The promoter effect of Co on the catalytic activity of the Cu oxide active phase supported on Al2O3 in the hydrogenolysis of glycerol

Article abstract of DOI:10.1039/c9nj03534f

The promoting effect of cobalt oxide on alumina-supported Cu oxide catalysts for the hydrogenolysis of glycerol was investigated. A series of Co(y)Cu/Al₂O₃ oxide catalysts with a fixed Cu loading of 7 wt% and a variable Co loading from 0 to 1.5 wt% was prepared by successive wet impregnation. The catalysts were characterized by X-ray diffraction (XRD), nitrogen sorption, temperature programmed reduction (TPR), ultraviolet-diffuse reflectance spectroscopy (UV-DRS), temperature programmed desorption of ammonia (TPD-NH₃), and X-ray photoelectron spectroscopy (XPS). The catalysts were tested in a batch reactor at 220 °C and 5 MPa of H₂. The activity of the catalysts on a per mole of Cu basis increased with Co content up to the plateau, and subsequently decreased at higher Co content. The increase of activity is attributed to the combined effect of the distortion of the octahedral geometry of CuO by the presence of CoO and the increase in the number of surface acid sites. In contrast, the subsequent decrease of activity at higher loading is related to the formation of less reducible, less acidic, and less active aluminates. The selectivity of products did not vary substantially with Co loading, indicative of the existence of active sites of similar characteristics on all the catalysts. Overall, all the catalysts favored the route comprising of C-O hydrogenolysis followed by hydrogenation, at the expense of the pathway involving C-C hydrogenolysis, leading to the predominant formation of 1,2-propanediol (1,2-PDO). This is probably due to the prevalence of metallic active sites over acidic sites on the surface, ensuring that partial removal of oxygen occurred without the loss of carbon atoms.

Full text of DOI:10.1039/c9nj03534f

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Escalona , New J. Chem., 2019, DOI: 10.1039/C9NJ03534F.
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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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DOI: 10.1039/C9NJ03534F
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Promoter effect of Co on the catalytic activity of Cu oxide active
phase supported on Al2O3 in the hydrogenolysis of glycerol
Received 00th January 20xx,
Accepted 00th January 20xx
C. Sepúlveda *a,h, K. Crucesa,h, J. Gajardoa, J. Seguela,h, R. Garcíaa, D. Salinasb**, J. L. G. Fierroc, I. T.
Ghampsond, R. Serpelle, N. Escalona f,g,h,i
DOI: 10.1039/x0xx00000x
The promoting effect of cobalt oxide on alumina-supported Cu oxide catalysts for the hydrogenolysis of glycerol was
investigated. A series of Co(y)Cu/Al2O3 oxide catalysts with a fixed Cu loading of 7 wt.% and a variable Co loading from 0 to
1.5wt.% was prepared by successive wet impregnation. The catalysts were characterized by X-ray diffraction (XRD),
nitrogen sorption, temperature programmed reduction (TPR), ultraviolet-diffuse reflectance spectroscopy (UV-DRS),
temperature programmed desorption of ammonia (TPD-NH3), and X-ray photoelectron spectroscopy (XPS). The catalysts
were tested in a batch reactor at 220°C and 5 MPa of H2. The activity of the catalysts on a per mole of Cu basis increased
with Co content up to plateau, and subsequently decreased at higher Co content. The increase of activity is attributed to
the combined effect of the distortion of the octahedral geometry of CuO by the presence of CoO and the increase in the
number of surface acid sites. In contrast, the subsequent decrease of activity at higher loading is related to formation of
less reducible, less acidic, and less active aluminates. Products selectivity did not vary substantially with Co loading,
indicative of the existence of active sites of similar characteristics on all the catalysts. Overall, all the catalysts favored the
route comprising of C-O hydrogenolysis followed by hydrogenation, at the expense of the pathway involving C-C
hydrogenolysis, leading to the predominant formation of 1,2-propanediol (1,2-PDO). This is probably due to the
prevalence of metallic active sites over acidic sites on the surface, ensuring that partial removal of oxygen occurred
without loss of carbon atoms.
selective hydrogenolysis of glycerol on bifunctional heterogeneous
catalyst produces 1,2-propanediol (1,2-PDO), 1,3-propanediol (1,3-
PDO), and ethyleneglycol (EG) 7-9. In particular, the conversion of
glycerol to 1,2-PDO has been extensively studied in order to
understand the kinetic behavior and its mechanisms, since 1,2-PDO
is an important raw material for the manufacture of polymers and
pharmaceuticals, as well as for the production of liquid detergents,
cosmetics, aromas and fragrances, personal care products, paints,
animal feed, antifreeze, among others 6.
1. Introduction
The production of biodiesel, considered an attractive
alternative to crude oil, by transesterification of biomass-derived
vegetable oils (triglycerides) has increased during the last decades.
