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nanomaterials
Article
Aqueous-Phase Hydrogenolysis of Glycerol over Re
Promoted Ru Catalysts Encapuslated in Porous
Silica Nanoparticles
Kuo-Tseng Li * and Ruey-Hsiang Yen
Department of Chemical Engineering, Tunghai University, 40704 Taichung, Taiwan; largestar790606@gmail.com
*Correspondence: ktli@thu.edu.tw; Tel.: +886-4-2359-0121
Received: 5 February 2018; Accepted: 7 March 2018; Published: 9 March 2018
Abstract:
Activity improvement of Ru-based catalysts is needed for efficient production of valuable
chemicals from glycerol hydrogenolysis. In this work, a series of Re promoted Ru catalysts
encapuslated in porous silica nanoparticles (denoted as Re-Ru@SiO
2
) were prepared by coating silica
onto the surface of chemically reduced Ru-polyvinylpyrrolidone colloids, and were used to catalyze
the conversion of glycerol to diols and alcohols in water. X-ray diffraction (XRD), scanning electron
microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption, X-ray photoelectron
spectroscopy (XPS) and temperature-programmed reduction (TPR) were used to characterize these
nanoparticles. Effects of Ru/Si atomic ratio, Re addition, glycerol and catalyst concentrations, reaction
time, temperature, and hydrogen pressure were investigated. Re addition retarded the reduction of
ruthenium oxide, but increased the catalyst reactivity for glycerol hydrogenolysis. Due to its greater
Ru content, Re-Ru@ SiO
2
showed much better activity (reacted at much lower temperature) and
more yields of 1,2-propanediol and overall liquid-phase products than Re-Ru/SiO
2
(prepared by
conventional impregnation method) reported before. The rate of glycerol disappearance exhibited
first-order dependence on glycerol concentration and hydrogen pressure, with an activation energy
of 107.8 kJ/mol. The rate constant increased linearly with increasing Ru/Si atomic ratio and catalyst
amount. The yield of overall liquid-phase products correlated well with glycerol conversion.
Keywords:
Re-Ru@SiO
2
nanoparticles; glycerol hydrogenolysis; 1,2-propanediol; 1,3-propanediol;
diols; alcohols
1. Introduction
Glycerol is the co-product of biodiesel manufacturing and has become one of the top 12 building
blocks of biorefinery [
1
,
2
]. The conversion of glycerol to other valuable chemicals is needed to solve
its oversupply problem [
3
,
4
]. Several useful C1–C3 diols and alcohols can be produced from glycerol
hydrogenolysis, via the reaction scheme shown in Figure 1[5,6].
Nanomaterials 2018,8, 153; doi:10.3390/nano8030153 www.mdpi.com/journal/nanomaterials
Nanomaterials 2018,8, 153 2 of 14
Figure 1. Reaction scheme of glycerol hydrogenolysis.
Among these diols/alcohols, 1,2-propanediol (1,2-PDO) is mainly used as a raw material for
producing unsaturated polyester resins, antifreeze fluid, solvent, and preservatives. 1,3-propanediol
(1,3-PDO) is used as the monomer for the production of polypropylene terephthalate (PPT), which is a
biodegradable polyester and has great potential for use in carpet and textile manufacturing. 1-propanol
(1-PrOH) is used mainly as a solvent, a printing ink, and a chemical intermediate for the production of
n-propyl acetate. 2-propanol (2-PrOH) is used as a solvent. Ethylene glycol (EG) is mainly used as an
antifreeze fluid and a raw material for polyethylene terephthalate (PET). Methanol is used for making
formaldehyde, methyl tert-butyl ether (MTBE), biodiesel and di-methyl ether [7].
Ruthenium-based catalysts, including Ru/C with an ion-exchanged resin [
8
,
9
], Ru on various
supports (SiO
2
, Al
2
O
3
, zeolites, graphite, carbon nanotubes, MCM-41) [
5
,
10
–
12
], Ru-Cu/TiO
2
[
13
],
Re-Ru on different supports (SiO
2
, Al
2
O
3
, carbon, ZrO
2
) [
14
,
15
] have been studied for catalyzing
glycerol hydrogenolysis to produce diols/alcohols in the presence of high pressure hydrogen. Ru/SiO
2
catalyst prepared with impregnation method showed low activity in the hydrogenolysis of glycerol [
16
].
Vasiliadou et al. obtained 20% glycerol conversion with 65% 1,2-PDO selectivity at 240
◦
C using a
Ru/SiO
2
catalyst prepared with wet impregnation method [
5
]. Ma and He obtained 51.7% glycerol
conversion and 25% 1,2-PDO yield at 160
◦
C, 8 MPa and 8 h, using a Re-Ru/SiO
2
catalyst prepared
by impregnating SiO
2
powder with aqueous solution of RuCl
3·
4H
2
O and HReO
4
[
15
]. Therefore,
activity improvement of Ru-based catalysts is needed for efficient production of diols/alcohols from
glycerol hydrogenolysis.
We have prepared palladium core-porous silica shell-nanoparticles (denoted as Pd@SiO
2
)
for catalyzing 4-carboxybenzaldehyde hydrogenation [
17
] and prepared copper core-porous silica
shell-nanoparticles (denoted as Cu@SiO
2
) for catalyzing glycerol hydrogenolysis in methanol [
18
].
These nanoparticles exhibited better performances than the corresponding catalysts prepared by
conventional impregnation methods. In addition, palladium nanoparticels encapsulated in porous
silica shell have been proved to be highly stable for CO oxidation due to the sintering prevention
effect of silica shell [
19
]. Although Cu@SiO
2
nanoparticles had 96.5% 1,2-PDO yield in methanol at
200
◦
C, they exhibited very low activity for catalyzing glycerol hydrogenolysis in water. It is desirable
to use environmental friendly solvent–water- and to decrease the reaction temperature for glycerol
hydrogenolysis. In addition, the use of Ru-based catalysts for glycerol hydrogenolysis can produce
several useful C1–C3 diols and alcohols simultaneously, as mentioned above.
In this work, glycerol hydrogenolysis was carried out in aqueous phase using Ru catalysts
encapuslated in porous silica nanoparticles (denoted as Ru@SiO
2
) promoted with rhenium oxide. The
Re-Ru@SiO
2
catalysts exhibited very high activity at 130
◦
C, which was significantly lower than that
(160
◦
C) used by Ma and He [
15
]. The use of lower reaction temperature resulted in greater yields of
1,2-propanediol and overall liquid-phase products.
Nanomaterials 2018,8, 153 3 of 14
2. Results and Discussion
2.1. Catalyst Characterization
Ru@SiO
2
and Re-Ru@SiO
2
nanoparticles with five different Ru/Si atomic ratios (in a range of
0.1–1.6) were prepared. The Re-Ru@SiO
2
nanoparticles with Ru/Si atomic ratios of 0.1, 0.2, 0.4, 0.8
and 1.6 were denoted as Re-01RuSi, Re-02RuSi, Re-04RuSi, Re-08RuSi, and Re-16RuSi, respectively.
Figure 2shows a scanning electron micrograph of a Re-04RuSi sample, indicating that the particles
have a size of 40–60 nm. Figure 3shows a transmission electron micrograph of calcined Ru@SiO
2
sample with a Ru/Si atomic ratio of 0.4, indicating that small ruthenium nanoparticles with a variety
of sizes are encapsulated in silica.
Nanomaterials 2018, 8, x FOR PEER REVIEW 3 of 13
2. Results and Discussion
2.1. Catalyst Characterization
Ru@SiO2 and Re-Ru@SiO2 nanoparticles with five different Ru/Si atomic ratios (in a range of
0.1–1.6) were prepared. The Re-Ru@SiO2 nanoparticles with Ru/Si atomic ratios of 0.1, 0.2, 0.4, 0.8
and 1.6 were denoted as Re-01RuSi, Re-02RuSi, Re-04RuSi, Re-08RuSi, and Re-16RuSi, respectively.
Figure 2 shows a scanning electron micrograph of a Re-04RuSi sample, indicating that the particles
have a size of 40–60 nm. Figure 3 shows a transmission electron micrograph of calcined Ru@SiO2
sample with a Ru/Si atomic ratio of 0.4, indicating that small ruthenium nanoparticles with a variety
of sizes are encapsulated in silica.
