CoZn-ZIF-derived ZnCo2O4-framework for the synthesis of alcohols from glycerol

Article abstract of DOI:10.1039/c8gc01768a

As a widely used solvent, fuel and intermediate, the demand for ethanol continuously increases. Currently, ethanol is produced via sugar fermentation and hydration of ethylene. It is of great significance to convert abundant glycerol to ethanol. Herein, we report a novel Co/ZnO catalyst with nano-sized Co particles embedded in ZnO plates, which was obtained through calcination and reduction of CoZn-ZIF precursor, and its excellent performance in the synthesis of ethanol from glycerol. The transformation process from CoZn-ZIF to Co/ZnO-ZIF was characterized by TG-DSC, XRD, SEM, TEM and XPS. Characterization results indicated that rhombic dodecahedral composite consisting ZnCo₂O₄ spinel formed after calcination of CoZn-ZIF precursor, which also led to the formation of small Co particles that embedded in ZnO plates during reduction. Under optimal conditions, the best yield of ethanol reached 1.45 g-ethanol per g-cat per h at 210 °C over Co/ZnO-ZIF. On the basis of a series of controlled experiments, the reaction mechanism for the formation of alcohols from glycerol was proposed.

Full text of DOI:10.1039/c8gc01768a

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CoZn-ZIF derived ZnCo2O4-framework for the synthesis of
alcohols from glycerol
Huaiyuan Zhao a, Yuanyuan Jiang a, Ping Chen a, Jie Fu b, Xiuyang Lu b, Zhaoyin Hou a,*
Received 00th January 2018,
Accepted 00th January 2018
DOI: 10.1039/x0xx00000x
As a widely used solvent, fuel and intermediate, the demand of ethanol increases continuously. Currently,
ethanol is produced via sugars fermentation and hydration of ethylene. It is of great significance to convert
the over-supplied glycerol to ethanol. Here, we report a novel Co/ZnO catalyst with nano sized Co particles
embedded in ZnO plates that was obtained through the calcination and reduction of CoZn-ZIF precursor, and
its excellent performance in the synthesis of ethanol from glycerol. The transformation process from CoZn-ZIF
to Co/ZnO-ZIF was characterized by TG-DSC, XRD, SEM, TEM and XPS. Characterization results indicated that
rhombic dodecahedral shaped composite consisting ZnCo2O4 spinel formed after calcination of CoZn-ZIF
precursor, which brought the formation of small Co particles that embedded in ZnO plates during reduction.
Under optimal conditions, the best yield of ethanol reached 1.45 g-ethanol/g-cat/h at 210 oC over Co/ZnO-ZIF.
On the basis of a series of controlled experiments, the reaction mechanism for the formation of alcohols from
glycerol was proposed.
www.rsc.org/
published in order to transform glycerol to valuable
products .6-11
In 2015, Hutchings et al found that glycerol could directly
1. Introduction
transform into methanol over MgO.12 In addition to methanol,
ethanol was also found in their experiments. What’s more,
Perosa and Tundo once reported that ethanol was generated
in the liquid phase hydrogenolysis of glycerol because of the
cleavage of carbon-carbon bond over Ni catalyst, especially at
high temperature and long reaction time.13 In the vapor phase
hydrogenolysis of glycerol, Ryneveld et al discovered that the
selectivity of methanol, ethanol and propanol increased
gradually with the increasing reaction temperature over
Ni/SiO2 and Ni/Al2O3 catalysts.14 And several works also
reported that ethanol was detected in the hydrogenolysis of
glycerol.15-20 According to these pioneering works, the
selective synthesis of ethanol as a main product from the
hydrogenolysis of glycerol would be possible.
Previous work in our laboratory found that
Ni2.4/Mg3.7Cr2.0O6.7 catalyst which derived from Ni-substituted
stichtite was efficient to produce ethanol from glycerol by a
“two-step hydrogenation” process, and the best yield of
ethanol attained 0.50 g-ethanol/g-cat/h at 250 °C. But the
formation of ethanol from glycerol was sensitive to reaction
temperature, and the selectivity of ethanol decreased sharply
when the reaction temperature was higher than 250 °C.21 At
the same time, it was also confirmed that the synergic effect
between Ni (hydrogenation active sites) and MgCr2O4
(dehydration active sites) in Ni2.4/Mg3.7Cr2.0O6.7 played a crucial
role in the selective formation of ethanol.
Ethanol is a widely used solvent, an intermediate of
manufacturing industries and a clean fuel. The largest single
use of ethanol is as an engine fuel and fuel additive. It was
reported that ethanol as a fuel can reduce harmful tailpipe
emissions, such as carbon monoxide, particulate matter,
oxides of nitrogen and other ozone-forming pollutants.1 And
now, the use of ethanol as fuel and fuel additive is widespread
all over the world, and more than 97% of gasoline in United
States and Brazil contains ethanol. In last September, China
planned to roll out the use of ethanol in gasoline nationally by
2020 similar to United States and Brazil, and it was predicated
that the fuel ethanol production in the mainland of China will
reach 4,200,000 ton in 2030.2 Currently, the main resource of
ethanol was the sugars fermentation and hydration of
ethylene.3,4
Glycerol is a by-product during the production of biodiesel,
the rapidly rising production of biodiesel has led to a serious
surplus of glycerol, and it was projected that the production of
glycerol would be six times more than demand in 2020, which
makes it become one of the most attractive platform
chemicals.5 In the past decade, many works have been
Metal-organic frameworks (MOFs) are porous materials with
organic ligands and metal atoms which have special physical
and chemical properties,22,23 and MOFs can be used as
sacrificial templates to construct various porous
nanostructures.24,25 Isomorphic MOFs was the ideal templates
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to build heterogeneous composite with controlled Kawazoe (HK) method was used to calculate pore size
composition and interaction between individual distributions from 0-2 nm and the Barrett-Joyner-Halenda (BJH)
components.26 In this work, nano Co particles that embedded method was used for pore size larger than
nm.
