Cu catalysts supported on CaO/MgO for glycerol conversion to lactic acid in alkaline medium employing a continuous flow reaction system

Article abstract of DOI:10.1039/d0ra06547a

The production of lactic acid (LA) from glycerol in alkaline medium was investigated using Cu catalysts supported on CaO, MgO and xCaO/MgO (x = 5, 10, 15 wt%), employing a continuous flow reaction system over a period of 30 h. In addition to assessing the effect of the composition of the catalytic support, the influence of the temperature (200-260 °C), NaOH/glycerol molar ratio (0.5-1.5), hydroxide type (NaOH and KOH), as well as the influence of concentration (10 and 20 vol%) and purity of glycerol was investigated. The catalysts were prepared by a wet impregnation method and characterized by XRF, XRD, N2 adsorption-desorption, H2-TPR and CO2-TPD. The catalytic tests showed that the use of NaOH results in higher yields to LA. Cu catalysts supported on xCaO/MgO exhibited better catalytic performance than the CuCa and CuMg catalysts. The LA yield increases with the increase of the reaction temperature from 200 to 240 °C, and then decreases with a subsequent increase to 260 °C. NaOH/glycerol molar ratios greater than 1.25 are not necessary, since high yield to LA (96.9%) was obtained in the catalytic test performed using a molar ratio of 1.25. The catalysts showed excellent stability without evidence of deactivation over the evaluated period. This journal is

Full text of DOI:10.1039/d0ra06547a

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Cu catalysts supported on CaO/MgO for glycerol
conversion to lactic acid in alkaline medium
employing a continuous flow reaction system
Cite this: RSC Adv., 2020, 10, 31123
Arthur M. Bruno, Thiago D. R. Simoes,
Mariana M. V. M. Souza
˜
*
and Robinson L. Manfro
The production of lactic acid (LA) from glycerol in alkaline medium was investigated using Cu catalysts
supported on CaO, MgO and xCaO/MgO (x ¼ 5, 10, 15 wt%), employing a continuous flow reaction
system over a period of 30 h. In addition to assessing the effect of the composition of the catalytic
support, the influence of the temperature (200–260 ꢀC), NaOH/glycerol molar ratio (0.5–1.5), hydroxide
type (NaOH and KOH), as well as the influence of concentration (10 and 20 vol%) and purity of glycerol
was investigated. The catalysts were prepared by a wet impregnation method and characterized by XRF,
XRD, N2 adsorption–desorption, H2-TPR and CO2-TPD. The catalytic tests showed that the use of NaOH
results in higher yields to LA. Cu catalysts supported on xCaO/MgO exhibited better catalytic
performance than the CuCa and CuMg catalysts. The LA yield increases with the increase of the reaction
temperature from 200 to 240 ꢀC, and then decreases with a subsequent increase to 260 ꢀC. NaOH/
glycerol molar ratios greater than 1.25 are not necessary, since high yield to LA (96.9%) was obtained in
the catalytic test performed using a molar ratio of 1.25. The catalysts showed excellent stability without
evidence of deactivation over the evaluated period.
Received 28th July 2020
Accepted 14th August 2020
DOI: 10.1039/d0ra06547a
rsc.li/rsc-advances
expected to direct the increased demand for LA in the coming
years. The global LA market size was valued at 1.29 billion USD
in 2016 and is expected to reach 9.8 billion USD in 2025.5
European demand for PLA in 2010–2012 was around of 25 000
tons per year, and could potentially reach 650 000 tons per year
in 2025.6
1. Introduction
2-Hydroxypropanoic acid, also named lactic acid (LA), is widely
used in the cosmetic industry as a moisturizer and in the textile
industry as a mordant (i.e. a chemical that enhances the color
durability). Moreover, LA is used in the dairy industry as a pH
regulator, preservative or even as an inhibitor of bacterial
spoilage. It is currently being used as a precursor of “green”
solvents such as ethyl lactate, and also employed in the
synthesis of poly lactic acid (PLA), which is increasingly used in
the production of biodegradable packaging.1 In addition, LA is
considered a platform molecule from which valuable industrial
oxygenated compounds are obtained such as: acrylic acid, 2,3-
pentanedione, pyruvic acid, propanoic acid, 1,2-propanediol
and acetaldehyde.2
Market studies conducted in the early 2000s pointed to
a growth of 19% per year in the consumption of LA for chemical
applications.3 The worldwide consumption of LA in 2005 was
estimated at 130 000–150 000 tons;3 in the year 2010 the
worldwide demand for LA was around of 300 000–400 000 tons.4
The global LA market is expected to grow from 714.2 kilotons in
2013 to 1960.1 kilotons in 2020.5 According to the report of
Global View Research (California, USA), the production of PLA is
The physical properties of PLA depend on the isomeric purity
of LA (L(+)- or D(ꢁ)). For example, L(+)-LA with high purity gives
PLA of high melting point and high crystallinity.7 Furthermore,
pure L(+)-LA can be polymerized to a high crystal polymer suit-
able for ber and oriented lm production and is expected to be
useful in the production of crystal liquids.8
Currently, commercial production of LA is based on chem-
ical synthesis or fermentation processes.4,9 Production by the
fermentation processes has the advantage that a pure isomers
(L(+)- or D(ꢁ)-LA) can be obtained by choosing an appropriate
strain of LA bacteria,10 while chemical synthesis always results
in a racemic mixture of LA.11 The fermentation has a high yield
(90%) into lactic acid but several problems are associated with
the process, such as the high cost of culture media, due to the
specic requirements of LA producing bacteria,12 inhibition by
the product,13 and high cost of purication process, besides
being a nonecological process.14 In addition, the fermentation
route may not be able to meet the growing demand for LA of the
chemical industry.15
´
Escola de Quımica, Universidade Federal do Rio de Janeiro (UFRJ), Centro de
Research of new alternatives for chemical routes to produce
LA has been of great interest in the last decades.16 One of the
Tecnologia, Bloco E, Sala 206, CEP 21941-909, Rio de Janeiro/RJ, Brazil. E-mail:
robinson@eq.ufrj.br; Tel: +55-21-39387643
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alternative production routes is the hydrothermal conversion of studies presented by Zhang et al.25 and Shimanouchi et al.26
glycerol to LA in alkaline medium which was rst described by which employed continuous ow reactor but with alkaline
Kishida et al.17 In this study Kishida used an autoclave reactor homogeneous catalysis. In previous studies of our group,27
and NaOH or KOH as catalyst and aer 90 min at 300 ꢀC an using a continuous-ow reaction system, it was observed that
aqueous solution of glycerol was converted into sodium lactate the catalyst with the highest basicity presents highest yields to
with yield higher than 90 mol% with 100% of glycerol conver- LA. A yield of LA around of 90% was obtained in the catalytic
sion, employing NaOH or KOH/glycerol molar ratio equal to tests carried out with 20 wt% Cu/MgO catalyst (NaOH/glycerol
molar ratio ¼ 1, 240 C/35 atm, WHSV ¼ 2 hꢁ1).
ꢀ
3.78.
High temperatures are required when homogeneous cata-
The production of LA from glycerol has been highlighted due
lytic process is used. The high temperature required for the to the growing production of biodiesel since glycerol generation
dehydrogenation of glycerol to glyceraldehyde may promote represents 10% of the total volume of biodiesel produced.28
a degradation of LA and pyruvaldehyde adversely affecting the Therefore, there is a surplus of glycerol in the world market and
selectivity to LA.18 To overcome this problem, Roy et al.19 the development of technologies that use glycerol besides giving
investigated the use of a suitable catalyst for dehydrogenation of it a noble destination can value the productive chain of bio-
glycerol in alkaline medium at low temperature. In this study, diesel, because the cost of the biodiesel's production is still
they observed that use of copper catalysts together with NaOH much higher than that of diesel fuel.29
provides a promising one-pot, low temperature route for glyc-
According to Renewables 2017 Global Status Report,30 the
erol conversion to LA. In this way, several investigations have world production of biodiesel in 2016 was 30.8 billion liters, and
been carried out focusing on the study of the combined use of may have generated 3.0 billion liters of glycerol, approximately.
heterogeneous catalysts in alkaline medium.9
Estimates for several countries show that the production of
Heterogeneous catalysts usually employed for hydrothermal biodiesel and glycerol will continue to increase worldwide due
conversion of glycerol into LA are based on noble metals such as to the mandatory use. For example, in Brazil the increase in the
Pt, Au and Pd. In addition to these metals, the use of copper has compulsory addition of biodiesel to diesel oil occurs since 2008,
also been studied, since copper is conventionally used as when it became mandatory to add biodiesel to diesel oil.
a catalyst for the dehydrogenation of ethanol.20 The glycerol Initially the mandatory blending of biodiesel to diesel oil was
dehydrogenation to glyceraldehyde is the rst step in the 2% (B2), and through successive increases over the years in 2017
production of LA from glycerol, hence the study of copper as reached 8% (B8) and from March of 2020 the Brazilian
catalyst has been evaluated besides the use of non-noble metals government increased the percentage of biodiesel in diesel oil
is more economical.
to 12% (B12). According to National Agency of Petroleum,
Arcanjo et al.21 investigated the use of Pt and Pd catalysts Natural Gas and Biofuels (ANP), the production of biodiesel in
supported on activated carbon. In this study it was obtained 2019 in Brazil was 5.9 billion liters,31 and there is an expectation
yield to LA of 68% and 74% using 10% Pd/C and 5% Pt/C as of an even higher production in 2020 due to the increase in
catalysts, respectively (molar ratio NaOH/glycerol ¼ 1.1, 230 ꢀC, demand for biodiesel promoted by the Brazilian government,
3 h). Ftouni et al.22 obtained aer 24 h of reaction at 180 ꢀC which aims to reach the B15 in 2023 according to the National
a yield of LA up to 84% employing 1.2% Pt/ZrO2 as catalyst and Energy Policy Council of the Ministry of Mines and Energy
NaOH/glycerol molar ratio equal to 1.8. Other studies using (CNPE).
