CuO/CeO2 catalysts for glycerol selective conversion to lactic acid

Article abstract of DOI:10.1039/c7dt04340f

Ceria supported copper oxide catalysts were produced by a deposition-precipitation method, at a high copper loading (up to >25 wt%). These materials demonstrated excellent properties for glycerol selective conversion to lactic acid, with a conversion reaching up to 87% with a selectivity to lactic acid of 74% (8 h reaction, 220 °C, under N₂ pressure). These catalysts also exhibited high stability upon 5 successive reaction cycles. The formation of a crystalline CuO phase was demonstrated in the nanocomposites at a high Cu loading, with elongated shaped particles formed on the cerium oxide surface. Such particles were however, not observed at low Cu loadings. XPS analysis revealed that Cu(ii) was the main Cu species on the fresh catalyst, and that this species was reduced to Cu(i) during the reaction. Complementary characterization over the spent catalyst clearly showed the morphological modifications of the CuO phase, however, did not impact significantly either glycerol conversion or selectivity to lactic acid upon recycling. For instance, apparently, the catalytic activity of CuO largely depends on the Cu(ii) species.

Full text of DOI:10.1039/c7dt04340f

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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CuO/CeO2 catalyst for glycerol selective conversion to lactic acid
Ruben Palacio,a Sebastian Torres,a Sébastien Royer,b Anne Sophie Mamede,b Diana Lópeza and
Diana Hernándeza,†
Received 00th January 20xx,
Accepted 00th January 20xx
Ceria supported copper oxide catalysts were produced by deposition-precipitation method, at high copper loading (up to >
25 wt%). These materials demonstrated excellent properties for the glycerol selective conversion to lactic acid, with
conversion reaching up to 87 % with a selectivity to lactic acid of 74 % (8 h reaction, 220 oC, under N2 pressure). These
catalysts also exhibited high stability upon 5 successive reaction cycles. The formation of crystalline CuO phase was
demonstrated in the nanocomposites at high Cu loading, with elongated shaped particles formed on cerium oxide surface.
Such particles were however, not observed at low Cu loadings. XPS analysis revealed that Cu(II) was the main Cu species
on fresh catalyst, and that this species was reduced to Cu(I) during the reaction. Complementary characterization over the
spent catalyst clearly showed morphological modifications of the CuO phase, however not impacting significantly either
glycerol conversion or selectivity to lactic acid upon recycling. For instance, the catalytic activity of CuO apparently largely
depends on the Cu(II) species.
DOI: 10.1039/x0xx00000x
www.rsc.org/
has been of great interest in the last decades 13
.
Introduction
One of these alternative production routes is the hydrothermal
conversion of glycerol to lactic acid which has been reported using
homogeneous catalysts 14-16. Therefore, the valorization of glycerol
2-hydroxypropanoic acid, also named as lactic acid, is a platform
molecule from which valuable industrial oxygenated compounds
such as acrylic and pyruvic acids, 1,2-propanediol, 2,3-
pentanedione, acrylate esters, lactate esters or linear polyesters,
can be produced 1-9. Lactic acid can also be used for obtaining
polylactic acid, which is currently widely used for producing
biodegradable polymers and plastics. Lactic acid is actually obtained
from chemical syntheses or fermentation processes 10, the latter
being the most widespread method for producing optically pure L-
(+)- or D-(-)-lactic acid 2, 11, 12. However, fermentation process shows
several drawbacks because of long-time production and process
operation at nearly neutral pH (pH between 5 and 7). For instance,
it requires the addition of calcium hydroxide or carbonate in order
to maintain the pH of the mixture at the optimal conditions.
Consequently, in the final stage of the production process
neutralization is required (using H2SO4) generating wastes. These
to lactic acid production, is of industrial interest since glycerol is the
14,
main sub-product of biodiesel production
17-22. During recent
years, worldwide production of biodiesel has considerably
21,
increased
23, leading to an important overproduction of crude
glycerol, with inherent high cost for its purification. The worldwide
production of biodiesel is evaluated at 37 billion gallons in 2016. For
24
instance, 4 billion gallons of crude glycerol are generated , and
volume is going to increase in parallel with the worldwide
production of biodiesel. This evolution stimulates research around
glycerol chemical transformation to produce high-added value
14, 17-20
chemicals or even energetic/fuel molecules
. The
hydrothermal conversion of glycerol to lactic acid is performed
using highly concentrated base, commonly NaOH, at reaction
temperatures between 250 and 300 oC 14-16. Such harsh reaction
conditions, i.e. high temperature and strong basic medium (KOH or
NaOH), induce a poor selectivity control during reaction in addition
to the need of severe process constraints due to the highly
corrosive reaction medium and waste post-treatment storage and
treatment. Such conditions are consequently not the more
appropriate for a sustainable production of valuable molecules 14, 16,
1,
7
constraints make the recovery of the final product expensive
.
Investigation of alternative chemical routes to produce lactic acid
25
.