in ZnO plates were synthesized through the controlled Thermogravimetric and differential scanning calorimeter (TG-
calcination and reduction of CoZn-ZIF ([M(CH3C3H2N2)2]n, M=Co DSC) analysis was carried out on Netzsch STA 409
2
a
and Zn) precursor and used in the continuous hydrogenolysis thermobalance. Analysis was measured from 25 to 700 °C at a
of glycerol to ethanol. It was found that this special structured heating rate of 10 °C /min in air flow (30 mL/min).
Co/ZnO-ZIF catalyst exhibited predominant activity and Transmission electron microscope (TEM) images were gotten
selectivity for the formation of ethanol at lower temperature by using an accelerating voltage of 200 kV (JEOL–2020 F).
(210 °C) among those tested Co/ZnO catalysts that prepared Elemental mapping was performed with FEI TECNAI F30
via co-precipitation, impregnation and solvent-free grinding.
microscope operated at 300 kV. X-ray photoelectron
spectroscopy (XPS) measurements were carried out on a
Kratos Axis Ultra DLD system with a base pressure of 10−9 Torr.
Temperature-programmed reduction (H2-TPR) of all
catalysts was carried out in the following process: 50 mg
sample was first pretreated at 500 °C for 1 h in Ar flow (30
2. Experimental
2.1. Catalyst preparation
CoZn-ZIF was synthesized according to the procedure that mL/min) and then cooled to room temperature. Finally, a
reported in reference.27 In a typical process, Co(NO3)2·6H2O reduction gas (10% H2/Ar mixture, 30 mL/min) was shifted and
and Zn(NO3)2·6H2O (molar ratio of Co/Zn = 1/1, total metal ion the reactor was heated to 700 °C at a heating rate of 15 °C
molar was 15 mmol) were dissolved in methanol (75 mL) to /min. Powder KOH was used to dry effluent and the amount of
form a clear solution. And then, the methanol solution of 2- hydrogen H2 (m/e = 2) consumption was detected and
recorded by a quadrupole mass spectrometer (OmniStarTM,
methylimidazole (2-MeIm) (75 mL, including 120 mmol 2-
MeIm) was added slowly into above solution. After vigorously GSD301, Switzerland).
stirring for
2 h, the mixture was incubated at room
2.3. Catalytic reaction
temperature for 24 h. The as-obtained precipitation was
centrifuged, washed with methanol for several times, and
dried at 60 °C overnight in vacuum. For convenience, the as-
prepared precursor was denoted as CoZn-ZIF.
Hydrogenolysis of glycerol was performed in a vertical fixed-
bed reactor and a back pressure regulator was used to control
the system pressure. The reactor was a stainless steel tube and
the internal diameter was 6 mm, the length was 540 mm. In a
typical run, 0.10 g catalyst (40-60 mesh) was charged in the
constant temperature part of the reactor and quartz sand was
packed in both ends. Before reaction, catalyst was in situ
reduced in pure H2 flow (80 mL/min) at 450 °C for 1 h.
Subsequently, the reactor was cooled to scheduled reaction
temperature and pressure adjusted to 2.0 MPa. 40 wt% of
glycerol aqueous solution was fed into the reactor
continuously at a flow rate of 0.02 mL/min. The reaction
products and unreacted glycerol were cooled in a condenser
(remained at 0 °C) and collected in a gas-liquid separator,
weighed and analyzed on a gas chromatograph (Shimadzu, 14B)
which was equipped with a FID and a 30-m capillary column
(DB-WAX 52 CB, USA). All products detected in the liquid were
verified by a gas chromatography-mass spectrometry system
(GC-MS, Agilent 6890) and quantified through an external
calibration method. Methane, CO and CO2 in gas phase were
analyzed in an on-line gas chromatograph (Shimadzu, 8A)
equipped with TCD detector. Every analysis was carried out
after a steady operation for 2 h, and the carbon balance was
calculated in all experiments.
Subsequently, the as-prepared CoZn-ZIF precursor was
heated to 400 °C and calcined at 400 °C in a flow of air for 3 h.
The calcined product was denoted as CoZnO-ZIF. Finally, the
calcined CoZnO-ZIF was further reduced in hydrogen flow (80
mL/min) at 450 °C for 1 h before the catalytic reaction, and
these reduced catalysts were denoted as Co/ZnO-ZIF. The bulk
composition of calcination sample was determined by
inductively coupled plasma-atomic emission spectroscopy
(ICP-AES; Thermo Elemental IRIS Intrepid), and these data
were summarized in Table 1.