copper-based catalysts demonstrate similar or even better
The objective of this study was to investigate the production
performances than noble metal catalysts. Yin et al.20 employing of LA from glycerol in alkaline medium, using copper catalyst
hydroxyapatite (HAP), MgO and ZrO2 as catalytic support for Cu supported on CaO/MgO at different calcium oxide loadings, in
(16 wt% Cu loading) obtained yield to LA of 81.9%, 80.1% and continuous ow reaction system over a period of 30 h. Accord-
32.0%, respectively (molar ratio NaOH/glycerol ¼ 1.1, 230 ꢀC, ing to the literature, there is evidence that the use of catalytic
2 h). The authors observed that Cu/MgO and Cu/HAP catalysts supports with basic characteristics presents better performance
exhibited higher LA yield because of higher basicity compared in glycerol transformation to LA compared to other catalytic
to Cu/ZrO2 catalyst. Shen et al.23 evaluated the effect of different- supports of lower basicity. Thus, we intend to develop a catalytic
sized Cu2O nanoparticles in the hydrothermal conversion system with high catalytic performance making possible the use
reaction of glycerol to LA and obtained a yield of 82.4% to LA of a lower NaOH/glycerol molar ratio. Additionally, copper has
aer 2 h (molar ratio NaOH/glycerol ¼ 1.2, 230 ꢀC). On the other been shown effective as catalyst for this reaction.19 Further-
hand, Yin et al.24 investigated the effect of different-sized Cu0 more, we evaluated the performance of the KOH and NaOH
nanoparticles and observed that the smaller metallic Cu0 bases, the effects of reaction temperature (200, 220, 240 and 260
nanoparticles have higher catalytic activity for the conversion of ꢀC), NaOH/glycerol molar ratio (0.5, 0.75, 1.0, 1.25, 1.5), glycerol
glycerol to lactic acid. This result was associated with the rapid concentration (10 and 20 vol%) and of the use of crude glycerol.
reaction over small-sized metallic Cu0 nanoparticles which
inhibits product degradation reactions and polymer-like
chemicals through the interaction between glycerol and
products.
2. Materials and methods
2.1. Characterization of catalysts
Most of the works in the literature of glycerol conversion to The supported copper catalysts (CuMg, CuCa, Cu5CaMg,
LA was carried out using batch reactor. Some exceptions are the Cu10CaMg and Cu15CaMg) were prepared by wet impregnation
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method with a loading of 20 wt% of CuO in calcined sample and
the CaO content in the mixed support (CaO/MgO) was 0, 5, 10,
15 and 100 wt%.
k ꢂ l
d ¼
(1)
b ꢂ cos q
˚
where d is the average diameter of the crystals (A), k is a constant
that depends on the particle shape (for sphere, k ¼ 0.94), l is the
wavelength of the X-ray source, b is the full width at half
maximum (FWHM) in radians and q is the diffraction angle.
The metallic dispersion (D) of the catalysts was estimated
according to Anderson32 (eqn (2)):
The MgO support was obtained from calcination of magne-
sium nitrate (Mg(NO3)2$6H2O) (Vetec), which was pre-calcined
ꢀ
in muffle at 350 C for 2 h with the aim of removing most of
ꢀ
the hydration water, and then calcined at 500 C for 3 h undꢁe1r
owing air (60 mL minꢁ1) using a heating rate of 10 C min
ꢀ
(rate used at all steps of the synthesis). Using the wet impreg-
nation methodology CaO was added to MgO in appropriate
amounts to produce catalysts with 5 wt%, 10 wt% and 15 wt% of
CaO in the calcined catalyst. To synthesize the mixed oxide
supports (CaO/MgO), appropriate quantities of calcium nitrate
(Ca(NO3)2$4H2O) (Vetec) were solubilized in 100 mL of distilled
and deionized water and added to the MgO. This mixture was
placed in a ask, which was coupled in a rotary evaporator (IKA
RV 10 Digital) and kept in rotation (120 rpm) for 1 h to
homogenize the mixture, and then the mixture was heated to
6 ꢂ Vm
D ¼
(2)
d ꢂ Am
where Vm is the atomic volume of the Cu (0.0118 nm3), d is the
average diameter of the crystals (in nm) and Am is the surface
area of a single atom of Cu (0.068 nm2).
From the Cu dispersion data, it is possible to determine the
Cu metal surface area per unit weight of catalyst (mꢁ2 gcat
)
ꢁ1
according to equation (eqn (3)):
D ꢂ AV ꢂ LCu
ꢀ
80 C under vacuum and maintained at this temperature until
SACu
¼
(3)
100 ꢂ WCu ꢂ NA
complete evaporation of the water. In sequence, the supports
were dried at 110 ꢀC overnight and subsequently calcined at
where D, AV, LCu, WCu, and NA are dispersion (%), Avogadro's
number, Cu content (wt%) in the catalyst, atomic weight of Cu
(63.5 g molꢁ1), and number of surface Cu atoms in unit surface
area (value commonly used in the literature 1.47 ꢂ 1019 atoms
per m2 (ref. 33)), respectively.34
The textural characteristics were determined by N2 adsorp-
tion–desorption at ꢁ196 ꢀC in a Micromeritics TriStar 3000. The
specic area of the samples was obtained using the Brunauer–
Emmett–Teller (BET) method and the pore volume obtained by
Barrett–Joyner–Halenda (BJH) method. Prior to the analysis the
samples were outgassed for 24 h at 250 ꢀC in vacuum. The
specic area of samples was calculated using the relative pres-
sure range 0.06–0.20 and the pore volume was determined by
BJH adsorption cumulative volume of pores between 2 nm and
60 nm of diameter.
700 C for 3 h under owing air (60 mL minꢁ1). Finally, aer
ꢀ
steps of preparation of the catalytic supports (Mg, 5CaMg,
10CaMg and 15CaMg), CuO was impregnated over these
supports employing adequate amounts of copper nitrate
(Cu(NO3)2$3H2O) (Vetec). The impregnation of the CuO was
carried out following the same procedure adopted to prepare
the mixed oxide supports, except the calcination temperature,
ꢀ
which was 500 C.
For the synthesis of the CuCa catalyst, calcium oxide (CaO)
was obtained by calcining calcium nitrate (Ca(NO3)2$4H2O –
ꢀ
Vetec), which was pre-calcined in a muffle at 700 C for 2 h, in
order to decompose the nitrates and remove the water present,
and in the sequence it was calcined at 700 ꢀC for 3 h under
owing air (60 mL minꢁ1). Then, the CaO was impregnated with
CuO following the methodology already described above.
The reducibility of the catalysts was analyzed by temperature-
programmed reduction (TPR), carried out in a microow reactor
operating at atmospheric pressure. TPR analysis was carried out
in a conventional apparatus equipped with a thermal conduc-
tivity detector (TCD). The samples (ꢃ50 mg) were rstly dehy-
drated at 150 ꢀC under argon (Ar) ow before the reduction. The
samples were then cooled at room temperature. Aerwards, tꢁh1e
2.2. Catalyst characterization
The chemical composition of the catalysts was determined by X-
ray uorescence (XRF) using a Rigaku Primini spectrometer,
equipped with X-ray generator tube of palladium.
ꢀ
ꢀ
heating was initiated up to 1000 C with a rate of 10 C min
,
using a mixture of 1.8% H2/Ar owing at 30 mL minꢁ1 through
the sample. Decomposition of the peaks was performed using
Gaussian shaped components. Agreement factors were in all
cases higher than 0.98.
X-ray powder diffraction (XRD) patterns were recorded in
a
Rigaku Miniex II X-ray diffractometer equipped with
˚
a graphite monochromator using CuKa radiation (l ¼ 1.54178 A),
X-ray tube operated at 30 kV with 15 mA and the diffractograms
obtained were compared with the database of the Joint
Committee on Powder Diffraction Standards (JCPDS). The
measurements were carried out with steps of 0.05ꢀ using
a counting time of 1 s per step and over the 2q range of 5ꢀ to 90ꢀ.
Reduced catalysts were analyzed aer ex situ reduction under the
same conditions used before the catalytic tests and spent cata-
lysts were analyzed without any other treatment aer reaction.
The average crystallite sizes of the Cu0 were calculated by the
Scherrer equation (eqn (1)), using the diffraction peak corre-
spondent to (200) plane at 2q ¼ 50.4ꢀ:
The catalyst basicity was investigated by temperature-
programmed desorption of carbon dioxide (CO2-TPD), which
was performed using a mass spectrometer QMG-220 (Pfeiffer).
The samples of the catalytic supports were previously treated at
150 ꢀC for 30 minutes under He ow (30 mL minꢁ1) before
adsorption of CO2. This prior treatment was not performed on
the catalysts, since they were rstly reduced by a mixture of 1.8%
H2/Ar up to 600 ꢀC at a heating rate of 10 ꢀC minꢁ1 with an
isothermal at 600 ꢀC for 30 min. Carbon dioxide adsorption was
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carried out at room temperature with 10% CO2/He (30 mL minꢁ1
)
ꢄ Selectivity to product i:
Selið%Þ ¼
for 30 min, followed by purging with He for 1 h. The desorption of
chemisorbed CO2 was realized by heating up to 1000 ꢀC at
20 ꢀC minꢁ1, under owing pure He (30 mL minꢁ1). The ratio m/z
¼ 44 was used for quantication of carbon dioxide. Decompo-
sition of the peaks was performed using Gaussian shaped
components considering each shoulder as a peak. Agreement
factors were in all cases higher than 0.98.
Mi ꢂ nCi
(5)
MgXlycerol ꢂ 3
where Mi is molar concentration of product i, nCi number of
carbon atoms of product i, MXglycerol is converted moles of
glycerol;
ꢄ Yield to product i:
Selið%Þ
Yið%Þ ¼ Xð%Þ ꢂ
(6)
100
2.3. Catalytic tests
The glycerol conversion to lactic acid was carried out in
a continuous system using a xed-bed reactor of Inconel 625,
with internal diameter of 0.5 cm and length of 15 cm. Aliquots
were collected hourly for further HPLC analysis. All catalytic
tests were performed during 30 h. However, before taking the
30 h of reaction, 2 h were taken for purging the reaction system.