Figure 2. Scanning electron micrograph of Re-Ru@SiO2 particles with a Ru/Si atomic ratio of 0.4.
Figure 3. Transmission electron micrograph of Ru@SiO2 particles with a Ru/Si atomic ratio of 0.4.
Figure 4 displays XRD patterns of (A) Re-04RuSi and (B) Re-08RuSi samples, which were
reduced with 5% hydrogen in argon at a heating rate of 1 °C/min to 200 °C, and then maintained at
200 °C for 4 h. The patterns exhibit the characteristic peaks of finely divided metallic ruthenium at 2θ
= 38°, 42°, 44°, 58°, 69° and 78°, which correspond to the reflection peaks of Ru(100), Ru(002),
Ru(101), Ru(102), Ru(110) and Ru(103), respectively [20]. Peak intensity of profile (A) is much smaller
than that of profile (B), suggesting that crystal size of Re-04RuSi is much smaller than that of
Re-08RuSi. The existence of RuO2 diffraction peak (2θ = 28°) in profile (B) suggests that Re-08RuSi
Figure 2. Scanning electron micrograph of Re-Ru@SiO2particles with a Ru/Si atomic ratio of 0.4.
Nanomaterials 2018, 8, x FOR PEER REVIEW 3 of 13
2. Results and Discussion
2.1. Catalyst Characterization
Ru@SiO2 and Re-Ru@SiO2 nanoparticles with five different Ru/Si atomic ratios (in a range of
0.1–1.6) were prepared. The Re-Ru@SiO2 nanoparticles with Ru/Si atomic ratios of 0.1, 0.2, 0.4, 0.8
and 1.6 were denoted as Re-01RuSi, Re-02RuSi, Re-04RuSi, Re-08RuSi, and Re-16RuSi, respectively.
Figure 2 shows a scanning electron micrograph of a Re-04RuSi sample, indicating that the particles
have a size of 40–60 nm. Figure 3 shows a transmission electron micrograph of calcined Ru@SiO2
sample with a Ru/Si atomic ratio of 0.4, indicating that small ruthenium nanoparticles with a variety
of sizes are encapsulated in silica.
Figure 2. Scanning electron micrograph of Re-Ru@SiO2 particles with a Ru/Si atomic ratio of 0.4.
Figure 3. Transmission electron micrograph of Ru@SiO2 particles with a Ru/Si atomic ratio of 0.4.
Figure 4 displays XRD patterns of (A) Re-04RuSi and (B) Re-08RuSi samples, which were
reduced with 5% hydrogen in argon at a heating rate of 1 °C/min to 200 °C, and then maintained at
200 °C for 4 h. The patterns exhibit the characteristic peaks of finely divided metallic ruthenium at 2θ
= 38°, 42°, 44°, 58°, 69° and 78°, which correspond to the reflection peaks of Ru(100), Ru(002),
Ru(101), Ru(102), Ru(110) and Ru(103), respectively [20]. Peak intensity of profile (A) is much smaller
than that of profile (B), suggesting that crystal size of Re-04RuSi is much smaller than that of
Re-08RuSi. The existence of RuO2 diffraction peak (2θ = 28°) in profile (B) suggests that Re-08RuSi
Figure 3. Transmission electron micrograph of Ru@SiO2particles with a Ru/Si atomic ratio of 0.4.
Figure 4displays XRD patterns of (A) Re-04RuSi and (B) Re-08RuSi samples, which were
reduced with 5% hydrogen in argon at a heating rate of 1
◦
C/min to 200
◦
C, and then maintained at
200
◦
C for 4 h. The patterns exhibit the characteristic peaks of finely divided metallic ruthenium at
2
θ
= 38
◦
, 42
◦
, 44
◦
, 58
◦
, 69
◦
and 78
◦
, which correspond to the reflection peaks of Ru(100), Ru(002),
Ru(101), Ru(102), Ru(110) and Ru(103), respectively [
20
]. Peak intensity of profile (A) is much smaller
Nanomaterials 2018,8, 153 4 of 14
than that of profile (B), suggesting that crystal size of Re-04RuSi is much smaller than that of Re-08RuSi.
The existence of RuO
2
diffraction peak (2
θ
= 28
◦
) in profile (B) suggests that Re-08RuSi sample is
not completely reduced during the reduction step at 200
◦
C. Peaks of Re species are not observed in
Figure 4, suggesting that Re species is highly dispersed [16].
Nanomaterials 2018, 8, x FOR PEER REVIEW 4 of 13
sample is not completely reduced during the reduction step at 200 °C. Peaks of Re species are not
observed in Figure 4, suggesting that Re species is highly dispersed [16].
Figure 4. X-ray diffraction (XRD) patterns of reduced Re-Ru@SiO2 particles with Ru/Si atomic ratios
of (A) 0.4 and (B) 0.8.
Catalyst reducibility was measured with a temperature-programmed reduction method using
hydrogen as the reductant. Figure 5 compares the TPR profiles of (a) 04RuSi (without Re addition),
(b) Re-01RuSi, (c) Re-02RuSi, (d) Re-04RuSi, (e) Re-08RuSi and (f) Re-16RuSi, indicating that RuO2
in these samples can be reduced completely below 350 °C and the reduction peak area increases
with increasing Ru/Si atomic ratio.
Figure 5. Temperature-programmed reduction (TPR) profiles of (a) Ru@SiO2 core-shell-particles
with a Ru/Si atomic ratio of 0.4 (without Re addition) and Re-Ru@SiO2 core-shell-particles with
Ru/Si atomic ratios of (b) 0.1, (c) 0.2, (d) 0.4, (e) 0.8 and (f) 1.6.
Profile (a) has a reduction peak at 170 °C, which is essentially the same as the reduction peak
temperature (167 °C) reported for a Ru/SiO2 catalyst prepared with the incipient wetness technique
using Ru(NO)(NO)3 solution [21], corresponding to the reduction of RuO2 formed after catalyst
calcination. Comparisons of profile (a) and profile (d) indicate that Re addition retards the reduction
of ruthenium oxide because peak reduction temperature (Tmax) increases from 170 °C in profile (a) to
193 °C in profile (d). This implies an interaction effect between the Ru and Re particles which
shifted the reduction peak to the higher temperature [22]. Shozi et al. [22] also found that the
presence of rhenium increased the reduction temperature of Cu-ZnO catalyst.
Profiles (e) and (f) in Figure 5 have large and broad reduction peaks, which is due to their greater
Ru content. Tmax of profiles (e) and (f) are 205 °C and 222 °C, respectively, which are higher than those
Figure 4.
X-ray diffraction (XRD) patterns of reduced Re-Ru@SiO
2
particles with Ru/Si atomic ratios
of (A) 0.4 and (B) 0.8.
Catalyst reducibility was measured with a temperature-programmed reduction method using
hydrogen as the reductant. Figure 5compares the TPR profiles of (a) 04RuSi (without Re addition),
(b) Re-01RuSi, (c) Re-02RuSi, (d) Re-04RuSi, (e) Re-08RuSi and (f) Re-16RuSi, indicating that RuO
2
in
these samples can be reduced completely below 350
◦
C and the reduction peak area increases with
increasing Ru/Si atomic ratio.
Nanomaterials 2018, 8, x FOR PEER REVIEW 4 of 13
sample is not completely reduced during the reduction step at 200 °C. Peaks of Re species are not
observed in Figure 4, suggesting that Re species is highly dispersed [16].
Figure 4. X-ray diffraction (XRD) patterns of reduced Re-Ru@SiO2 particles with Ru/Si atomic ratios
of (A) 0.4 and (B) 0.8.
Catalyst reducibility was measured with a temperature-programmed reduction method using
hydrogen as the reductant. Figure 5 compares the TPR profiles of (a) 04RuSi (without Re addition),
(b) Re-01RuSi, (c) Re-02RuSi, (d) Re-04RuSi, (e) Re-08RuSi and (f) Re-16RuSi, indicating that RuO2
in these samples can be reduced completely below 350 °C and the reduction peak area increases
with increasing Ru/Si atomic ratio.
