Study of hydrotalcite-supported transition metals as catalysts for crude glycerol hydrogenolysis

Article abstract of DOI:10.1016/j.mcat.2019.02.008

Hydrotalcites containing Fe, Ni-, Zn- and Cu were synthesized by the coprecipitation method. These catalysts were characterized and employed for the hydrogenolysis of pure glycerol. The catalyst with the best performance (HDT-Cu) was used to carry out the hydrogenolysis of crude glycerol. The best yield (74.1%) for the real, low purity (62%) glycerol was obtained at 3.4 MPa of H₂ and 200 °C for 24 h of reaction. The regeneration capacity of the catalysts was also studied in order to develop an economically feasible process for industrial application.

Full text of DOI:10.1016/j.mcat.2019.02.008

Molecular Catalysis 468 (2019) 9–18
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Study of hydrotalcite-supported transition metals as catalysts for crude
glycerol hydrogenolysis
a
b
c
c
b,⁎
A. López , J.A. Aragón , J.G. Hernández-Cortez , M.L. Mosqueira , R. Martínez-Palou
b
a
Gerencia de Separación de Hidrocarburos, Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, 07730, Ciudad de México
Gerencia de Transformación de Biomasa, Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, 07730, Ciudad de México
c
Gerencia de Desarrollo de Materiales y Productos Químicos, Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, 07730, Ciudad de México
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydrotalcites containing Fe, Ni-, Zn- and Cu were synthesized by the coprecipitation method. These catalysts
were characterized and employed for the hydrogenolysis of pure glycerol. The catalyst with the best performance
Hydrotalcite
Crude glycerol
Hydrogenolysis
Transition metals
(
HDT-Cu) was used to carry out the hydrogenolysis of crude glycerol. The best yield (74.1%) for the real, low
purity (62%) glycerol was obtained at 3.4 MPa of H and 200 °C for 24 h of reaction. The regeneration capacity of
2
the catalysts was also studied in order to develop an economically feasible process for industrial application.
1
. Introduction
In recent years, the interest in the use of biodiesel as an alternative
1,2-PDO is used not only in the synthesis of unsaturated polyester
resins and antifreezes, but also in the pharmaceutical, food, cosmetic,
detergent, flavoring, personal care, paint and animal food industries
[25,26]. On the other hand, the main application of 1,3-PDO is the
manufacture of polyester fibers with high commercial interest [27].
In recent years, glycerol hydrogenolysis has received significant
attention, and most research works on this topic have been developed
by using high purity commercial glycerol and very expensive catalysts
with high loads of noble metals (i.e. Pd and Pt); unfortunately, the
possibility of regenerating and reusing these catalysts in order to de-
velop a process that is economically feasible for industrial application
has not been explored. Since glycerol purification is a highly de-
manding energy process that greatly increases the cost of this raw
material from about $1000 for 200–400/metric ton [28], the devel-
opment of processes for the hydrogenolysis of crude glycerol as it is
obtained from biodiesel production with technical and economically
feasible reaction conditions is needed [29].
to fossil fuels has increased due to the problems stemming from the
energy crisis and the environmental deterioration being faced by the
world population. With the use of biodiesel, it is possible to reduce the
emissions of carbon monoxide, sulfur, aromatic hydrocarbons, among
other solid particles, which are mainly attributed to environmental
deterioration [1]. As the production of biodiesel is more expensive than
that of fossil diesel, the governments of many countries subsidize it to
stimulate this fuel production branch [2].
In biodiesel production via transesterification of the triglycerides
present in animal or vegetable origin oils, about 10% of crude glycerol
is obtained as a by-product [3,4]. The increase in the biodiesel pro-
duction has generated a glycerol surplus, causing a decrease in its price
due to the production-demand effect. As a result, research aimed at
transforming glycerol into other chemicals with higher added value has
considerably increased, which contributes significantly to the profit-
ability of the biodiesel production process [5].
Different noble and transition metals (e.g. Pt, Pd, Rh, Ir, Ni, and Cu)
[15–24] and different supports (e.g. SiO
2
, ZnO, Cr
2
O
3
, MgO, Al
2
O ,
3
Currently, there are several routes for the generation of higher-
added-value products from glycerol such as oxidation [6], dehydration
TiO , zeolites and hydrotalcites) [30–37] have been used in glycerol
2
hydrogenolysis, but in most cases, this reaction has been carried out
using highly pure glycerol and the possibility of regenerating and re-
using these catalysts in order to develop a process that is economically
feasible for industrial application has not been explored.
[
7], pyrolysis [8], esterification [9], and etherification [10], among
others [11–14].
One of the glycerol valorization strategies that has received more
attention is a hydrogenolysis reaction that produces mainly 1,2- and
In the present work, hydrotalcites containing Fe, Ni-, Zn- and Cu
were synthesized by the coprecipitation method. These catalysts were
1
,3-propanediols (1,2-PDO and 1,3-PDO), which are low-toxicity and
high-added-value chemicals [15–24].
characterized by N -adsorption, X-ray diffraction, scanning electron
2
⁎
Corresponding author.