However, glycerol is obtained as a by-product during this process
and constitutes about 10 wt.% of the total production, generating
an annual accumulation of 1 million tons worldwide 2, which is
Noble metals such as Ru, Rh and Pt have been widely used
as catalysts for the hydrogenolysis of glycerol due to their high
activity towards propanediols 8, 10. However, due to the high cost of
3
approximately 65% of the total glycerol produced . Moreover, the
purity of this compound is around of 80%, and the remaining 20% is
heavily contaminated by free fatty acids, water, inorganic salts,
these metals non-noble metals that are active for hydrogenolysis
3
unreacted triglycerides, methanol, among others
. These
13-15
reactions such as copper 7, 9, 11, 12, nickel
and cobalt 16, 17have
properties limit their direct use in certain applications and hinder
their disposal as waste, requiring more sophisticated purification
processes that ultimately raise the cost of biodiesel production and
cause the closure of some industries. For this reason, the catalytic
conversion of glycerol to produce a number of hydrocarbons,
aldehydes and alcohols has been reported to be one of the most
effective ways to address these issues. The conversion of glycerol to
received increased attention for glycerol conversion.
In particular, copper has been one of the most reported
metallic catalysts for hydrogenolysis of glycerol due to its high
hydrogenation activity 12, 18, low activity towards C-C bond rupture,
and high efficiency for the hydrogenation of C=O bonds to produce
propanediols 7, 9, 19The last year the effect of Re as promoter of Cu
and Ni supported catalysts in hydrogenolysis of glycerol have been
higher value-added products includes
a series of selective
20, 21
study
, showing an increase of selectivity to lower alcohols as
processes, such as etherification, dehydration to acrolein, glycerol
methanol, ethanol and 1-propanol, by Re addition. Similar behavior
in the same reaction was observed on Mo and W metal supported
catalysts, but with major selectivity to lower alcohols in comparison
to 1,2-PDO, it was attributed to an increase of Bronsted acid sites22.
Theses researches indicated that Re, Mo and W metal actives sites
are no efficient to promoted the selectivity to 1,2-PDO, as a high
added-value compound from hydrogenolysis of glycerol.Cobalt
catalysts have received less attention but have been shown to be
active in several hydrogenolysis reactions in which more than one
2,
acid oxidation and hydrogenolysis to propanediols
4-6. The
a
Universidad de Concepción, Facultad de Ciencias Químicas, Edmundo Larenas 129,
Casilla 160C, Chile
Universidad del Bio-Bio, Facultad de Ciencias, Departamento de Química, Avenida
b
Collao 1202, Casilla 5C, Chile
c Instituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, 28049 Madrid, Spain.
d Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Departamento de Ingeniería y Gestión de la Construcción, Escuela de Ingeniería,
Pontificia Universidad Católica de Chile, Santiago, Chile
e
C-C rupture is required 16. Cobalt is also used industrially to
16
promote dehydration and also for Fischer-Tropsch synthesis
.
f Departamento de Ingeniería Química y Bioprocesos, Escuela de Ingeniería, Pontificia
Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, Macul, Santiago, Chile.
g Facultad de Química y Farmacia, Pontificia Universidad Católica de Chile.
h Millennium Nuclei on Catalytic Processes Towards Sustainable Chemistry (CSC), Chile
I Unidad de Desarrollo Tecnológico, Universidad de Concepción, Coronel, Chile.
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°
There is a continuing need to explore other active phases as part of h, and finally calcined at 300 C for 2 h. Then, aqueous solution of
View Article Online
concerted efforts towards developing effective catalysts for the Cu(NO3)2·3H2O, corresponding to 7 wt.% oDf OCIu: ,10w.1a0s39i/mCp9NreJg0n3a53te4dF
conversion of glycerol to high value-added products.
following the same procedure described, except that the samples
One of the potential alternatives to metallic catalysts is metal were calcined at 500 oC for 2 h to obtain Co(y)Cu/Al2O3 oxide
oxide catalysts, particularly copper or cobalt oxide catalysts due to catalysts. A monometallic Cu/Al2O3 oxide catalyst was also prepared
their propensity to cleave C-O bonds. Application of metal oxide and used as reference following the same procedure and condition
catalysts also avoids the necessity to use hydrogen for catalyst pre- described for the impregnation of copper. The Cu and Co contents
treatment. If partial metal reduction is required, the presence of H2 in the catalysts were determined by atomic absorption
in the reactant feed can potentially partially reduce surface oxides spectroscopy using a Thermo Scientific ICE 3000 model series, and
under experimental conditions. In addition, the oxygen vacancies are presented in Table 1.
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created 23, 24 could provide adequate acidity on the surface 25 which
19, 26
could favor selectivity towards acetol, a precursor to 1,2-PDO
.
2.2 Characterization of supports and catalysts
The BET specific surface area (SBET) and total pore
This acidity was on display when CuO/Al2O3 catalysts were studied
for dimethyl ether (DME) production via methanol volume (Vp)of the catalysts and supports were determined from
dehydration: the results revealed that unbound CuO particles were nitrogen sorption measurement at -196 ºC using Micromeritics
dispersed on the surface of fresh CuO/Al2O3 catalysts, providing the TriStar II 3020 equipment. Prior to the measurements, the samples
active sites for methanol decomposition 27. In an unrelated study, were degassed at 300 °C for 2 h under vacuum. The BET specific
Mao et al.28 studied the catalytic hydrogenation of CO2 to DME on surface area was calculated from the adsorption branch of the
CuO-ZnO-ZrO2/HZSM-5 catalysts and reported that the selectivity isotherms in the range 0.05 ≤ P/P°≤ 0.25, while the total pore
and yield to DME were directly related to CuO species, and that volume was recorded at P/P0 = 0.99.
