cellulosic biomass to value-added lactic acid with heterogeneous catalysts
has attracted much attention recent years. While both solid Lewis acids and bases have been extensively
studied, the role of redox catalysts for the production of lactic acid is barely understood. Herein, the
LaCoO3 perovskite metal oxides with strong redox properties and a good stability in hydrothermal media
were used as the catalysts for the conversion of a variety of cellulosic biomass to lactic acid. The effects of
reaction conditions such as reaction temperature, catalyst loading, and gas atmosphere were investigated.
At the optimum conditions, the yields of approximately 40%, 38%, and 24% lactic acid were obtained from
glucose, xylose and cellulose, respectively. The key intermediates such as pyruvic acid were used as the
probe reactants to explore the reaction mechanism. Unlike Lewis acid or base catalysed sugar conversion
reactions, the redox pathway might start from the oxidative decarboxylation of aldose sugars and the
lattice oxygen atoms in the LaCoO3 perovskite structure participate the redox cycles in the conversion
of sugars to lactic acid. Lastly, the stability of the LaCoO3 catalyst in hydrothermal reaction media was
Received 1 August 2015
Received in revised form
30 November 2015
Accepted 21 December 2015
Available online xxx
Keywords:
Glucose
Xylose
Cellulose
Lactic acid
Perovskite
Redox catalysis
discussed.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
assisted method [11]. However, strong homogeneous mineral base
catalysts can cause severe corrosion, and thus limiting their use in
With the gradual depletion of fossil fuels and the increasing
concern of greenhouse gas emissions linked to climate change,
using biomass resources instead of petroleum to manufacture com-
modity chemicals has attracted growing interest in recent years
[1,2]. Carbohydrates, the most abundant biomass compounds, are
a sustainable feedstock which can be used to produce bio-based
platform chemicals [3]. Lactic acid (LA) is such a sugar derived plat-
form chemical that can be converted into a wide range of useful
compounds [3–5]. Its global production capacity is over 500,000
tons per year and continues to grow [5]. Currently, the commercial
production of LA is via fermentation processes which have draw-
backs such as slow kinetics, poor scalability, and large amounts of
waste [6,7].
lactic acid production. Therefore, the development of highly effec-
tive heterogeneous catalysts for converting carbohydrates to LA is
highly desirable. Chambon et al. reported that the yields of 27 mol-
% and 18 mol-% of LA were produced from cellulose over tungstated
zirconia (ZrW) and tungstated alumina (AlW), respectively, in sub-
critical water [12]. Taarning’s group used Sn-beta as the Lewis acid
catalyst and obtained a 26C-% yield of LA from glucose at in aque-
ous media [13]. Our group found that the yields of 42 mol-% and
30 mol-% of LA were obtained from xylose and xylan, respectively,
using the ZrO2 catalyst [14]. Wang et al. used CuO as an oxidant to
convert glucose to lactic acid in the yield of 59 mol-% in alkaline
solution [15]; however, CuO was reduced to Cu after the reac-
tion. Dong’s group recently reported a 67.6C-% yield of LA from
cellulose with the erbium-exchanged montmorillonite K10 cata-
lyst [16]. However, significant leaching of Er ions was observed.
Several research groups [13,17–19] reported the conversion of car-
bohydrate biomass into lactic acid derivatives in organic solvents, in
which leaching of metal oxide catalysts may be mitigated. Liu et al.
found that with the solid base catalyst, MgO, ∼29.5 mol-% methyl
lactate was yielded from glucose in the methanol solvent [20].
However, most heterogeneous catalysts reported in literature for
chemo-catalytic conversion of biomass to LA or lactates are based
Various chemo-catalytic processes have been studied other than
the industrial fermentation to produce lactic acid [8,9]. Jin’s group
synthesized lactic acid with a 27% yield from glucose in a 2.5 M
NaOH hydrothermal solution [10]. Epane et al. produced lactic acid
in a 75C-% yield using KOH as the catalyst via the microwave-
∗
Corresponding author. Tel.: +1 7757844697; fax: +1 7753275059.
E-mail address: HongfeiL@unr.edu (H. Lin).
http://dx.doi.org/10.1016/j.cattod.2015.12.003
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Yang, et al., Effect of redox properties of LaCoO3 perovskite catalyst on production of lactic acid
from cellulosic biomass, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2015.12.003

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on acid/base properties. Long-term stabilities of these catalysts
are still uncertain due to leaching or deactivation in hydrothermal
media.
flow was switched to 10% hydrogen in argon at 50 mL/min and
the TPR was performed from 50 ◦C to 800 ◦C with the temperature
ramp of 30 ◦C/min. Then, the sample was cooled to 30 ◦C in helium
flow before the gas flow was switched to 10% oxygen in helium at
50 mL/min. TPO experiment was performed from 50 ◦C to 800 ◦C
with the same temperature ramp of 30 ◦C/min.
Temperature-programmed desorption (TPD) of adsorbed NH3
on the catalysts was carried out using the same Chemisorption
Analyser. In a typical NH3 TPD experiment, the catalyst was first
degassed at 900 ◦C for 1 h and then cooled to 100 ◦C in a helium flow.
Afterwards, the gas was switched to 10% ammonia in helium at the
flow rate of 50 mL/min to allow NH3 adsorption for 40 min. The
physisorbed NH3 was removed under helium flow for 30 min. The
NH3 TPD was performed from 50 ◦C to 800 ◦C with the temperature
ramp of 30 ◦C/min.