E-mail address: rpalou@imp.mx (R. Martínez-Palou).
https://doi.org/10.1016/j.mcat.2019.02.008
Received 16 January 2019; Received in revised form 8 February 2019; Accepted 12 February 2019
2468-8231/ © 2019 Published by Elsevier B.V.

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
microscopy, Fourier transform infrared spectroscopy and CO
2
tem-
with spectral resolution of 4 cm−1, 50 scans and equipped with a DTGS
detector. The samples were analyzed in the form of self-supporting ta-
blets in absorbance-transmittance to analyze the adsorption-desorption
perature programmed desorption, and evaluated in the hydrogenolysis
of pure glycerol. The best performance catalyst (Cu-HDT) was used to
study the reaction conditions of the hydrogenolysis of crude glycerol.
Finally, the regeneration and reusability capacities of the catalysts were
studied in order to develop a feasible process for the industrial pro-
duction of glycerol-added-value products from the environmental and
economic viability standpoints.
of CO
600 °C.
Temperature Programmed Desorption (TPD) of CO
in an Autosorb-1 of Quantachrome. Prior to CO adsorption, all cata-
lysts ( 100 mg) were heated up to 500 °C under helium as carrier gas
flow rate 25 cm h ). The CO
2
during the thermal treatment from room temperature (25 °C) to
2
was carried out
2
˜
3
−1
(
2
temperature adsorption occurred at
2
. Experimental section
50 °C for 30 min and the corresponding desorption patterns of deso-
−1
rption were obtained by heating (10 °C min ) up to a final tempera-
ture of 500 °C. The gas mixture was analyzed by means of a Mass
Spectrometer equipment by Pfeiffer Prisma.
2.1. Crude glycerol characterization
Crude glycerol was supplied by the Biodiesel Pilot Plant of Sinaloa
University, Mexico. The following analyses, in triplicate, were carried
out on the crude glycerol by using the following standard test methods:
heavy metal and sodium (D-5856), ashes (D-482-07), moisture content
2.4. Hydrogenolysis reactions
The glycerol reaction was carried out in a custom-designed, stainless
steel, 100-mL Parr reactor, model 4598, equipped with a thermoelectric
couple and magnetically stirred. An aqueous solution of glycerol
(10.0 mL at 60 wt. % of pure glycerol, or crude glycerol without dilu-
tion) was mixed with the corresponding catalyst in a 1:12 ratio (w/w).
(
D-203-08), pH (E-70-90), and specific gravity (D-1298-12b). Glycerol
contents were determined by the iodometric-periodic acid method
conducted according to the AOCS Official Method Ca14-56. Mono-, di-
and triesters were identified by gas chromatography (GC) analysis
(
D6584-10a).
The reactor was purged with H at 3.4 MPa and placed in oil bath
2
preheated at 200 °C and kept at that temperature (180 or 200 °C, spe-
2.2. Preparation of catalysts
cified in each case) for a maximal time of 24 h with vigorous stirring
(
250 rpm) at a pressure range of 0–3.4 MPa (monitored by an electronic
All Nitrate salts (Sigma-Aldrich) were used as metal sources: mag-
transducer).
Each evaluated catalyst was calcined in air atmosphere at the cor-
responding temperature, conditioned in N flow (70 mL/min) at 120 °C
for 1 h in order to eliminate humidity and finally activated by reduction
at 350 °C with H flow (70 mL/min) for 3 h.
nesium nitrate (Mg (NO
3
)
2
·6H
2
O, 98%), zinc nitrate (Zn (NO
3
)
2
·6H O,
2
9
8%), copper nitrate (Cu (NO
NO ·6H O, 98.5%), iron nitrate (Fe(NO
minum nitrate (Al (NO ·9H O, 98%).
3
)
2
·2.5H
2
O, 98%), nickel nitrate (Ni
2
(
3
)
2
2
3
)
3
·9H O, 98%) and alu-
2
3
)
3
2
2
The materials were synthesized by coprecipitation at constant pH,
which consisted in the preparation of 3 M metal solutions from the
corresponding nitrate salts and a second KOH solution used as pre-
cipitating agent; both solutions were added dropwise into a glass re-
actor to keep pH 10. In all the samples, the content of the divalent
The reactant solution was cooled to room temperature after reaction
and analyzed by GC on an HP-6890 chromatograph with an Innowax
column (30 m × 320 μm × 0.5 μm), using a flame ionization detector
(FID); 1-butanol was used as the internal standard in the samples pre-
pared with 100 μL of the reaction product diluted with 900 μL of me-
thanol. The areas under the curve of the chromatograms were trans-
formed into weight percentages by means of calibration curves for each
of the likely reaction products as 1,2- and 1,3-propanodiols (1,2-PDO
and 1,3-PDO), ethyleneglycol (EG), propionic acid (PA), and glycer-
aldehyde (GA).