As references, several Co/ZnO catalysts with similar
composition were prepared in the traditional co-precipitation
(denoted as Co/ZnO-CP), impregnation (denoted as Co/ZnO-IM)
and solvent-free grinding28 (denoted as Co/ZnO-SF). The
preparation procedures of these catalysts were described in
detail in the supplementary information section. These
catalysts were also calcined and reduced under the same
conditions as that of Co/ZnO-ZIF.
2.2. Characterizations
Powder X-ray diffraction (XRD) patterns of precursor,
calcined catalysts and reduced catalysts were collected on a
Rigaku D/WAX-2500 diffractometer which uses Cu Kα radiation
o
3. Results and discussion
(1.5406 Å) with a step of 0.02o and 5 /min from 5 to 80o at
room temperature. Scanning electron microscope (SEM) 3.1. The structure of CoZn-ZIF precursor
images were detected on Leo Evo Series SEM (VP 1430,
Powder XRD analysis confirmed that the prepared CoZn-ZIF
Germany) at an accelerating voltage of 3 kV. Platinum was
used to coat samples in order to avoid charging. N2 adsorption
was tested at its normal boiling point using an ASAP 2010
analyzer (Micromeritics) after pretreated at 150 °C for 4 h in
vacuum. Brunner-Emmet-Teller (BET) method was used to
precursor exhibited the same diffraction pattern with original
ZIF-8 ([Zn(2-MeIm)2]n, JCPDS 00-062-1030) (Fig. 1A), which
indicated the homogenous substitution of Co and Zn due to
their similar ionic radius and electronegativity.29 The SEM
image (Fig. 1B) showed clearly that the synthesized CoZn-ZIF
crystals had a rhombic dodecahedral outline.
calculate specific surface area (P/P0
MOFs and 0.05-0.3 for other samples) and the Horvath-
=
0.0-0.01 for
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Fig. 1 XRD pattern (A) and SEM image (B) of CoZn-ZIF precursor.
N2 sorption isotherm of fresh CoZn-ZIF was type I model and
there was a rapid rise of N2 uptake at 0–0.01 of P/P0 (Fig. 2),
which was the feature of microporous material. The calculated
BET surface area and pore volume of CoZn-ZIF were 1528 m²/g
and 0.64 cm³/g, respectively. These data indicated that the
synthesized CoZn-ZIF had
a well-distributed microporous
channels at 0.67 nm and there were some pores with the
diameter of 0.99 nm, which was similar to the previous
report.30
3.2. Synthesis of Co/ZnO-ZIF from CoZn-ZIF
The thermal stability of fresh CoZn-ZIF precursor was
detected via TG-DSC analysis (Fig. 3A). It was found that there
were two major weight losses during the calcination of CoZn-
ZIF, the first weight loss at 20-300 °C was attributed to the
volatilization of water and methanol in the pores of CoZn-ZIF,
and the second loss at 300-470 °C with a strong exothermal
signal was the combustion of (2-MeIm)−. The weight of sample
was no obvious change in the following calcination process.
The weight loss of CoZn-ZIF between 300 and 470 °C was
57.1%, which was consist with the chemical compositions of
M(2-MeIm)2 (M=Co and Zn) transforming into metal oxide. In
order to obtain the nanobox structure, the thermal
decomposition temperature was determined as 400 °C. In this
temperature, the ligand (2-MeIm)− was decomposed slowly
and the rhombic dodecahedron morphology could be retained.
Fig. 3B showed the XRD patterns of calcined CoZnO-ZIF and
those reference catalysts prepared in different methods. It
Fig.
3
TG and DSC curve (A) of CoZn-ZIF precursors under air
atmosphere; XRD patterns (B) of CoZnO-ZIF, CoZnO-CP, CoZnO-IM and
CoZnO-SF.
could be found that original structure of CoZn-ZIF precursor
collapsed and mainly spinel ZnCo2O4 (JCPDS 00-023-1390)
formed in the calcined CoZnO-ZIF. But in those reference
catalysts prepared via co-precipitation (CoZnO-CP),
impregnation (CoZnO-IM) and solvent-free grinding (CoZnO-
SF), separated ZnO (JCPDS 01-079-5604) and Co3O4 (JCPDS 00-
043-1003) were also detected besides ZnCo2O4. These results
indicated that the homogenously dispersed cobalt/zinc ions in
the framework of CoZn-ZIF precursor were more favorable for
the formation spinel structured ZnCo2O4, in which cobalt/zinc
ions remained their atomic dispersion.
SEM images and TEM images (Fig. 4A-D) further disclosed
that the CoZn-ZIF derived sample still remained their original
rhombic dodecahedral shape. But the faces of calcined CoZnO-
ZIF became concave and rough, the edge was prominent
because of the decomposition of organic ligand and the
oxidation of cobalt/zinc ions.31 The corresponding elemental
mapping revealed that Co and Zn were uniformly distributed in
the rhombic dodecahedral composite (Fig. 4E) and the
corresponding spectra was showed in Fig. S1. High-resolution
TEM image at the edge of these composite (Fig. 4F) clearly
showed that those spinel ZnCo2O4 particles stacked tightly and
the exposed crystal faces were different seeing from the lattice
fringe. That is, CoZnO-ZIF was a porous and hollow structure
which was built by spinel ZnCo2O4 particles. The SEM images of
reference catalysts were illustrated in Fig. S2 of the supporting
information.