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Recently, heterogeneous catalysts were proposed for the glycerol acid selectivity of 85.6 % (reaction performed at 140 C, 1.4 MPa of
to lactic acid reaction 26, 27. Noble metal containing materials were N2, with NaOH to Glycerol molar ratio of 1.1) 37. Better activities are
evidenced to effectively catalyze the selective conversion of obtained, with palladium-copper formulation (CuPd/rGO), with
glycerol into lactic acid. Glycerol hydrogenolysis, either under H2- conversion of glycerol to 53.1 % after 16 h reaction, but with no
free or inert atmosphere, is one of the most studied synthetic improvement in lactic acid selectivity. Recently, Co3O4/CeO2
strategies. When using monometallic Ru, Pt particles 26 or bimetallic catalysts were also proposed for the selective conversion of glycerol
PtRu, AuRu over carbon 27, and in presence of NaOH, selectivity into lactic acid (250 oC and 60 bar of N2 38. Co3O4/CeO2 catalysts
)
toward lactic acid reaches up to 62 % at 25 % glycerol conversion on present good activity, with >70 % conversion reached after 8 h
Pt/C at 200 oC and under 40 bar of H2. Good selectivity and activity reaction, while the highest selectivity in LA is reported at 79.8 %
are also reported for supported AuRu catalyst (selectivity toward over the catalyst prepared using the impregnation method.
lactic acid of 60 % at 21 % conversion). Dispersing platinum over However, these catalysts demonstrated poor stability in reaction,
basic support, like with Pt/CaCO3, reaches up to 55 % lactic acid
selectivity at 46 % glycerol conversion 28. Therefore, titanium oxide
and zirconium oxide are also reported as potential supports for
platinum, obtaining high selectivity to lactic acid (reaction
performed at 180 oC and at a NaOH to Glycerol molar ratio of 1.8)
29. Iridium based catalysts, Ir/C, are also proposed for the glycerol
conversion in alkaline medium (1 M NaOH, 180 °C). Use of Ir/C
allows to produce both 1,2-propanediol and lactic acid. The yield
can be modulated by controlling reaction atmosphere from inert
(He) to reducing (H2) 4, 30. Yields of 48 % in lactic acid (under He) and
63 % in 1,2-propanediol (under H2) are reached, at glycerol
conversion of 90 %. ZnO-supported Pt, Pd, Rh or Au are also
proposed as catalysts for this reaction. A most efficient system
(Rh/ZnO) in alkaline medium and under H2 atmosphere, presents
lactic acid selectivity up to 68 % at ~100% glycerol conversion
(reaction performed at 200 °C, under 20 bar of H2, and basic
conditions) 31. Under base-free medium, lactic acid selectivity up to
66.6 % at 13.6 % glycerol conversion is reported when using
bimetallic AuPd particles supported on TiO2 under acidic conditions
(160 oC, under 1 MPa O2 pressure) 32. With metal-acidic bifunctional
catalyst, Pt/Sn supported on MFI-type zeolite, lactic acid selectivity
of 80.5 % at 89.8 % glycerol conversion is reported (100 °C, under
with an activity loss of ∼45 % after three catalytic cycles.
In this work, CuO/CeO2 composites are prepared using deposition-
precipitation of copper precursor over preformed ceria, at different
copper loadings. Properties of composites are evaluated for the
selective conversion of glycerol into lactic acid, in alkaline media.
Extended characterization of fresh catalysts and of post-catalytic
reaction highlight significant modification upon reaction of the
copper active phase. Despite material evolution, catalyst presents
excellent stability upon reaction cycling.
Experimental section
CeO2 synthesis
Ceria support is prepared by precipitation. For this purpose, 9.03 g
of Ce(NO3)·6H2O are dissolved in 240 mL of ultrapure H2O, then
19.2 g of NaOH are added, and the resulting mixture was stirred
during 1 h at 20 °C. The mixture is thereafter transferred into a
Teflon-lined stainless-steel autoclave for ageing at 100 oC during 14
h. The precipitate is recovered by filtration, extensive washing with
ultrapure H2O, before being dried at 80 °C for 24 h.
0.62 MPa of O2 pressure) 33
.
CuO/CeO2 synthesis
39, 40
Following a similar procedure as described in previous works
,
Literature dealing with the selective conversion of glycerol into
lactic acid over non-noble metal containing catalysts is, in contrast,
3.0 g of as-synthesized CeO2 powder is dispersed in 25 mL of
ultrapure water. The pH of the suspension is adjusted to 9.0 by
adding 0.25 M Na2CO3 solution. Thereafter, 10 mL of a solution
containing dissolved Cu(NO3)2·3H2O precursor, at a concentration
adjusted to obtain the desired Cu loading, is added dropwise
simultaneously with Na2CO3 solution to maintain the pH at 9.0. The
resulting suspension is stirred at room temperature for 1.5 h, and
the precipitate is recovered by filtration, washing with ultrapure
H2O and air-drying at 80 oC. After drying, the powder is calcined at
400 oC for 4 h (applied temperature increase rate of 2 oC min-1).
Samples are denoted: XCuO/CeO2, where X indicates the metal
loading, as determined using atomic absorption spectroscopy.
34-
relatively limited. Reaction was studied over Cu-based catalysts
36. When using supported Cu, CuO or bulk Cu2O (at 240 oC, under 14
bar of N2 pressure, with NaOH to Glycerol molar ratio of 1.1), high
lactic acid selectivity is obtained, 78-80 %, but with conversion
significantly impacted by the nature of the support (75 % over silica,
98 % over Al2O3) 34. Authors evidenced that oxide phases (CuO and
Cu2O) are more efficient than metallic copper for the glycerol
conversion. Despite the excellent catalytic properties for these
systems, stability of the silica and alumina supports in alkaline
conditions is not adequate for long term reaction. MgO and
hydroxyapatite (HAP) are consequently proposed as potential
support for metallic copper, due to their better stability under
alkaline conditions 36. Cu/HAP displays highest selectivity toward
lactic acid (up to 90 %) at high glycerol conversion (91 %) when
reaction is performed at 230 oC and under N2 atmosphere.