The period among 9 and 22 h of reaction was not analyzed
(overnight). Although the rst 2 h of reaction have been dis-
carded due to the purging of the reaction system, in the rst
hours the results showed some uctuation; however, aer
overnight period the results were much more stable, making it
possible a consistent analysis of results.
Turnover frequencies (TOF) values were calculated from the
average formation rate (mol hꢁ1) of LA obtained between 23 and
30 h of reaction divided by the number of metallic sites (mol),
which was determined using the number of moles of Cu present
in the mass of catalyst employed in the reaction multiplied by
the dispersion, eqn (7).
À
Á
LAðg Lꢁ1Þ
ꢂ volumeðL hꢁ1Þ=molLAðg molꢁ1Þ
average
À
TOF hꢁ1
¼
Á
ꢁ1
mol of Cu mol gcat ꢂ 1:25 gcat ꢂ dispersion
(7)
The catalysts were reduced in situ using a mixture of 30% H2/
N2 with a total ow of 90 mL minꢁ1 and a heating rate of
10 ꢀC minꢁ1 up to 600 ꢀC for 30 min. The nal reduction
temperature was determined based on the results of the TPR
analysis. It was evaluated reaction variables such as hydroxide
3. Results and discussion
3.1. Characterization of catalysts
The chemical composition of the catalysts calcined at 500 ꢀC/3 h
is presented in Table 1. The theoretical composition of CuO for
all catalysts was 20 wt%. For the CuMg and CuCa catalysts, MgO
and CaO were the only supports used, respectively. However, the
catalysts Cu5CaMg, Cu10CaMg and Cu15CaMg were synthe-
sized with different CaO loads, i.e. 5, 10 and 15 wt%, respec-
tively. The real chemical composition of the catalyst was very
similar to the nominal ones, with only minor differences that
may be attributed to losses during synthesis and/or due to the
imprecision of the equipment. Thus, it is observed that the
synthesis method is effective in the preparation of catalysts with
the desired chemical composition.
ꢀ
ꢀ
type (KOH and NaOH), temperature (200 C/20 atm, 220 C/25
ꢀ
ꢀ
atm, 240 C/35 atm and 260 C/45 atm), NaOH/glycerol molar
ratio (0.5, 0.75, 1.0, 1.25 and 1.5), glycerol concentration (10 and
20 vol%), as well as the use of crude glycerol with 85% of purity
determined by HPLC analysis. The reagents were injected to the
reactor by an HPLC pump (Eldex 1SAM). The pressure in the
reactor was controlled using a diaphragm-type back pressure
regulator (GO Regulator valve model BP-3). The mass of catalyst
used was 1.25 g and the feed rate of 0.041 mL minꢁ1, repre-
senting a weight hourly space velocity (WHSV) equal to 2 hꢁ1
and as the catalyst bed is distributed in approximately 6 cm in
the reactor, the residence time of glycerol is 29 min.
Fig. 1 shows the XRD patterns of the catalytic supports,
catalysts calcined at 500 ꢀC/3 h, catalysts reduced at 600 ꢀC/
30 min, and spent catalysts aer reactions (30 h). Except for
CuCa catalyst, all the diffractograms of the catalytic supports,
calcined and reduced catalysts exhibited the MgO phase with
reections at 2q ¼ 36.9ꢀ, 42.9ꢀ, 62.3ꢀ, 74.6ꢀ and 78.6ꢀ (JCPDS 45-
0946). Fig. 1(C)–(E) show that aer impregnation of MgO with
CaO and subsequent calcination at 700 ꢀC, the catalytic
supports display a CaO crystalline phase with main diffraction
peaks at 2q ¼ 32.2ꢀ, 37.3ꢀ, 53.8ꢀ, 64.1ꢀ and 67.3ꢀ (JCPDS 48-
1467), and the intensity of the diffraction peaks increases with
increasing CaO load. With these diffractograms, it is observed
that the calcination temperature was effective in producing
a crystalline CaO phase.
The liquid phase was analyzed in a Shimadzu Prominence
HPLC with a Bio-Rad Aminex HPX-87 H column (300 ꢂ 7.8 mm)
at 60 C, using 0.01 M H2SO4 as eluent at 0.6 mL minꢁ1, and
ꢀ
refractive index (RID) and ultraviolet (UV) detectors. Samples for
analysis were prepared by diluting aliquots of the reaction
mixture in an aqueous solution of 0.01 M H2SO4, with dilution
factor of the 1 : 100, thus, the nal pH was very close to eluent of
HPLC. The catalyst performance in terms of glycerol conver-
sion, selectivity and yield to products i, was calculated accord-
ing to eqn (4)–(6):
ꢄ Glycerol conversion:
in
glycerol
out
glycerol
M
ꢁ M
glycerol
Xð%Þ ¼
ꢂ 100
(4)
in
M
Mahdavi and Monajemi35 prepared CaO/MgO/Al2O3 catalysts
in
where M
is the molar concentration of glycerol in the feed, by co-precipitation and obtained CaO and MgO crystalline
glycerol
out
M
glycerol
is the molar concentration of glycerol aer the phases by calcination temperature of 700 ꢀC for 5 h. On the
reaction;
other hand, it is usual to use higher calcination temperatures
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Table 1 Chemical composition of the catalysts calcined, and textural properties of the support and calcined catalysts at 500 ꢀC/3 h
Catalyst
CuO (wt%)
CaO (wt%)
MgO (wt%)
SBET (m2 gꢁ1) Cat/Supa
Vpore (cm3 gꢁ1) Cat/Supa
Dpore (nm) Cat/Supa
CuMg
CuCa
Cu5CaMg
Cu10CaMg
Cu15CaMg
20.6
21.0
21.8
21.2
19.4
—
79.0
5.7
9.8
15.2
79.4
—
72.5
69.0
65.4
16/20
<10/<10
12/13
20/10
10/15
0.036/0.028
0.006/0.023
0.023/0.036
0.073/0.027
0.031/0.028
13.1/5.7
12.9/27.4
11.8/18.1
18.6/16.0
19.5/7.1
a
Cat ¼ catalyst; Sup ¼ support.
(ꢃ800 ꢀC) due to the high stability of calcium carbonate aer impregnation of the catalytic supports with CuO and
(CaCO3).36 However, our synthesis process had the formation of subsequent calcination at 500 ꢀC/3 h the formation of a new
CaCO3 minimized. It is observed in Fig. 1(A) and (C)–(E), that phase of CuO takes place, with diffraction peaks at 2q ¼ 35.5ꢀ
Fig. 1 XRD patterns of the catalytic support, catalysts calcined, catalysts reduced and catalysts spent in the reactions performed at 240 ꢀC/35
atm for 30 h with NaOH/glycerol molar ratio ¼ 0.75. (A) CuMg, (B) CuCa, (C) Cu5CaMg, (D) Cu10CaMg and (E) Cu15CaMg.
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and 38.7ꢀ (JCPDS 48-1548). The low intensity of the peaks of support also showed a complete transformation of the crystal-
CuO phase may be associated with a great dispersion of the CuO line structure of the supports. Thus, it can be seen that aer the
phase37 or it results from a phase with low crystallinity. In reactions, the Cu5CaMg, Cu10CaMg and Cu15CaMg catalysts
addition, it is noted that the addition of CuO promotes exhibited a mixture of several phases, such as Mg(OH)2, MgCO3,
a reduction in the intensity of the diffraction peaks of the CaCO3, and in addition to these, a phase known as dolomite
catalytic support, which may be related to a reduction in its (CaMg(CO3)2) (JCPDS 75-1758) was also observed. Therefore, the
crystallinity. Yin et al.20 synthesized copper catalysts supported catalytic support of all catalysts is completely transformed into
on MgO and observed a loss of the crystallinity of the support different crystalline phases during the reaction.
with the increase of copper concentration in the catalysts, which
It is observed that for both CuMg and CuCa catalysts, the
was associated with high copper dispersion. Aer the addition metallic phase of copper (Cu0) is maintained even aer the 30 h
of CuO over the Cu5CaMg catalyst, the CaO phase is no longer of reaction. Therefore, it is noted that these catalysts have
observed, probably due to the loss of crystallinity associated excellent stability for the metallic phase. However, the dif-
with the low CaO content.
fractograms of the spent Cu5CaMg, Cu10CaMg and Cu15CaMg
The diffraction pattern of the calcined CuCa catalyst catalysts exhibited a new phase of copper oxide (Cu2O) with low
(Fig. 1(B)), in addition to presenting the characteristic peaks of intensity which displays main diffraction peaks at 2q ¼ 36.4ꢀ,
the CaO phase, showed that the addition of CuO to the support 42.3ꢀ and 61.3ꢀ (JCPDS 65-3288). Although there is an overlap of
generated a new crystalline phase of calcium copper oxide diffraction peaks in these spent catalysts, it is still possible to
(CaCuO2), which has diffraction peaks at 2q ¼ 16.8ꢀ, 28.3ꢀ, observe the peaks associated with metallic copper at positions
33.1ꢀ, 34ꢀ, 36.1ꢀ, 43.8ꢀ and 56.5ꢀ (JCPDS 48-0197). From the 2q ¼ 43.3ꢀ and 50.4ꢀ. Therefore, despite the formation of Cu2O,
composition of this catalyst and the conditions used in calci- the Cu0 phase is still observed aer the reactions.
nation, the formation of this two phases (CaO and CaCuO2) is in
From the diffractograms of the reduced catalysts it was
accordance with the results predicted by Roth et al.,38 which possible to determine the Cu average crystallite size and Cu
studied the phase equilibrium diagram for the binary system dispersion, employing the Scherrer equation and Anderson
CaO–CuO. Kaewpanha et al.39 prepared catalysts with different correlation, respectively, besides the metallic surface area
CuO loading supported in scallop shells (>99% CaO) by the wet (Table 2). These results were determined using the diffraction
impregnation method. Aer the calcination of the catalysts at peak at 2q ¼ 50.4ꢀ, which corresponds to (200) crystalline plane,
650 ꢀC/3 h, it was obtained a mixture of phases, including CaO, due to the absence of interference with other crystalline phases.