2
+
3+
cation was 15 wt. % with M /M molar ratio of 3. The mixture was
aged for 24 h under vigorous stirring. The synthesized materials were
washed and centrifuged to eliminate the presence of salts, then dried
and calcined at 600 °C for 5 h.
2.3. Characterization of catalysts
The conversion of glycerol and selectivity of products were calcu-
lated by using the following expressions:
The chemical compositions of Al, Mg, Cu and K were determined by
moles of glycerol(in)
moles of glycerol(out)
moles of glycerol(in)
the EPA 6010C method using a Perkin-Elmer Plasma 400 piece of
Conversion(%) =
Selectivity(%) =
× 100
equipment. Each sample was disintegrated in HNO at 10% before
3
carrying out the analysis. For the quantification of carbon, nitrogen and
hydrogen, a Carlo Erba EA1108 elemental analyzer was used.
moles of product
moles of all product
× 100
The gas adsorption/desorption isotherms are the most used for the
characterization of textural properties of porous materials, such as
surface area, pore volume and pore size distribution. Nitrogen adsorp-
tion was measured at its normal boiling point (- 196 °C) using an ASAP
3. Results and discussion
2
4
405 analyzer (Micromeritics) after pretreating samples at 250 °C for
h in vacuum. The BET surface area and pore structure were calculated
We decided to synthesize catalysts by using transition metals such as
Fe, Ni, Cu and Zn supported on hydrotalcites (HDTs) in order to pro-
duce scalable, stable, highly active and low cost catalysts to be used for
more than one reaction cycle because it is well known that Cu-based
catalysts can show low stability and suffer from severe deactivation in
this kind of reaction [31].
from the adsorption branches.
XRD patterns were collected on an
X SIEMENS D5005 dif-
fractometer with CuK 1 signal 2 = 1.5416 Å with angular interval of 4-
α
7
0° in 2θ.
SEM images were obtained by means of a field emission piece of
equipment JEOL JSM-6700 F, working at an acceleration voltage of
HDT-supported catalysts have attracted much attention in the last
year because these materials have shown many potential applications
as effective catalysts for transforming biomass-derived molecules with
environmental protection [38–40].
5
kV, and a JEOL JSM6400 piece of equipment in the 30–35 kV range.
In some cases, the SEM technique was combined with energy dispersive
X-ray analysis (SEM-EDX). The samples were metallized with gold or
palladium to create the necessary contrast.
3.1. Characterization of catalysts
Fourier-transform infrared spectroscopy (FTIR) was used to analyze
the synthesized material HDT-Cu-15; the infrared spectra were obtained
by employing a Thermo-NICOLET spectrometer model MAGNA 560
XRD patterns of the synthesized catalysts containing 7% of the
added metal are presented in Fig. 1, where the presence of the
10

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
Table 1
Textural properties of the calcined catalysts (600 °C).
a
b
b
Pore diameterb
Å
Sample
Ratio
Surface area
Pore volume
2
+
/M3+
2
3
M
m /g
cm /g
HDT-Fe
HDT-Ni
HDT-Cu
HDT-Zn
2.5
2.5
3.1
2.5
286.4
248.9
204.1
234.4
0.91
0.51
0.37
0.79
127.0
82.61
73.15
134.3
a
b
Determined by atomic absorption.
Based on the isotherm desorption branch.
area because this element has a lower ionic radius than Cu and Zn. This
fact could be probably attributed to the synthesis or maturation of the
material.
In Table 1, the main results of the characterization of the family of
basic catalysts are shown. It is observed that the value of the surface
2
2+
3+
area is greater than 200 m /g; the metal molar ratio M /M
de-
termined by atomic absorption is in the range from 2.5 to 3, thereby
Fig. 1. XRD patterns of the synthesized catalysts.
characteristic plane signals of the HDT-type materials is shown: (003),
2
+
2+
confirming the presence of the divalent cation (M
final catalyst.
+ Mg ) in the
HDT-Cu presents the smallest volume and pore diameter, so pre-
sumably these parameters play a key role in the activity of the catalyst.
(
006), (009), (015), (0012), (110) and (1013). It is observed that all the
samples present only the hydrotalcite phase, since they display char-
acteristic peaks of a natural hydrotalcite with the exception of HDT-Zn-
The N adsorption-desorption isotherms of the HDT-Ni, HDT-Cu,
2
HDT-Fe and HDT-Zn materials are shown in Fig. 3, where they are all
type IV isotherms according to the IUPAC classification, which indicates
that these are materials with relatively large pores. In addition, hys-
teresis loops formed by the capillary condensation produced in the
mesopores and by the limits of adsorbed quantity at high relative
pressure are characteristic in this type of isotherms.