Conversion of glycerol over reduced graphene oxide supported
copper (Cu/rGO) reached only 9.6 % after 6 h reaction with a lactic
Bulk CuO and Cu2O synthesis
For comparison, bulk CuO and Cu2O were synthesized, materials
41
were synthesized from modified reported works for CuO
and
of
42
Cu2O
. Briefly, CuO was obtained by dissolving 3.01 g
Cu(NO3)2⋅3H2O in 120 mL of deionized water. Then, 72 mL of NH3
solution were added. The resulting mixture was transferred into a
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o
Teflon-lined stainless-steel autoclave for ageing at 130 C during 14 pressure is immediately raised to 62 bar with N2, and the stirring
h. For Cu2O, 80.57 g of polyvinylpyrrolidone were dissolved in 90 mL was settled to 650 rpm (initial time for reaction, to). Samples for
polyethylene glycol at 40 oC. Thereafter, 10 mL of a solution analysis are taken out periodically during the catalytic reaction
(0.35 mL for each sample), and immediately quenched with 1.0 mL
H2SO4 aqueous solution (0.5 M). Isopropanol is added as internal
standard (0.05 mL). Samples are filtered, and reaction products are
analyzed by HPLC (Agilent 1200 series chromatograph) using a
refractive index (RI) detector, equipped with an ICSep ICE-COREGEL-
containing 4.54 g of Cu(NO3)2⋅3H2O were added and the mixture
was heated at 180 oC with a reflux system and stirred for 8 h. The
powder was recovered by centrifugation and washed with
deionized water.
87H3 column (mobile phase 0.005 M H2SO4, flow 0.5 mL min-1
-
Characterization
Tcolumn = 70 oC). Periodically (usually after 3 h reaction), gas phase
samples are analyzed. Gas phase analysis is performed by GC, using
an Agilent Technologies instrument model 6890N, equipped with a
TCD and a FID detector, and using a Carboxen 1010 PLOT fused
silica capillary column (30 m X 0.53mm) and an Agilent DB-PETRO
column (50 m length, 0.2 mm diameter) for product separation.
The copper loading in XCuO/CeO2 composites is determined by
Atomic Absorption Spectroscopy using a Thermo instrument 3000
series. For analysis, composites are dissolved in aqua regia.
The specific surface areas are determined using N2 physisorption at
77 K, using a Micromeritics ASAP 2020 instrument. Before analysis,
the samples are heated under vacuum at 250 oC during 12 h. The
Brunauer-Emmett-Teller (B.E.T.) method is applied to determine
the specific surface area, in P/P0 range of 0.05 to 0.35.
The conversion of glycerol (GLY) is calculated according to Equation
1:
X
GLY (%) = ((nGLY, in - nGLY, out)/nGLY, in) x 100
(1)
XRD patterns are collected on a Panalytical, Empyrean Serie 2
diffractometer operating at 40 kV and 40 mA equipped with a solid
state Pixcel-3D detector. Scans are recorded for 2θ ranging from 10°
to 90°, with a step size of 0.01° and a step time of 10 s. Phase
identification is performed by comparison with references from
JCPDS database.
Where nGLY, in and nGLY, out are the GLY initial number of moles and
the residual GLY number of moles at time t, respectively.
The selectivity is calculated using Equation 2:
Sproduct, i (%) = (nproduct, i /(nGLY, in - nGLY, out)) x 100 (2)
The XPS analyses were performed using a Kratos Analytical AXIS
UltraDLD spectrometer. A monochromatic aluminium source (Al Kα =
1486.6 eV) was used for excitation. The analyser was operated in
constant pass energy of 40 eV using an analysis area of
approximately 700 μm x 300 μm. Charge compensation was applied
to compensate for the charging effect occurring during the analysis.
The adventitious C1s (285.0 eV) binding energy (BE) was used as
internal reference. Quantification and simulation of the
experimental photopeaks were carried out using CasaXPS software.
Quantification took into account a non-linear Shirley background
Where nproduct, and nGLY, and nGLY, represent the number of
out
i
in
moles of product formed and the GLY initial number of moles and
the residual GLY number of moles, respectively.
The product yield is calculated using Equation 3:
Y
product, i (%) = (nproduct, i /nGLY, in) x 100
(3)
Where nGLY, is the initial amount of moles of GLY and nproduct,
in
i
subtraction 43
.
represent the number of moles of product formed at time t.
SEM images are obtained using a JEOL JSM-6490 LV instrument.
Samples are fixed on a graphite ribbon, and surface is coated with
Results and discussion
gold for analysis (using a DENTON VACUUM Desk IV equipment). Structural characterization of catalysts
The microscope is equipped with an EDS analyzer, allowing to
Table 1 shows the experimental Cu loading in the CuO/CeO2
composites, which is observed to range between 5.7 wt.% to 26.1
wt.%. Surface area of composites is evolving with the copper
loading. At low loadings, 5.7 wt.% and 9.9 wt.%, the specific surface
area remains close to that of the parent CeO2 (113-127 m2 g-1 for
the two composites, vs. 118 m2 g-1 for CeO2). At higher Cu loadings,
17.8 wt.% and 26.1 wt.%, the specific surface area decreases with
the increase in copper loading. For the highest loading, 26.1 wt.%,
the composite presents a surface area of only 84 m2 g-1, that is ∼
30% below the surface area displayed by the ceria support. This
evolution suggests that at low loading, copper phase remains well
dispersed throughout the ceria surface, allowing to maintain high
surface area. When the loading increases, a fraction of the copper
perform element mapping on material surface.