The Bruker EQUINOX 55 Fourier transform infrared spec-
troscopy (FT-IR) spectrometer with a MCT detector was used for
the characterization. The sample was loaded in a high-temperature
reaction cell (Harrick) located within a DRIFT accessory (Praying
MantisTM, Harrick). The sample was degassed at 550 ◦C for 1 h
under helium in order to remove adsorbed water. After that, FT-
IR spectra (with a resolution of 2 cm−1) were collected from the
sample at 120 ◦C. For pyridine adsorption, small aliquots of pyridine
were subsequently exposed to the sample at 120 ◦C for 15 min. Prior
to the measurement the weakly adsorbed pyridine was removed by
flowing helium at 250 ◦C for 1 h, and the spectra were then collected
at 120 ◦C.
Scanning electron microscopy with energy-dispersive X-ray
spectroscopy (SEM-EDS) analyses have been carried out using a FEI
Quanta 400 scanning electron microscope (Hillsboro, OR) equipped
with a Thermo NSS-UPS-SEM-INORAN System SIX (Thermo Fisher
Scientific, Waltham, MA) for X-ray microanalysis (EDS). For EDS
analysis, the samples were placed on a stub with carbon tape and
the SEM imaging was run at 15 kV. Aqueous solutions collected after
reactions were analyzed by an Optima 3000 DV inductively coupled
plasma atomic emission spectrometer (ICP-AES) (Acme Analytical
Laboratory). All samples for ICP-AES analysis were acidified with
5% Optima HNO3.
The stability issue of transition metal oxide materials in
hydrothermal media has been the concern plaguing the aqueous-
phase catalysis. Only few metal oxides (such as ZrO2, TiO2 and
␣-Al2O3) exhibit acceptable stabilities in certain biomass con-
version reactions in subcritical water [21–24]. Developing mixed
metal oxide catalysts could improve the stability of heteroge-
neous catalysts. For instance, perovskite materials, composed of
mixed transition metal ions with lanthanides, exhibit better stabil-
ities compared to single metal oxides in certain catalytic reactions
[25,26]. The redox properties of perovskite metal oxides catalysed
a variety of reactions including CO oxidation [27], CO2 reform-
ing [28–31], NO oxidation [32–34], and oxidation of hydrocarbons
[25,35–37]. Recently, Deng et al. found that the perovskite metal
oxides, such as LaMnO3, LaCoO3, LaFe1-xCuxO3, etc., were efficient
catalysts for the wet aerobic oxidation of lignin to aromatic aldehy-
des [38–40]. Escalona et al. used La1-xCexNiO3 to convert guaiacol
to cyclohexanol with a yield of 60% [41]. Li et al. [42] found that Cr
modified LaCo0.8Cu0.2O3, catalysed the production of furfural from
xylan at 160 ◦C. To the best of our knowledge, the application of
perovskite oxide catalysts in the LA production is rarely reported.
Herein, we report that the LaCoO3 perovskite oxide is an efficient
heterogeneous catalyst for the conversion of cellulosic biomass to
LA. For the first time, we found that the redox properties of the
LaCoOx catalyst, which is distinctly different from the acid/base
properties of the prevailing catalysts for the LA production, play
a central role in the conversion of a variety of C3–C6 aldose sugars
and their polysaccharides to LA in subcritical water.
2. Experimental
2.1. Materials
The following reagents and products were used as received:
d-(+)-xylose (99%), d-(+)-glucose (Reagent ACS Grade) and
microcrystalline cellulose were purchased from Acros Organics. l-
(+)-lactic acid (98%), lanthanum nitrate hexahydrate (98%), cobalt
nitrate hexahydrate (98%), fructose (99%), sucrose (99%), lan-
thanum(III) oxide (99.9%), cobalt (II, III) oxide (99.5%) and citric acid
(99%) were purchased from Sigma–Aldrich.
2.4. Catalytic reaction procedure
Reactions were carried out by suspending the catalyst in a solu-
tion of biomass substrates in DI water in a 100 mL stirred Parr
microreactor. In each reaction, a glass liner was used to prevent
reactants from contacting the metal reactor walls. The liner was
loaded and placed inside the reactor, and the reactor was charged
with 400 psi N2. The reactor was heated at a ramp rate of 10 ◦C/min
until the desired set temperature was reached. During the reaction,
mixing was achieved through an internal propeller operating at 750
RPM. Once the set temperature was attained, the reactor was held
at the set temperature for 1 h and then quenched in an ice bath for
fast cooling. The reactor was then immediately disassembled and
the solid residue was collected for calculation of mass conversion.
The liquid and solid fractions were separated using a centrifuge.
The liquid fraction was collected and used for subsequent chemical
analysis and the solid residues were dried over night at 110 ◦C. The
mass balance was calculated and ensured to be between 98% and
102%.
2.2. Catalyst preparation
In the typical synthesis, 0.2 mol La(NO3)3·6H2O and 0.2 mol
Co(NO3)2·6H2O were dissolved into 50 mL DI water and stirred for
10 min. Then, 0.6 mol citric acid was added into the above solu-
tion and stirred for another 30 min. After that, the solution was
placed into a wide mouth container and dried at 110 ◦C in an oven
overnight. The dried powder was collected and heated to 200 ◦C at
10 ◦C/min for 30 min, then heated to 900 ◦C with the temperature
ramp of 3 ◦C/min, and finally calcined at 900 ◦C for 4 h.
2.3. Catalyst characterization
A PANalytical X’Pert PRO diffractometer with graphite-filtered
Cu K␣ radiation was used to determine the crystalline phases of the
as-synthesized LaCoO3 powder over the 2ꢀ values from 10◦ to 90◦
with the step size of 0.02◦.