N2 adsorption analysis also disclosed that the calcined
CoZnO-ZIF was a porous and hollow architecture with highest
surface area and bigger pore volume among those reference
catalysts prepared in different methods (see Table 1 and Fig. 5).
Fig. 2 N2 adsorption–desorption isotherm and pore volume distribution
(insert) of CoZn-ZIF.
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volume of CoZnO-ZIF reached 97.7 m²/g and 0.30 cm³/g,
respectively. While CoZnO-CP, CoZnO-IM and CoZnO-SF had a
type I adsorption isotherm, and their surface area decreased
to 74.2, 39.7 and 30.2 m²/g, respectively.
The H2-TPR profiles of CoZnO-ZIF and those reference
catalysts were shown in Fig. 6. It could be found that there
were mainly two reduction peaks detected in all the samples.
The initial reduction temperature (260 °C) and the center
temperature of the first peak (330 °C) were similar in all
samples, but the outline and the center temperature of the
second peak varied obviously. The calculated ratios of
hydrogen consumption of these two peaks in CoZnO-ZIF,
CoZnO-CP, CoZnO-IM and CoZnO-SF were 2.1, 2.6, 2.4 and 2.9,
respectively. According to above characterization results and
published works,32-34 the first hydrogen consumption peak
could be ascribed to the reduction of Co3+ in ZnCo2O4 (in CoZn-
ZIF) and/or mixed ZnCo2O4+Co3O4 (in reference catalysts) to
Co2+, and the second hydrogen consumption peak was the
reduction of Co2+ to Co0. The calculated ratio of hydrogen
consumption (2.1) and the successive reduction profile of
CoZnO-ZIF also indicated that mainly ZnCo2O4 formed. These
results fitted well with above XRD analysis.
The binding energy of Co 2p3/2 in CoZnO-ZIF, CoZnO-CP,
CoZnO-IM and CoZnO-SF was shown in Fig. 7A. The spectra of
Co 2p3/2 were deconvoluted into two peaks at 780.2 and 781.3
eV, which could be assigned to that of Co2+ and Co3+.35,36
These results further indicated that mainly spinel ZnCo2O4
formed in CoZnO-ZIF, while both Co3O4 and spinel ZnCo2O4
coexisted in those reference samples. At the same time, the
asymmetric spectra of O1s in these samples could be
deconvoluted into three peaks centered at 529.3, 530.4 and
531.5 eV, which were attributed to the oxygen in Co3O4,37
ZnCo2O4 38 and ZnO,39 respectively (Fig. 7B). These results were
Fig. 4 SEM (A,B) and TEM images (C,D,F) of CoZnO-ZIF; element mapping of
an individual nanobox (E).
It was found that CoZnO-ZIF showed a typical type IV
adsorption isotherm and a H3-type hysteresis loop at a relative
pressure of 0.45–1.0. The calculated BET surface area and pore
Fig. 5 N2 adsorption–desorption isotherms of CoZnO-ZIF,
CoZnO-CP, CoZnO-IM and CoZnO-SF.
Fig. 6 H2-TPR profiles of CoZnO-ZIF, CoZnO-CP, CoZnO-IM
and CoZnO-SF.
Table 1. The composition and textural properties of CoZnO.
Co:Zn compositiona
(molar ratio)
1.1 : 1
Surface Area
Pore Volume
(cm³/g)
0.30
Pore diameter
(nm)
8.9
Sample
(m²/g)
97.7
74.2
39.7
30.2
CoZnO-ZIF
CoZnO-CP
CoZnO-IM
CoZnO-SF
1.0 : 1
0.9 : 1
1.0 : 1
0.61
0.14
0.11
26.2
12.5
12.1
a Calculated from ICP analysis.
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Fig. 7 The binding energy of Co 2p3/2 (A) and O1s (B) in CoZnO samples.
consistent with above characterizations, which further confirmed respectively. These data indicated that spinel ZnCo2O4 intermediate
that mainly ZnCo2O4 formed in the process of CoZn-ZIF calcination was favorable for the formation of highly dispersed Co particles.
under air flow.
The surface composition of these samples before and after
reduction was analyzed via XPS and summarized in Table 2. It was
found that the detected mole ratio of surface Co/Zn increased from
1.12 (in CoZnO-ZIF, before reduction) to 1.42 (in Co/ZnO-ZIF, after
reduction), which meant that Co dispersed mainly on the surface of
reduced Co/ZnO-ZIF, and/or the particle size of Co was small. On the
other hand, the mole ratio of Co/Zn decreased sharply from 0.83 (in
CoZnO-SF, before reduction) to 0.42 (in Co/ZnO-SF, after reduction),
which inferred that partial Co would be covered by ZnO and/or the
particle size of Co enlarged after reduction.