CuO and CaCuO2. The formation of CaCuO2 was not observed All catalysts exhibited similar crystallite size values, around
in the Cu5CaMg, Cu10CaMg and Cu15CaMg catalysts, which 14 nm, demonstrating that both MgO and CaO supports have
may be related to the low CaO concentration added to the a good capacity of stabilizing the active phase, avoiding the
catalyst and/or due to the strong interaction between CaO and sintering of copper particles during the CuO reduction. Another
MgO.
factor that may have contributed to this result may be associ-
Aer catalyst reduction, it can be observed the absence of ated with the dispersion of the CuO phase over the catalytic
diffraction peaks associated with CuO and CaCuO2 phases, support, which is in agreement with the XRD results.
which demonstrates that the conditions used for activation of
The catalysts also showed similar values of dispersion and
the catalysts are effective. The transformation of the CuO and metallic surface area, since these properties are intrinsically
CaCuO2 phases is accompanied by the formation of a new related to the crystallite size of Cu. The metallic dispersion
crystalline phase of metallic copper (Cu0), with main diffraction values are in the rangeꢁo1f 7.0 to 8.1%, with a metallic surface
peaks at 2q ¼ 43.3ꢀ, 50.4ꢀ and 74.1ꢀ (JCPDS 04-0836).
area around 8 m2 gcat . Zhang and He40 prepared Cu/CaO
Fig. 1 also shows the diffractograms of the catalysts used in catalysts with low loading of Cu (about 2 wt%) by the wet
the reactions aer 30 h reaction at 240 ꢀC/35 atm, using NaOH/ impregnation method and obtained Cu particle size of 12.9 nm
glycerol molar ratio ¼ 0.75 and 10 vol% glycerol solution. The and dispersion of 9%. Using the same methodology, Moreira
diffractogram data for all the used catalysts were multiplied by et al.27 obtained for the Cu/MgO (15 wt%) catalyst average Cu
2, to make the visualization of the crystalline phases present crystallite size of 12 nmꢁw1 ith a dispersion of 6.3% and metallic
clearer. The XRD of the spent CuMg catalyst (Fig. 1(A)) showed surface area 7.5 m2 gcat
.
that the MgO phase was completely converted into two new
The dispersion of the active phase (Cu) may be inuenced by
crystalline phases: (1) magnesium hydroxide (Mg(OH)2), with several factors, including the calcination temperature, copper
main diffraction peaks at 2q ¼ 18.6ꢀ, 38.0ꢀ, 50.8ꢀ, 58.6ꢀ and loading and the synthesis methodology. Nagaraja et al.41 evalu-
62.0ꢀ (JCPDS 07-0239) and (2) magnesium carbonate (MgCO3), ated the inuence of the synthesis methodology on Cu/MgO
with main diffraction peaks at 2q ¼ 32.6ꢀ, 42.9ꢀ and 53.8ꢀ catalysts. Among the different methodologies used, coprecipita-
(JCPDS 08-0479). In other studies of conversion of glycerol to LA tion, wet impregnation and solid–solid methods, coprecipitation
in alkaline medium, a complete transformation of the MgO method presented the highest copper dispersion (8.0%). Reddy
phase into Mg(OH)2 was also observed.20,27 The XRD of the spent et al.42 obtained for the Cu/MgO catalyst (20 wt%) prepared by
CuCa catalyst showed that the CaO phase was completely coprecipitation method an average crystallite size of 64 nm. The
transformed into calcium carbonate (CaCO3), which has main conditions used in the preparation of the catalyst may have
diffraction peaks at 2q ¼ 23.2ꢀ, 29.5ꢀ, 36.1ꢀ, 39.5ꢀ, 43.3ꢀ, 47.6ꢀ contributed to the big crystallite size. Although it used lower
and 48.6ꢀ (JCPDS 72-1650). The catalysts composed by mixed temperatures in calcination (450 ꢀC) and reduction (280 ꢀC) the
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Table 2 Copper crystallite size before reaction, metallic dispersion (D), metallic surface area (SACu), reduction degrees of Cu0 (RD) calculated
from H2-TPR results, amount of desorbed CO2, proportion of weak, medium and high strength basic sites and density of basic sites derived from
CO2-TPD analysis
Cu crystallite sizea
(nm)
Db
(%)
SACu
RDc
(%)
No. of basic sitesd
(mmol CO2 per gcat)
W : M : H strength basic
sitese (%)
Density of basic sites
(m2 gcat
)
(mmol CO2 per m2)
ꢁ1
Catalyst
CuMg
CuCa
Cu5CaMg
Cu10CaMg
Cu15CaMg
14.6 ꢅ 1.0
14.1 ꢅ 1.4
14.4 ꢅ 1.5
14.8 ꢅ 1.9
12.9 ꢅ 1.8
7.1
7.4
7.2
7.0
8.1
7.6
8.0
8.1
7.7
8.1
97
101
73
80
70
203
126
189
273
183
100 : 00 : 00
00 : 00 : 100
29 : 00 : 71
17 : 00 : 83
02 : 00 : 98
12.7
42.0
15.8
13.7
18.3
a
Calculated by the Scherrer equation using the (200) plane of Cu (2q ¼ 50.4ꢀ). b Estimated according to Anderson (eqn (2)). c Reduction degree (%)
d
¼ H2 uptake (experimental)/H2 uptake (theoretical) ꢂ 100. Amount of basic sites calculated from Gaussian deconvolution of CO2-TPD proles.
e
Ratio of weak : medium : high strength basic sites (%), calculated from CO2-TPD proles considering CO2 desorption peak (weak: peak below
400 ꢀC; medium: peak between 400–600 ꢀC; high: above 600 ꢀC).
high time employed in calcination (5 h) and reduction (3 h) may average pore diameter of the materials comprises the range of
have contributed to the formation of larger particles.
Effects of sintering processes on the catalysts aer reaction catalysts have a wide range of pore distribution.
mesoporous materials (2–50 nm).44 Thus, we observed that the
was possible to evaluate only for the CuCa catalyst since the
Fig. 2 shows the proles of temperature-programmed
other spent catalysts showed Cu diffraction peaks overlapping reduction of the catalysts calcined at 500 ꢀC for 3 h. It is
with other crystalline phases, which made the determination of observed that all catalysts showed peaks of reduction in the
ꢀ
ꢀ
the Cu crystallite size inadequate. Although a previous study range of 365 C to 685 C. However, the peaks observed in the
ꢀ
ꢀ
carried out by the group has shown that the sintering process range of 570 C to 685 C were associated with the decompo-
can be considered negligible,27 it was obtained an average Cu0 sition process CaCO3, which is formed due to CaO carbon-
crystallite size of 51 nm for the CuCa spent catalyst, this is an ation45 and/or due to the decomposition of residual carbonate,
indication that this catalyst has undergone sintering processes despite calcination at 500 ꢀC, which is in accordance with
during the reaction.
previous studies.46,47 In addition, Kalinkin et al.48 observed that
Table 1 presents BET surface area, pore volume and pore at room temperature, both CaO and Ca(OH)2 can react with
diameter obtained from N2 adsorption isotherms of the humidity and CO2 of the air, forming amorphous CaCO3 on the
supports and catalysts aer calcination at 500 ꢀC/3 h. It is surface of the particles, which is in accordance with the XRD
observed that both the supports and the catalysts showed low results of the calcined CuCa catalyst (Fig. 1(B)) and with TPR
values of specic area, in the range of 10 to 20 m2 gꢁ1, except for results, since the decomposition of CaCO3 occurs in a range of
CaO and CuCa, which presented specic areas below 10 m2 gꢁ1
,
480 ꢀC to 650 ꢀC.49 The peaks obtained in the range of 365 ꢀC to
with values of 5 and 3 m2 gꢁ1, respectively. The low specic 470 ꢀC were related to the reduction of CuO, which is above the
areas may be associated with the methodology employed in the range described in the literature (190 ꢀC to 350 ꢀC) for reducing
synthesis, as well as with the fact that the oxides of Cu, Ca and of bulk CuO,47,50–52 and this may indicate the existence of
Mg are traditionally materials that have a low specic area. Low a stronger interaction between the copper phases and the
values of specic area have also been observed by other authors. catalytic supports.
Zhang et al.40 obtained for the Cu/MgO and Cu/CaO catalysts
specic areas of 24.2 m2 gꢁ1 and 0.6 m2 gꢁ1, respectively.
Manovic and Anthony43 obtained a specic area of 1.69 m2 gꢁ1
for Cu/CaO (45 wt%) and Nagaraja et al.41 obtained 18 m2 gꢁ1 for
Cu/MgO prepared by wet impregnation method.
The N2 adsorption–desorption isotherms of the supports and
catalysts exhibited a pattern type III, without hysteresis, with the
exception of the supports of the CuMg and Cu15CaMg catalysts,
which showed a pattern more similar to type IV with H3 hyster-
esis loop. In the type III isotherm, the adsorbed amount tends to
innity when P/P0 tends to 1, corresponding to the adsorption in
multiple overlapping layers and occurs in non-porous or mac-
roporous solids. The characteristics of the type IV isotherm are
hysteresis loop, which is associated with capillary condensation
in the mesopores and limiting uptake over a range of high P/P0.44
Although most synthesized materials did not exhibit isotherms
characteristic of mesoporous materials, it is observed that the
Fig. 2 H2-TPR profiles of calcined catalysts.