Catalytic test
Catalytic reactions are conducted in a 250 mL batch autoclave
reactor from Parr, equipped with mechanical stirring. 100 mL of 0.6
M aqueous NaOH solution and 6.3 g of glycerol (GLY) aqueous
solution (85 wt.%) are mixed into the autoclave. Under these
conditions, the GLY concentration in the reaction mixture is 5.0
wt.%, and the NaOH/GLY molar ratio is 1.0. After a homogeneous
stirring is stabilized, 0.6 g of catalyst is added, and the autoclave is
sealed, flushed with N2 and heated up under autogenous pressure
o
until the reaction temperature, 220 C. After reaching the reaction
temperature, the autogenous pressure is 20 bar. The autoclave
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phase is suggested to be present as a bulk phase, engendering a
sensible, but reasonable, decrease in global surface area.
Table 1. Textural properties of CeO2 and CuO/CeO2 composites
after calcination at 400 oC.
Composite
CeO2
SBET (m2 g-1)
118
113
127
97
Figure 1. X-ray diffraction patterns recorded for calcined CeO2 and
5.7CuO/CeO2
9.9CuO/CeO2
17.8CuO/CeO2
26.1CuO/CeO2
CuO/CeO2.
84
Surface characterization of catalysts
Analysis of cerium by XPS of 5.7CuO/CeO2 sample is presented in
Figure 2a. Comparable Ce 3d spectra are obtained in terms of
binding energy values and relative intensities of the components
over all copper supported catalysts (see Figure 11a for fresh
26.1CuO/CeO2). For instance, indicating that the chemical
environment of the surface Ce(IV) species does not significantly
change after deposition of the copper species. The Ce 3d XPS
spectrum of 5.7CuO/CeO2 composite (Figure 2a) shows the
characteristic photopeaks of the 3 d5/2 (labelled v, v’’, v’’’) and the 3
Wide-angle XRD patterns are shown in Figure 1. The XRD pattern of
CeO2 powder, after calcination at 400 oC, exhibits only diffraction
peaks characteristic of the face-centered cubic CeO2, fluorite-type
structure, with space group Fm3m (JCPDS 01-075-0076). After
deposition and calcination step, all obtained composites, whatever
the copper loading, exhibit the same characteristic diffraction peaks
of the face-centered cubic CeO2 structure. It indicates that the ceria
structure is preserved regardless of the Cu deposition and thermal
activation (Figure 1). For low loading composites, 5.7 wt.% and 9.9
wt.%, no additional reflections can be observed on the
diffractograms. This indicates that at these loadings, copper phase:
(i) remains amorphous, or (ii) forms a crystalline phase not visible
d
3/2 (labelled u, u’’, u’’’) spin-orbit components 47-49. The spectrum is
characterized by components at 917.1 (u’’’) and 898.7 (v’’’) ± 0.1 eV,
which are characteristic of the Ce(3d94f0)O(2p6) state. The
components at lower binding energies, labelled u (901.6
± 0.2 eV), v
(883.0 0.2 eV), and u’’ (908.1 0.1 eV), v’’ (889.3 0.1 eV), are
±
±
±
characteristic of the Ce(3d94f2)O(2p4) and Ce(3d94f1)O(2p5) states,
respectively 47, 48. The signal, at 917.1 eV, is associated to the Ce
3d3/2 state distinctive of the Ce(IV) ions (Figure 2a).
by XRD, that means at a size below the detection limits (∼4 nm or
below). Even if on the basis of this result we cannot conclude on
the formation of copper crystalline supported phase, it is evident
from previously discussed surface area trend that the deposited
phase is in a high dispersion state even if crystalline or not. Such
result is coherent with previously presented results from the
literature 44. At higher Cu loadings, 17.8 wt.% and 26.1 wt.%,
additional reflections of low intensity, positioned at 35.6° and 38.9°,
are observed (Figure 1). These two reflections are originated from
the monoclinic CuO structure, with space group C2/c (JCPDS 01-
44-46
074-1021) 39
,
.
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Morphology of catalysts
SEM images recorded for selected materials are presented in Figure
3. Pure CeO2 shows an aggregate-like morphology of particles (not
shown), with wide size population for the aggregates from few
micrometers to several tenths of micrometers. This morphology is
maintained after deposition of copper, with, at low copper loading,
exactly comparable SEM images recorded (Figures 3a and 3b, for
loadings of 5.7 wt.% and 9.9 wt.% respectively). A significant
evolution of the large scale morphology is observed at higher
copper loading (Figures 3c and 3d, for loadings of 17.8 wt.% and
26.1 wt.% respectively). Indeed, elongated-like particles presenting
irregular surface form on the surface of the CeO2 aggregates. It
seems that CuO forms clusters-like nucleus from which elongated
shaped particles grow up from close centers and split into several
directions. The coverage of the ceria aggregate is increasing with
the copper loading.