Figure 5. Temperature-programmed reduction (TPR) profiles of (a) Ru@SiO2 core-shell-particles
with a Ru/Si atomic ratio of 0.4 (without Re addition) and Re-Ru@SiO2 core-shell-particles with
Ru/Si atomic ratios of (b) 0.1, (c) 0.2, (d) 0.4, (e) 0.8 and (f) 1.6.
Profile (a) has a reduction peak at 170 °C, which is essentially the same as the reduction peak
temperature (167 °C) reported for a Ru/SiO2 catalyst prepared with the incipient wetness technique
using Ru(NO)(NO)3 solution [21], corresponding to the reduction of RuO2 formed after catalyst
calcination. Comparisons of profile (a) and profile (d) indicate that Re addition retards the reduction
of ruthenium oxide because peak reduction temperature (Tmax) increases from 170 °C in profile (a) to
193 °C in profile (d). This implies an interaction effect between the Ru and Re particles which
shifted the reduction peak to the higher temperature [22]. Shozi et al. [22] also found that the
presence of rhenium increased the reduction temperature of Cu-ZnO catalyst.
Profiles (e) and (f) in Figure 5 have large and broad reduction peaks, which is due to their greater
Ru content. Tmax of profiles (e) and (f) are 205 °C and 222 °C, respectively, which are higher than those
Figure 5.
Temperature-programmed reduction (TPR) profiles of (a) Ru@SiO
2
core-shell-particles with a
Ru/Si atomic ratio of 0.4 (without Re addition) and Re-Ru@SiO
2
core-shell-particles with Ru/Si atomic
ratios of (b) 0.1, (c) 0.2, (d) 0.4, (e) 0.8 and (f) 1.6.
Profile (a) has a reduction peak at 170
◦
C, which is essentially the same as the reduction peak
temperature (167
◦
C) reported for a Ru/SiO
2
catalyst prepared with the incipient wetness technique
using Ru(NO)(NO)
3
solution [
21
], corresponding to the reduction of RuO
2
formed after catalyst
calcination. Comparisons of profile (a) and profile (d) indicate that Re addition retards the reduction of
ruthenium oxide because peak reduction temperature (T
max
) increases from 170
◦
C in profile (a) to
193 ◦C in profile (d). This implies an interaction effect between the Ru and Re particles which shifted
Nanomaterials 2018,8, 153 5 of 14
the reduction peak to the higher temperature [
22
]. Shozi et al. [
22
] also found that the presence of
rhenium increased the reduction temperature of Cu-ZnO catalyst.
Profiles (e) and (f) in Figure 5have large and broad reduction peaks, which is due to their greater
Ru content. T
max
of profiles (e) and (f) are 205
◦
C and 222
◦
C, respectively, which are higher than those
in profiles (b) to (d) (in a range of 190
◦
C to 200
◦
C). It was reported in the literature that the reduction
temperature of unsupported RuO
2
is 217
◦
C [
23
]. The higher reduction temperatures of profiles (e)
and (f) suggest that some RuO
2
enclosed in Re-08RuSi and Re-16RuSi nanoparticles do not contact
with silica shell, and they have a reduction temperature similar to that of unsupported Ru species.
Pore size distribution analyses of Re-04RuSi and Re-08RuSi samples indicated that these particles
were mesoporous materials with a major pore diameter of 3.8 nm. Pore volume decreased with
increasing Ru/Si atomic ratio (pore volumes were 0.6, 0.4, 0.35 and 0.33 cm
3
/g for Ru/Si atomic ratios
of 0.2, 0.4, 0.8, and 1.6, respectively), suggesting that most pores were generated in the silica shell
due to burning out of organic material (PVP). It is known that fine pores in the range of 1 to 10 nm in
radius account for most of the surface area [
24
], therefore, the decrease of pore volume with increasing
Ru/Si ratio resulted in the decrease of catalyst surface area (surface areas were 405.2, 353.6, 230.9 and
188.3 m
2
/g for Ru/Si atomic ratios of 0.2, 0.4, 0.8, and 1.6, respectively). Scanning electron micrograph
images of catalysts indicated that catalyst particle size decreased continuously with increasing Ru/Si
atomic ratio. This is contrary to the results of surface area measurements (surface area decreased with
increasing Ru/Si atomic ratio), indicating that internal surface is more important than the external
surface for determining the total surface area. The addition of Re species on Ru@SiO
2
resulted in the
significant decrease of surface area. For example, surface areas decreased from 420 m
2
/g (without Re
addition) to 353.6 m2/g (with Re addition) when Ru/Si atomic ratio = 0.4.
Figure 6shows the Ru 3d spectrum for Re-Ru@SiO
2
nanoparticles, which has one set of doublets
with a spin-orbit splitting of 3.8 eV. The first peak of Ru 3d is located at about 282.3 eV, which is similar
to that (282.4 eV) reported before for a Ru/SiO
2
catalyst [
25
], suggesting that Re addition does not
modify the electronic properties of ruthenium.
Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 13
in profiles (b) to (d) (in a range of 190 °C to 200 °C). It was reported in the literature that the reduction
temperature of unsupported RuO2 is 217 °C [23]. The higher reduction temperatures of profiles (e) and
(f) suggest that some RuO2 enclosed in Re-08RuSi and Re-16RuSi nanoparticles do not contact with
silica shell, and they have a reduction temperature similar to that of unsupported Ru species.
Pore size distribution analyses of Re-04RuSi and Re-08RuSi samples indicated that these
particles were mesoporous materials with a major pore diameter of 3.8 nm. Pore volume decreased
with increasing Ru/Si atomic ratio (pore volumes were 0.6, 0.4, 0.35 and 0.33 cm3/g for Ru/Si atomic
ratios of 0.2, 0.4, 0.8, and 1.6, respectively), suggesting that most pores were generated in the silica
shell due to burning out of organic material (PVP). It is known that fine pores in the range of 1 to 10
nm in radius account for most of the surface area [24], therefore, the decrease of pore volume with
increasing Ru/Si ratio resulted in the decrease of catalyst surface area (surface areas were 405.2,
353.6, 230.9 and 188.3 m2/g for Ru/Si atomic ratios of 0.2, 0.4, 0.8, and 1.6, respectively). Scanning
electron micrograph images of catalysts indicated that catalyst particle size decreased continuously
with increasing Ru/Si atomic ratio. This is contrary to the results of surface area measurements
(surface area decreased with increasing Ru/Si atomic ratio), indicating that internal surface is more
important than the external surface for determining the total surface area. The addition of Re species
on Ru@SiO2 resulted in the significant decrease of surface area. For example, surface areas decreased
from 420 m2/g (without Re addition) to 353.6 m2/g (with Re addition) when Ru/Si atomic ratio = 0.4.
Figure 6 shows the Ru 3d spectrum for Re-Ru@SiO2 nanoparticles, which has one set of doublets
with a spin-orbit splitting of 3.8 eV. The first peak of Ru 3d is located at about 282.3 eV, which is
similar to that (282.4 eV) reported before for a Ru/SiO2 catalyst [25], suggesting that Re addition does
not modify the electronic properties of ruthenium.
Figure 6. Ru 3d XPS spectra of the Re-Ru@SiO2 nanoparticles.
Figures 7 and 8 show a scanning electron micrograph and a transmission electron micrograph
of used Re-04Ru@SiO2 nanoparticles, respectively. A comparison of Figures 7 and 2 indicates that
particle size does not change significantly after hydrogenolysis. Figure 8 shows that small
ruthenium nanoparticles with a variety of sizes remain highly dispersed in silica, indicating that the
catalyst is stable under the glycerol hydrogenolysis conditions.
Figure 7. Scanning electron micrograph of spent Re-04RuSi catalyst.
Figure 6. Ru 3d XPS spectra of the Re-Ru@SiO2nanoparticles.
Figures 7and 8show a scanning electron micrograph and a transmission electron micrograph
of used Re-04Ru@SiO
2
nanoparticles, respectively. A comparison of Figures 2and 7indicates that
particle size does not change significantly after hydrogenolysis. Figure 8shows that small ruthenium
nanoparticles with a variety of sizes remain highly dispersed in silica, indicating that the catalyst is
stable under the glycerol hydrogenolysis conditions.