The conversion (X), solid residue yield (S.R.) and the product
yield C-% (Y) are calculated using the following equations:
H2 temperature-programmed reduction (TPR) and O2
temperature-programmed (TPO) on the catalysts were car-
ried out using a Micromeritics AutoChem II 2920Chemisorption
Analyzer. In a typical H2 TPR experiment, the catalyst was first
degassed in helium at 900 ◦C for 1 h, following which the temper-
ature was decreased to 30 ◦C in helium flow. Afterwards, the gas
ꢀ
ꢁ
mass of carbon in unreacted biomass
mass of carbon in total biomass
X = 1 −
× 100%
ꢀ
ꢁ
Weight of total dried solid residue − Weight of solid catalyst
S.R. =
× 100%
Initial weight of biomass
Please cite this article in press as: X. Yang, et al., Effect of redox properties of LaCoO3 perovskite catalyst on production of lactic acid
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3
mass of carbon in product
mass of carbon in total biomass
Y =
× 100%
2.5. Stability study
For the catalyst stability tests, the spent catalyst was collected
and dried overnight in an oven at 100 ◦C, and was further calcined in
air at 550 ◦C for 6 h before reuse. For each run, there was a ∼2% cat-
alyst weight loss during the collection and transfer of the catalyst.
The as-received regenerated catalysts were used for the subsequent
reaction tests without adding supplemental fresh catalyst for all the
catalyst stability tests. We kept the same amount of glucose load-
ing for the first 3 times reuses, while slightly lowered the feeding
amount of glucose to maintain the same molar ratio of glucose to
catalyst in the 4th and 5th runs as that in the initial run.
2.6. Chemical analysis/characterization of products
Fig. 1. XRD spectra of the LaCoO3 structures: (a) fresh synthesized sample; (b) after
1 h hydrothermal treatment; (c) after 1 h reaction. Reaction conditions: 1 mmol glu-
cose, 4 mmol catalyst, 20 g DI water, 200 ◦C, 400 psi initial pressure N2, 1 h. For
hydrothermal treatment, no glucose was added.
The aqueous phase samples after reaction were filtered through
a 0.45 micron syringe filter then diluted 10 times with the internal
standard solution (200 ppm benzoic acid solution). High perfor-
mance liquid chromatography (HPLC) was used for quantitative
chemical analysis using a Shimadzu HPLC LC-10A system equipped
with a UV–vis detector (Shimadzu SPD 10-AV) and a refractive
index detector (Shimadzu RID-10A). For analysis of organic acids
and reaction intermediates, the samples were separated in an
Aminex HPX-87H Ion Exclusion column (Bio-Rad), using 5 mM
H2SO4 as the mobile phase at a flow rate of 0.7 mL/min, and a col-
umn temperature of 55 ◦C. The wavelength of the UV–vis detector
was set at 208 nm and 290 nm.
Qualitative gas chromatography coupled with mass spectrome-
ter (GC–MS) analysis was performed for identification of unknown
aqueous-phase products. The silylation derivatization method was
reported before [14]. The derivatized samples were analysed by
the Agilent 6890 series GC–MS equipped with a DB5-MS column
(30 m × 0.25 mm ID, 0.25 um film thickness) and an Agilent 5973
mass selective detector. The column temperature was maintained
at 80 ◦C for 2 min then ramped at 10 ◦C/min to 260 ◦C and held at
260 ◦C for 2 min.
0.1% by adding the LaCoO3 catalyst. The amount of solid residue
decreased to 6.3% (weight percentage), which may be ascribed to
the lower production yield of HMF, a notorious humin precursor
in hydrothermal media. La2O3 and Co2O3 metal oxides with the
same moles of La and Co ions as those of the LaCoO3 catalyst were
used for comparison. The yield of LA over Co2O3 reached 31.3%,
which was just a little lower than that of LaCoO3. However, after
the first time usage, there was a 16.2% weight loss of Co2O3 due to
the Co leaching, while there was almost no weight loss of LaCoO3.
La2O3, a weak solid base catalyst, gave only 18.8% LA yield. Approx-
imately 20.0% of La2O3 dissolved in the hydrothermal media after
a one-time use. (Entries 2 and 3, Table 1) Compared to the similar
glucose oxidation reactions over CuO [15], the significant difference
between LaCoO3 and CuO is that no LA was produced over CuO in
pH-neutral aqueous solutions, even at elevated temperatures.
3.2. Reaction temperature effect on lactic acid yield
3. Results and discussion
The temperature dependence of glucose conversion with the
LaCoO3 catalyst was shown in Fig. 2. The conversion of glucose
3.1. Catalytic effect of LaCoO3 on glucose conversion
The crystalline rhombohedral perovskite structure of the as-
synthesized LaCoO3 catalyst was confirmed by the X-ray diffraction
characterization (Fig. 1a). The characteristic spectrum of the sam-
ple showed the diffraction lines located at 2ꢀ = 32.98◦, 33.38◦ and
47.58◦ (JCPDS data no. 48-0123). No peaks related to bulk Co3O4 or
La2O3 phases were detected.