decrease of CO2 adsorption capacity of the samples was related to
the reducibility of CuO. These studies have demonstrated that calcined sample was carried out in
Temperature programmed reduction (TPR) of the
quartz cell using
a
a
reducible CuO active phase can catalyze hydrogenation routes and Micromeritics AutoChem II equipment equipped with a thermal
weaker acid sites formed on the surface favour dehydration conductivity detector. Approximately 20 mg of the sample was
reactions, both of which are key pathways during the heated at 110 °C under He for 0.5 h, cooled to room temperature
hydrogenolysis of glycerol.
under He, and then heated under 5% H2/Ar (50 mLmin-1) from 25 °C
Critical catalyst parameters need to be investigated in order to to 1050 °C (10 °C min-1). The consumption of hydrogen was
assess the viability of reducible metal oxide active phase for glycerol determined after internal calibration of the TCD.
hydrogenolysis. One of these parameters is the ability of a
XPS measurements were performed using a VG Escalab
hemispherical
promoter to modify the acidity, nature and generation of active 200R electron spectrometer equipped with
a
sites of metal oxide catalysts. According to reported by electron analyzer and Mg Kα (1253.6 eV) X-ray source. The samples
bibliography, Cu oxide is active for hydrogenation/dehydration were reduced in-situ at 400 °C under H2for 4h and transferred from
reactions, and Co has been reported as a good promoter for several the pre-treatment chamber to the spectrometer. The intensity of
hydrogenation reactions. It should be pointed out that mixed the peaks was estimated from the integral of each peak after
copper-cobalt oxides have been applied in a variety of important subtracting an S-shaped background and fitting the curve to a
catalytic reactions such as synthesis gas conversion to higher combination of Gaussian/Lorentzian functions. Atomic ratios were
alcohols, carbon monoxide oxidation, reaction oxygen evolution calculated by using peak areas normalized on the basis of
(EOR), Fischer-Tropsch synthesis, among others. Mixed Cu-Co acquisition parameters, sensitivity and transmission factors
oxides have also been used in thermoelectric power generation provided by the manufacturer, and determined from the
materials 29
Therefore, considering the more efficiency C-O cleavage of Cu factors, with a precision of ±7%.
.
corresponding peak intensities, corrected with tabulated sensitivity
and Co in hydrotreatment reaction, compared to other transition
Diffuse Reflectance UV-VIS (DRS) measurements of well-
metal catalysts, the aim of the current work is to provide insights dried, calcined samples were carried out using a Lambda 35
into how Co oxide can improve the performance of CuO/Al2O3 spectrophotometer, equipped with a diffuse reflectance sphere for
catalysts for the hydrogenolysis of glycerol. An investigation into powder analysis. The UV-DR spectra were recorded in the
the role of Co oxide in altering the reactivity of CuO active phase wavelength range 200-900 nm at ambient conditions. The Kubelka-
will lead to a greater understanding of the viability of this class of Munk's equation was used to calculate the intensities (IOH/IOHd) ratio
catalysts and provide useful guidance for their use for the between Cu2+ octahedral species (IOH) and octahedral distorted Cu2+
hydrogenolysis of glycerol.
species (IOHd) as a function of Co content.
Powder X-ray diffraction (XRD) patterns were obtained
using a D4 Endeavor Bruker AXS diffractometer equipped with a
nickel-filtered CuKα1 radiation (λ= 1.5418Å). The standard scan
parameters were 1° per min for the 2θ range from 10 to 90°.
as Identification of the phases was achieved by reference to ICDD files.
Experimental
2.1 Catalyst preparation
The
CoOx(y)CuO/Al2O3
catalysts,
denoted
Co(y)Cu/Al2O3, were prepared by successive wetness impregnation In order to increase identification sensibility of the phases, the
using aqueous solution of Co(NO3)2·6H2O (Merck) and experimental patterns was performed using Match! software.
Cu(NO3)2·3H2O (Sigma Aldrich) as precursor salts and γ-Al2O3 (SPH Quantitative phase analysis was carried out using the Rietveld
50 LA) as support. Prior to impregnation, the support was crushed refinement of phase scale factors 30. Prior to analysis of support
and sieved to 80 and 125 μm particle size. Aqueous solutions of materials, precursor material and catalyst samples, XRD equipment
different amounts of Co(NO3)2·6H2O,corresponding to Co nominal shape function and X-ray source parameters were refined using XRD
loadings of 0.8, 1.0,1.3 and 1.5 wt.%, was first impregnated into the data obtained from a pure corundum standard sample.