3.3. The properties of Co/ZnO
XRD (Fig. S3) and XPS (Fig. S4) analysis of the reduced catalysts
confirmed that both spinal ZnCo2O4 and Co3O4 could be reduced in
hydrogen flow at 450 oC for 1 h. Metallic Co (JCPDS 01-023-4651)
and ZnO (JCPDS 01-079-5604) were detected in these reduced
samples. According to the half-width of Co (111), the calculated
crystallize size of Co NPs in reduced Co/ZnO-ZIF, Co/ZnO-CP,
Co/ZnO-IM and Co/ZnO-SF was 10.3, 15.8, 14.8 and 23.9 nm,
Table 2. Surface composition of catalysts before and after reduction.a
Surface composition (mole ratio)
After reduction
Before reduction
Catalyst
Co
Zn
Co/Zn
1.12
0.90
0.82
0.83
Co
Zn
Co/Zn
1.42
0.78
0.87
0.42
CoZnO-ZIF
CoZnO-CP
CoZnO-IM
CoZnO-SF
23.6
22.6
21.1
22.4
21.1
25.1
25.7
27.0
41.5
28.0
30.3
17.3
29.2
36.0
34.8
41.3
a Calculated from survey scan results of XPS.
Fig. 8 TEM images of Co/ZnO-ZIF (A, A1, A2) and Co/ZnO-SF (B, B1); Line-EDS of Co/ZnO-ZIF (A3) and Co/ZnO-SF (B2).
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A
B
Fig. 9 Schematic diagram of prepared Co/ZnO- ZIF (A) and Co/ZnO- SF (B).
The surface enriched and small sized Co particles in reduced the most active and selective one for the formation ethanol (see
Cu/ZnO-ZIF were clearly identified in its high resolution TEM images Table S1).
(Fig. 8A) and corresponding Line-scan EDS analysis (Fig. 8A3). Both
These results indicated that the performance of Co/ZnO catalysts
the lattice fringes ascribed to ZnO (see Fig. 8A1, in red square), Co for the hydrogenolysis of glycerol depended strongly on its
(see Fig. 8A2, in green square) and Line-scan EDS analysis confirmed structure. Above characterizations indicated that small sized Co
that small Co NPs were embedded in the surface of thin ZnO plate. particles were enriched on the surface of thin ZnO plate during the
On the other hand, HRTEM analysis (Fig. 8B and 8B1) and reduction of Co/ZnO-ZIF (see Table 2, Fig. 8). And this special
corresponding Line-scan EDS analysis (Fig. 8B2) also confirmed that structure (Co NPs embedded in ZnO plate, see Fig. 9A) enhanced
Co particles in reduced Cu/ZnO-SF were completely sandwiched the accessibility of glycerol and hydrogen to the surface of Co,
between ZnO plates and partial surface of Co particles was covered which accelerated the hydrogenolysis of glycerol.19,40 On the
with thick ZnO. On the basis of above results, the proposed contrary, the lowest activity of Co/ZnO-SF could be attributed to
structure of reduced Cu/ZnO-ZIF and Cu/ZnO-SF was depicted in Fig. that Co particles were sandwiched between ZnO plates and partial
9A and Fig. 9B, respectively.
surface of Co was covered by ZnO (see Fig. 9B).
3.4. Synthesis alcohols over Co/ZnO catalysts prepared in
different methods
3.5. Synthesis alcohols over Co/ZnO-ZIF under varied conditions
The performance of Co/ZnO-ZIF for the synthesis of alcohols
Table
3
summarized the results of Co/ZnO catalysts that under varied reaction temperature (190 to 250 oC) was shown in
synthesized in different methods (with the same weight) for the Fig. 10. It was found that the conversion of glycerol reached 92.7%
continuous hydrogenolysis of glycerol in a vertical fixed-bed reactor at 190 oC, and mainly 1,2-PDO was detected in the reaction mixture
at 210 oC, 2.0 MPa. It was found that Co/ZnO-ZIF showed an (the selectivity of 1,2-PDO was 41.2%). The selectivity of ethanol
outstanding performance, the detected conversion of glycerol increased quickly from 21.9 (at 190 oC) to 57.9% (at 210 oC) with the
reached up to 98.8% with a 57.9% selectivity of ethanol and a 94.2% increasing temperature, and then decreased slightly under higher
selectivity of mono alcohol (including methanol, ethanol, 1-PO and temperature (>220 °C) with a complete conversion of glycerol.
2-PO). Among those reference catalysts, the conversion of glycerol Similar tendency was also detected over Co/ZnO-CP, Co/ZnO-IM
over Co/ZnO-CP and Co/ZnO-IM exceeded 96%, but the selectivity and Co/ZnO-SF catalysts, but the selectivity of ethanol passed its
of ethanol decreased to 33.7 and 37.1%, respectively. On the other maximum at 250, 230 and 250 °C, respectively (see Fig.S5).These
hand, mainly 1,2-PDO formed over Co/ZnO-SF and the selectivity of results indicated that high temperature was more favourable for
ethanol decreased sharply to 6.0% under the same condition.
the hydrogenation and the cleavage of C-C and C-O.14 Thus, an
Those controlled experiments with equivalent catalyst surface appropriate reaction temperature was necessary for the selective
areas presented in the reactor also confirmed that Co/ZnO-ZIF was hydrogenolysis of glycerol to ethanol.
Table 3. The performance of Co/ZnO catalysts for hydrogenolysis of glycerol.