Glucose conversions were compared with and without the
LaCoO3 catalyst at 200 ◦C in the pH neutral aqueous solutions
(Entries 1 and 4, Table 1). The catalytic effect of the LaCoO3 cat-
alyst was obvious. Without a catalyst, the yield of LA was only 0.9%,
while a 29.1% yield of 5-hydroxymethylfurfural (HMF) was formed
which might be attributed to the acidic property of hydrothermal
water [15]. Other carboxylic acids with short carbon chains like gly-
colic acid and acetic acid were produced with the total yield of less
than 5%. In contrast, with the LaCoO3 catalyst and under the oth-
erwise identical reaction conditions, the yield of LA reached 39.5%.
The carbon distribution of other acids also changed considerably:
more acetic acid, ∼4%, was produced compared to the catalyst-free
reaction and especially, the yield of pyruvic acid was noticeable at
2.7%. Meanwhile, the yield of HMF decreased significantly to only
Fig. 2. Effect of reaction temperature on the glucose conversion over the LaCoO3
catalyst in hydrothermal media. Reaction conditions: 1 mmol glucose, 0.5 mmol
catalyst, 20 g DI water, 400 psi initial pressure of N2, 1 h.
Please cite this article in press as: X. Yang, et al., Effect of redox properties of LaCoO3 perovskite catalyst on production of lactic acid
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Table 1
Conversions of different cellulosic biomass with and without the different catalystsa.
b
Entry
Carbon yields of aqueous products (%)
Substrate
Catalyst
S. R. (%)
Gluconic
Pyruvic
Glycolic
Lactic
Formic
Acetic
HMF
Furfural
1
2
3
4
5
6
7
8
Glucose
Glucose
Glucose
Glucose
Glucose
Fructose
Fructose
Glyceraldehyde
Glyceraldehyde
DHAc
/
22.2
25.0
12.8
6.3
17.0
16.6
10.4
39.0
26.4
34.1
29.3
21.2
5.3
0.9
0.1
0.5
0.0
0.0
0.3
0.0
–
–
–
–
–
0.4
0.2
0.3
2.7
0.1
0.2
1.4
0.6
2.1
0.6
0.4
0.0
0.4
0.0
0.5
1.8
4.2
3.2
5.7
2.3
1.7
2.3
3.2
4.1
0.0
1.6
2.3
5.1
0.8
3.3
0.9
18.8
31.3
39.5
14.3
9.8
19.8
28.9
52.6
31.9
29.8
6.7
1.9
1.1
1.5
1.1
2.7
2.4
0.6
2.3
3.0
1.4
0.8
1.6
1.2
3.6
2.4
0.0
2.8
1.4
3.9
0.2
3.4
2.6
3.2
5.9
0.8
4.5
0.6
3.4
3.6
0.8
29.1
1.2
1.4
0.1
2.4
23.1
1.4
–
–
–
–
–
1.1
0.3
0.5
0.4
0.5
1.1
0.6
–
–
–
–
32.4
0.5
3.0
0.1
La2O3
Co2O3
LaCoO3
Co(NO3)2
/
LaCoO3
/
LaCoO3
/
LaCoO3
/
LaCoO3
/
9
10
11
12
13
14d
15d
DHAc
Xylose
Xylose
Cellulose
Cellulose
–
0.2
1.6
37.9
4.6
24.9
–
24.7
0.3
9.8
13.6
LaCoO3
Reaction conditions: 1 mmol biomass substrate, 4 mmol catalyst, 20 g DI water, 200 ◦C, 400 psi initial pressure of N2, 1 h.
S.R. represents the mass yield of solid residue.
a
b
c
DHA represents 1,3 dihydroxyacetone.
Reaction temperature is 240 ◦C.
d
increased from 85% to 98% as the reaction temperature increased
(Figs. S1–S3). The trace amounts of gluconic and xylonic acids, the
characteristic intermediates produced from the partial oxidation
of glucose and xylose, respectively, were also observed. Other C3
and C4 acids, such as hydroxypropionic and hydroxybutyric acids,
were present as well. We found that the yield of LA from C5 xylose
were very close to that from C6 glucose under the same reaction
conditions. Most previous literature reports showed a retro–aldol
condensation reaction pathway over the base or Lewis acid catal-
ysed glucose conversion [14], in which one C6 glucose molecule
can be cleaved to form two C3 compounds, and theoretically the
yield of LA from C6 glucose can be twice higher than that from C5
xylose. The very similar LA yields from both glucose and xylose in
the current study suggested a possible new reaction pathway in
which retro–aldol condensation of the sugars is not the initial step
of the LA formation.
from 160 ◦C to 200 ◦C, while the yield of LA enhanced from 16.0% to
34.1% and the yield of glycolic acid also increased from 3.1% to 6.4%.
However, further increasing the temperature to 240 ◦C decreased
the yield of LA to 27.4%, which might be ascribed to the degrada-
tion of LA at higher temperatures in hydrothermal media. We thus
carried out a series of LA stability tests at different temperatures,
as shown in Table 2. To our surprise, in the presence of the LaCoO3
catalyst, 12% of LA was converted at 200 ◦C while the conversion of
LA only slightly increased to 17% at 240 ◦C, suggesting that LA was
relatively stable in subcritical water with the LaCoO3 catalyst at
<240 ◦C. Therefore, other side reactions might cause the decrease
of the LA yield from glucose as the temperature increased from
200 ◦C to 240 ◦C.
It is well known that catalytic conversion of cellulose is much
more challenging than converting water-soluble sugars with het-
erogeneous catalysts due to sluggish depolymerisation of cellulose.