7
, which presents other non-characteristic peaks of the hydrotalcite
phase, attributed to unidentified impurities.
The sharp and symmetrical peaks at 11.7, 23.0, 60.6 and 61.8° are
designated to the planes of the laminar structure (003), (006), (110)
and (113). By comparing these peaks, it is observed that the intensity of
the characteristic peak (003) increases with the replacement of the
metals iron, nickel, copper and zinc, respectively, in the environment
with laminar octahedral brucite coordination, indicating an increase in
the structural integrity by assuming that the stacking of the layers is
trilaminar rhombohedral.
The hysteresis loops that these materials present in the multilayer
zone are type H3. Loops or H3-type hysteresis loops are associated with
plate-shaped materials, giving rise to pores in the form of a slit. The
closure points of the hysteresis loops were produced at a relative
pressure of 0.5 in HDT-Fe, 0.6 in HDT-Ni, 0.4 in HDT-Cu and 0.8 in
HDT-Zn; based on this, it was found that there was an increase in the
relative pressure as the ionic radius of the metals increased, with the
exception of HDT-Cu. Then, as the relative pressure of the closure point
increased, the pore diameter also increased.
3.2. Nitrogen adsorption-desorption
The adsorption-desorption isotherms of the catalysts are presented
in Fig. 2. By analyzing the characterization results of the surface
properties of the different HDT-type catalytic materials, an area de-
crease can be observed (Table 1) with respect to the increase in the
cation ionic radius (ionic radius, Fe < Ni < Cu < Zn). However, an
exception is shown for HDT-Ni, since it displays an area value that is
smaller than those obtained with materials containing Cu and Zn. It was
expected that the catalyst containing Ni could have a greater surface
3.3. Fourier transform infrared spectroscopy (FTIR)
Within the FTIR spectra of the HDL-type materials, three general
types of vibration zones were found: area of vibrations of molecular
elongation of the hydroxyl groups, zone of the vibrations of the octa-
hedral sheets and vibrations of the interlaminar anions. Fig. 3 shows the
FTIR spectra of the HDT-Ni, HDT-Cu, HDT-Fe and HDT-Zn materials.
The HDT-Zn spectrum presents the typical broad band at around
−
1
3
612 cm , which is attributed to the tension vibration mode of hy-
drogen carbonates that are formed in the presence of basic hydroxyl
−1
group [41]. The shoulder observed at 3000 cm
is assigned to the
hydrogen bridge between water and carbonates in the interlaminar
−1
space while the low intensity band at 1643 cm
shows the water de-
formation mode (δHOH). Due to the thermal treatment, the carbonate
bands underwent a reorganization in the interlaminar space of sym-
−
1
metry D3h to C3v or C2v, so that the band that appears at 1367 cm
decreases in intensity, which can be assigned to interlaminar carbonates
(
united both in the form of a chelate or as a bidentate); two peaks are
−1 3- 2+
observed at around 1510 and 1400 cm
teractions [42].
due to CO
2
and Mg
in-
As it can be seen, the HDT-Ni and HDT-Fe materials show very si-
−
1
milar spectra, presenting a barely visible band at around 3700 cm
,
which is attributed to the tension mode of hydrogen carbonates that are
formed in the presence of basic hydroxyls [41]. Very intense broad
−1
bands are seen at around 3530 cm , which are attributed to the vi-
brations of physisorbed water, elongation vibration bands of OH-
Fig. 2. N adsorption-desorption isotherms of the catalysts.
2
11

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
Fig. 3. FTIR spectroscopy (300 °C) of the catalysts studied in the 4000–2500 cm−1 (A) and 1750-1000 cm−1 regions (B).
structural groups and MeOH vibrations in hydroxycarbonates. The
Table 2
−
1
shoulder observed at around 3000 cm
is assigned to the hydrogen
Basic site density and amount of desorbed CO by TPD.
2
bridge between water and carbonates in the interlaminar space, which
occurs because the calcination treatment is of low intensity. On the
/m2
Sample
μmol CO
2
/g cat.
μmol CO
2
−
1
other hand, there is a band at around 1575 cm , characteristic of an
HDT-Fe
HDT-Ni
HDT-Cu
HDT-Zn
57
0.20
0.40
0.63
0.18
101
128
43
asymmetric vibration band (OeCeO in a monodentate adsorbed species
of CO
2
on an O -basic site). Bands at around 1510, 1400 and
2
−
1
1
360 cm
above.
In the case of the HDT-Cu material, it only shows wide bands of very
are also observed, which have already been explained
−
1
low intensity at around 3448 cm
and
000 cm , which were already discussed above. The bands with very
low intensity are also observed at wave numbers around 1510, 1400
a shoulder at around
−
1
3
−
1
and 1360 cm
CO -TPD for the calcined samples in the range 30–500 °C is shown
in Fig. 4. The amount of CO
.