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The CuMg catalyst exhibited a single reduction peak at
415 ꢀC, indicating a homogeneous copper oxide phase and with
strong interaction with the MgO support. Nagaraja et al.53 ob-
tained for Cu/MgO (16 wt%) catalyst prepared by the wet
impregnation a reduction prole similar to that found in this
study, with
a
maximum reduction peak at 400 ꢀC,
approximately.
As it can be seen in Fig. 1(B), all Cu added to the pure CaO
support led to the formation of the CaCuO2 phase, which
showed a great stabilization for Cu, since the reduction prole
of the CuCa catalyst had the highest temperatures. Halasz
et al.54 investigated different types of cuprates through TPR
analysis and observed that different cuprates, including
Ca2CuO3, required higher reduction temperatures than CuO. Su
et al.55 observed for Cu/CaO (10 wt%) catalyst a single reduction
peak between 400–450 ꢀC assigned to the reduction of Ca2CuO3
cuprate. The Cu5Ca, Cu10Ca and Cu15Ca catalysts showed
main reduction peaks at lower temperatures, 395 ꢀC, 375 ꢀC and
365 ꢀC, respectively. This may be indicative of a small increase
in Cu dispersion because the higher dispersion offers larger
reactive surface area and a higher concentration of defects
where the reduction can start at a lower reduction tempera-
ture.51 However, it is observed that the Cu10Ca catalyst has
a secondary reduction peak at 460 ꢀC while the Cu15Ca catalyst
ꢀ
has a small peak at 450 C. A high CuO reduction temperature
may also indicate less dispersion of CuO species,52 which is in
agreement with the dispersion data presented in Table 2.
Although the Cu10Ca catalyst has the main reduction peak at
375 ꢀC, it has a second reduction peak at a higher temperature,
thus, this result indicates that the catalyst has two distinct CuO
phases.
Fig. 3 CO2-TPD profiles of the reduced catalysts.
ꢀ
of CO2 desorption (50–400 C) is commonly observed in cata-
lysts supported on MgO.27,40,58,59 On the other hand, the CuCa
catalyst mainly exhibits strong basic sites, with a narrower CO2
desorption band at higher temperature, in the range of 650 to
685 ꢀC. It is widely accepted in the literature that CaO presents
strong basic sites with CO2 desorption in the range of 500–
The reduction degree (RD) of CuO shown in Table 2 was
determined from the TPR analysis considering the consump-
ꢀ
tion of H2 presented by the peaks in the range of 365–470 C,
and it was assumed that Cu2+ is reduced in one step (Cu2+
/
Cu0) without formation of Cu2O. The CuMg and CuCa catalysts
practically showed a complete reduction of CuO, while the
Cu5CaMg, Cu10CaMg and Cu15CaMg catalysts demonstrated
degrees of reduction in the range of 70 to 80%. It is observed
that low CaO load (5–15 wt%) promotes a decrease in the
reduction degree. Similar results were observed by Xue et al.,56
where it was evaluated the inuence of CaO addition on Cu/
ZSM-5 catalysts. In this study it was observed that the reduc-
ibility of CuO decreases with the CaO addition, and this effect
was associated with the coverage of the surface of CuO by CaO
and the interactions between CaO and CuO, which are unfa-
vorable for the reduction of CuO.
750 C.40,55,58,60
ꢀ
Catalysts composed of mixed oxides (Cu5CaMg, Cu10CaMg
and Cu15CaMg) exhibited a wide range of CO2 desorption
ꢀ
temperatures (75–850 C), due to the presence of both types of
sites: weak and strong basic, characteristic of the MgO and CaO
phases, respectively. With successive increase in CaO load in
the catalyst, it is observed that the strong basic sites become
predominant, while the weak basic sites become less intense.
Tauq-Yap et al.58 observed this same behavior for CaO–MgO
catalysts and this loss of weak basic sites was attributed to the
dispersion of CaO on the surface of MgO, preventing the
adsorption of CO2 on basic sites of MgO.
Table 2 shows the total amount of basic sites per gram of
catalyst (mmol CO2 per gcat), the proportion of weak, medium
and high strength basic sites and the densities of basic sites for
each catalyst. Although the distribution of the strength of the
basic sites was different among the catalysts, they presented
a similar amount of basic sites, in the range of 130 to 270 mmol
CO2 per gcat. The CuMg and CuCa catalysts showed values
similar to those obtained by Zhang et al.,40 which obtained for
the Cu/CaO and Cu/MgO catalysts prepared by wet
ꢀ
The basicity of the catalysts reduced at 600 C/30 min was
evaluated by CO2-TPD analysis and the desorption proles are
shown in Fig. 3. The peaks were deconvoluted and classied
into three regions of temperature, according to the classica-
57
ꢀ
ꢀ
´
tion used by Aramendıa et al.: up to 400 C, 400–600 C and
above 600 ꢀC, which were related to weak, medium and high
strength basic sites, respectively. The CuMg catalyst displays
ꢀ
onlyꢀweak basic ꢀsites, with 3 peaks of CO2 desorption (135 C,
ꢀ
190 C and 255 C), the main peak is located at 255 C. These
results are in accordance with the literature, since a wide range
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impregnation 183 and 148 mmol CO2 per gcat, respectively. 96.5% and 95.1%, respectively. Although the conversion of
Although CaO promotes the formation of new strong basic sites glycerol did not present signicant differences, the average
in catalysts supported on CaO/MgO, there was no signicant selectivity to LA was higher in the reaction performed with
increase in the number of total basic sites, and this may be NaOH, which presented 90.4% of selectivity to LA, while the
associated with a low proportion of CaO added and/or with the reaction carried out with KOH displayed an average selectivity to
reduction of weak basic sites of the MgO. Furthermore, LA of 80.8%. Only 1,2-propanediol (1,2-PD) was obtained as
analyzing the strength distribution of the basic sites of catalysts, byproduct in all catalytic tests and the average selectivity ob-
it is observed that the formation of strong basic sites is tained was 12.4% (NaOH) and 7.4% (KOH). Since the conver-
progressive with the increase in the CaO load, and concomi- sion of glycerol obtained using the NaOH and KOH bases was
tantly there is a reduction in the amount of weak basic sites. The practically the same and above 95%, the yield are close to the
catalysts showed a density of basic sites in the range of 12 to selectivity values. Thus, it was obtained the highest yield to LA
18 mmol CO2 per m2, except for the CuCa catalyst, which showed (87.2%) in the catalytic test performed with NaOH, while the
a density of 42 mmol CO2 per m2. The density obtained for the test performed with KOH showed a lower yield, 76.8%. The yield
CuCa catalyst may be overestimated because there is an inac- to 1,2-PD was of 11.9% (NaOH) and 7.0% (KOH). Our results
curacy in determining the specic area for materials with very indicate that NaOH has a better performance than KOH,
low surface area. However, in general, it is observed that the although Shen et al.63 has observed the opposite. This difference
addition of CaO to the catalysts promotes an increase in the may be associated with the reaction system used, once that
density of basic sites.
Shen et al.63 used a batch system without heterogeneous
catalysts.
It is widely accepted in the literature that the hydrothermal
conversion of glycerol to LA has as initial step the dehydroge-
3.2. Catalytic tests
3.2.1. Inuence of the base. The alkaline medium is widely nation of glycerol to glyceraldehyde. Subsequently glyceralde-
reported in the literature as a factor of great importance in the hyde undergoes dehydration to form 2-hydroxypropanal, which
hydrothermal reactions of glycerol conversion into lactic acid. A through a molecular rearrangement, known as keto–enol tau-
previous study by our group evaluated the effect of the absence tomerization, transforms the hydroxyl present in the 2-hydrox-
of the base on the reactions, as well as the effect of the reactions ypropanal into a ketone forming pyruvaldehyde, and nally the
in alkaline medium without catalyst at 240 ꢀC.27 In the catalytic pyruvaldehyde is converted to LA (lactate) by a mechanism
test performed without catalyst in alkaline medium (NaOH) no similar to the rearrangement of benzylic acid (intra molecular
formation of any product was observed and the conversion Cannizzaro reaction);17,22,27,64–67 these steps are shown in Fig. 4.
glycerol was practically null. Sharninghausen et al.61 also noted According to Ftouni et al.,22 the rst step in the process that
that there is no conversion of glycerol when the reaction at produces glyceraldehyde occurs on the metallic sites, while the
115 ꢀC is carried out in alkaline medium (KOH) without catalyst. other steps occur in solution in the presence of hydroxyl group
On the other hand, in the reaction performed employing (OHꢁ). However, on the metallic sites the 2-hydroxypropenal
copper-based catalyst (Cu/ZnO) without alkaline medium, no and pyruvaldehyde may also be hydrogenated producing 1,2-
production of LA was detected, but a yield of 8% for 1,2-pro- PD.66,68 Furthermore, among transition metals, copper-based
panediol (1,2-PD) with glycerol conversion of 12% was ob- catalysts are efficient in glycerol hydrogenolysis under an
tained.27 Thus, we can observe that the concomitant use of atmosphere rich in H2 due to their high activity in the cleavage
alkaline medium and catalyst is necessary to obtain high yields of C–O bonds associated with the low activity of cleavage of C–C
of LA from the hydrothermal conversion of glycerol. This is in bonds.69,70
agreement with Zavrazhnov et al.,62 where it was obtained an
3.2.2. Inuence of the catalytic supports. The catalysts are
increase in the yield to LA from 4.7% to 39.4%, due to the of great importance in the hydrothermal conversion of glycerol
ꢀ
concomitant use of NaOH and Cu catalyst (230 C; 3 h).