However, a relatively high yield of LA was obtained from cellulose
at 240 ◦C using the LaCoO3 catalyst. In fact, the best yield of LA,
∼24.9%, in our study was higher than those using the solid Lewis
acid catalysts such as ZrW and AlW [12]. The promotion function
of LaCoO3 in the conversion of cellulose might be ascribed to the
lanthanide(III) ions that have a high affinity to oxygen contain-
ing functional groups in cellulose. Thus the lanthanide–saccharide
3.3. Catalytic effect of LaCoO3 on the conversion of other
cellulosic biomass
The catalytic conversions of other carbohydrate biomass feed-
stocks, including C3 aldose (glyceraldehyde), C5 aldose (xylose) and
polysaccharide (cellulose), were also performed using the LaCoO3
catalyst, as shown in Entries 8, 9, 12–15, Table 1. Both C3 and C5
sugars can be converted to LA as the main product with the yields of
52.6% and 37.9% respectively, together with a considerable amount
of by-products including pyruvic acid, glycolic acid, acetic acid and
formic acid, which were confirmed by the GC/MS characterization
Table 2
Conversion of lactic acid (LA) and gluconic acid (GA) under the different reaction conditions with and without the LaCoO3 catalysta.
Entry
Carbon yields of aqueous products (C-%)
Substrate
Catalyst
Atmosphere
Temp.(C) Conv.(%)
Pyruvic
Glycolic
Lactic
Formic
Acetic
Acrylic
HMF
1
2
3
4
5
6
7
8
LA
LA
LA
LA
LA
LA
LA
LA
LA
LA
GA
GA
/
N2
N2
N2
N2
N2
N2
10% O2
10% O2
100% O2
100% O2
N2
160
160
200
200
240
240
160
160
240
240
160
160
1.6
12.1
3.3
12.3
2.0
16.7
2.2
0.3
0.0
0.4
0.0
0.6
1.2
1.1
0.8
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.8
–
–
–
–
–
–
–
–
–
–
0.4
5.2
0.1
0.4
0.5
0.2
0.5
0.0
0.2
0.2
0.0
0.0
0.4
1.0
0.1
2.5
0.2
2.8
0.0
2.8
0.8
4.6
0.0
45.3
1.6
4.2
0.0
0.0
0.1
0.0
0.4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
–
–
–
–
–
–
–
–
–
–
0.3
0.1
LaCoO3
/
LaCoO3
/
LaCoO3
/
LaCoO3
/
16.6
9
100
100
3.3
13.9
10
11
12
LaCoO3
/
LaCoO3
N2
a
Reaction conditions: 1 mmol biomass substrate, 0.5 mmol catalyst, 20 g DI water, 400 psi initial N2 pressure, 1 h.
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complexes formed between the hydrated lanthanide ions and the
sugar units may facilitate the depolymerization of cellulose [43,44].
Furthermore, it is interesting to find that C6 fructose, an iso-
mer of glucose, produced a much lower amount of LA than glucose
over the LaCoO3 catalyst. (Entries 6–7, Table 1) Similarly, the C3
ketose (1,3 dihydroxyacetone), the isomer of glyceraldehyde, pro-
duced a similar amount of LA over the LaCoO3 catalyst than without
catalyst, suggesting LaCoO3 did not catalyze the conversion of dihy-
droxyacetone to LA. (Entries 10–11, Table 1) Note that with Lewis
acid or base catalysts, ketoses, which are prone to retro–aldol con-
densation, are the key intermediates in the production of LA from
glucose or xylose [13,14]. However, in this study, the LaCoO3 cat-
alyst seemed to poorly catalyze the conversion of ketoses to LA.
These results motivated us to explore the different reaction mech-
anism to produce LA from sugars with the LaCoO3 catalyst.
3.4. Redox reaction mechanism
Fig. 3. FTIR spectra of adsorbed pyridine on LaCoO3 and Co2O3 materials.
Thus far, many researchers have studied the reaction pathways
of the formation of LA from sugars under hydrothermal conditions
[9,10,15,45–47]. It is widely accepted that either base or Lewis acid
catalysts are able to catalyse the isomerization of aldoses to ketoses
and then the retro–aldol condensation to lower-molecular weight
compounds. For instance, our previous study found that the ZrO2
catalyst containing Lewis acid/base sites promoted the retro–aldol
condensation of xylose, as well as the intramolecular Cannizzaro
reaction of pyruvaldehyde, leading to the formation of LA [14].
In contrast, although the LA yield over LaCoO3 is comparable
to those over Lewis acidic zeotype catalysts [13], the Lewis acidic
property of the LaCoO3 catalyst was ruled out according to the pyri-
dine desorption FTIR spectra as shown in Fig. 3. Moreover, the NH3
TPD result (Fig. S4) also confirmed that the LaCoO3 did not show
Brønsted acidic property either. The basicity of the LaCoO3 catalyst
was negligible, as seen in the CO2 TPD curve (Fig. S5). Instead, the
redox properties of the LaCoO3 were dominant. According to the
TPR analysis, the hydrogen intake profile of LaCoO3 (Fig. 4a) dis-
played two peaks between 300 and 500 ◦C and 500–700 ◦C which
suggested a two-step reduction process: the reduction of Co3+ to
Co2+ at 300–500 ◦C and the further reduction of Co2+ to Co0 at ∼
600 ◦C. Reverse transformation of Co ion states was observed by the
TPO analysis (Fig. 4b), in which the peaks at 200–400 ◦C indicate the
transition of Co0 to Co2+ and peaks at 700–800 ◦C show the oxida-
tion from Co2+ to Co3+. The results are coincident with literature
reports on the characterization of LaCoO3 in liquid phase reaction
[48]. The TPR profile of Co2O3 shows three main peaks with the
maxima at ∼360, 420 and 480 ◦C, indicating the reduction of Co3+
to Co2+, coexisting Co2+ and Co3+ ions as in Co3O4, and the further
reduction of Co2+ to Co0 at ∼ 500 ◦C. The TPO analysis correspond-
ingly showed three main peaks with the maxima at ∼390, 510 and
550 ◦C, indicating the transition of Co0 to Co2+, coexisting Co2+ and
Co3+ ions, and the oxidation of Co2+ back to Co3+. The results show
that the H2 uptake capacity of Co2O3 was less than that of LaCoO3
on the basis of the same moles of Co ions in both materials, which
may explain why lower amounts of LA from glucose were produced
over Co2O3 than over LaCoO3 at the same Co loading.