Figure 2. XPS analysis of CuO/CeO2 composites calcined at 400 oC:
Ce 3d spectrum of 5.7CuO/CeO2 composite (a); Cu 2p spectra for
5.7CuO/CeO2 and 9.9CuO/CeO2 composites (b) and for
17.8CuO/CeO2 and 26.1CuO/CeO2 composites (c).
In low copper loading composites (Figure 2b), Cu(II) species are
identified by a main Cu 2p3/2 photopeak at 934.2
± 0.1 eV and its
Figure 3. SEM images of a) 5.7CuO/CeO2, b) 9.9CuO/CeO2, c)
49
shake-up satellite structure (labelled S) located at 940-945 eV
.
17.8CuO/CeO2 and d) 26.1CuO/CeO2 composites.
The Cu(II) oxide spectral decomposition, following the fitting
parameters proposed by Biesinger et al. 49, evidences the presence
EDS-mapping analyses, performed on presented SEM images, are
provided in Figure 4. Over 5.7CuO/CeO2 (Figures 4a-d-g),
homogeneity in O, Ce and Cu species dispersion throughout the
aggregate surface is clearly observed. For higher copper loadings
17.8CuO/CeO2 and 26.1 CuO/CeO2 samples, EDS-mapping shows
that O, Ce and Cu are always observed throughout the whole
surface of the observed grain. However, a difference appears
compared with samples displaying lower copper content. Indeed,
intensity of the Cu species signal appeared more intense on the
areas where elongated particles are observed (Figures 4h and 4i, for
17.8CuO/CeO2 and 26.1CuO/CeO2 respectively). Accordingly, CuO
clusters having different sizes are present on the CeO2 aggregate
surface at high Cu loadings, demonstrating an enrichment of the
external surface of aggregate by the CuO phase when loading in
composite increases.
of an additional component at lower binding energy at 933.0
± 0.1
eV. In order to discriminate between the copper species, e.g. Cu(0)
and Cu(I), the modified Auger parameter (2p3/2, L3M45M45) is
calculated 49, showing values of 1849.1 and 1849.3
± 0.2 eV for
5.7CuO/CeO2 and 9.9CuO/CeO2, respectively. These values indicate
the presence of Cu(I) species. Consequently, both Cu(I) and Cu(II)
species are identified on the surface of low copper content
composites. For composites with higher Cu loadings (17.8 and 26.1
wt.%), Cu(II) species are identified by a main Cu 2p3/2 photopeak at
934.3
Auger parameter (2p3/2, L3M45M45) values of 1849.3 and 1849.4
0.2 eV are obtained for 17.8CuO/CeO2 and 26.1CuO/CeO2
±
0.1 eV and its shake-up satellite (Figure 2c) 49. Modified
±
49
respectively, therefore indicating the presence of Cu(I) species
.
For instance, the speciation of copper shows no differences with
the copper loading, Cu(II) and Cu(I) are present in all composites
regardless of the copper loading, with a distribution of around 60%
and 40%, respectively.
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56 % for 26.1CuO/CeO2 (Figure 5a). Increasing the reaction time
(from 0.25 h to < 3 h) induces a rapid increase in glycerol conversion
that reaches 19 % for 5.7CuO/CeO2, while it reaches 78 % for
26.1CuO/CeO2. A rapid adsorption and conversion of the GLY at the
catalyst surface then occurs over highly loaded Cu catalysts. For
26.1CuO/CeO2, a maximum conversion of 87.3 % is measured after
8 h of reaction. Significant evolution in catalyst activity with copper
loading is obtained, as already reported for Cu-containing MgAl LDH
50. For comparison purpose, glycerol conversion of 100 % and LA
o
selectivity of 94.6 % is reported over 30 wt.% CuO/ZrO2 (at 180 C,
NaOH/GLY = 1 and 1.4 MPa N2) 35. Then, satisfying activities are
obtained for Cu based catalysts when copper content is higher or
equal to 17.8 wt.% (Table 2).
Figure 4. SEM-EDS mapping analyses. Oxygen in yellow, Cerium in
green blue, Copper in brown for a-d-g) 5.7CuO/CeO2, b-e-h)
17.8CuO/CeO2 and c-f-i) 26.1CuO/CeO2 composites.
Catalytic reaction
Results of catalytic reaction (activity and selectivity in reaction) are
presented in Figure 5, and gathered in Table 2. Without catalyst and
under the selected reaction conditions, the glycerol conversion
reached only 30.5 % after 8 h reaction, showing a rate of glycerol
Figure 5. a) Glycerol conversion and b) Lactic acid yield as a function
of the reaction time for copper containing catalysts (Reaction
conditions: 220 oC, 100 mL of NaOH 0.6 M, 5 wt.% GLY, 0.6 g of
catalyst). Δ 5.7CuO/CeO2,
26.1CuO/CeO2.
○ 9.9CuO/CeO2, ▲17.8CuO/CeO2, ●
transformation rather limited. Lactic acid is formed with
a
selectivity of 51.5 % and pyruvaldehyde appears as secondary
product with a selectivity of 10.7 % (Table 2). When pure ceria is
used as catalyst, no significant modification of glycerol conversion is
obtained however, lactic acid selectivity reached 74.7 %, indicating
that CeO2 might have an influence on product selectivity, especially
in lactic acid formation (Table 2).