Conversion
(%)
Production selectivity in liquid phase (%)
Carbon balance
(%)b
Catalyst
1,2-PDO
Ethanol
PO
Othersa
Co/ZnO-ZIF
Co/ZnO-CP
Co/ZnO-IM
Co/ZnO-SF
93.5
95.1
94.6
98.2
98.8
96.8
97.7
73.6
4.8
57.9
33.7
37.1
6.0
36.0
27.1
24.7
6.6
1.3
11.0
28.2
26.3
66.7
11.9
20.7
Reaction conditions: catalyst, 0.10 g; 2.0 MPa, 210 oC；40 wt.% of glycerol aqueous solution 0.02 mL/min, and H2/glycerol = 40 (mol) in feed; data acquisition
after steady operation for 2h.
a Including methanol, ethylene glycol, acrolein, acetol and acetaldehyde etc. b(All carbon atoms detected in liquid products)/(All carbon atoms in feed) ×
100%.
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Fig. 10 Hydrogenolysis of glycerol at varied temperature over Co/ZnO-ZIF.
Reaction conditions: catalyst 0.10 g; 2.0 MPa; 40 wt.% of glycerol aqueous
solution, 0.02 mL/min, and H2/glycerol = 40 (mol) in feed; data acquisition
after steady operation for 2h.
Fig. 11 Hydrogenolysis of glycerol with varied concentration of glycerol in
feed over Co/ZnO-ZIF.
Reaction conditions: catalyst 0.10 g; 2.0 MPa, 210 oC; glycerol aqueous
solution, 0.02 mL/min, and H2/glycerol = 40 (mol) in feed; data acquisition
after steady operation for 2h.
Fig. 11 showed the performance of Co/ZnO-ZIF for the velocity (WHSV) at 210 °C and 2.0 MPa. It was found that (1) the
hydrogenolysis of glycerol with different concentration of glycerol conversion of glycerol increased quickly from 24.9% (at 0.015 h) to
in feed. It was found that the conversion of glycerol decreased 98.8% (at 0.076 h) and then remained higher than 99%. (2) Acetol
slightly from 99.3 to 87.7% when the concentration of glycerol in was the first intermediate during the hydrogenolysis of glycerol and
feed was increased from 20 to 80 wt.%. The selectivity of ethanol it transformed rapidly. The detected selectivity of acetol decreased
and 1-PO decreased severely with the increasing concentration of sharply from 47.8% (at a 24.9% conversion of glycerol) to 1.6% (at a
glycerol, and mainly 1,2-PDO formed when using 80 wt.% glycerol 95.4% conversion of glycerol). (3) The detected selectivity of 1,2-
as feedstock. These results might be attributed to the fact that high PDO and EG went up with the conversion of glycerol at first, reached
viscosity of glycerol would bring mass transfer limitations41 and the their maximum (63.0% and 16.2%, at a 74.3% conversion of
consecutive hydrogenolysis of 1,2-PDO to ethanol was retarded. In glycerol) and then decreased. (4) The selectivity of ethanol and 1-PO
this case, 40 wt.% aqueous solution of glycerol was optimal and the increased continuously along with the conversion of glycerol and
best yield of ethanol reached 1.45 g-ethanol/g-cat/h.
reached their maximum (57.9% and 28.6%, respectively) at a 98.8%
Hydrogenolysis of glycerol over Co/ZnO-ZIF under different conversion of glycerol. After that, the selectivity of ethanol and 1-
hydrogen pressure indicated that the conversion of glycerol was PO decreased slightly with the increasing contact time, which could
less sensitive to hydrogen pressure (see Fig. S6). But the selectivity be attributed to that it would be the further decompose of ethanol
of ethanol increased quickly from 29.1 (at 0.5 MPa) to 57.9% (at 2.0 and 1-PO to methanol, CO, CH4 and/or CO2. The best yield of
MPa) with the increasing pressure at first, and then decreased
slightly at higher hydrogen pressure (>2.5 MPa). The selectivity of
1,2-PDO decreased continuously from 28.2 to 4.1% with the
increasing selectivity of 1-PDO (from 19.6 to 34.6%) in the tested
0.5-3.0 MPa.
The molar ratio of H2/glycerol in feed also exhibited significant
influence on the production distribution during the hydrogenolysis
of glycerol over Co/ZnO-ZIF (see Fig.S7). It was found that the
conversion of glycerol excessed 93% in all experiments. The
selectivity of ethanol increased quickly from 12.7 to 58.1% with the
increasing ratio of hydrogen in feed, and the selectivity of 1-PO also
increased steadily from 10.2 to 29.5%. On the other hand, the
selectivity of 1,2-PDO decreased from 59.2 to 3.9%. These results
indicated high flow H2, as a carrier gas, could transfer reactants and
products timely, which was beneficial to the glycerol hydrogenolysis.
At the same time, that large amount of H2 could speed up the deep
hydrogenation.15
Fig. 12 Hydrogenolysis of glycerol over Co/ZnO-ZIF at various WHSV.
Reaction conditions: catalyst, 0.10 g; 2.0 MPa; 210 °C; 40 wt% of
3.6. Possible mechanism of glycerol hydrogenolysis over Co/ZnO-
glycerol aqueous solution with the feed of 0.005-0.1 mL/min, and
H2/glycerol = 40 (mol) in feed; data acquisition after steady operation
for 2 h. WHSV (weight hourly space velocity) = mass of aqueous feed
solution/hour/mass of catalyst.