2
2
desorbed throughout in the temperature
range was calculated from the TPD curves.
In Table 2, the basic site density and amount of desorbed CO
TPD are presented.
2
by
The relationship between glycerol conversion and CO basic site
2
density can be seen in Table 2. A remarkable correlation between the
catalyst basicity and glycerol conversion in the hydrogenolysis reaction
can be observed (Fig. 5).
3.4. Pure glycerol hydrogenolysis with HDT type catalysts
Fig. 5. Correlation between the basicity of the HDT-Me catalysts and glycerol
The activity of the HDT-Metal catalysts in the hydrogenolysis of
conversion in the hydrogenolysis reaction.
aqueous glycerol was tested at 200 °C for 24 h, 250 rpm of stirring and a
glycerol/catalyst ratio of 12:1 w/w after screening several temperature,
H
2
pressure, reaction time and glycerol/catalyst ratio conditions.
The results concerning the conversion and selectivity of the glycerol
hydrogenolysis as a function of the reaction time are shown in Figs. 6a
and b, which were evaluated at a hydrogen pressure of 3.4 MPa and
different reaction temperatures.
As shown in Fig. 6a, the reaction rate increases when temperature
also increases. In the first two hours of reaction at 180 °C, only a con-
version of 24% was achieved, instead of a conversion of 67% at 200 °C.
Thus, after 24 h of reaction, glycerol conversions of 78 and 97% were
reached at 180 and 200 °C, respectively. In addition, it can be said that
the conversion of glycerol increased rapidly in the first 6 h, for both
temperatures; after this time, the conversion rate decreased, which can
be attributed to the glycerol concentration diminishing in the reaction
mixture. This fact is in good agreement with S. Xia et al., who stated
that in addition to catalyst basicity, glycerol conversion also depends
strongly on the concentration present in the reaction mixture [43].
Fig. 4. CO TPD curves of the HDT-Me catalysts.
2
12

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
Fig. 6. Evaluation of the temperature effect on the hydrogenolysis reaction of crude glycerol. Conversion of crude glycerol (a) and selectivity towards 1,2-PDO (b).
Table 3
be attributed to the fact that when the metal load is increased, also the
number of active sites is raised for the dehydrogenation to happen,
which is the initial step in the mechanism by the basic route.
Thus, HDT-Cu-15 was selected as the catalyst for studying the crude
glycerol hydrogenolysis.
Quantification of liquid products obtained in the pure glycerol hydrogenolysis
using HDT catalysts containing Fe, Ni, Cu and Zn.
a
Catalyst
Conversion
Selectivity
Yield
(
%)
(%)
1
0
,2-PDO
1,3-PDO
EG
Acetol
Fe-HDT
Ni-HDT
Cu-HDT
Zn-HDT
50.6
47.8
66.0
0
0
0
0
0
0
100
0
0
3.5. Characterization of HDT-Cu-15
100
95.7
0
0
47.8
63.1
0
4.3
0
0
0
3.5.1. N adsorption-desorption isotherm of HDT-Cu-15
2
A study of the textural properties of this catalyst shows a surface
a
Experimental conditions: 200 °C, 24 h, Pr = 3.4 MPa, 250 rpm of stirring,
glycerol/catalyst ratio of 12:1 w/w.
2
3
area of 166.34 m /g, pore volume of 0.479 cm /g and pore size of
1
7
15.25 Å.
For HDT-Cu-15, the surface area decreased with respect to HDT-Cu-
2+
In Table 3, the results of the pure glycerol hydrogenolysis using HDT
catalysts containing Fe, Ni, Cu and Zn are summarized.
2+
2+
(Table 3) as a function of the Cu
ion increase in the Cu /Mg
ratio, attributed to the fact that the order of the crystals decreases with
the increase in copper content.
In this family of catalysts, it can be seen that iron is a cation that
preferentially tends to perform deep oxidation reactions by breaking
CeC bonds, and despite having conversions greater than 50%, it does
not have the desired selectivity. On the other hand, Zn did not present
any catalytic activity under these evaluation conditions; Ni-HDT
showed a total selectivity to 1,2-PDO with moderated yield and Cu-HDT
was the catalyst with higher reaction yield. In the case of Fe-HDT, the
conversion of glycerol was totally directed to the formation acetol.
Fig. 7 shows the results obtained when evaluating the Cu content
effect on the hydrotalcite type material. When the metal content in-
creases from 7 to 15 wt. %, the conversion of glycerol is improved by
The N adsorption-desorption isotherm of HDT-Cu-15 is shown in
2
Fig. 8, which is also type IV according to the IUPAC classification, in-
dicating that it is a material with relatively large pores. In this type of
isotherms, hysteresis loops are characteristic, which are formed by the
capillary condensation produced in the mesopores. The hysteresis loop
is presented from the relative pressure of 0.7, giving the hysteresis the
H3 type form that is associated with plate-shaped materials.