Carbon balance values raging from 90 % to 97 % are obtained for
CuO/CeO2 catalysts, showing that the formation of sub-products
through side reactions, and the surface adsorption without
transformation phenomenon remain limited under the reaction
conditions. From a general point of view, only three mains products
are detected (lactic acid, LA; pyruvaldehyde, PYR; 1,2-Propanediol,
1,2-PDO), at quantities allowing to their quantification. This is
reflected by carbon balance, at the end of the reaction at values
always above 90 to 97 % and confirming the limited formation of
sub products or adsorbed molecules on the catalyst surface.
Table 2. Catalyst activity and selectivity after 8 h reaction.
Catalyst
XGLY
(%)
S (%)
PYR
C balance Unknown
(%)
(%)
LA
1,2-
PDO
1.9
without
catalyst
30.5 51.5 10.7
89
11
GLY conversion leads to production of LA as main product (Figure 7
and Table 2), with limited production of PYR and 1,2-PDO. Lactic
acid yield steeply increases during the 1.5-2 first hours of reaction
(Figure 5b). After 7 h of reaction, a maximum LA yield of 68 % is
obtained for 26.1CuO/CeO2 while it slightly decreases for longer
reaction times (65 % after 8 h reaction). High selectivities toward
lactic acid are then obtained with all the catalysts (from 55.2 % to
80.3 % SLA, Table 2), selectivities above 70 % being reached at high
copper loading. PYR is identified as coproduct, since the beginning
of the reaction, and whatever the copper content in the catalyst
CeO2
27.5 74.7 11.3
27.4 55.2 10.7
3.7
1.1
3.9
3.7
3.2
97
91
96
92
90
3
9
5.7CuO/CeO2
9.9CuO/CeO2
45.6 71.1
6.3
3.5
5.8
4
17.8CuO/CeO2 70.3 80.3
26.1CuO/CeO2 87.3 74.4
PYR: pyruvaldehyde; 1,2-PDO: 1,2-propanediol. Other products identified: hydroxyacetone (HYD), glyceraldehyde (GLY),
acetic acid (AA). Carbon balance (C balance) = ((total number of moles of C in the products)/total number of C introduced
in the reactor) x 100. Unknown: 100 – C balance.
8
10
Typical GLY conversion curves obtained are presented in Figure
5(a). The glycerol conversion at the beginning of the reaction (0.25
h) increases with the copper loading, from 12 % for 5.7CuO/CeO2 to
(Figure
6 and Table 2). However, the selectivity to PYR is
significantly affected by the copper loading. Then, at low copper
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loading, SPYR close to 11 % is obtained (value close to those obtained The gas phase is also analyzed, after 3 h reaction. The only product
without catalyst or with ceria alone). When copper loading is identified in the gas phase is hydrogen, as classically reported
4
higher, SPYR goes to stabilize at ±5 %, the decrease of the PYR during glycerol hydrogenolysis over noble-metal catalysts or over
selectivity being in favor of the increase in lactic acid selectivity. A reduced Cu species supported on carbon nanofibers (CNF) 53. These
high Cu loading in the catalyst (17.8 wt.% and higher), 1,2-PDO is results show that the CuO/CeO2 catalysts are highly selective to
also identified as reaction product (Figure 6 and Table 2), with liquid products, among those mainly lactic acid is identified.
selectivity always below 5 %. The presence of 1,2-PDO is not
51
surprising since CuO is active in the hydrogenolysis of glycerol or
hydrogenolysis of polyols such as sorbitol 52, leading to the
production of this molecule. Considering the absence of significant
increase in catalytic activity in terms of glycerol conversion between
the blank and CeO2, and also between CeO2 and 5.7CuO/CeO2,
suggest that no significant interaction between Ce4+/Ce3+ and
Cu2+/Cu+/Co0 redox couples is awaited, with limited impact on the
catalytic process. Then, although ceria modified selectivity toward
LA when compared to the blank, with CuO/CeO2 based catalysts,
the role of ceria seems to confer stability in reaction and to provide
an adequate surface to stabilize copper oxide phase. Significant
glycerol conversion and lactic acid selectivity are observed at high
copper loading.
For the most active catalyst, 26.1CuO/CeO2, product yield is
presented as function of reaction time in Figure 6 (only identified
products are considered). Lactic acid is formed since the beginning
of the reaction and yield increased with glycerol conversion;
pyruvaldehyde and 1,2-propanediol followed a similar behavior
while glyceraldehyde is only detected at high glycerol conversion.
Figure 7. Chromatograms obtained after 8 h reaction on the
different CuO/CeO2 catalysts (220 oC, 100 mL of NaOH 0.6 M
solution, 5 wt.% Glycerol, 0.6 g of catalyst).
Table 3 summarizes some representative results available in the
literature for the conversion of glycerol over copper-containing
catalysts. The catalytic performance reported in this work is among
the best available in the literature for copper-based catalyst. The
use of ceria allows to reach better performances than using silica,
magnesium oxide, and alumina as support. Only the use of zirconia
and hydroxyapatite as support allows to reach comparable
performances than those reported in this work.
Figure 6. Glycerol conversion and product yield as function of the
reaction time for 26.1CuO/CeO2 catalyst (Reaction conditions: 220
oC, 100 mL of NaOH 0.6 M, 5 wt.% GLY, 0.6 g of catalyst).
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Table 3. Catalytic performance of different Cu or CuO-based 66.6 % on CuO while it was 34.9% on Cu2O. For instance, these
catalysts.
results demonstrate that CuO catalyst, with Cu(II) species, are
highly active for glycerol conversion into lactic acid as it was the
main product formed during the reaction. These results
demonstrated that Cu(II) species might play a key role on the
glycerol activation and its subsequent selective conversion to lactic
acid.
a.