Nanomaterials 2018,8, 153 6 of 14
Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 13
in profiles (b) to (d) (in a range of 190 °C to 200 °C). It was reported in the literature that the reduction
temperature of unsupported RuO2 is 217 °C [23]. The higher reduction temperatures of profiles (e) and
(f) suggest that some RuO2 enclosed in Re-08RuSi and Re-16RuSi nanoparticles do not contact with
silica shell, and they have a reduction temperature similar to that of unsupported Ru species.
Pore size distribution analyses of Re-04RuSi and Re-08RuSi samples indicated that these
particles were mesoporous materials with a major pore diameter of 3.8 nm. Pore volume decreased
with increasing Ru/Si atomic ratio (pore volumes were 0.6, 0.4, 0.35 and 0.33 cm3/g for Ru/Si atomic
ratios of 0.2, 0.4, 0.8, and 1.6, respectively), suggesting that most pores were generated in the silica
shell due to burning out of organic material (PVP). It is known that fine pores in the range of 1 to 10
nm in radius account for most of the surface area [24], therefore, the decrease of pore volume with
increasing Ru/Si ratio resulted in the decrease of catalyst surface area (surface areas were 405.2,
353.6, 230.9 and 188.3 m2/g for Ru/Si atomic ratios of 0.2, 0.4, 0.8, and 1.6, respectively). Scanning
electron micrograph images of catalysts indicated that catalyst particle size decreased continuously
with increasing Ru/Si atomic ratio. This is contrary to the results of surface area measurements
(surface area decreased with increasing Ru/Si atomic ratio), indicating that internal surface is more
important than the external surface for determining the total surface area. The addition of Re species
on Ru@SiO2 resulted in the significant decrease of surface area. For example, surface areas decreased
from 420 m2/g (without Re addition) to 353.6 m2/g (with Re addition) when Ru/Si atomic ratio = 0.4.
Figure 6 shows the Ru 3d spectrum for Re-Ru@SiO2 nanoparticles, which has one set of doublets
with a spin-orbit splitting of 3.8 eV. The first peak of Ru 3d is located at about 282.3 eV, which is
similar to that (282.4 eV) reported before for a Ru/SiO2 catalyst [25], suggesting that Re addition does
not modify the electronic properties of ruthenium.
Figure 6. Ru 3d XPS spectra of the Re-Ru@SiO2 nanoparticles.
Figures 7 and 8 show a scanning electron micrograph and a transmission electron micrograph
of used Re-04Ru@SiO2 nanoparticles, respectively. A comparison of Figures 7 and 2 indicates that
particle size does not change significantly after hydrogenolysis. Figure 8 shows that small
ruthenium nanoparticles with a variety of sizes remain highly dispersed in silica, indicating that the
catalyst is stable under the glycerol hydrogenolysis conditions.
Figure 7. Scanning electron micrograph of spent Re-04RuSi catalyst.
Figure 7. Scanning electron micrograph of spent Re-04RuSi catalyst.
Nanomaterials 2018, 8, x FOR PEER REVIEW 6 of 13
Figure 8. Transmission electron micrograph of spent Re-04RuSi catalyst.
2.2. Hydrogenolysis of Glycerol
Glycerol hydrogenolysis was carried out with five Re-Ru@SiO2 catalysts mentioned above, five
reaction temperatures (110–160 °C), six reaction times (2–12 h), five hydrogen pressure (0–1100
psig), three catalyst amounts (0.315–1.26 g) and five glycerol concentrations (10–50 wt %). The
effects of these variables on glycerol conversion and diol/alcohol yields or selectivities were
investigated.
Figure 9 presents the relationship between glycerol conversion/product selectivity and Ru/Si
atomic ratio under the following conditions: temperature = 130 °C, pressure = 1100 psig, glycerol
concentration = 40 wt %, reaction time = 8 h, catalyst weight = 0.63 g. With the increase of Ru/Si
atomic ratio from 0.1 to 1.6, glycerol conversion increases continuously (from 23.9% to 87.6%), which
should be due to the increase of Ru content in the catalyst. But 1,3-PDO selectivity decreases
continuously (from 11% to 0.9%) with increasing Ru/Si ratio, suggesting that it is more difficult to
remove the middle OH group in the glycerol molecules when the catalyst is crowded with Ru atoms.
1,2-PDO selectivity is around 50% and overall liquid-phase product (the combination of diols and
alcohols) selectivity is greater than 92% for Ru/Si atomic ratio ≤0.4, but decrease rapidly after the
further increase of Ru/Si ratio. This can be explained in terms of the competitive adsorption between
reactant (glycerol, a triol) and liquid-phase products (diols and alcohols). At low Ru/Si ratio and low
glycerol conversion, most Ru sites are covered by glycerol molecules because the adsorption
strength typically is in the order: triol > diol > alcohol. At high Ru/Si ratio and high conversion, more
Ru sites are available for adsorbing liquid-phase products and gaseous products (H2, CO or CO2) are
produced via C-C bond cleavage of the adsorbed diols/alcohols.
Figure 9. Influence of Ru/Si atomic ratio on glycerol conversion and product selectivity.
Ru / Si Atomic Ratio
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Conversion or Selectivity , ( % )
0
20
40
60
80
100
Conversion
Liquid Product
1,2 PDO
PA
1,3 PDO
IPA
Figure 8. Transmission electron micrograph of spent Re-04RuSi catalyst.
2.2. Hydrogenolysis of Glycerol
Glycerol hydrogenolysis was carried out with five Re-Ru@SiO
2
catalysts mentioned above, five
reaction temperatures (110–160
◦
C), six reaction times (2–12 h), five hydrogen pressure (0–1100 psig),
three catalyst amounts (0.315–1.26 g) and five glycerol concentrations (10–50 wt %). The effects of these
variables on glycerol conversion and diol/alcohol yields or selectivities were investigated.
Figure 9presents the relationship between glycerol conversion/product selectivity and Ru/Si
atomic ratio under the following conditions: temperature = 130
◦
C, pressure = 1100 psig, glycerol
concentration = 40 wt %, reaction time = 8 h, catalyst weight = 0.63 g. With the increase of Ru/Si atomic
ratio from 0.1 to 1.6, glycerol conversion increases continuously (from 23.9% to 87.6%), which should be
due to the increase of Ru content in the catalyst. But 1,3-PDO selectivity decreases continuously (from
11% to 0.9%) with increasing Ru/Si ratio, suggesting that it is more difficult to remove the middle
OH group in the glycerol molecules when the catalyst is crowded with Ru atoms. 1,2-PDO selectivity
is around 50% and overall liquid-phase product (the combination of diols and alcohols) selectivity
is greater than 92% for Ru/Si atomic ratio
≤
0.4, but decrease rapidly after the further increase of
Ru/Si ratio. This can be explained in terms of the competitive adsorption between reactant (glycerol, a
triol) and liquid-phase products (diols and alcohols). At low Ru/Si ratio and low glycerol conversion,
most Ru sites are covered by glycerol molecules because the adsorption strength typically is in the
order: triol > diol > alcohol. At high Ru/Si ratio and high conversion, more Ru sites are available for
adsorbing liquid-phase products and gaseous products (H
2
, CO or CO
2
) are produced via C-C bond
cleavage of the adsorbed diols/alcohols.
Nanomaterials 2018,8, 153 7 of 14
Nanomaterials 2018, 8, x FOR PEER REVIEW 6 of 13
Figure 8. Transmission electron micrograph of spent Re-04RuSi catalyst.
2.2. Hydrogenolysis of Glycerol
Glycerol hydrogenolysis was carried out with five Re-Ru@SiO2 catalysts mentioned above, five
reaction temperatures (110–160 °C), six reaction times (2–12 h), five hydrogen pressure (0–1100
psig), three catalyst amounts (0.315–1.26 g) and five glycerol concentrations (10–50 wt %). The
effects of these variables on glycerol conversion and diol/alcohol yields or selectivities were
investigated.