to LA in an alkaline medium, since they promote the conversion
Initially it was evaluated the inuence of the NaOH and KOH of glycerol due to the metallic sites decreasing the use of high
bases and the choice of these bases was due to their strong basic temperatures in the reaction.22 The performance of the catalysts
character. Furthermore, among all types of base, the NaOH is was evaluated considering the glycerol conversion, selectivity
the most widely used in the hydrothermal conversion reaction and yield to LA and 1,2-PD; additionally, turnover frequencies
of glycerol to LA with or without catalysts and according to the (TOF) values were also determined. In order to make a more
study presented by Shen et al.63 KOH showed higher activity accurate comparison, Table 3 shows the average values ob-
than NaOH in the hydrothermal conversion of glycerol to LA. tained between 23 h and 30 h of reaction, since it is the period in
The reaction conditions used to evaluate the effect of hydrox- which a relative reaction stability was observed. The catalytic
ides (NaOH and KOH) on the reaction of LA production from tests were performed using 10 vol% glycerol solution; NaOH/
ꢀ
glycerol in alkaline medium, employing continuous ow reac- glycerol molar ratio ¼ 0.75; temperature/pressure ¼ 240 C/35
tion system were: 10 vol% glycerol solution; base/glycerol molar atm; feed ow ¼ 0.041 mL minꢁ1, 1.25 g of catalyst (WSHV ¼
ratio ¼ 1; temperature/pressure ¼ 240 ꢀC/35 atm; CuMg catalyst 2 hꢁ1). In order to observe greater differences between the
(1.25 g) and feed ow ¼ 0.041 mL minꢁ1 (WSHV ¼ 2 hꢁ1).
catalysts, it was decided to employ an intermediate value of
Both hydroxides (NaOH or KOH) exhibited high glycerol NaOH/glycerol molar ratio (0.75). Fig. 5 shows the glycerol
conversions, with an average (23–30 h) glycerol conversion of conversion obtained for the different catalysts during 30 h of
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Fig. 4 Reaction pathways for glycerol conversion into lactic acid (adapted from Kishida et al.;17 Liu and Ye66).
catalytic evaluation. It is observed that all catalysts showed high With the addition of CaO to the catalyst, high strength basic
conversions of glycerol aer stabilization, above 80%, while sites are created; however, an increase in CaO loads drastically
catalysts supported on CaO/MgO showed even higher conver- reduce proportion of weak strength basic sites, in this way, it is
sions, above 90%. It is observed that the synthesis of catalysts observed that a balance between weak and high strength basic
supported on mixed oxides (CaO/MgO) promotes a synergistic sites should be maintained to obtain greater selectivities to LA,
effect, increasing the catalytic activity. As shown in Table 3, the as well as superior glycerol conversions. Therefore, as shown in
highest average glycerol conversions were obtained in the Table 2, a greater balance in the distribution of the basic sites is
catalytic tests performed with the Cu5CaMg and Cu15CaMg obtained in the Cu5CaMg catalyst.
catalysts (96.4%). Furthermore, it is observed that all catalysts
have excellent catalytic stability, because during the evaluated distribution of the basic sites has a behavior opposite to LA
period, no catalytic deactivation was observed. selectivity. The highest selectivity to 1,2-PD was obtained in the
Regarding the selectivity to 1,2-PD, it is observed that the
Fig. 6(A) shows the selectivity to LA and 1,2-PD obtained catalytic test carried out with the CuMg catalyst (36.5%), which
during 30 h of reaction using different catalysts. It is observed presents only weak strength basic sites. Therefore, the effect of
that the catalysts composed of CaO/MgO have higher selectiv- basic properties in relation to selectivity to 1,2-PD is not clear.
ities to LA than the CuMg and CuCa catalysts, the same The selectivities obtained for 1,2-PD may be associated with the
behavior observed in the glycerol conversion. However, the aviability of metallic Cu0 (Fig. 4). Thus, the results presented by
highest selectivity is obtained with the addition of low CaO load the CuCa and CuMg catalysts may be related to the greater
(5 wt%), and subsequent additions seem to demonstrate reduction degree obtained (100%). In addition, CuMg and CuCa
a reduction in selectivity to LA, since the average selectivity spent catalysts have not shown the formation of the Cu2O phase
decreased with successive increases in CaO (Table 3). Thus, the
highest average selectivity to LA was obtained in the catalytic
test performed with the Cu5CaMg catalyst (78.0%). This
behavior may be associated with the basic sites of catalysts.
Table 3 Average (23–30 h) conversion of glycerol (X), selectivity and
yields to products and TOF
Selectivitya
(%)
Yielda (%)
1,2-PD TOFa,b (hꢁ1
)
Catalyst
Xa (%) LA
1,2-PD LA
CuMg
CuCa
Cu5CaMg
Cu10CaMg 91.0
Cu15CaMg 96.4
81.9
87.8
96.4
63.9 36.5
52.3 29.9
51.9 21.0
75.2 19.9
66.0 23.0
65.7 26.7
8.0
7.5
10.7
10.2
10.7
59.1 23.9
78.0 20.7
72.5 25.3
68.1 27.7
a
Reaction condition: 240 ꢀC/35 atm; 10 vol% glycerol solution; NaOH/
glycerol molar ratio ¼ 0.75; 1.25 g catalyst; feed ow 0.041 mL minꢁ1
.
Fig. 5 Glycerol conversion obtained from the different synthesized
catalysts. Reaction condition: 1.25 g catalyst, 240 ꢀC/35 atm, 10 vol%
glycerol solution, NaOH/glycerol molar ratio ¼ 0.75 and feed flow ¼
0.041 mL minꢁ1 (WHSV ¼ 2 hꢁ1).
b
TOF values were based on the average rate of LA production (23–30
h). TOF ¼ LA produced (mol hꢁ1)/metallic Cu (mol).
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(Fig. 1). All catalysts, except CuCa, showed sum of selectivities (CaO–MgO) have reduction degree in the range of 70–80% and
(LA and 1,2-PD) close to 100%, this is an indication that gaseous this incomplete reduction did not negatively impact the results
products are not formed. The CuCa catalyst showed a selectivity of the catalytic tests, in accordance with the ndings of Roy
sum of 83%, indicating that there may have been a greater et al.19 and Yang et al.72 Li and Li73 evaluated different unsup-
conversion of glycerol to gaseous products and/or the genera- ported copper salts and bulk CuO in the reaction of glycerol
tion of some unidentied liquid byproduct.
conversion to LA in alkaline medium employing batch reactor.
Fig. 6(B) shows the yield to LA and 1,2-PD obtained in the Although the CuBr2 salt was reduced during the reaction, the
catalytic tests performed with the different catalytic supports. results obtained were slightly better than CuO at reaction
ꢀ
The Cu5CaMg catalyst showed the highest average yield to LA temperature above 155 C.
(ꢃ75%), since it obtained high glycerol conversion and the
The turnover frequencies (TOF) values were calculated
highest average selectivity to LA among the catalysts. The according to eqn (7) and the results are shown in Table 3. Cata-
Cu10CaMg and Cu15CaMg catalysts showed similar LA yields, lytic tests performed with CuMg and CuCa catalysts exhibited the
around 65%, while CuMg and CuCa catalysts showed LA yields lowest TOF values. On the other hand, the catalytic tests carried
in the range of 52%. As shown in Fig. 1, the diffractograms of out using the Cu catalysts supported on CaO/MgO exhibited an
the Cu5CaMg, Cu10CaMg and Cu15CaMg catalysts aer reac- increase in the TOF value, but without a signicant distinction
tion (spent) showed that a partial oxidation of Cu occurs, among the Cu5CaMg, Cu10CaMg and Cu15CaMg catalysts. This
forming Cu2O. However, this did not result in a catalytic deac- shows that the association of weak and high strength basic sites
tivation, because relatively stable values of LA yield were ob- has a higher catalytic performance, than only weak (MgO) or high
tained (aer overnight period).
(CaO) strength basic sites. Thus, it is demonstrated that an
This may indicate that the Cu oxide phase also has some addition of CaO greater than 5 wt% is not necessary, since the
catalytic activity in the reaction, which is in agreement with the maximum catalytic performance is achieved in the smallest
literature.66,71 Moreover, the catalysts supported on mixed oxide proportion. In addition, the highest LA yield was also obtained
with the Cu5CaMg catalyst.
Similar results were observed by Yin et al.20 in the hydrothermal
conversion of glycerol to LA. Although the Cu(16)/MgO catalyst
exhibits almost double of basic sites, the better catalytic perfor-
mance was obtained with Cu(16)/HAP catalyst, which may be
associated with a wide distribution of basic sites (100–661 ꢀC)
presented by the Cu(16)/HAP catalyst, while the catalyst Cu(16)/
MgO showed a more concentrated (202–396 ꢀC) distribution of
basic sites. Liu and Ye66 evaluated the inuence of basic oxides
(MgO, CaO and SrO) employing a batch reactor. It was observed
that the catalytic activity of these oxides followed the same order of
the basic strength (SrO > CaO > MgO); however, the performance
of SrO is slightly better than CaO, not justifying its use because SrO
is much more expensive than CaO.
3.2.3. Inuence of temperature. The effect of the reaction
temperature for glycerol conversion into LA was evaluated in the
range 200–260 ꢀC with step of 20 ꢀC, and the results are shown
in Table 4 and Fig. 7.
The catalytic tests were performed employing the Cu5CaMg
catalyst, which showed the best results in previous studies,
Table 4 Average (23–30 h) conversion of glycerol (X), selectivity and
yields to products
Selectivitya
(%)
Yielda (%)
Temperature (ꢀC)/pressure
(atm)
Xa (%) LA
1,2-PD LA 1,2-PD
200/20
220/25
240/35
260/46
72.0
97.4
96.4
90.8
61.4 16.3
44.2 11.7
63.2 15.3
75.2 19.9
55.7 20.2
64.9 15.7
78.0 20.7
61.3 22.3
Fig.
6 Lactic acid (filled symbols) and 1,2-propanediol (empty
symbols) selectivity (A) and yields (B) obtained for the different catalysts
evaluated. Reaction condition: 1.25 g catalyst, 240 ꢀC/35 atm, 10 vol%
glycerol solution, NaOH/glycerol molar ratio ¼ 0.75 and feed flow ¼
0.041 mL minꢁ1 (WHSV ¼ 2 hꢁ1).
a
Reaction conditions: 1.25 g Cu5CaMg catalyst; 10 vol% glycerol
solution; NaOH/glycerol molar ratio ¼ 0.75; feed ow 0.041 mL minꢁ1
.