support, kept at room temperature for 24 h, dried at 120 °C for 12
2 | J. Name., 2012, 00, 1-3
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푛푝푟표푑푢푐푡푗
Temperature programmed desorption of ammonia (TPD-
View Article Online
DOI: 10.1039/C9NJ03534F
푆푒푙푒푐푡푖푣푖푡푦(푚표푙%) = 푛
× 100
(3)
퐺푙푦푐 푐표푛푣푒푟푡푒푑
NH3) was performed using a TPR/TPD Micromeritics 2900 system
equipped with a thermal conductivity detector (TCD). Prior to the
analysis, the samples were dried under He (50 mL min-1) at 100 °C
for 0.5 h. The TPD-NH3 measurements were performed by pulsing
NH3 at 120 °C until saturation of the catalyst surface. Then, the
samples were cooled to room temperature and purged under He to
remove physically adsorbed NH3. Once the baseline was stabilized,
the sample was heated to 700 ºC by a ramp of 10 °C min-1 under
helium. The total amount of moles of desorbed ammonia in the
samples (catalysts and supports) was obtained by the standardized
DTP-ammonia analytical areas per catalyst mass. In addition, the
semi-quantitative comparison of the distribution of acidic sites
available on the surface was determined from the integration by
Gaussian deconvolution of the ammonia desorption signals
according to the temperature range at which the desorption signal
appears; weak acid sites between 0-300 oC, moderate acid sites
Where nproductj is the moles of product j (mol) and nGlycconverted is the
moles of glycerol converted (mol).
2. Results and discussion
3.1 Catalysts and support Characterization
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Table 1 summarizes the textural properties of the γ-Al2O3
support, Cu/Al2O3 and Co(y)Cu/Al2O3 oxide catalysts. The BET
surface area of the promoted Co(y)Cu/Al2O3 oxide catalysts ranged
from 213 to 225 m2 g-1, which corresponds to a 29-33% decrease of
SBET of the support. The promoted catalysts presented similar
specific surface area and pore volume as the non-promoted
catalyst, indicating that the small quantities of Co did not have a
significant effect on the textural properties of the catalysts. Also,
Table 1 shows that pore volume decreases slightly with Cu and Co
deposition, suggesting that there was no significant pore mouth
blockage.
between 300-500 oC and strong acid sites at > 500 oC 31
.
2.3. Catalytic tests
Hydrogenolysis of glycerol was performed in a 300 mL
batch reactor (4848 Parr model). The glycerol (> 99 % purity, Sigma
Aldrich) concentration used was 10 mol L-1 in deionized water. This
concentration was selected after rigorous study of the optimal
glycerol amount that needs to be used to avoid the high vapor
pressure of water and the burning of glycerol. Similar high glycerol
concentration has been reported by several authors 32-34. Also, the
mass of catalyst used has been experimentally verified to keep the
reaction under kinetic regime without any diffusional limitations. In
this context, the liquid reactant feed consisted of an aqueous
glycerol solution (80 wt.%). About 0.5 g of oxide catalyst was added
to the reactant. Then, the system was closed and N2 was bubbled
through the solution for 10 min to remove air from the system. Still
under N2, the reactor was heated to the reaction temperature of
220 oC with continuous stirring. When this temperature was
reached, H2 was introduced into the reactor to 5 MPa which was
kept constant during the course of the experiment. Condensed
samples were taken periodically during the reaction and were
identified and quantified using a Perkin Elmer Autosystem XL gas
chromatograph with a flame ionization detector equipped with a
capillary column (Nukol, 30 m x 0.53 mm x0.5μm).
Figure
1 shows the XRD patterns of the Cu/Al2O3 and
Co(y)Cu/Al2O3 catalysts.
Table 1: Textural properties and metal contents of Cu/Al2O3
and Co(y)Cu/Al2O3 catalyst samples
Metal Content
(wt.%)*
SBET
Vp
Samples
(m2 g-1)
(cm3g-1)
Cu
Co
Al2O3
319
229
0.35
0.31
Cu/Al2O3
Co(0.8)Cu/Al2O3
Co(1.0)Cu/Al2O3
Co(1.3)Cu/Al2O3
Co(1.5)Cu/Al2O3
7.8
7.6
7.2
6.7
0.8
1.0
1.3
1.5
225
227
215
213
0.31
0.33
0.31
0.31
*Real Cu and Co content obtained from ICP analysis
The conversion of glycerol (XGly) was calculated from:
0
퐺푙푦 ― 푛푖퐺푙푦
푛0퐺푙푦
퐺푙푦(%) = 푛
× 100
Co(1.5)Cu/Al2O3
Co(1.3)Cu/Al2O3
Co(1.0)Cu/Al2O3
Co(0.8)Cu/Al2O3
푋
(1)
where nºGly is the initial moles of glycerol in the solution (mol) and
niGly is the moles of glycerol at time i(mol). The specific rate for the
total conversion of glycerol was calculated from the initial slope of
the conversion vs. reaction time plot. The intrinsic rate was
calculated from the specific rate according to the following
equation:
Cu/Al2O3
N
(2)
rI  rS
nCu
20
40
60
80
Where rI is the intrinsic rate (molecules of glycerol converted per Cu
atom per second), rS is the specific rate (moles of glycerol converted
per gram of catalyst per second), nCu represents the total number
of Cu atoms per gram of catalyst, and N is Avogadro’s number. The
product distribution was calculated from Eq. (3) when the reaction
was completed.