Figure 9 presents the relationship between glycerol conversion/product selectivity and Ru/Si
atomic ratio under the following conditions: temperature = 130 °C, pressure = 1100 psig, glycerol
concentration = 40 wt %, reaction time = 8 h, catalyst weight = 0.63 g. With the increase of Ru/Si
atomic ratio from 0.1 to 1.6, glycerol conversion increases continuously (from 23.9% to 87.6%), which
should be due to the increase of Ru content in the catalyst. But 1,3-PDO selectivity decreases
continuously (from 11% to 0.9%) with increasing Ru/Si ratio, suggesting that it is more difficult to
remove the middle OH group in the glycerol molecules when the catalyst is crowded with Ru atoms.
1,2-PDO selectivity is around 50% and overall liquid-phase product (the combination of diols and
alcohols) selectivity is greater than 92% for Ru/Si atomic ratio ≤0.4, but decrease rapidly after the
further increase of Ru/Si ratio. This can be explained in terms of the competitive adsorption between
reactant (glycerol, a triol) and liquid-phase products (diols and alcohols). At low Ru/Si ratio and low
glycerol conversion, most Ru sites are covered by glycerol molecules because the adsorption
strength typically is in the order: triol > diol > alcohol. At high Ru/Si ratio and high conversion, more
Ru sites are available for adsorbing liquid-phase products and gaseous products (H2, CO or CO2) are
produced via C-C bond cleavage of the adsorbed diols/alcohols.
Figure 9. Influence of Ru/Si atomic ratio on glycerol conversion and product selectivity.
Ru / Si Atomic Ratio
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Conversion or Selectivity , ( % )
0
20
40
60
80
100
Conversion
Liquid Product
1,2 PDO
PA
1,3 PDO
IPA
Figure 9. Influence of Ru/Si atomic ratio on glycerol conversion and product selectivity.
Reaction rates (based on unit catalyst weight),
−
r
A
’, were calculated from conversion data (shown
in Figure 9) according to the following equation:
−rA’ = moles of glycerol fed ×conversion/(catalyst weight ×reaction time) (1)
Re-04RuSi catalyst had a reaction rate of 0.017 mol/h
·
g catalyst at 130
◦
C and 1100 psig, which
is 8.4 times that (0.00206 mol/h
·
g catalyst at 160
◦
C and 1160 psia) calculated from the data of Ma
and He [
15
]. That is, Re-Ru@SiO
2
catalysts exhibit much greater activity than Re-Ru/SiO
2
catalyst
prepared by conventional impregnation method.
Re addition significantly increased catalyst activity and PDO selectivity. Under the reaction
conditions of Figure 9, the reaction rate of the Re-04RuSi catalyst was 30% greater than that of the
corresponding Ru@SiO
2
catalyst (without Re addition). The former had a 1,2-PDO selectivity of 50.3%
and a 1,3-PDO selectivity of 4.5%, which are better than those (38.9% and 1%, respectively) of the
latter. These are consistent with the results of previous papers, which demonstrated that Re addition
to Ru, Pt and Ir catalysts enhanced glycerol C–O hydrogenolysis activities and increased 1,3-PDO
selectivity compared to the corresponding monometallic catalysts [
14
,
26
]. Chia et al. studied cyclic
ether hydrogenolysis on Rh-Re catalysts, the combination of their NH
3
TPD data and density function
theory (DFT) calculation results suggested that hydroxyl groups on rhenium atoms are acidic, due to
the strong binding of oxygen atoms by rhenium [
27
]. It is possible that the glycerol hydrogenolysis
proceeds by the hydrogenolysis of the alkoxide species on Re with hydrogen species on the Ru metal
surfaces, similar to the mechanism proposed by Amada et al. for the hydrogenolysis of 1,2-propanediol
to propanols on Rh-ReOx/SiO2catalysts [28].
Figure 10 shows the effect of hydrogen pressure in glycerol hydrogenolysis at 130
◦
C and 8 h
using Re-08RuSi catalyst.
Nanomaterials 2018,8, 153 8 of 14
Nanomaterials 2018, 8, x FOR PEER REVIEW 7 of 13
Reaction rates (based on unit catalyst weight), −rA’, were calculated from conversion data
(shown in Figure 9) according to the following equation:
−rA’ = moles of glycerol fed × conversion/(catalyst weight × reaction time) (1)
Re-04RuSi catalyst had a reaction rate of 0.017 mol/h·g catalyst at 130 °C and 1100 psig, which is
8.4 times that (0.00206 mol/h·g catalyst at 160 °C and 1160 psia) calculated from the data of Ma and
He [15]. That is, Re-Ru@SiO2 catalysts exhibit much greater activity than Re-Ru/SiO2 catalyst
prepared by conventional impregnation method.
Re addition significantly increased catalyst activity and PDO selectivity. Under the reaction
conditions of Figure 9, the reaction rate of the Re-04RuSi catalyst was 30% greater than that of the
corresponding Ru@SiO2 catalyst (without Re addition). The former had a 1,2-PDO selectivity of
50.3% and a 1,3-PDO selectivity of 4.5%, which are better than those (38.9% and 1%, respectively) of
the latter. These are consistent with the results of previous papers, which demonstrated that Re
addition to Ru, Pt and Ir catalysts enhanced glycerol C–O hydrogenolysis activities and increased
1,3-PDO selectivity compared to the corresponding monometallic catalysts [14,26]. Chia et al.
studied cyclic ether hydrogenolysis on Rh-Re catalysts, the combination of their NH3 TPD data and
density function theory (DFT) calculation results suggested that hydroxyl groups on rhenium atoms
are acidic, due to the strong binding of oxygen atoms by rhenium [27]. It is possible that the glycerol
hydrogenolysis proceeds by the hydrogenolysis of the alkoxide species on Re with hydrogen species
on the Ru metal surfaces, similar to the mechanism proposed by Amada et al. for the hydrogenolysis
of 1,2-propanediol to propanols on Rh-ReOx/SiO2 catalysts [28].
Figure 10 shows the effect of hydrogen pressure in glycerol hydrogenolysis at 130 °C and 8 h
using Re-08RuSi catalyst.
Pressure (psi)
0 200 400 600 800 1000
Conversion and Yield , ( % )
0
20
40
60
Conversion
Liquid Product Yield
1,2 PDO Yield
PA Yield
1,3 PDO Yield
IPA Yield
Figure 10. Influence of hydrogen pressure on glycerol conversion and product yield for the
Re-08RuSi catalyst.
With the increase of pressure from 0 to 1100 psig, glycerol conversion increases from 4.9% to
72.2%, but the maximum 1,2-PDO yield (31.6%) and the maximum yield of overall liquid-phase
products (59.3%) occur at 800 psig. The decrease of overall liquid-phase product selectivity (from
98.6% at 800 psig to 72.3% at 1100 psig) should be due to the low glycerol coverage on Ru sites.
The effects of reaction temperature (in a range of 110–160 °C) on glycerol conversion and
diol/alcohol selectivies are presented in Table 1 for Re-08RuSi catalyst, which were obtained at 1100
psi, 8 h and 0.63 g Re-08RuSi catalyst. Table 1 shows that glycerol conversion increases with
increasing reaction temperature and reaches 100% at 160 °C. However, the selectivities of overall
liquid-phase product, 1,2-PDO and 1,3-PDO decrease with increasing reaction temperature and
reached 0% at 160 °C. That is, all liquid products were converted to gaseous products (CO, CO2 and
hydrogen, as identified by GC analyses) at 160 °C and 1100 psi for the Re-08 RuSi catalyst.
Figure 10.
Influence of hydrogen pressure on glycerol conversion and product yield for the
Re-08RuSi catalyst.
With the increase of pressure from 0 to 1100 psig, glycerol conversion increases from 4.9% to 72.2%,
but the maximum 1,2-PDO yield (31.6%) and the maximum yield of overall liquid-phase products
(59.3%) occur at 800 psig. The decrease of overall liquid-phase product selectivity (from 98.6% at
800 psig to 72.3% at 1100 psig) should be due to the low glycerol coverage on Ru sites.
The effects of reaction temperature (in a range of 110–160
◦
C) on glycerol conversion and
diol/alcohol selectivies are presented in Table 1for Re-08RuSi catalyst, which were obtained at 1100 psi,
8 h and 0.63 g Re-08RuSi catalyst. Table 1shows that glycerol conversion increases with increasing
reaction temperature and reaches 100% at 160
◦
C. However, the selectivities of overall liquid-phase
product, 1,2-PDO and 1,3-PDO decrease with increasing reaction temperature and reached 0% at
160
◦
C. That is, all liquid products were converted to gaseous products (CO, CO
2
and hydrogen, as
identified by GC analyses) at 160 ◦C and 1100 psi for the Re-08 RuSi catalyst.