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10 vol% glycerol solution, NaOH/glycerol molar ratio ¼ 0.75, 1.25 g temperature/pressure on LA selectivity. However, when evalu-
of catalyst and feed rate of 0.041 mL minꢁ1 (WHSV ¼ 2 hꢁ1).
ating the average selectivity to LA presented in Table 4, it is
Table 4 shows the average conversion of glycerol (X) obtained noted that the selectivity increases with the increase in
in the period of 23–30 h of reaction. The lowest glycerol temperature/pressure up to 240 ꢀC. On the other hand, the
conversion (72%) was obtained at the lowest reaction temper- selectivity to 1,2-PD shows an almost linear relationship with
ature, and with subsequent increase in reaction temperature the increase in reaction temperature. This behavior may be
ꢀ
(220–260 C) the glycerol conversion rises above 90%. Thus, to associated with the hydrogenolysis of glycerol to 1,2-PD in an
obtain high conversions of glycerol it is necessary to perform alkaline medium, a process widely studied in the literature.74,75
the catalytic tests at temperatures above 220 ꢀC. The increase in Aer reaching the maximum selectivity to LA at 240 ꢀC, it is
pressure could negatively affect the conversion of glycerol, as noted that the increase in temperature to 260 ꢀC contributes
was observed by Yang et al.72 In this study was evaluated the negatively. This reduction in LA selectivity is in accordance with
18
´
´
effect of pressure on the reaction of glycerol conversion to LA Ramırez-Lopez et al.: they observed that the increase in
ꢀ
(NaOH/glycerol ¼ 1; 180 C; 8 h), and it was observed that the temperature may promote the thermal decomposition of LA, as
glycerol conversion reduced from 100% to 87.7% with an well as its intermediates. These effects are more evident when
increase in pressure from 14 bar to 20 bar.
assessing yields of the products, Fig. 7(B). With successive
ꢀ
Fig. 7(A) shows that the highest selectivity to LA was obtained increases in reaction temperature from 200 to 240 C, the LA
in the catalytic test carried out at 240 ꢀC/35 atm, which pre- yield increases from 45 to 75%, respectively. However, in the
sented values of LA selectivity close to 80%. Selectivity to LA in catalytic test performed at 260 ꢀC/46 atm there is an accentuated
the range of 60 to 65% was obtained in the other conditions reduction in LA yield, with values in the range of 55%. The 1,2-
studied. Evaluating the Fig. 7(A), besides the reduction in PD yield also increases with rising temperatures, however,
selectivity to LA, it is not possible to observe the effect of a maximum is reached (ꢃ20%) at the highest temperatures
ꢀ
(240–260 C).
Roy et al.19 also observed an increase in the conversion of
glycerol and in the yield to LA with the increase of the reaction
ꢀ
temperature from 200 to 240 C using Cu/SiO2 and batch reac-
tion system. Li et al.76 evaluated the effect of temperature in
a smaller range (100–175 ꢀC), using commercial Cu–Zn–Al
catalyst (63.5 wt% Cu) in batch reactor.
It was observed in the temperature range 100–145 ꢀC that the
glycerol conversion and LA yield increase rapidly with
increasing reaction temperature, obtaining at 145 ꢀC close to
100% of glycerol conversion with LA yield around of 90% (4 h of
reaction; 1.2 g catalyst; NaOH/glycerol ¼ 1.5). It is not possible
to make a direct and precise comparison with our results, since
the reaction conditions and the reaction system are different;
however, we observed that it is possible to obtain high LA yields
if the reaction conditions were adequate.
Yin et al.77 studied effect of reaction temperature on the
conversion of glycerol into LA over Ni/graphite catalyst. With
increasing the reaction temperatures from 180 to 230 ꢀC, the
conversions of glycerol increased from 34% to 95% and the
yield of LA increased from 32% to 88%, approximately.
However, in the catalytic test performed at 250 ꢀC, although the
conversion of glycerol was 100%, a reduction in the yield to LA
(ꢃ80%) was obtained. Thus, the appropriate reaction temper-
ꢀ
ature should be less than 260 C and among the temperatures
ꢀ
evaluated, 240 C proved to be the most suitable.
3.2.4. Inuence of the NaOH/glycerol molar ratio. Fig. 8(A)
and (B) shows the selectivity and yield, respectively, to LA and
1,2-PD in different NaOH/glycerol molar ratios at 240 ꢀC/35 atm
using Cu5CaMg catalyst and 10 vol% glycerol solution. The
average conversion of glycerol was determined and the results
are shown in Table 5. It is observed that the molar ratio of 0.5
showed the lowest conversion of glycerol. When increasing the
ratio to 0.75, a large increase in the conversion of glycerol is
observed, above 90%, suggesting that the base concentration
has a positive inuence on the conversion of glycerol. Yin et al.77
Fig.
7 Lactic acid (filled symbols) and 1,2-propanediol (empty
symbols) selectivity (A) and yields (B) at different temperatures/pres-
sures (200 ꢀC/20 atm; 220 ꢀC/25 atm; 240 ꢀC/35 atm; 260 ꢀC/46 atm).
Reaction conditions: 1.25 g Cu5CaMg catalyst, 10 vol% glycerol solu-
tion, NaOH/glycerol molar ratio ¼ 0.75 and feed flow ¼ 0.041
mL minꢁ1 (WHSV ¼ 2 hꢁ1).
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in agreement with our results, since conversions close to 100%
were obtained in NaOH/glycerol molar ratios greater than 1.25.
Analyzing Fig. 8(A), it is observed that the lowest selectivities
to LA were obtained in the catalytic test performed with NaOH/
glycerol molar ratio of 0.50. With the increase of the molar ratio
to 0.75 and 1.00, it was observed an increase in LA selectivity
(75–80%), but without much distinction between them.
However, in the catalytic tests carried out with NaOH/glycerol
molar ratios of 1.25 and 1.50, a signicant increase in selec-
tivity to LA was obtained, both reaching values close to 100%.
Regarding the selectivity to 1,2-PD, it is noted that with the
increase in the molar ratio (0.75 to 1.5) the formation of 1,2-PD
is continuously suppressed, reaching values below 1% (Table 5).
Therefore, it is observed that the raise of the NaOH/glycerol
molar ratio, besides promoting an increase in the conversion
of glycerol and LA selectivity, minimizes the formation of 1,2-PD
that is the main secondary reaction.
According to the literature, the addition of base is consid-
ered as a factor of great importance to initiate the reaction of
transformation of glycerol to LA and to improve selectivity to LA.
Some studies that evaluated the relationship between base and
glycerol agree that the addition of bases, such as NaOH and
KOH, potentiate the formation of LA.72,78,79
From Fig. 4, which shows reaction pathways for glycerol
conversion into LA in alkaline medium, it is observed that the
base (OHꢁ) has a fundamental role in two steps, rstly in the
dehydration of glyceraldehyde to 2-hydroxypropenal and pos-
teriorly in the nal step of the transformation of pyruvaldehyde
in LA, through a molecular rearrangement also known as intra
Fig.
8 Lactic acid (filled symbols) and 1,2-propanediol (empty
symbols) selectivity (A) and yields (B) at different NaOH/glycerol molar molecular Cannizzaro reaction. Thus, we observed that NaOH/
ratios (0.5, 0.75, 1.0, 1.25 and 1.5). Reaction conditions: 240 ꢀC/35 atm,
glycerol molar ratios greater than 1.0, are necessary to obtain
high selectivities to LA as well as to reduce secondary products.
1.25 g Cu5CaMg catalyst, 10 vol% glycerol solution and feed flow ¼
0.041 mL minꢁ1 (WHSV ¼ 2 hꢁ1).
Fig. 8(B) shows the yields obtained during 30 h of reaction
under different NaOH/glycerol molar ratios and in Table 5 the
average values (23–30 h) are presented. The lowest yield to LA
obtained an increase in the conversion of glycerol from 94.1%
was obtained in the catalytic test performed with the lowest
to 99.0% with an increase in the NaOH/glycerol molar ratio
NaOH/glycerol molar ratio (0.5), which is directly related to the
from 1.0 to 1.5, in reactions performed at 230 ꢀC using a batch
poor results of glycerol conversion and selectivity to LA, ob-
system with a reaction time of 2 h, results that are similar to that
tained in this condition. Catalytic evaluations performed with
obtained in this study. According to the literature, higher base/
NaOH/glycerol molar ratios of 0.75 and 1.0, a considerable gain
glycerol molar ratios reduce the concentration of glyceralde-
in yield was obtained (ꢃ20%). Greater gains in yield to LA were
obtained when molar ratios greater than 1.0 were used, reach-
ing yields close to 100% in the catalytic tests carried out with
molar ratios of 1.25 and 1.5, Table 5. The yield to 1,2-PD shows
the opposite behavior, since the increase in the concentration of
NaOH in the reaction medium promotes a reduction in the yield
to 1,2-PD; thus, it is observed that there is a shi of the reaction
towards LA (Fig. 4), consequently minimizing the secondary
reaction of 1,2-PD production.
hyde that indirectly favors the conversion of glycerol,19 which is
Table 5 Average (23–30 h) conversion of glycerol (X), selectivity and
yields to products
Selectivitya (%)
Yielda (%)
LA
NaOH/glycerol
molar ratio
Xa (%)
LA
1,2-PD
1,2-PD
3.2.5. Inuence of concentration and purity of glycerol.
The effect of glycerol concentration on the reaction of trans-
formation of glycerol to LA was investigated using 10% and
20 vol% glycerol solutions (1.37 M and 2.7 M). In order to clearly
observe the effect of the glycerol concentration over the reaction, it
was decided to perform this evaluation using the NaOH/glycerol
molar ratio ¼ 0.75, which represents a NaOH molar concentra-
tion of 1 M and 2 M, for glycerol solutions at 10% and 20 vol%,
0.5
0.75
1.0
1.25
1.5
72.5
96.4
91.9
99.8
99.8
65.4
78.0
75.9
97.1
98.8
16.5
20.7
16.0
1.2
47.4
75.2
69.7
96.9
98.6
11.9
19.9
14.7
1.2
0.7
0.7
a
Reaction conditions: 240 ꢀC/35 atm; 10 vol% glycerol solution;
Cu5CaMg catalyst (1.25 g); feed ow 0.041 mL minꢁ1
.