To clarify the possible redox role of LaCoO3 during the con-
version of glucose to LA, the effect of different types of reaction
atmospheres on the LA yield was further investigated. We found
that the LA yield decreased significantly from 16.0% to 4.1% by
increasing the O2 partial pressure from 0% to ∼5%, while at the same
time the glycolic acid and formic acid yields increased significantly,
as shown in Fig. 5a. Further increasing the O2 partial pressure con-
tinually decreased the LA yields. Under the 100% O2 atmosphere,
only a negligible amount of LA was synthesized, compared to the
yields of 22.7% glycolic acid, 8.7% formic acid and 6.9% acetic acid.
On the other hand, without the LaCoO3 catalyst, regardless of the O2
partial pressure, LA was formed only in trace amounts. Using xylose
as the feedstock, we also observed the same trend, as shown in
Fig. 5b. LA was thus used as the probe to test its own stability under a
wide range of O2 partial pressures, as seen in Table 2. At 240 ◦C, LA is
relatively stable under N2 atmosphere and ∼17% LA was converted
over the LaCoO3 catalyst. In contrast, under 100% O2 atmosphere,
Fig. 4. (a) H2 temperature programmed reduction (TPR) and (b) O2 temperature programmed oxidation (TPO) of LaCoO3 (solid line) and Co2O3 (dot line) materials.
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Fig. 5. Effect of O2 partial pressure on the conversion of (a) glucose and (b) xylose over the LaCoO3 catalyst. Reaction conditions: 1 mmol glucose or xylose, 2 mmol catalyst,
20 g DI water, 160 ◦C, 400 psi initial pressure, 1 h.
LA was fully converted and 45% acetic acid was produced over the
LaCoO3 catalyst. Very interestingly, adding a small amount of H2 in
N2 atmosphere seemed to have a minor promotional effect, or at
least no inhibition effect like O2, on the conversion of glucose to LA
in the presence of high loadings of the LaCoO3 catalyst. Compared
to the pure N2 atmosphere, the LA yield slightly changed from 39.5%
to 40.1% with adding 5.5% of H2 into N2. The conversion of glucose
to LA over the LaCoO3 catalyst might proceed through an initial step
of partial oxidation of glucose which was performed better in N2
than in O2. Under the pure N2 atmosphere, LaCoO3 is the possible
weak oxidant that selectively oxidized glucose.
We thus propose
a possible redox reaction mechanism
(Scheme 1) with the hypothesis that oxygen for the oxidation reac-
tions during the conversion of glucose to LA might be stemmed
from the lattice oxygen atoms in the LaCoO3 perovskite structure,
similar to the classical Mars and Van Krevelen (MvK) mechanism,
i.e., lattice oxygen of metal oxide takes part in oxidation reac-
tions, which is widely accepted in gas-phase oxidation processes
but rarely reported in aqueous-phase oxidation processes [49,50].
Here, the oxygen atoms in the lattice of LaCoOx might participate
in the redox cycles of the transformation of glucose and its derived
intermediates. For instance, LaCoO3 firstly oxidizes the aldehyde
group in glucose to form gluconic acid, which is different from
isomerization and retro–aldo condensation as the initial step of
acid/base catalysed glucose conversion reactions [14]. The catalyst
itself is reduced from LaCoO3 to LaCoO2.5 and the correspond-
ing cobalt valence states changed from Co3+ to Co2+. Afterwards,
gluconic acid undergoes an oxidative decarboxylation [51] and is
transformed to xylose, which was detected by the GC–MS (Fig. S1b).
Xylose repeats the oxidation step to form xylonic acid and then
undergoes oxidative decarboxylation to produce C4 aldose, which
is then oxidized to hydroxybutyric acid. Finally hydroxypropionic
acid is formed by the alternate oxidative decarboxylation and oxi-
dation. Hydroxypropinoic acid can continue to react in the same
fashion to further produce glycolic acid and then formic acid, espe-
cially with the presence of strong oxidants like O2, as we observed
that under higher O2 partial pressures, more glycolic and formic
acids, rather than LA, were produced from glucose. On the other
hand, hydroxypropinoic acid can also undergo a dehydration reac-
tion at elevated temperatures to produce pyruvic acid, which was
also detected by the GC–MS. It is possible that over the reduced
perovskite structure, LaCoO2.5, pyruvic acid is reduced to LA. Mean-
while, Co2+ is oxidised back to Co3+ and complete the catalytic cycle
of Co3+ → Co2+ → Co3+, while the perovskite structure is unchanged.
Fig. 6. Effect of catalyst loading amount on the product yields of the glucose conver-
sion over the LaCoO3 catalyst. Reaction conditions: 1 mmol glucose, 20 g DI water,
200 ◦C, 400 psi initial pressure of N2, 1 h.