Table 1. Effects of reaction temperature on glycerol conversion and diol/alcohol selectivities.
Reaction Temp. (◦C) 110 120 125 130 160
conversion (%) 27.42 34.59 45.06 72.16 100.00
methanol sel. (%) 0.65 0.36 0.49 0.34 0
isopropanol sel. (%) 2.59 2.19 3.43 3.44 0
ethanol sel. (%) 3.94 5.15 5.83 7.89 0
n-propanol sel. (%) 15.94 19.12 8.18 16.42 0
1.2-PDO sel. (%) 63.36 58.29 64.77 35.16 0
EG sel. (%) 4.50 5.76 6.64 4.72 0
1.3-PDO sel. (%) 8.53 8.05 2.89 4.31 0
Overall liquid product sel. (%)
99.50 98.92 92.23 72.27 0
Reaction conditions: 18.52 g glycerol, 27.78 g water, t = 8 h, P = 1100 psi, 0.63 g Re-08RuSi catalyst.
It is known that glycerol can be either converted to diols/alcohols via hydrodeoxygenation (based
on the pathways shown in Figure 1), or can be used to produce hydrogen through aqueous phase
reforming (APR) [
29
–
31
]. In Table 1, PDO selectivity improved significantly by using low reaction
temperature. For example, the decrease of reaction temperature from 130
◦
C to 110
◦
C resulted in the
increase of 1,2-PDO selectivity from 35.2% to 63.4% and the increase of 1,3-PDO selectivity from 4.3%
to 8.5%. The results suggest that the activation energies for PDO conversion to alcohols are higher
than those for PDO formation from glycerol. That is, the formers are more temperature sensitive
than the latters. At 160
◦
C, the disappearance of diols/alcohols completely and the generation of
Nanomaterials 2018,8, 153 9 of 14
gaseous products (CO, CO
2
and hydrogen) only indicate that APR (C–C bond breaking) favors at
higher reaction temperatures [
29
]. The production of hydrogen through the aqueous-phase reforming
(APR) of glycerol is considered as a promising catalytic process and the results of Table 1indicate that
Re-08RuSi catalyst is also a very good catalyst for producing hydrogen at mild reaction condition. In
Table 1, methanol selectivity is less than 1% for all conversions, indicating that the O–H bond rupture
(hydrogenolysis) is the fundamental prerequisite for C–C cleavage [
29
]. Re-08RuSi catalyst was reused
at 120
◦
C three times for testing its stability, glycerol conversion decreased slightly from 34.6% (the 1st
time use) to 32.8% (the 3rd time use), which indicates that Re-Ru@SiO
2
catalyst is stable under the
glycerol hydrogenolysis conditions.
Kinetic study was carried out with Re-08RuSi and Re-04RuSi catalysts. To determine reaction
rate parameters, the following differential equation was established to describe the reaction system
in a batch reactor by assuming a pseudo-first-order rate equation for glycerol (denoted as A)
hydrogenolysis:
−dCA/dt = k CA(2)
Integration of Equation (2) yields
ln(1 −X) = kt (3)
Experimental results of Re-08RuSi and Re-04RuSi catalysts at 130
◦
C and 800 psig were plotted
according to Equation (3), and a straight line passing through zero was obtained, as illustrated
in Figure 11. Therefore, the rate of glycerol disappearance is first-order with respect to glycerol
concentration. The rate equation (based on unit volume of reaction solution) can be written as
−rA=kCA(4)
with
k = A exp(−E/RT) (5)
Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 13
Time ( h )
024681012
- ln ( 1 - X )
0.0
0.4
0.8
1.2
Re-08RuSi
Re-04RuSi
Figure 11. Test of pseudo-first-order kinetic model for Re-04RuSi and Re-08RuSi catalysts at 130 °C
and 800 psig.
The rate constants k at 130 °C obtained from Figure 11 are 0.11/h and 0.072/h for Re-08RuSi and
Re-04RuSi, respectively. The higher rate constant obtained with the former (Re-08RuSi) should be
mainly due to its greater Ru content, as shown in Figure 12, which presents the effects of Ru/Si
atomic ratio and Re-08RuSi catalyst amount on rate constant k. It is interesting to note that k
increases linearly with the increase of Ru/Si atomic ratio and with the increase of Re-08RuSi catalyst
amount, suggesting that nearly all Ru sites have identical activity for catalyzing glycerol
hydrogenolysis.
For the Re-08RuSi catalyst, frequency factor A and activation energy E were 1.06 × 1013/h and
107.8 kJ/mol, respectively, which were obtained from an Arrhenius plot of lnk versus 1000/T. In the
region of pressure ≥800 psig, rate constant increased linearly with hydrogen pressure (a straight line
passing through zero was obtained by plotting k versus pressure), which suggests that glycerol
hydrogenolysis is first order with respect to hydrogen pressure.
Ru / Si Atomic Ratio
0.00.40.81.21.6
Ru / Si Atomic Ratio
0.00.40.81.21.6
1st order rate constant k , ( 1/h )
0.00
0.05
0.10
0.15
0.20
0.25
k vs. Ru/Si atomic ratio
Regression of k vs. Ru/Si ratio
k vs. catalyst weight
Regression of k vs. catalyst weight
Figure 12. Pseudo-first-order rate constant k (at 130 °C and 800 psi) versus Cu/Si atomic ratio and
Re-08RuSi catalyst weight.
Figure 11.
Test of pseudo-first-order kinetic model for Re-04RuSi and Re-08RuSi catalysts at 130
◦
C
and 800 psig.
The rate constants k at 130
◦
C obtained from Figure 11 are 0.11/h and 0.072/h for Re-08RuSi and
Re-04RuSi, respectively. The higher rate constant obtained with the former (Re-08RuSi) should be
mainly due to its greater Ru content, as shown in Figure 12, which presents the effects of Ru/Si atomic
ratio and Re-08RuSi catalyst amount on rate constant k. It is interesting to note that k increases linearly
Nanomaterials 2018,8, 153 10 of 14
with the increase of Ru/Si atomic ratio and with the increase of Re-08RuSi catalyst amount, suggesting
that nearly all Ru sites have identical activity for catalyzing glycerol hydrogenolysis.
Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 13
Time ( h )
024681012
- ln ( 1 - X )
0.0
0.4
0.8
1.2
Re-08RuSi
Re-04RuSi
Figure 11. Test of pseudo-first-order kinetic model for Re-04RuSi and Re-08RuSi catalysts at 130 °C
and 800 psig.
The rate constants k at 130 °C obtained from Figure 11 are 0.11/h and 0.072/h for Re-08RuSi and
Re-04RuSi, respectively. The higher rate constant obtained with the former (Re-08RuSi) should be
mainly due to its greater Ru content, as shown in Figure 12, which presents the effects of Ru/Si
atomic ratio and Re-08RuSi catalyst amount on rate constant k. It is interesting to note that k
increases linearly with the increase of Ru/Si atomic ratio and with the increase of Re-08RuSi catalyst
amount, suggesting that nearly all Ru sites have identical activity for catalyzing glycerol
hydrogenolysis.
For the Re-08RuSi catalyst, frequency factor A and activation energy E were 1.06 × 1013/h and
107.8 kJ/mol, respectively, which were obtained from an Arrhenius plot of lnk versus 1000/T. In the
region of pressure ≥800 psig, rate constant increased linearly with hydrogen pressure (a straight line
passing through zero was obtained by plotting k versus pressure), which suggests that glycerol
hydrogenolysis is first order with respect to hydrogen pressure.
Ru / Si Atomic Ratio
0.00.40.81.21.6
Ru / Si Atomic Ratio
0.00.40.81.21.6
1st order rate constant k , ( 1/h )
0.00
0.05
0.10
0.15
0.20
0.25
k vs. Ru/Si atomic ratio
Regression of k vs. Ru/Si ratio
k vs. catalyst weight
Regression of k vs. catalyst weight
Figure 12. Pseudo-first-order rate constant k (at 130 °C and 800 psi) versus Cu/Si atomic ratio and
Re-08RuSi catalyst weight.