2º)
Figure 1. X-ray diffraction patterns for calcined of the Cu/Al2O3 and
Co(y)Cu/Al2O3 catalysts
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Figure 3 shows the H2-TPR profiles of the catalysts. The reducibility
View Article Online
Characteristic diffraction peaks of CuO at 2 = 35.0 o and
of the catalysts provides useful informationDoOnI:t1h0e.10sp39e/cCie9sNJp0re35s3e4nFt
on the surface of the catalyst and insights into the nature of the
active sites, particularly since it is assumed that in-situ reduction
occurred during glycerol hydrogenolysis reaction at 220 °C. The TPR
39.0 o were detected, assigned to base-centered monoclinic phase
(JCPDS PDF: 48-1548), and for γ-Al2O3 at 2 = 37.6o, 45.7o and 67.0o
(JSPDS PDF: 10-0425) were observed 35-37. Other species such as
spinel CuAl2O4, which is typically formed after calcination at 400 oC,
or CuCo2O4 which is usually formed after calcination below 300 °C
o
profile of Cu/Al2O3 presented one main peak at 200 C, assigned to
41, 42
the reduction of highly dispersed CuO species to Cu0
;
furthermore, a shoulder appears at 240 oC which is attributed to the
reduction of Cu+ to Cu0 43, 44, or to the reduction of larger particles of
Cu2+ to Cu0 43. The TPR profile of a reference Co(1.5%)/Al2O3 sample
39
38, were not identified. However, Lee et al. have reported that
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diffraction peaks of CuCo2O4 nanoparticles appear at 2= 19.0°,
37.0° and 38.7°, corresponding to (111) (311) and (222) planes,
respectively. Thus, the absence of these peaks could be due to the
formation of highly-dispersed species, or the masking of diffraction
peaks by broad peaks of the γ-Al2O3 support. Therefore, to further
understand the effect of Co addition on the structure of supported
Cu catalysts, quantitative phase analysis by Rietveld refinement was
carried out (the results are summarized in Table 2). From the
experimental analysis, some sharp peaks appeared to be
superimposed on top of the smooth pattern of the nano-crystalline
γ-alumina support. There were no sharp peaks observed on the
other end of the diffraction angle. However, the pattern obtained
after subtraction of the pattern of the γ-Al2O3 support from that of
the sample could tentatively be identified as a
spinel structure, such as CuCo2O4. Therefore, the CuCo2O4 structure
was modelled using the structure reported by Wyckoff 40. Table 2
shows that the fraction CuCo2O4 increased as the concentration of
Co increased in the samples. The overall trends can be seen in
Figure 2. Thus, based on Rietveld analysis, the addition of Co to
Cu/Al2O3 created a new phase (CuCo2O4) whose relative fraction
increased with Co content. It is assumed that this new phase
probably improved the surface distribution and stability of Cu
species in the promoted catalyst.
o
presented a low-intense peak at 280 C, assigned to the reduction
of Co2+ to Co0 45-47. There was no evidence of the step-wise
reduction of cobalt oxide (Co3O4CoO Co0) that is commonly
observed for catalysts with high Co content (≥10 wt.%) 45, 46. For the
Co(y)Cu/Al2O3 catalysts, the reduction peaks associated with Cu
species were the most prominent while those corresponding to Co
species were not visible. The absence of the reduction peak for Co
species may be due to an overlap with the reduction signals of Cu2+
species, suggesting that Cu promoted the reduction of CoO species.
48
Kaluza et al. reported that during reduction of supported Co-Cu
catalysts, adsorbed atomic hydrogen could migrated from Cu0 to Co
oxide species and promote reduction to Co0 via a possible spillover
mechanism. The reduction profile of the Co(1.5)Cu/Al2O3 catalyst
shows a weak but obvious feature around 325 °C. This may be
linked to the reduction of CuCo2O4, as suggested by XRD results.
The profiles of the Co(y)Cu/Al2O3 catalysts show a weak feature
around 750 °C, attributed to the reduction of aluminates species
(CuAl2O4).
24
22
20
18
Table 2. Rietveld refinement results (relative phase fractions)
for samples of different Co content (wt %)
Co
0.8%
1.276
3.081
788
1.0%
1.273
2.947
771
1.3%
1.231
2.936
784
1.5%
1.241
2.911
719
Sigma
Rwp %
intensity
Al2O3
16
Co(1.5)Cu/Al O
2
3
14
12
10
08
06
04
02
Co(1.3)Cu/Al O
2
99.960
99.436
99.591
99.459
3
CuCo2O4
0.040
0.329
0.409
0.541
Co(1.0)Cu/Al O
2
3
Co(0.8)Cu/Al O
2
3
Cu/Al O
2
3
Co/Al O
2
3
150
300
450
600
750
900
Temperature (ºC)
Figure 3. H2-TPR of Co/Al2O3, Cu/Al2O3 and Co(y)Cu/Al2O3 catalysts
Table 3 lists XPS results of reduced catalysts. The analysis
provides complementary information on the presence of cobalt
and/or Cu aluminates species as well as gives insight into the
stability of the oxides. The binding energy (BE) of Cu 2p3/2 is
observed at 932.7 0.1 (eV), attributed to Cu0 species 49-51. Based
on A parameter from XPS measurements, a signal at 1851.1 ± 0.1
(eV) was detected, indicating only Cu0 species is distinguished on
the surface of the catalysts, in agreement with conclusion drawn by
Figure 2. Rietveld refined from experimental X-ray diffraction
calculation for Co(y)Cu/Al2O3 catalyst
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previous work52. This result suggests a complete reduction of Cu performed in order to analyze the effect of the presence of Co on
species under the experimental condition used. The Co 2p3/2 signal the coordination symmetry of Cu oxide spDeOciIe:s1.0.F10ig3u9r/eC9SN1J0s3h5o3w4Fs
View Article Online
that the main bands observed are between 350 and 450 nm which
is associated with the octahedral geometry (hexacoordinated
symmetry) of Cu2+ species 57-59. Other bands observed between
600-900 nm are attributed to d-d electronic transition of Cu2+
revealed a doublet with BE at 778.0  0.1 and 780.1  0.1 (eV),
47, 53
assigned typically to Co0 and Co2+ species, respectively
.