By comparing the isotherms of the HDT-Cu-7 (Fig. 2) and HDT-Cu-
1
5 (Fig. 8) materials, it is observed that the hysteresis loops of the
2+ 2+
calcined materials, which also have a higher Cu /Mg ratio, occur at
higher relative pressures; these results indicate that there is a large
1
2.1%, maintaining the selectivity towards 1,2-PDO. These results can
Fig. 7. Conversion of glycerol (XGLY), selectivity to 1,2-PDO (S1,2-PDO) and yield of 1,2-PDO (Y1,2-PDO) of the glycerol hydrogenolysis catalyzed by HDT-Cu.
13

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
The profile only shows a single peak between 180 and 230 °C, which
2
+
indicates the consumption of hydrogen used in the reduction of Cu
ions located in the laminar structure of the material and the peak ob-
served at 510 °C can be considered as impurity due to its minimal in-
tensity.
3
.5.6. FTIR
The FTIR spectrum of the HDT-Cu-15 calcined material, synthesized
by the coprecipitation method, is shown in Fig. 12. As it can be seen,
−1
the spectrum shows the typical broadband at 3448 cm
with low in-
tensity, because it decreases due to dehydroxylation caused by the
−
1
calcination treatment. The shoulder observed at around 3000 cm
is
assigned to the hydrogen bridge between water and carbonates in the
interlaminar space, which due to the calcination treatment is of low
intensity. On the other hand, as it is known, with the thermal decom-
position of the material, water is lost physisorbed in the sheets, which is
−
−
1
1
demonstrated with the disappearance of the bands at 1643 cm
(
corresponding to the water deformation vibration), at 1739 cm
Fig. 8. N adsorption-desorption isotherm of HDT-Cu-15.
2
(
water deformation vibration restricted in the interlaminar space) and
−1 3−
at 3050 cm
(H
2
O−CO
2
interaction in the interlaminar space).
Due to the thermal treatment, the carbonate bands underwent a
reorganization in the interlaminar space of symmetry D3h to C3v or C2v
−1
so that the band that appears at 1374 cm
decreases in intensity,
which can be assigned to interlaminar carbonates (united both in the
form of a chelate or a bidentate), and two peaks are observed at around
−1
3−
2+
1
513 and 1406 cm
due to the CO
2
and Mg
interactions [40].
−1
The presence of a low intensity band at around 1458 cm
is also
observed, which is attributed to carbonates adsorbed after thermal
decomposition. Therefore, it can be said that the applied calcination
temperature of 600 °C is not sufficient to completely eliminate hydroxyl
and carbonates, even if the hydrotalcite phase is destroyed. This fact is
in agreement with the results of the TGA analyses, which indicate the
presence of a first stage of carbonate loss in the form of CO (accom-
2
panied by water vapor in minimal form) that is carried out before
00 °C. The IR studies indicate that above this temperature there are
still highly refractory carbonates that can still be observed at up to
000 °C, since CO is still being released at this temperature.
6
1
2
Fig. 9. Pore diameter distribution of HDT-Cu-15.
The intensity of the carbonate losses due to the formation of CO
2
was quantified from the TGA studies in the 50–600 °C range. From this
number of pore channels with slot opening, formed due to the ag-
gregation of plate-like layers (Fig. 9).
analysis, a total loss of 13.91% of the mass (2.7 mg) is observed at
600 °C. The released gases were analyzed by FTIR and CO bands are
2
observed in the spectra, which helped calculate the amount of carbo-
nates, obtaining a result of 2.19 mg, which agrees with the TGA result.
The FTIR analysis (Fig. 14) shows the bands assigned to metal
carbonates that evaporate before 600 °C, leaving residual carbonates.
After the heat treatment, the sample was allowed to return to room
3
.5.2. Scanning Electron Microscopy (SEM)
SEM images were obtained to investigate the morphology of the
calcined HDT-Cu-15 catalyst, which are shown in Fig. 10 at different
magnifications. The micrographs show that the mixed oxide materials
calcined at 600 °C kept the laminar structure with similar morphology
to that of a fresh hydrotalcite material.
temperature in a CO
2
atmosphere to observe the regeneration of the
−
1
band (1700–1200 cm ) forming carbonates of the same species (the
shape of the bands is the same) as the degraded ones. The analysis of
the areas of this signal indicates the loss of 13.91% of the initial mass.
3
.5.3. Energy dispersive X-ray (EDX)
2
The recovered area is 187.51 m after exposure to CO
2
. Using the area
The HDT-Cu-15 material was analyzed by EDX. The results are
ratio data, the sample recovered 47.21 wt. % due to the readsorption of
CO in the form of carbonates.
shown in Fig. 11, where as expected, in the calcined material, only Mg,
Al, Cu and O are present.