Figure 12.
Pseudo-first-order rate constant k (at 130
◦
C and 800 psi) versus Cu/Si atomic ratio and
Re-08RuSi catalyst weight.
For the Re-08RuSi catalyst, frequency factor A and activation energy E were 1.06
×
10
13
/h and
107.8 kJ/mol, respectively, which were obtained from an Arrhenius plot of lnk versus 1000/T. In
the region of pressure
≥
800 psig, rate constant increased linearly with hydrogen pressure (a straight
line passing through zero was obtained by plotting k versus pressure), which suggests that glycerol
hydrogenolysis is first order with respect to hydrogen pressure.
Figure 13 shows the effect of glycerol conversion on the selectivities of overall liquid-phase
products, 1,2-PDO and 1,3-PDO for Re-08RuSi catalyst, which were obtained at 130
◦
C using the
Re-08RuSi catalyst with the variations of glycerol concentration (20–50 wt %), hydrogen pressure
(400–1100 psi), reaction time (2–12 h) and catalyst amount (0.32–1.26 g). All data points essentially fall
on a single curve. The selectivities of overall liquid-phase products and 1–2 PDO are not sensitive
to the change of glycerol conversion when glycerol conversion is
≤
60.2%, but decrease rapidly at
the higher conversion range. 1,3-PDO selectivity decreases significantly when glycerol conversion is
greater than 30%, suggesting that 1,3-PDO is easier than 1,2-PDO to turn into alcohols. The results are
consistent with the activity order (1,3-PDO~glycerol > 1,2-PDO~1-propanol) proposed by Peng et al.
for a Pt/Al2O3catalyst [30].
Nanomaterials 2018,8, 153 11 of 14
Nanomaterials 2018, 8, x FOR PEER REVIEW 10 of 13
Figure 13 shows the effect of glycerol conversion on the selectivities of overall liquid-phase
products, 1,2-PDO and 1,3-PDO for Re-08RuSi catalyst, which were obtained at 130 °C using the
Re-08RuSi catalyst with the variations of glycerol concentration (20–50 wt %), hydrogen pressure
(400–1100 psi), reaction time (2–12 h) and catalyst amount (0.32–1.26 g). All data points essentially
fall on a single curve. The selectivities of overall liquid-phase products and 1–2 PDO are not
sensitive to the change of glycerol conversion when glycerol conversion is ≤60.2%, but decrease
rapidly at the higher conversion range. 1,3-PDO selectivity decreases significantly when glycerol
conversion is greater than 30%, suggesting that 1,3-PDO is easier than 1,2-PDO to turn into alcohols.
The results are consistent with the activity order (1,3-PDO~glycerol > 1,2-PDO~1-propanol)
proposed by Peng et al. for a Pt/Al2O3 catalyst [30].
Comparisons of Figure 12 (rate constant increases linearly with increasing Ru/Si ratio) and the
surface area data (surface area decreases with increasing Ru/Si ratio) indicate that Ru content is
much more important than surface area for determining catalyst activity.
Glycerol Conversion ( % )
10 20 30 40 50 60 70 80 90
Selectivity (%)
0
20
40
60
80
100
Total liquid phase product
1,2-PDO
1,3-PDO
Figure 13. Influence of glycerol conversion on selectivities to overall liquid-phase product, 1,2-PDO
and 1,3-PDO for Re-08RuSi catalyst.
3. Materials and Methods
3.1. Catalyst Preparation
Ru@SiO2 nanoparticles with five different Ru/Si atomic ratios were prepared by coating slica
onto the surface of Ru-polyvinylpyrrolidone (PVP) colloids, according to the well-known Stober
method, which included hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol,
using ammonia as catalyst to initiate the reaction. The Ru-PVP colloids were synthesized by
chemical reduction of Ru3+ in an alkaline environment using formaldehyde as reducing agent and
PVP as protecting agent [17,18]. The calcined Ru@SiO2 nanoparticles were then impregnated to
incipient wetness with NH4ReO4 aqueous solution to obtain Re-Ru@SiO2 catalysts. The starting
materials were RuCl3·xH2O (containing 42 wt % Ru, Seedchem, Melbourne, Australia), NH4ReO4
(Sigam-Aldrich, St. Louis, MO, USA), polyvinylpyrrolidone (molecular weight = 8000; ACROS, Geel,
Belgium), tetraethyl orthosilicate (ACROS), NaOH, NH4OH (Showa Chemicals, Tokyo, Japan),
formaldehyde (Scharlan Chimie, Barcelona, Spain), ethanol, and acetone (Echo Chemical, Tou Fen,
Taiwan).
The main steps for catalyst preparation were as follows [17,18]:
1. Prepare an aqueous solution of RuCl3 and PVP, using 2 g RuCl3.xH2O, 96 mL de-ionized water
Figure 13.
Influence of glycerol conversion on selectivities to overall liquid-phase product, 1,2-PDO
and 1,3-PDO for Re-08RuSi catalyst.
Comparisons of Figure 12 (rate constant increases linearly with increasing Ru/Si ratio) and the
surface area data (surface area decreases with increasing Ru/Si ratio) indicate that Ru content is much
more important than surface area for determining catalyst activity.
3. Materials and Methods
3.1. Catalyst Preparation
Ru@SiO
2
nanoparticles with five different Ru/Si atomic ratios were prepared by coating slica
onto the surface of Ru-polyvinylpyrrolidone (PVP) colloids, according to the well-known Stober
method, which included hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol,
using ammonia as catalyst to initiate the reaction. The Ru-PVP colloids were synthesized by chemical
reduction of Ru
3+
in an alkaline environment using formaldehyde as reducing agent and PVP as
protecting agent [
17
,
18
]. The calcined Ru@SiO
2
nanoparticles were then impregnated to incipient
wetness with NH
4
ReO
4
aqueous solution to obtain Re-Ru@SiO
2
catalysts. The starting materials were
RuCl
3·
xH
2
O (containing 42 wt % Ru, Seedchem, Melbourne, Australia), NH
4
ReO
4
(Sigam-Aldrich,
St. Louis, MO, USA), polyvinylpyrrolidone (molecular weight = 8000; ACROS, Geel, Belgium),
tetraethyl orthosilicate (ACROS), NaOH, NH
4
OH (Showa Chemicals, Tokyo, Japan), formaldehyde
(Scharlan Chimie, Barcelona, Spain), ethanol, and acetone (Echo Chemical, Tou Fen, Taiwan).
The main steps for catalyst preparation were as follows [17,18]:
1.
Prepare an aqueous solution of RuCl
3
and PVP, using 2 g RuCl
3·
xH
2
O, 96 mL de-ionized water
and 8.2 g PVP.
2. Add 3.3 g formaldehyde and 0.59 g NaOH to the above solution to synthesize Ru-PVP colloids.
3. Wash the Ru-PVP colloids with acetone three times and then dry the products obtained.
4.
Prepare and sonicate an aqueous solution containing Ru-PVP colloids, ethanol (213 mL), NH
4
OH
(10.9 mL) and de-ionized water (34.4 mL).
5.
Add TEOS (amount depends on Ru/Si atomic ratio) and agitate the resulting solution at room
temperature for 24 h.
6. Collect the Ru@SiO2nanoparticles by washing, centrifuging, and drying.
7. Calcine the sample at 400 ◦C for 4 h.
Nanomaterials 2018,8, 153 12 of 14
8.
Incipient impregnation of 2 g Ru@SiO
2
particles with an aqueous solution containing 0.2 g
NH4ReO4.
9. Calcine the Re-Ru@SiO2sample at 400 ◦C for 4 h.
10. Reduce the particles in a gas of 5% hydrogen in 95% argon at a heating rate of 1
◦
C/min to 200
◦
C,
and maintain at 200 ◦C for 4 h.
3.2. Catalyst Characterization
A Micromeritics surface area analyzer and porosimetry system (model ASAP 2020) was
used to determine catalyst-specific surface area and pore size distribution by nitrogen adsorption.