However, other studies related to CuCo2O4 nanocrystals and spinel
structures have reported that the BE at 780.0 eV is associated with
the presence of Co3+ 54-56
.
corresponding
to
the
distorted
octahedral
geometry
(pentacoordinated symmetry) 57-59. In order to determine the effect
of Co oxide species on the symmetry of CuO species, the ratio of the
intensities (IOh/IOhD) between octahedral Cu2+ species (IOh) and
octahedral distorted Cu2+ species (IOhD) as a function of Co content
was calculated (Figure 5). It can be observed that the (IOh/IOhD)
ratio decreased after Co addition, reaching a minimum value at 1.0
wt.% of Co, and then increased at higher Co content. The decrease
of (IOh/IOhD) ratio suggests a distortion of octahedral symmetry of
CuO species by the presence of Co oxide, probably due to a CoOx-
CuOx interaction. This behavior agrees with previous results by
Xiao et al.60, in which a partial substitution or insertion of CoO in Cu
octahedral oxide structure was proposed to have occurred. This
Therefore, the co-existence of Co2+ surface species and Co3+
associated to CuCo2O4 structures cannot be ruled out. Table 3 also
shows that the fraction of Co species in metallic state increased
with Co loading from 0.8 to 1.3 wt.%, and then decreased at a
higher loading. This indicates that Co becomes easier to reduce due
to weaker Co-Cu interaction as Co content increases, as suggested
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by TPR results. In fact, Medina et al. proposed that at low Co
content, Co2+ species interact strongly with the support, hindering
complete reduction to metallic Co. Conversely, larger, more-easily
reducible particles are formed at higher Co loadings. No shifts in
binding energies were observed, indicating the absence of charge
transfer. This is reasonable due to the marginal difference of
electronegativity values between Cu (1.90) and Co (1.88).
61
behavior was also similarly observed by Wang et al. for Co-Ce
oxide catalysts.
In contrast, the increase of (IOh/IOhD) ratio observed at
higher Co content could be attributed to a decrease in the
interaction between CoOx and CuOx species. Another possible
reason could be inferred from XRD quantitative phase analysis
results.
For this reason, it appears that an ensemble effect was
Table 3. XPS binding energies and surface atomic ratios for Cu/Al2O3 and
Co(y)Cu/Al2O3 reduced catalysts.
Al2p
(eV)
Cu2p3/2
(eV)
Co2p3/2
(eV)
--
O1s
(eV)
Oxide catalysts
Cu/Al2O3
Cu/Al
0.021
0.060
Co/Al
932.7
778.0 (61)
780.1 (39)
778.1 (66)
780.1 (34)
778.0 (72)
780.1 (28)
778.1 (68)
780.1 (32)
Co(0.8)Cu/Al2O3
74.5
74.5
74.5
74.5
932.8
932.7
531.4
531.4
531.5
531.5
0.024
0.041
0.054
0.060
Co(1.0)Cu/Al2O3
Co(1.3)Cu/Al2O3
Co(1.5)Cu/Al2O3
0.058
0.064
0.063
932.7
932.8
operational upon the addition of Co to Cu/Al2O3, consistent with
UV-DRS results which showed changes to the coordination
symmetry and geometry of copper oxide species. Figure 4 shows an
increase in the Co/Al atomic ratio, while the Cu/Al atomic ratio
remained almost unchanged. This is consistent with the loadings
and indicates that Co and Cu species are well dispersed on the
surface and in the bulk of the catalysts.
Figure 5. (IOh/IOhD) ratio of octahedral Cu2+species (IOh) and
octahedral distorted Cu2+ species (IOhD) as a function of Co content
for Cu/Al2O3 and Co(y)Cu/Al2O3 catalysts
Figure 6 shows the TPD-NH3 profiles of Cu/Al2O3 and
Co(y)Cu/Al2O3 oxide catalysts. After deconvolution of the
desorption profile with a Gaussian function, acid sites distribution
was calculated from areas spanning different ranges of
temperature: weak acid sites(<300 °C), medium acid sites (between
300 and 500 °C)and strong acid sites (> 500 °C ) 31. Figure 6 shows
that, except for Co(1.5)Cu/Al2O3, the addition of Co significantly
Figure 4. Co/Al and Cu/Al atomic ratio obtained from XPS
measurements for Cu/Al2O3 and Co(y)Cu/Al2O3 catalysts
Figure S1 (supplementary information) shows the UV-DRS
spectra of Cu/Al2O3 and Co(y)Cu/Al2O3oxide catalysts. This was
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increased the intensity of NH3 desorption peaks in comparison to
Cu/Al2O3.