2
3
.6. Crude glycerol hydrogenolysis
3
.5.4. Transmission Electron Microscopy (TEM)
Another analysis that was carried out to obtain more information
The composition of crude glycerol utilized in this study is shown in
about the morphological characterization was Transmission Electron
Microscopy (TEM), which is presented in Fig. 12.
Table 4.
As it can be seen in Table 4, the raw sample used in this study had
It can be confirmed in Fig. 12 that the calcined HDT-Cu-15 material
still presents part of the laminar structure. Fig. 12a shows the cross-
section of the layered platelet from which the gallery structure can be
found, which can be observed with higher magnification in Fig. 12b.
low glycerol content and was accompanied by water, methanol and
mono, di and triglycerides as main impurities, as well as high content of
sodium due to the catalyst (NaOH) employed in biodiesel production.
According to the results with pure glycerol, HDT-Cu-15 was selected
as the catalyst to study the hydrogenolysis of crude glycerol. The pro-
cess was evaluated at various pressures (0–3.4 Mpa) and temperatures
(180 and 200 °C). The results are summarized in Table 5.
3.5.5. Temperature programed reduction (H2-TPR)
Fig. 13 illustrates the H -TPR profile of the calcined HDT-Cu-15.
2
14

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
Fig. 10. SEM of HDT-Cu-15 at a) 10,000X, b) 20,000X, a) 50,000X and b) 100,000X.
Fig. 11. EDX of HDT-Cu-15.
15

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
Fig. 12. TEM of HDT-Cu-15.
Table 4
Characterization of crude glycerol utilized in this study.
Parameter
Unit
Content
Glycerol content
%w/w
%w/w
%w/w
%w/w
%w/w
%w/w
–
62.10
3.60
Monoester content
Diester content
0.09
Triester content
1.08
Moisture content
Ash Content (550 °C)
pH at 25 °C(10% water)
Specific gravity (25 °C)
Heavy metals
4.92
2.99
10.6
–
0.989
11.54
888.12
ppm
Sodium content
ppm
The obtained results according to Table 5 indicate that the best
yields were produced at 3.4 Mpa of H pressure at 200 °C for 24 h. The
2
maximum yield obtained for real loads was 72% of 1,2-PDO with a
glycerol conversion of 74.1%, contrast with the results obtained with
pure glycerol (89.7% of 1,2-PDO and 92% of conversion, Table 4). The
results with crude glycerol are significantly high if we consider the
Fig. 13. H -TPR profile of the calcined HDT-Cu-15.
2
Fig. 14. FTIR of HDT-Cu-15.
16

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
Table 5
3.7. Analysis of the gas phase in the hydrogenolysis reaction
Results of the catalytic evaluation of HDT-Cu-15 in crude glycerol hydro-
genolysis.
The composition of the compounds present in the gas phase in the
hydrogenolysis reaction of crude glycerol is presented in Table 6.
As it can be seen, the compound with the highest presence in all the
reactions is carbon dioxide, which is attributed to the fact that as it is
crude glycerol and when it reaches reaction temperatures, cracking of
compounds present in the reaction mixture begins. In addition, a
minimal presence of hydrocarbons, mostly paraffins, can be seen.
The performance of the HDT-Cu-15 catalyst is comparable with that
reported in recently published articles about glycerol hydrogenolisys
b
Temperature
H
2
Pressure
Conversion
Selectivity
Yield
(
°C)
(Mpa)
(%)
(%)
1
,2-PDO
EG
N.Ia
2
2
2
2
1
1
00
00
00
00
80
80
0
89.6
92.9
90.6
97.2
72.9
81.6
28.6
57.0
59.3
74.1
50.4
55.5
5.5
7.8
6.8
3.7
5.0
3.3
65.9
35.3
33.9
22.3
44.6
41.2
25.6
52.9
53.8
72.0
36.7
45.3
0.7
2.0
3.4
2.0
3.4
(
Table 7). In previous studies, higher selectivity to 1,2-propanediol with
a
b
lower glycerol conversion or vice versa has been reported using pure
glycerol and higher temperature and/or pressure. The work of Zhu et al.
Unidentified products including acetol, ethanol and others.
Experimental conditions: 24 h, 250 rpm of stirring, glycerol/catalyst ratio
of 12:1 w/w.
[47] being an exception, in that work, the addition of suitable B
2
O to
3
Cu/SiO catalyst pronouncedly enhanced the catalyst activity and sta-
2
Table 6
bility for glycerol hydrogenolysis. The catalyst 3CuB/SiO
to complete conversion with 98.0% of 2-PDO selectivity.
2
afforded up
Composition of the gas phase in the hydrogenolysis reaction (24 h) of crude
glycerol using the HDT-Cu-15 catalyst.