This work
Catalyst
Cu
wt.%
60
CGLY
(%)
SLA
(%)
79.7
78.6
78.1
82.1
85
NaOH/GLY
Atmosphere
T
time
(h)
6
Ref.
(oC)
240
240
240
160
160
180
230
230
230
220
Cu/SiO2
CuO/Al2O3
Cu2O
75.2
97.8
93.6
43.4
67.5
100
99
1.1
1.1
1.1
1
N2/14 bar
N2/14 bar
N2/14 bar
N2/1.4 MPa
N2/1.4 MPa
N2/1.4 MPa
N2
34
34
34
35
35
35
36
36
36
a
13
6
---
6
CuO/ZrO2
CuO/ZrO2
CuO/ZrO2
Cu/MgO
Cu/ZrO2
20
6
30
1
6
30
94.6
83
1
8
16
1.1
1.1
1.1
1
8
16
64
50
N2
8
Cu/HAP
16
99
87
N2
8
CuO/CeO2
26.1
87.3
74.4
N2/42 bar
8
In order to elucidate the impact of the NaOH concentration, the
reaction is performed at half NaOH concentration (0.3 M, given a
NaOH/GLY mole ratio of 0.5). Under lower NaOH concentration,
notable decrease in glycerol conversion is observed (Figure 8a),
being divided by a factor ∼2 at the end of the reaction. Decreasing
the NaOH concentration also impacts the lactic acid yield (Figure
8b). For instance, OH- ions concentration significantly impacts the
GLY conversion rate over CuO/CeO2 catalysts while selectivity to LA
(90 %) does not apparently depend on the OH- concentration. This
result slightly differs from precedent results from the literature,
where the presence of a base in the reaction medium was reported
to mediate the conversion of glycerol while it allows to increase
selectivity towards lactic acid 53
.
Figure 9. Glycerol conversion on bulk Cu2O and CuO.
Stability of the 26.1CuO/CeO2 catalyst upon reaction cycling is
evaluated. Recyclability test is performed recovering the catalyst
after the first run. Recovered solid is washed, dried and calcined at
400 oC during 4 h. Test is performed adjusting the glycerol weight to
the mass of the recovered catalyst, keeping constant the NaOH/GLY
mole ratio equal to 1.0. Evolutions of the glycerol conversion and
selectivity to lactic acid with reaction cycle are plotted in Figure 10.
While glycerol conversion of 88% is measured for the first reaction
cycle, it goes to stabilize to 81±3 % since the second cycle. Then,
during the four last cycles, the conversion remains stable. This
stability in conversion is denoting of a satisfying stability of the
CuO/CeO2 catalyst under the reaction conditions, with no significant
irreversible poisoning/sintering of the active sites. In addition,
selectivity to lactic acid, for all cycles measured at 90±1%, confirms
that the active sites are not modified during reaction and retains
Figure 8. (a) Glycerol conversion on 26.1CuO/CeO2 catalyst as a
function of reaction time, at NaOH concentration of ● 0.6 M, 0.3
M, (b) Lactic acid yield on 26.1CuO/CeO2 catalyst as a function of
reaction time at NaOH concentration of ● 0.6 M, 0.3 M.
○
○
Catalytic reaction was performed with bulk CuO or Cu2O (Figure 9).
Reaction was performed under similar conditions as described for
CuO/CeO2 catalyst. The amount of CuO or Cu2O was adjusted in
order to keep the ratio Glycerol/Cu constant. The results show
glycerol conversion increased at low reaction times on both
catalysts, demonstrating oxidized copper species, Cu(II) or Cu(I), are
active for converting glycerol. However, conversion of glycerol on
CuO is significantly enhanced, after 5 min reaction the glycerol
conversion reached 28.3% on CuO while it was only 10.7% on Cu2O,
and it continued a rapid increase on CuO being almost twice the
conversion on Cu2O. After 2 h reaction glycerol conversion reached
their initial properties even after
5 reaction cycles. Glycerol
conversion, when using Co3O4/CeO2 catalysts 38, reaches 85.7%
(when the catalyst is prepared by impregnation) and 74.4 % (when
the catalyst is prepared by deposition-precipitation). For instance,
using CeO2 support, Co3O4-based catalysts exhibited lower activity
and required more demanding reaction conditions than CuO-based
catalysts. Besides, Co3O4/CeO2 catalyst deactivated in the third
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catalytic cycle. Nevertheless, with both Co3O4/CeO2 and CuO/CeO2
catalysts, lactic acid is the main product formed.
26.1CuO/CeO2 to 1849.0
±
0.2 eV for spent 26.1CuO/CeO2. These
results therefore indicate that reaction induces a modification of
the Cu(I)/Cu(II) surface ratio, in favor of the Cu(I) species. In
contrast, reaction of glycerol to lactic acid performed on CuO/ZrO2
catalysts, with a Cu loading around 30 wt.%, reached 59.1 % with a
lactic acid selectivity of 89.5 % but conversion decreased to 24.1 %
during the third catalytic run and authors stated that Cu2+ was
reduced to metallic Cu (0) 35. Besides ZrO2 also suffered structural
changes. Authors reported that during the second catalytic run,
Cu(0) was the active species but deactivated during the third
catalytic cycle because of the Cu(0) particles growing. In other
research work, Cu-based catalysts were reduced before the
catalytic test 36. Authors reported that metallic copper species (Cu0)
were active in converting glycerol to lactic acid. It was also showed
that catalysts were active after the catalytic test without
pretreatment.