Interestingly, the intensity of the desorption signals
decreased after further increase of Co content above 1.0 wt.%.
Table 4 gives quantitative evidence of the effect of variable Co Hydrogenolysis of glycerol over Cu-based catalysts have been
loading on acidity. Total amount of acid sites increases with Co reported in the literature
loading, goes through a maximum at a nominal Co content of 1.0 principal routes: (i) C-C bond breaking, in which glycerol is
wt%, and decreases at higher loadings. The increase in total acid converted to ethyleneglycol, and subsequently to methanol by
sites is probably related to the presence of adjacent Co2+ sites hydrogenation; and (ii) C-O bond cleavage(dehydration of glycerol)
through the formation of CuCo2O4. In contrast, the decrease of total to form hydroxyacetone (acetol) and 3-hydroxypropanal (3-HPA) as
View Article Online
2.2 Catalytic activity of CuO/Al2O3DOaIn: d10.1C0o3O9/xCC9uNOJ0/3A5l32O4F3
catalysts
7, 9, 12, 19, 33
and it proceeds through two
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49
50
51
52
53
54
55
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57
58
59
60
acid sites at higher Co content could be linked to formation of less-
intermediates, which are then further hydrogenated to form 1,2-
acidic aluminates (CoCuAl2O3). This is most evident on thecatalyst PDO and 1,3-PDO, respectively (1.2-PDO and 1.3-PDO can undergo
with the highest Co content (Co(1.5)Cu/Al2O3), in which a drastic additional hydrogenation to produced propanols). The intermediate
decrease in total amount of total acid sites could be related to the product, 3-hydroxypropanal, can also be dehydrated to 2-propenal
presence of CoCuAl2O3 species, as deduced from TPR result.
(acrolein) and propanoic acid1, 19, 33, as shown in Scheme 1. Table 5
summarizes the catalytic activity of the Cu/Al2O3 and Co(y)Cu/Al2O3
oxide catalysts, represented by initial intrinsic rate (expressed as
molecule of glycerol converted per atom of Cu per time). Figure 7
illustrates the result.
Co(1.5)Cu/Al2O3
------ Co(1.3)Cu/Al2O3
Co(1.0)Cu/Al2O3
Co(0.8)Cu/Al2O3
Cu/Al2O3
Table 5. Catalytic activity and selectivity of Cu/Al2O3 and Co(y)Cu/Al2O3
catalysts in hydrogenolysis of glycerol at 220 °C and 5 MPa H2
Catalysts
Intrinsic
Selectivity
rate
(calculated at 10 % of glycerol conversion)
(* 10-3 molec
gly Cu at-1 s1)
1,2-
acetol
1-P
Ethy
PA
oths
PDO
Cu/Al2O3
3.9
4.2
6.9
7.0
4.1
65
79
61
78
87
20
15
24
19
12
6.5
6.0
5.0
1.0
1.0
--
--
--
--
8.5
--
Co(0.8)Cu/Al2O3
Co(1.0)Cu/Al2O3
Co(1.3)Cu/Al2O3
Co(1.5)Cu/Al2O3
--
2.0
--
8.0
--
2.0
--
--
--
150
300
450
600
Temperature (oC)
The addition of Co increased the catalytic activity, reached a
maximum with Co content of 1.0and 1.3 wt.%, and then decreased
at a higher Co loading. The increase in activity could be explained by
two complementary effects. The effect is related to the presence of
CuCo2O4 species, as obtained from Rietveld refinement of XRD
patterns. The formation of CuCo2O4 species is alluded to as the
reason for the highest amount of acid sites on the Co(1.0)Cu/Al2O3
catalyst. This increase of acidity could favour dehydration of
8.0
Figure 6 TPD-NH3 profiles of Cu/Al2O3 and Co(y)Cu/Al2O3 catalysts
Regardless of the change in the total amount of acid sites, it can be
estimat
Table 4 Total acidity and acid sites distribution on calcined
ed from
Cu/Al2O3 and Co(y)Cu/Al2O3 catalysts (x10-4mol NH3 g-1)
Table 4
Catalysts
Weak
Medium
Strong
Total
that
weak
acid
sites
account
for 55-
68 % of
Cu/Al2O3
Co/Al2O3
Co(0.8)Cu/Al2O3
Co(1.0)Cu/Al2O3
Co(1.3)Cu/Al2O3
Co(1.5)Cu/Al2O3
0.97
4.50
7.79
9.73
7.88
0.69
0.29
9.38
2.54
5.14
2.82
0.37
0.18
3.22
1.07
2.23
1.69
0.20
1.4
17.1
11.4
17.1
12.4
1.3
6.0
4.0
2.0
0.0
the total acid sites. Thus, weak acid sites are mainly responsible for
any acid-catalysed reactions during the hydrogenolysis of glycerol.
1.5
1.0
0
0.8
1.3
Co wt % in Co(y)Cu/Al2O3 catalysts
glycerol to acetol. The second effect is related to changes of
coordination symmetry and geometry of the copper oxide species
observed by DRS-UV. The presence of multiple adjacent Co sites on
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the surface of Cu/Al2O3(as deduced from XPS results) could favour presence of acid sites, probably associated with Co2+, and XRD
View Article Online
the hydrogenation step, specifically to the formation of 1,2-PDO.
quantitative analysis reveal the existence of
(CuCo2O4).