To test our hypothesis that LaCoO3 serves as the weak oxidant,
we examined the effects of different catalyst loading amounts on
the LA yield. As shown in Fig. 6, by increasing the molar ratio of
LaCoO3 to glucose from 0.5:1 to 4:1, the LA yield increased from
∼33% to ∼40%. However, as the catalyst to biomass molar ratio
further increased to 10:1, the LA yield decreased significantly to
∼32%. These results reasonably support our hypothesis: stoichio-
metrically one mole of LaCoO3 only provides 0.5 moles of oxygen,
but there are multiple oxidation steps during the conversion of glu-
cose to LA, and thus excess catalyst loadings are needed to achieve
a high yield of LA. However, as the molar ratio of catalyst to glu-
cose was too high, the excess LaCoO3 might prevent LaCoO2.5 from
reducing pyruvic acid, the key step to produce LA in our proposed
mechanism.
To further illustrate this key redox step, pyruvic acid was used as
a probe reactant to find out whether it would be reduced over the
LaCoO3 catalyst which was pre-reduced with the H2 flow. We found
that in the pure N2 atmosphere, a yield of 16.5% LA was obtained
from pyruvic acid without adding any catalyst at 200 ◦C, while 21.3%
glycolic acid was the dominant product. In contrast, 36.8% LA was
produced as the dominant product over the pre-reduced LaCoO3
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Scheme 1. Proposed redox reaction pathway for the formation of lactic acid from glucose with the LaCoO3 catalyst.
catalyst. In addition, when supplementary H2 was added into the
2 atmosphere, a 56.9% LA yield was obtained, as shown in Table 3.
ble products compared to other C1 and C2 acids. However, this
proposed redox reaction mechanism well explained why the pro-
duction yields of LA from xylose were nearly the same as those
from glucose. On the other hand, ketose such as fructose, which
possesses the end ketone group instead of the end aldehyde group,
is recalcitrant to the oxidative decarboxylation and thus hardly
produces as a high amount of LA as glucose over the LaCoO3 catalyst.
N
This is in contrast to the conversion of pyruvic acid to LA with the
fully oxidized LaCoO3, over which only a 3.6% LA yield was obtained.
We also used gluconic acid as the probe reactant to validate the
proposed reaction mechanism. At 200 ◦C, 14% gluconic acid was
converted, with LA and acetic acid as the dominant products (Entry
12, Table 2). The probe reaction results suggest that the proposed
reaction pathway is reasonable although the actual chemistry and
kinetics could be much more complex in this reaction system. Simi-
larly, other aldoses such as xylose undergo the same redox cycles to
produce LA. The stepwise oxidative decarboxylation of the aldoses,
with the different carbon numbers ranging from six to four, peels
off the carbon in the end aldehyde group to form CO2. According
to our proposed redox reaction mechanism, ideally, 1 mmol of glu-
cose will produce 1 mmol of LA and 3 mmol of CO2. In the typical
glucose conversion experiment, 3.12 mmol of CO2 were produced
from 1 mmol of glucose, as seen in Fig. S6. The carbon balance of this
reaction was close to 100%. As this reaction proceeds, CO2 would
be the only end product. Indeed, it is unclear why LA is more sta-
3.5. Stability of LaCoO3 catalyst
We found that the stability of either La2O3 or Co2O3 was poor
under hydrothermal conditions, while the perovskite structure of
LaCoO3 stabilized La and Co ions to some extent. Our catalyst sta-
bility was examined by five consecutive usages of the fresh and
spent LaCoO3 catalysts for the conversion of glucose to LA. The
spent catalyst after each use was regenerated by calcination in
air at 550 ◦C for 6 h in order to remove any biomass derived solid
residues. Fig. 7 shows that the yield of LA decreased from 39.5% to
33.3% after the first usage. The activity of the spent LaCoO3 cata-
lyst was restored after the regeneration and remained fairly stable
Table 3
Conversion of pyruvic acid with and without the different LaCoO3 catalysts under inert or reductive atmospheres.
Entry
Catalyst
B/Cb
Atm.
Carbon yields of aqueous products (C-%)a
Glycolic
Lactic
Formic
Acetic
1
2
3
4
5
–
–
N2
N2
N2
21.3
3.8
24.5
19.7
0.0
16.5
36.8
3.6
17.2
56.9
0.8
0.1
0.0
0.7
0.0
2.5
3.2
1.6
3.3
4.8
R. LaCoO3
O. LaCoO3
–
1/2
1/2
–
c
c
H2 and N2
H2 and N2
R. LaCoO3
1/2
a
Reaction conditions: 1 mmol pyruvic acid, 2 mmol pre-reduced (R.) or fully oxidized (O.) LaCoO3 catalyst, 200 ◦C, 400 psi initial pressure, 1 h.
B/C represents the mole ratio of biomass substrate to catalyst.
A mixture of H2 and N2 atmosphere with 2 mmol H2.
b
c
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∼40% LA using the LaCoO3 catalyst. Therefore the leached Co ions
did not dominantly catalyse the LA formation during the catalytic
conversion of sugars in the presence of the LaCoO3.
The changes in particle size, surface composition, and sur-
face area were dynamic in the reactive environment. We found
the mean crystallite size of LaCoO3 decreased from ∼77.2 nm to
∼48.3 nm (caculated from the XRD spectra) after the 1st reaction.
The decrease of the size could certainly increase the suface area.