3.8. Regeneration of the catalyst
Pressure H
2
(Mpa)
3.4
2.0
0.7
0
3.4
2.0
Temperature (°C)
Compound
200
200
200
200
180
180
weight %
The reuse of the HDT-Cu15 catalyst was evaluated in the hydro-
genolysis reaction of crude glycerol at 200 °C and 3.4 MPa. After several
studies, it was found that the catalyst could be efficiently recovered by a
vacuum filtering process and then washed with n-hexane and finally
conditioned by calcination and reduction. Once recovered and condi-
tioned, the catalytic material was put in reaction with a fresh load of
crude glycerol.
Methane
Ethane
0.04
…
0.03
0.02
0.03
0.06
0.02
0.01
0.13
0.01
…
0.03
0.02
0.02
…
0.07
0.03
0.00
…
0.04
0.11
0.10
1.69
0.18
…
0.02
0.02
0.01
0.02
0.02
0.01
0.06
0.01
…
Propane
Iso-Butane
n-Butane
0.08
0.03
0.10
0.07
…
0.02
0.01
0.12
0.08
…
0.02
0.01
….
2
,2-Dimethylpropane
Iso-Pentane
n –Pentane
n –Hexane
…
Fig. 15 shows the catalytic activity of the catalyst after 24 h of re-
action for various numbers of catalyst regeneration cycles. As it can be
observed, Cycle 2 shows catalytic activity that is similar to the one in
Cycle 1; on the other hand, Cycles 3 and 4 show glycerol conversion
drops of 61.75 and 67.78%, respectively while the selectivity towards
…
0.01
…
0.14
0.06
…
…
2
1
-Methylpentane
-Hexene
…
0.04
…
…
…
…
…
0.01
97.74
1.95
…
…
…
Carbon dioxide
Non-identified
94.26
5.42
96.93
2.72
94.31
5.55
97.68
0
97.83
2.00
1
,2-PDO remained practically constant for 4 regeneration cycles.
Although it has been well described that this type of copper catalyst
large amount of impurities present in the crude glycerol used in this
study.
is easily deactivated [29], in our case, the catalyst showed good sta-
bility only for the first and second reaction cycles, after third cycle, the
activity fell remarkably. According to the results shown in Fig. 13, the
process developed for the catalyst regeneration could be used for at
least two reaction cycles without considerable 1,2-PDO yield loss and at
least for four cycles of reuse without losing considerable selectivity in
the case of the crude glycerol used in the present study which had fairly
low purity.
Interestingly, the hydrogenolysis of crude glycerol showed a marked
selectivity to the formation of 1,2-PDO even in the absence of initial
pressure of hydrogen, which indicates that the hydrogen necessary to
hydrogenate the intermediates to propanediols is taken from the gly-
cerol molecule; in addition to crude glycerol, there is a remnant of
methanol used in the production process of biodiesel that can partici-
pate as a hydrogen donor molecule [43].
According to our studies, the metal content in the catalyst did not
change during the recycling processes and the catalyst performance
Table 7
Comparison of the results obtained in this work with recent sources about glycerol hydrogenolysis using Cu-based catalyst.
Catalyst
Reaction temp. (°C)
Reaction pressure (Mpa)
Glycerol type
Conversion (%)
Selectivity to 1,2-PDO
Reference
[37] a
(
mol.%)
Cu/Ni/TiO
2
230
180
210
230
220
225
200
180
220
200
200
3.5
3.0
3.0
7.0
1.5
5.0
5.0
0.1
5.0
3.4
3.4
pure
pure
pure
pure
pure
pure
pure
pure
pure
pure
crude
100
82.2
98.2
92.2
81.4
78.8
79.0
98.0
84.0
93.3
92.0
74.1
b
Cu-HDT
80.0
95.1
96.1
74.3
83.0
100
[39]
c
b
b
b
d
e
b
Cu-HDT
[43]
Cu/Zn/Al
Cu-HDT
[44]
[45]
CuO/ZnO
[46]
3
CuB/SiO
2
[47]
Cu-SBA-15
90.0
61.0
89.7
97.2
[48]
Cu/Al
2
O
3
[49]
b
Cu-HDT
Cu-HDT
This work
This workb
a
b
c
d
e
Fixed-bed flow reactor, using 2-propanol as hydrogen source, 10 wt% glycerol in 2-propanol, nitrogen flow rate, 50 mL·min−1
.
Reactions in batch using H2.
using ethanol as hydrogen source, 0.087 mol ethanol and 0.022 mol glycerol, 0.25 g catalyst, 3.0 MPa N , 10 h.
2
−1
−1
−1
Fixed-bed flow reactor, H
Fixed-bed flow reactor, H
2
2
flow rate 150 mL min , WHSV = 0.075 h
.
−1
flow rate 30 mL min , WHSV = 1.03 h
.
17

A. López, et al.
Molecular Catalysis 468 (2019) 9–18
Fig. 15. Behavior of glycerol conversion (x), 1,2-PDO selectivity (S) and 1,2-PDO yield (Y) during four regeneration cycles and catalyst reuses.
decrease seems to be related to the contamination of the catalyst active
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