Although the surface concentration of these sites differs
DOI: 10.10m39i/xCe9dNJ0o3x5id34eFs
6
depending on Co content, the nature of the sites remains largely
unaffected. This is probably one of the reasons why product
selectivity did not vary substantially with Co content. Another
reason could be the relative activity of the multiple sites that
participate in the transformation of glycerol. It is not yet clear
whether the coexistence of these sites promotes or hinders each
other. For example, metallic Co and Cu are both hydrogenation sites
but Cu/Al2O3 and Co(1.0)Cu/Al2O3 have similar selectivity towards
1.2-PDO formation. Therefore, it is not certain whether these sites
are beneficial to the formation of 1.2-PDO. Another example is the
role of acidity on acetol formation, an intermediate to 1.2-
PDO.Kinetic and spectroscopic analysis of individual pathways are
on-going to shed more insight into the role of these sites on the
main reaction routes.
7
8
9
Figure 7. Catalytic activity as a function of Co content on
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Co(y)Cu/Al2O3 catalysts
In general, the yields of products over all the catalysts
were similar. It can be observed that hydrogenolysis of glycerol led
to 1.2-PDO as the principal product, acetol as an intermediate
product, and 2-propanol (2-P) as
a minor product. Other
productsbserved in trace amounts include propanoic acid (PA), 1,3-
PDO, acetic acid, propanal and ethylene glycol (Ethy). But,
selectivity calculated at a glycerol conversion of 10%, present
differences after Co oxide addition on the Cu/Al2O3 catalysts, which
Scheme 1: Reactions network for hydrogenolysis of glycerol over
1
metal-based catalysts reported by Gandarias et al.
are evidenced in Figure 8 and Table 5. Selectivity to1.2-PDO ranges
from 61-87%, and the selectivity to acetol ranges from 12-24%,
indicating that Co(y)Cu/Al2O3 catalysts significantly favor the
consecutive C-O bond cleavage and hydrogenation pathway over
the other pathways. Figure 8 shows that no clear trend can be
observed with Co content. This is not unexpected because the main
products are formed sequentially. According to previous studies,
the initial transformation of glycerol to acetol is catalyzed by acid
sites, while
Others
Prop. Ac.
Ethy
1-prop
acetol
1,2-PDO
100
80
60
40
20
0
the subsequent hydrogenation to 1.2-PDO is catalyzed by active
19, 33, 62,
metallic sites
63. Characterization results indicate the
presence of these sites on the Co(y)Cu/Al2O3 catalysts. XPS results
indicate the presence of metallic Cu and Co, UV-DRS and TPR results
hint at adjacent Cu and Co atoms, TPD-NH3 results show the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
1.3
1.5
0
0.8
1.0
Co wt. % in Co(y)Cu/Al2O3 catalysts
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Figure 8. Product distribution calculated at 10% conversion of
glycerol on Cu/Al2O3 and Co(y)Cu/Al2O3 catalysts
9.
10.
11.
12.
Conclusions
This study showed two main effects. First, alumina-
supported Cu catalysts are active for the hydrogenolysis of glycerol.
Second, the addition of CoO to CuO/Al2O3 had a beneficial effect on
catalytic activity. However, the effect of Co loading on activity was
parabolic. Characterization results show that the addition of
appropriate amount of Co to Cu/Al2O3 leads to the formation of
adjacent Co and Cu atoms, increase in the number acid sites, and
formation of mixed oxides (CuCo2O4), leading to enhanced activity.
On the other hand, a high Co loading (1.5 wt.%) decreases acidity
and promotes the formation of aluminates (CoCuAl2O3), leading to
13.
14.
15.
16.
diminished activity. All the catalysts exhibited
a substantial
preference for C-O hydrogenolysis to acetol and the subsequent
hydrogenation to 1.2-PDO due to presence of metallic and acidic
sites. The lack of consistent trend of product selectivity as a
function of Co loading is linked to the existence of the same type of
active sites on the catalysts, regardless of Co content.
17.
18.
19.
20.
Conflicts of interest
There are no conflicts to declare
21.
22.
23.
24.
Acknowledgements
The authors thank CONICYT-Chile for FONDECYT Nº 11130376. The
authors also gratefully acknowledge the financial support under
CONICYT PIA CCTE AFB170007, FONDEQUIP EQM 120096 and
150103 grants. This work was also supported by funding from
Millennium Science Initiative of the Ministry of Economy,
Development and Tourism, Nuclei on Catalytic Processes towards
Sustainable Chemistry (CSC) grant.
25.
26.
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DOI: 10.1039/C9NJ03534F
Table of contents
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The CuCo2O4 species formed by CoO addition on CuO/Al2O3 had a beneficial effect on
catalytic activity in hydrogenolysis of glycerol
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CuCo2O4
CoOx
CoCuAl2O3O4
CuO/Al2O3
ri (molec gly atCu-1 s-1)
1.5
1.3
1.0
0
0.8
Glycerol
1,2-PDO
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0
4