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respectively. This evaluation was carried out using the Cu5CaMg LA was reduced from 97.1% (pure glycerol) to 90.2%, while the
catalyst (1.25 g), feeding ow of 0.041 mL minꢁ1 (WHSV ¼ 2 hꢁ1
)
selectivity to 1.2-PD increased from 1.2% (pure glycerol) to
3.1%. Ftouni et al.22 evaluated the use of crude glycerol in
and 240 ꢀC/35 atm.
The average conversion of glycerol obtained for the two a batch reactor at 180 ꢀC using NaOH/glycerol molar ratio ¼ 1.1
concentrations was very close to each other, 96.4% and 94.7%, and Pt/ZrO2 catalyst. Aer 24 h of reaction it was observed that
respectively to the concentrations of 10 and 20 vol%. However, the selectivity to LA and 1,2-PD did not change, however the
the increase in glycerol concentration to 20 vol% promoted glycerol conversion was signicantly impacted with the use of
a loss of selectivity to LA, presenting an average selectivity of crude glycerol, which represented a reduction in the yield to LA
59.3%, which represents a reduction of approximately 20 points of 72.2% (pure glycerol) to 52.5%, approximately. We also
in relation to the catalytic test performed with 10 vol% glycerol observed a reduction in yield to LA by using crude glycerol,
solution (78.0%), while the average selectivity to 1,2-PD showed however it was less pronounced, from 96.9% (pure glycerol) to
a small increase to 24.4%. Therefore, the reduction of the 88.7%. This may suggest that the impact of impurities is less on
selectivity to LA observed with the increase in the concentration the catalytic activity of copper than on platinum. Long et al.81
of glycerol may be associated with the increase in the generation using 1.5 M sodium silicate as catalyst in a batch reactor at
of 1,2-PD, as well as an increase in the glycerol conversion to 300 ꢀC, a slight reduction in LA yield was observed with the use
18
´
´
gaseous products, since high values of glycerol conversion were of crude glycerol. Ramırez-Lopez et al. evaluated the hydro-
obtained, and no new liquid byproducts were observed in the thermal conversion of glycerol to LA at 280 ꢀC with NaOH/
chromatograms. These results are in agreement with that glycerol molar ratio of 1.1 and did not observe differences
observed by Yin et al.,77 where they obtained a reduction in the between the tests performed with crude and pure glycerol.
carbon balance from 96.8% to 83.8%, in the catalytic tests
carried out with glycerol concentration of 3.6 vol% (0.5 M) and
21.9 vol% (3.0 M), respectively. This reduction in the carbon
4. Conclusions
balance was related to the decomposition reactions of the Cu catalysts supported on CaO/MgO in different proportion was
18
´
´
products, with generation of CO2. Ramırez-Lopez et al. also prepared by wet impregnation, containing 20 wt% of CuO. The
observed a reduction in selectivity and yield in LA with XRD analysis showed that the calcination temperature of 700 ꢀC
increased concentration of glycerol in the reaction medium (2.5 was effective in the formation of the crystalline CaO phase, as
3.5 M). The authors related this result to an increase in the well as demonstrated that the conditions employed in the
concentration of intermediate products and LA in the reaction reduction of CuO were adequate. Concerning the Cu average
medium and this has a negative effect, since it promotes an crystallite size and Cu dispersion, there were no signicant
increase in decomposition rates.
differences among the catalysts, which showed values in the
The LA yield showed values around 56% for the catalytic test range of 14 nm and 7%, respectively. The XRD of the spent
developed using a 20 vol% glycerol solution, representing catalysts showed that new crystalline phases are formed aer
a reduction of approximately 20 points, compared to the test the catalytic test due to the transformation of the crystalline
performed with 10 vol% glycerol solution (75.2%). However, the structure of the support, however, without promoting any loss
most concentrated glycerol solution has been shown to posi- of performance for the catalysts. In addition, it is noted that
tively affect the yield of 1,2-PD, which may be associated with although the Cu2O formation occurs, the metallic copper phase
a higher hydrogenation rate of 2-hydroxypropenal and/or remains aer the reaction. The textural properties showed that
pyruvaldehyde.
As previously reported in the synthesis of 1,2-PD, the pres- values (<20 m2 gꢁ1).
ence of impurities such as salts, ash, methanol and water could The H2-TPR analysis showed that the maximum reduction
the specic areas of all catalysts were similar and with low
affect the catalyst activity.80 Thus, in order to evaluate whether peaks of CuO of all catalysts were in the range 365 to 470 C,
crude glycerol from biodiesel production could be used directly values above the range considered in the literature for the CuO
for synthesizing LA, a reaction using a crude glycerol obtained reduction. This fact was associated with a strong interaction
from a plant producing biodiesel from soybean oil was per- between copper and supports. Through the CO2-TPD analysis it
formed in the following conditions: 12 vol% crude glycerol was observed that the combination of the supports (CaO and
solution (equivalent to 10 vol% pure glycerol solution), 240 ꢀC/ MgO) results in a catalyst with a wider distribution of basic
35 atm, NaOH/glycerol molar ratio of 1.25, feed ow of 0.041 sites, and the most balanced proportion between weak and high
ꢀ
mL minꢁ1, 1.25 g of Cu5CaMg catalyst (WHSV ¼ 2 hꢁ1).
strength basic sites was obtained with the Cu5CaMg catalyst. In
The conversion of crude glycerol showed high values with an addition, it was observed that Cu catalysts with different CaO
average (23–30 h) conversion of 98.4%, very similar to that ob- loads (5, 10 and 15 wt%) showed an increase in the density of
tained with pure glycerol (99.8%). Therefore, the impurities basic sites in relation to the CuMg catalyst.
present in the crude glycerol did not promote catalytic deacti-
The catalytic tests showed that NaOH presents higher
vation, since the conversion remained constant throughout the selectivities to LA than KOH, although both present high glyc-
evaluated period (30 h). Usually such impurities (not analyzed) erol conversion. The Cu5CaMg, Cu10CaMg and Cu15CaMg
are salts, residual alcohol, soap, and fats. However, the selec- catalysts showed superior catalytic performance in relation to
tivity to LA and 1,2-PD products was affected by the impurities the CuMg and CuCa catalysts, and this improved performance
present in the crude glycerol. The average (23–30 h) selectivity to was associated with the wide distribution of basic sites.
31136 | RSC Adv., 2020, 10, 31123–31138
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Although TOF analysis demonstrated that mixed support cata- 10 Y. J. Wee, J. N. Kim and H. W. Ryu, Food Technol. Biotechnol.,
lysts do not have much distinction among them, they had 2006, 44, 163–172.
higher values (ꢃ35%) than CuMg and CuCa catalysts. There- 11 H. Oh, Y. J. Wee, J. S. Yun, H. H. Seung, S. Jung and
fore, it is observed that an addition of CaO greater than 5 wt% is H. W. Ryu, Bioresour. Technol., 2005, 96, 1492–1498.
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exhibited the best results in terms of glycerol conversion, 14 D. Rathin and T. Shih-Perng, Fuels Chem. from Biomass, 1997,
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molar ratio demonstrated to be a relevant factor to achieve 15 L. Shen, Z. Yu, D. Zhang, H. Yin, C. Wang and A. Wang, J.
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however, it was observed that in the molar ratio of 1.25 it was 16 A. Corma Canos, S. Iborra and A. Velty, Chem. Rev., 2007,
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increase in the concentration of glycerol in the feed from 18 C. A. Ramırez-Lopez, J. R. Ochoa-Gomez, M. Fernandez-
´
´
´
´
´
´
10 vol% to 20 vol% showed to be disadvantageous to the reac-
tion, due to the loss of selectivity to LA. In the catalytic test
performed with crude glycerol, under the optimized conditions,
Santos, O. Gomez-Jimenez-Aberasturi, A. Alonso-Vicario
and J. Torrecilla-Soria, Ind. Eng. Chem. Res., 2010, 49,
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it was observed that it is possible to replace the rened glycerol 19 D. Roy, B. Subramaniam and R. V. Chaudhari, ACS Catal.,
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2011, 1, 548–551.
20 H. Yin, C. Zhang, H. Yin, D. Gao, L. Shen and A. Wang, Chem.
Eng. J., 2016, 288, 332–343.
Conflicts of interest
´
´
21 M. R. A. Arcanjo, I. J. Silva, E. Rodrıguez-Castellon,
A. Infantes-Molina and R. S. Vieira, Catal. Today, 2017,
279, 317–326.
There are no conicts to declare.
22 J. Ftouni, N. Villandier, F. Auneau, M. Besson, L. Djakovitch
and C. Pinel, Catal. Today, 2015, 257, 267–273.
23 L. Shen, H. Yin, H. Yin, S. Liu and A. Wang, J. Nanosci.
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24 H. Yin, H. Yin, A. Wang, L. Shen, Y. Liu and Y. Zheng, J.
Nanosci. Nanotechnol., 2017, 17, 1255–1266.
Acknowledgements
The authors thank the GreenTec/EQ/UF for providing BET
analysis, FAPERJ (#E-26/202.751/2019 and #E-26/010.100619/
2018) and CNPq (#400706/2016-4) for the nancial support
granted to carry out this work.
25 G. Zhang, F. Jin, B. Wu, J. Cao, Y. S. Adam and Y. Wang, Int. J.
Chem. React. Eng., 2012, 10, 1–22.
26 T. Shimanouchi, S. Ueno, K. Shidahara and Y. Kimura,
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