Figure 10. Evolution of the glycerol conversion (XGLY, black bars) and
selectivity to lactic acid (SLA, grey bars. Reported in terms of the
total identified products) with reaction cycle number.
Properties of CuO/CeO2 catalysts after reaction
After reaction, 26.1CuO/CeO2 catalyst was recovered by
centrifugation, washed with ultrapure water, dried and
characterized using ICP, XPS, SEM-EDS mapping.
Cu content in the spent 26.1CuO/CeO2 catalyst is analyzed using
atomic absorption spectroscopy. The Cu loading did not change
(26.2 wt.%), thus indicating that there was not leaching during the
catalytic reaction. It is also unlikely that Cen+ suffered leaching as
the reaction basic medium is similar to the medium of the CeO2
synthesis. Then, the absence of leaching comforts catalytic results
demonstrating the good stability performance of CuO/CeO2
catalysts under reaction conditions applied.
Figure 11. XPS Cu 2p spectra for fresh 26.1CuO/CeO2 and spent
26.1CuO/CeO2 composites.
XPS characterization of fresh and spent 26.1CuO/CeO2 catalyst
shows that cerium characteristics do not evolve during the catalytic
reaction (Figure 11a), with the Ce 3d spectrum of spent
26.1CuO/CeO2 catalyst being similar to that of the fresh catalyst (in
terms of binding energy and relative intensities of the components).
As previously discussed, CeO2 signals do not change by the presence
of copper and CeO2 exhibited limited activity in converting glycerol
although but modified lactic acid selectivity (Table 2). The XPS
characterization indicates several modifications on the Cu 2p
photopeaks (Figure 11b). The intensity of the Cu(II) component at
934.2 eV decreases while the intensity of the Cu(I) component at
933.0 eV increases. The intensity of the satellite centered between
945 and 940 eV are the most affected, with a significant decrease in
intensity. Moreover, there is no significant change of the modified
Since some modifications of the surface properties of the material
are observed by XPS, SEM-EXDS analysis is performed. Images show
that the CeO2 support morphology/aggregate size did not change
upon catalytic reaction, and the support is always observed in the
form of several micrometers in size aggregates of small particles of
CeO2. Modification of CuO phase morphology is observed. Initially,
elongated CuO particles were observed (Figure 3). Such elongated
particles growing perpendicularly to the ceria aggregate surface are
no more observed in the spent composite (Figure 12) indicating a
redistribution of the copper inside the material upon reaction, with
formation of aggregated copper oxide phase (with decreased
average oxidation state, almost on the surface as analyzed by XPS).
Despite, EDS-mapping analyses indicate that, on a micrometric
Auger parameter (2p3/2, L3M45M45): from 1849.4
± 0.2 eV for fresh
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scale, O, Ce and Cu species are always observed throughout the
material surface (Figure 13), the intensity of the Cu species
CuO/CeO2 catalysts were prepared at different composition
using deposition-precipitation method. Characterization
appearing more intense where aggregates on the surface of the evidenced the formation of CuO phase in the materials, with
particle from pseudo-spherical shape at low loadings to
elongated shape formed on the CeO2 surface (i.e. at high CuO
loadings, 17.8 and 26.1 wt.%). Analysis of the surface
CeO2 are observed.
SEM results obtained on the spent catalyst showed significant
modifications of the active phase morphology. The modification of evidenced the formation of Cu(II) and Cu(I) species at
comparable ratios whatever the copper loading. The catalysts
present excellent catalytic properties for the selective glycerol
to lactic acid oxidation in basic conditions, with observed
increases in conversion degree and selectivity to lactic acid
with copper loading increasing in the composite. Then,
selectivity slightly above 90% is achieved at conversion degree
close to 90% over the highest copper oxide content.
Remarkable stability of the catalyst in recycling conditions is
obtained, with selectivity to lactic acid maintained upon 5
cycles with limited conversion degree decreased. Catalyst
copper phase shape did not result in catalytic activity decrease
between successive reaction cycles. Measured stability can
originate from (i) a rapid change of morphology during first cycle,
making the impact of morphology change not visible on the activity
between two cycles, or (ii) no significant modification of active site
accessibility between the initial and the final morphology.
post-reaction
characterization
showed
morphological
modifications of the copper oxide phase on the ceria support,
with the disappearance of the elongated-shaped particles in
benefit to aggregate-like type particles. Also, surface
characterization showed Cu(II) reduction into Cu(I) species
during reaction. Despite these modifications, CuO/CeO2
catalyst remains highly active for the selective production of
lactic acid in aqueous basic medium.
Figure 12. Representative SEM images obtained for 26.1CuO/CeO2
composite after reaction.
Conflicts of interest
The authors stated “There are no conflicts to declare”.
Acknowledgements
The authors gratefully acknowledge the Universidad de
Antioquia and COLCIENCIAS for funding the project
111565842346. S. Royer and A.-S. Mamede acknowledge
Chevreul institute (FR 2638), Ministère de l’Enseignement
Supérieur et de la Recherche, Région Nord – Pas de Calais and
FEDER for supporting and funding partially this work
.
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Graphical abstract