ZIF
Fig. 12 exhibited the performance of Co/ZnO-ZIF for
hydrogenolysis of glycerol under different weight hourly space
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ethanol reached 1.45 g-ethanol/g-cat/h. The performance of CoZnO-ZIF. The best selectivity of ethanol over Co/ZnO-ZIF catalyst
Co/ZnO-SF under various WHSV was showed in Fig. S8 and it was reached 57.9% (the yield of ethanol reached 1.45 g-ethanol/g-cat/h)
found that the product distribution changed in similar tendency. at 210 °C, and ethanol was relative stable under the reaction
And the calculated product distribution under iso-conversion level condition. Those controlled experiments with varied weight hourly
of glycerol over Co/ZnO-ZIF and Co/ZnO-SF were summarized and space velocity, different starting materials and other experiments
compared in Table S2. Table S3 summarized the product suggested that ethanol formed mainly via the consecutively
distribution under iso-conversion level of glycerol over different hydrogenation of glycerol (via 1,2-PDO intermediate). At the same
catalysts at different temperature. It was found that the selectivity time, a parallel routine via the cracking (of glycerol to EG) and
of detected products over these catalysts was similar.
hydrogenation (of EG) also occurred on Co/ZnO-ZIF.
At the same time, a series of controlled experiments, in which
1,2-PDO, EG, 1-PO, 2-PO and ethanol were used as the staring
materials, were carried out in order to explore reaction mechanism
of ethanol synthesis from glycerol over Co/ZnO-ZIF. The results of
these experiments were summarized in Table S4. It was found that
(1) when 1,2-PDO was used as the starting material, the product
distribution was similar with that of glycerol. (2) Mainly ethanol
formed in the consecutive hydrogenolysis of EG. (3) 1-PO and 2-PO
could be further hydrogenated to ethanol due the cleavage of C-C
bond, but the detected conversion was quite low. (4) Under the
reaction condition, ethanol (in feed) was relative stable compared
with glycerol, 1,2-PDO and EG.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (Contract Nos. 21773206, 21473155,
21273198) and Natural Science Foundation of Zhejiang Province
(Contract No. LZ12B03001).
On the basis of the above results and the mechanism previously
proposed in literatures,14,21,42,43 we thought that mechanism for
ethanol synthesis from glycerol could be depicted in the following
Scheme 1. Acetol was the direct dehydration production of glycerol,
and then it converted to 1,2-PDO quickly via hydrogenation on the
surface of Co/ZnO-ZIF. 1,2-PDO could be consecutively
hydrogenated to ethanol, 1-PO, 2-PO and methanol. At the same
References
1
2
3
4
G.A. Mills, E.E. Ecklund, Annu. Rev. Energ. Env., 2003, 12, 47-
80.
R. He, Z. Zhao, P. Liu, Z. Li, ACS Sustain. Chem. Eng., 2018, 6,
4633-4647.
C.A. García, F. Manzini, J.M. Islas, Renew. Sust. Energ. Rev.,
2017, 72, 1199-1207.
M. Karimi, B. Jenkins, P. Stroeve, Renew. Sust. Energ. Rev.,
2014, 40, 400-421.
time,
a parallel reaction from glycerol would also occur
simultaneously, in which EG formed directly from the cracking of
glycerol, and EG could be hydrogenated to ethanol. Controlled
experiments over Co/ZnO-ZIF further indicated that ethanol was
relative stable under the reaction condition (see Table S4).
5
6
7
J.J. Bozell, Science, 2010, 329 522-523.
P. Gallezot, Chem. Soc. Rev., 2012, 43 1538-1558.
C.H. Zhou, J.N. Beltramini, Y.X. Fan, G.Q. Lu, Chem. Soc. Rev.,
2008, 39, 527-549.
4. Conclusion
8
9
B.Katryniok, H.Kimura, P.Fongarland, F.Dumeignil, Green
Chem., 2011, 13, 1960-1979.
D. Sun, Y. Yamada, S. Sato, W. Ueda, Green Chem., 2017,19,
3186-3213.
In summary, we found that a novel structured Co/ZnO catalyst
could be synthesized via the controlled pyrolysis and reduction of
CoZn-ZIF precursor and it showed the best performance for the
10 A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green
Chem., 2008,10, 13-30.
continuous hydrogenolysis of glycerol in
a fixed-bed reactor
comparing with Co/ZnO catalysts prepared by co-precipitation,
impregnation and solvent-free grinding. Characterization indicated
that the mainly spinel ZnCo2O4 formed in calcined CoZnO-ZIF, and
the initial rhombic dodecahedral structure with high surface area
and big pore volume remained. Highly dispersed Co NPs that
embedded in thin ZnO plates formed during the reduction of
11 D. Sun, Y. Yamada, S. Sato, W. Ueda, Appl. Catal. B: Environ.
2016, 193, 75-92.
12 M.H. Haider, N.F. Dummer, D.W. Knight, R.L. Jenkins, M.
Howard, J. Moulijn, S.H. Taylor, G.J. Hutchings, Nat. Chem.
2015, 7, 1028-1032.