However, since the BET surface area of the fresh LaCoO3 mate-
rial was already rather low (Table S1), the effect of surface area
change might be minimal. Note that the LA yield decreased from
39.5% to 33.3% after the 1st use and then remained stable through-
out the subsequent re-use of the spent catalyst, as shown in Fig. 7,
suggesting that the dominant factor might be leaching instead of
the changes in particle size or surface area and leaching might be
reversible under the reaction condition.
4. Conclusions
Fig. 7. Stability of the LaCoO3 catalyst in the production of lactic acid from glucose
in hydrothermal media. Reaction conditions: 1 mmol glucose, 4 mmol catalyst, 20 g
DI water, 200 ◦C, 400 psi initial pressure of N2, 1 h. For the 4th and 5th reactions, the
feed amounts of glucose were slightly adjusted in order to keep the same ratio of
catalyst to glucose.
In summary, we demonstrate that the LaCoO3 perovskite cat-
alyst can effectively catalyse the conversion of cellulosic biomass
to lactic acid in hydrothermal media. The reaction temperature,
gas atmosphere, and catalyst loading had profound effects on the
production yield of lactic acid from glucose. C5 to C6 aldose sug-
ars including xylose and glucose yielded a similar amount of lactic
acid, up to ∼40%, at 200◦C and the initial pressure of 400 psi N2,
while the highest yield of lactic acid from cellulose was ∼24% at
240◦C, in hydrothermal media. Much lower amounts of HMF and
humin products were obtained from glucose with the LaCoO3 cat-
alyst. A possible redox reaction mechanism, which involved the
key steps such as the oxidative decarboxylation of aldose sugars
and the reduction of pyruvic acid with the aid of the Co3+ and Co2+
redox cycles in the LaCoO3 perovskite structure, was proposed for
the conversion of aldose sugars to lactic acid. Unlike Lewis acid or
base catalysed reactions in which the production of lactic acid was
initialized by the retro–aldol condensation of the sugars, the lat-
tice oxygen atoms in the LaCoO3 perovskite metal oxides acted as
the weak oxidants in the selective conversions of aldoses to lactic
acid under inert or even mild reductive atmosphere. The leach-
ing of LaCoO3 occurred during the glucose conversion reactions in
hydrothermal media but the catalyst was found to be relatively
stable in the short term.
through the following usages after the first reuse in the hydrother-
mal reaction system. In contrast, the yield of LA decreased to less
than 20% with the un-regenerated catalyst. During the reaction, the
LaCoO3 catalyst acted as an oxidant initially; after reaction, it was
mostly reduced while only part of LaCoO3 were restored to the oxi-
dized state by the reduction of pyruvic acid. That is why the catalyst
activity is low if we reuse the spent LaCoO3 catalyst without regen-
eration. However, after being calcined in air, the spent catalyst was
fully oxidized again and thus the catalytic activity was restored.
Hence, unlike other catalysts, LaCoO3 in this reaction is similar to a
recyclable solid reactant.
The XRD analysis shows that after the 1-h treatment in the pH
neutral hydrothermal medium at 200 ◦C, the crystalline structure
of LaCoO3 was almost unchanged (Fig. 1b). However, after the 1-h
glucose conversion reaction, a slight collapse of crystalline struc-
ture of the LaCoO3 catalyst was observed, as shown in Fig. 1c. The
pH of the aqueous solution decreased to 3.75 after the reaction due
to the formation of carboxylic acids, which might accelerate the
leaching of metal ions. Elemental analysis by EDS confirmed that a
small amount of Co ions in the LaCoO3 was leached out after the
reaction compared with the fresh sample, as shown in Fig. S7. Addi-
tionally, the leaching of Co and La during the typical reaction (after
the first use of the LaCoO3 catalyst in Fig. 7) was characterized by
the inductively coupled plasma atomic emission spectroscopy (ICP-
AES). Approximately 2.4 mol% Co ions and 1.5 mol% La ions were
leached out from the LaCoO3 perovskite catalyst after the reaction.
Yet the Co and La ions leached out from the Co2O3 and La2O3 cat-
alysts were ∼11.4 mol% and ∼15.0 mol% respectively in the control
reactions under the otherwise identical conditions. Although the
structure of perovskite could not completely prevent the leaching
of Co and La ions, it significantly retarded the leaching process.
The leached Co ions might be the homogeneous Lewis acid
which could potentially catalyse the production of LA from sug-
ars. The comparative tests using Co2+ as catalyst were thus carried
out to determine whether or not the catalytic performance was
stemmed from the leached Co ions. Different product distribu-
tions and LA yields were observed when using the equal moles
of Co(NO3)2 and LaCoO3 as the catalysts, respectively (Entry 5,
Table 1). The Co2+ ion promoted the dehydration of glucose as
a noticeable yield of HMF (2.4%) was produced which was not
observed by using the LaCoO3 catalyst. Yet the yield of LA with the
Co(NO3)2 catalyst was ∼14% which was much lower than that of
Acknowledgements
We gratefully thank Prof. Glenn Miller for the useful discussion,
Dr. Stevens Spain at the Shared Instrumentation Laboratory (SIL)
of Chemistry department at University of Nevada Reno for the help
on chemical analysis, Prof. Wei Fan at University of Massachusetts
Amherst for the help on FTIR characterization, Dr. Jingji Zhang for
useful discussions on the catalyst synthesis, and Mr. Reed Nicholas
Liu for proofreading. This material is based upon work supported
by the National Science Foundation under Grant no. CBET 1337017.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.12.
003.
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Please cite this article in press as: X. Yang, et al., Effect of redox properties of LaCoO3 perovskite catalyst on production of lactic acid
from cellulosic biomass, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2015.12.003