13 A.P. And, P. Tundo, Ind. Eng. Chem. Res. 2005,44, 8535-8537.
14 E.V. Ryneveld, A.S. Mahomed, P.S.V. Heerden, M.J. Green, H.B.
Friedrich, Green Chem., 2011, 13, 1819-1827.
15 M.L. Shozi, V. Dasireddy, S. Singh, D.J. Morgan, H.B. Friedrich,
ACS Sustain. Chem. Eng.,2016, 4, 5752-5760.
16 X.F. Lin, Y.H. Lv, Y.Y. Xi, Y.Y. Qu, C.G. Liu, Energy Fuels, 2014, 28,
3345-3351.
17 Y. Nakagawa, K. Tomishige, Catal. Sci. Technol., 2011, 1, 179-
190.
18 S.Y. Yang, D.S. Park, J. Yi, Catal. Sci. Technol., 2014, 4, 3191-
3202.
19 S. Xia, R. Nie, X. Lu, L. Wang, P. Chen, Z. Hou, J. Catal., 2012,
296,1-11.
20 S. Zhu, Y.L. Zhu, S.L. Hao, H.Y. Zheng, Y.W. Li, Green Chem.,
2012, 14, 2607-2616.
21 L. Zheng, H. Zhao, J. Fu, X. Lu, Z. Hou, Appl. Clay. Sci., 2018,
153, 54-60.
22 S.L. James, Chem. Soc. Rev., 2003, 32, 276-288.
Scheme 1. Proposed mechanism for the formation of ethanol from
glycerol.
8 | Green Chem., 2018, 00, 1-8
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DOI: 10.1039/C8GC01768A
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23 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M.
473, 68-73.
O'Keeffe, O.M. Yaghi, Science, 2002, 295, 469-472.
24 S. Dang, Q.-L. Zhu, Q. Xu, Nat. Rev. Mater., 2017, 3, 17075-
34 N.N. Madikizela-Mnqanqeni, N.J. Coville, J. Mol. Catal. A
Chem., 2005, 225, 137-142.
17088.
35 B.J. Tan, K.J. Klabunde, P.M.A. Sherwood, J. Am. Chem. Soc.,
1991,113, 855–861.
36 N.S. Mcintyre, D.D. Johnston, L.L. Coatsworth, R.D. Davidson,
J.R. Brown, Surf. Interface. Anal., 1990, 15, 265-272.
37 L. Formaro, R. Giannantonio, C. Pastorelli, R. Carli, J. Phys.
Chem., 1992, 96, 3197-3200.
25 W. Xia, A. Mahmood, R. Zou, Q. Xu, Energy Environ. Sci.,
2015, , 1837-1866.
8
26 L. Zheng, X. Li, W. Du, D. Shi, W. Ning, X. Lu, Z. Hou, Appl.
Catal. B: Environ., 2017, 203, 146-153.
27 G. Huang, Q. Li, D. Yin, L. Wang, Adv. Funct. Mater., 2017, 27
,
1604941-1604947.
28 W. Du, L. Zheng, X. Li, J. Fu, X. Lu, Z. Hou, Appl. Clay. Sci., 2016,
123, 166-172.
38 J. Cheng, Y. Lu, K. Qiu, H. Yan, X. Hou, J. Xu, L. Han, X. Liu, J.-K.
Kim, Y. Luo, Phys. Chem. Chem. Phys., 2015, 17, 17016-17022.
39 E.C. Onyiriuka, J. Non-Cryst. Solids., 1993, 163, 268-273.
40 I. Melián-Cabrera, M.L. Granados, J.L.G. Fierro, J. Catal., 2002,
210, 285-294.
29 X. Wang, F. Li, W. Li, W. Gao, Y. Tang, R. Li, J. Mater. Chem. A,
2017, 5, 17982-17989.
30 L. Yang, L. Yu, M. Sun, C. Gao, Catal. Commun., 2014, 54, 86-
41 E. S. Vasiliadou and A. A. Lemonidou, Chem. Eng. J., 2013, 231,
103–112.
90.
31 Y. Lü, W. Zhan, Y. He, Y. Wang, X. Kong, Q. Kuang, Z. Xie, L.
42 E. van Ryneveld, A.S. Mahomed, P.S. van Heerden, M.J. Green,
Zheng, ACS Appl. Mater.Inter., 2014,
6, 4186-4195.
C. Holzapfel, H.B. Friedrich, Catal. Sci. Technol., 2014, 4, 832-
32 M. Haneda, Y. Kintaichi, N. Bion, H. Hamada, Appl. Catal. B:
Environ., 2003, 46, 473-482.
33 C.W. Tang, C.B. Wang, S.H. Chien, Thermochim. Acta., 2008,
837.
43 R. Mane, S. Patil, M. Shirai, S. Rayalu, C. Rode, Appl. Catal. B:
Environ., 2017, 204, 134-146.
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Graphical abstract
CoZn-ZIF derived ZnCo2O4-framework for the synthesis of alcohols from glycerol
Huaiyuan Zhao, Yuanyuan Jiang, Ping Chen, Jie Fu, Xiuyang Lu, Zhaoyin Hou
A novel Co/ZnO catalyst obtained from CoZn-ZIF precursor is highly active and selective for the synthesis of
ethanol from glycerol.
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