A field emission scanning electron microscope (JEOL JSM-7000F) and a transmission electron
microscope (JEOL JEM2100F) were used to observe catalyst particle size and morphology. Catalyst
crystalline structure was examined by X-ray diffraction (XRD) crystallography on a Shimadzu
XRD-6000 diffractometer with Cu K
α
radiation. Catalyst reducibility was studied with a
temperature-programmed reduction (TPR) method; details are given elsewhere [18].
3.3. Reaction Studies
Hydrogenolysis of glycerol to diols/alcohols was carried out with a 600 mL stirred reactor made
of stainless steel (supplied by Parr Instruments Co., Moline, IL, USA). In a typical run, 18.5 g (0.2 gmol)
glycerol (Alfa Aesar, Ward Hill, MA, USA), 0.63 g Re-Ru@SiO
2
nanoparticles prepared above, and
27.8 g water (i.e., 40 wt % glycerol aqueous solution) were mixed together and charged into the reactor.
The agitator speed was set at 500 rpm, hydrogen was introduced into the reactor at a desired pressure,
and the reaction mixture was then heated to the desired temperature. At the end of the reaction,
the component compositions were determined with a Shimadzu (Kyoto, Japan) high performance
liquid chromatography (model: LC-10A) equipped with a 250 mm long C-18 column and a UV
detector (wavelength was set at 254 nm). The glycerol conversion was defined as (moles of glycerol
reacted)/(moles of glycerol fed to the reactor)
×
100%, product yield was defined as(moles of product
obtained)/(moles of glycerol fed to the reactor)
×
100%, product selectivity was defined as (moles of
product obtained)/( moles of glycerol reacted ) ×100%.
4. Conclusions
Re-Ru@SiO
2
nanoparticles with five different Ru/Si atomic ratios were prepared by coating
silica on PVP-stabilized nano-ruthenium colloids, followed by incipient impregnating with NH
4
ReO
4
aqueous solution. Mesoporous structure was generated due to the burning out of the PVP molecules.
The increase of Ru/Si atomic ratio resulted in the increase of catalyst activity but the decrease of pore
volume, surface area and particle size. Re addition improved Ru@SiO
2
performance but retarded RuO
2
reduction. The rate of glycerol disappearance was first–order with respect to glycerol concentration,
and the rate constant (with an activation of 107.8 kJ/mol) increased linearly with increasing Ru/Si
atomic ratio, catalyst amount and hydrogen pressure when pressure
≥
800 psig. Reaction rate at 130
◦
C
for a Re-Ru@SiO
2
catalyst (with a Ru/Si atomic ratio of 0.4) was 8.4 times that (at 160
◦
C) reported
previously for a Re-Ru/SiO
2
catalyst prepared with conventional impregnation method, which was
ascribed to the greater Ru content and smaller size of the former. Under isothermal condition, product
selectivity correlated well with glycerol conversion. 98.6% overall liquid-phase product selectivity
(including 52.5% 1,2-PDO selectivity and 3.5% 1,3-PDO selectivity) at 60.2% glycerol conversion was
obtained at 130
◦
C for the Re-Ru@SiO
2
catalyst with a Ru/Si atomic ratio of 0.8. The use of lower
reaction temperature and higher hydrogen pressure improved PDO selectivity.
Acknowledgments:
The authors are grateful to ROC Ministry of Science and Technology for financial support
(MOST 102-2221-E029-021-MY2) and Yuan-Shiuan Lee for XPS measurements.
Author Contributions:
K.-T.L. and R.-H.Y. conceived and designed the experiments; R.-H.Y. performed
experiments; K.-T.L. and R.-H.Y. analyzed the data; K.-T.L. contributed reagents/materials/analysis tools; K.-T.L.
wrote the paper.
Nanomaterials 2018,8, 153 13 of 14
Conflicts of Interest:
The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
References
1.
Fernado, S.; Adhikari, S.; Chandrapal, C.; Murali, N. Biorefineries: Current status, challenges, and future
direction. Energy Fuels 2006,20, 1727–1737. [CrossRef]
2.
Johnson, D.T.; Taconi, K.A. The glycerin glut: Options for the value-added conversion of crude glycerol
resulting from biodiesel production. Environ. Prog. 2007,26, 338–348. [CrossRef]
3.
Pagliiaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Pina, C.D. From glycerol to value-added products.
Angew. Chem. Int. Ed. 2007,46, 4434–4440. [CrossRef] [PubMed]
4.
Zhou, C.H.; Beltramini, J.N.; Fan, Y.X.; Lu, G.Q. Chemoselective catalytic conversion of glycerol as a
biorenewable source to valuable commodity chemicals. Chem. Soc. Rev.
2008
,37, 527–549. [CrossRef]
[PubMed]
5.
Vasiliadou, E.S.; Heracleous, E.; Vasalos, I.A.; Lemonidou, A.A. Ru-based catalysts for glycerol
hydrogenolysis—Effect of support and metal precursor. Appl. Catal. B Environ. 2009,92, 90–99. [CrossRef]
6.
Schlaf, M. Selective deoxygenation of sugar polyols to
α
,
ω
-diols and other oxygen content reduced
materials—A new challenge to homogeneous ionic hydrogenation and hydrogenolysis catalysis. Dalton
Trans. 2006,39, 4645–4653. [CrossRef] [PubMed]
7. Sulivan, C.J. Ullmanns Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 2005.
8.
Miyazawa, T.; Kusunoki, Y.; Kunimori, K.; Tomishige, K. Glycerol conversion in the aqueous solution under
hydrogen over Ru/C + an ion exchange resinand its reaction mechanism. J. Catal.
2006
,240, 213–221.
[CrossRef]
9.
Miyazawa, T.; Koso, S.; Kunimori, K.; Tomishige, K. Development of a Ru/C catalyst for glycerol
hydrogenolysis in combination with an ion-exchange resin. Appl. Catal. A Gen.
2007
,318, 244–251.
[CrossRef]
10.
Jin, S.; Xiao, Z.; Li, C.; Williams, C.T.; Liang, C. Hydrogenolysis of glycerol over HY zeolite supported Ru
catalysts. J. Energy Chem. 2014,23, 185–192. [CrossRef]
11.
Vanama, P.K.; Kumar, A.; Ginjupalli, S.R.; Komandur, V.R.C. Vapor-phase hydrogenolysis of glycerol over
nanostructured Ru/MCM-41 catalysts. Catal. Today 2015,250, 226–238. [CrossRef]
12.
Gallegos-Suarez, E.; Guerrero-Ruiz, A.; Rodriguez-Ramos, I.; Arcoya, A. Comparative study of the
hydrogenolysis of glycerol over Ru-based catalysts supported on activated carbon, graphite, carbon
nanotubes and KL-zeolite. Chem. Eng. J. 2015,262, 326–333. [CrossRef]
13.
Salazzr, J.B.; Falcone, D.D.; Pham, H.N.; Datye, A.K.; Passos, F.B.; Davis, R.J. Selective production of
1,2-propanediol by hydrogenolysis of glycerol over bimetalic Ru-Cu nanoparticles supported on TiO
2
.Appl.
Catal. A Gen. 2014,482, 137–144. [CrossRef]
14.
Ma, L.; He, D.; Li, Z. Promoting effect of rhenium on catalytic performance of Ru catalysts in hydrogenolysis
of glycerol to propanediol. Catal. Commun. 2008,9, 2489–2495. [CrossRef]
15.
Ma, L.; He, D. Influence of catalyst pretreatment on catalytic properties and performances of Ru-Re/SiO
2
in
glycerl hydrogenolysis to propanediol. Catal. Today 2010,149, 148–156. [CrossRef]
16.
Tamura, M.; Amada, Y.; Liu, S.; Yuan, Z.; Nakagawa, Y.; Tomishige, K. Promoting effect of Ru on Ir-ReO
x
/SiO
2
catalyst in hydrogenolysis of glycerol. J. Mol. Catal. A Chem. 2014,388–389, 177–187. [CrossRef]
17.
Li, K.T.; Hsu, M.H.; Wang, I. Palladium core-porous silica shell-nanoparticles for catalyzing the
hydrogenation of 4-carboxybenzaldehyde. Catal. Commun. 2008,9, 2257–2260. [CrossRef]
18.
Li, K.T.; Wang, C.H.; Wang, H.C. Hydrogenolysis of glycerol to 1,2-propane diol on copper core–porous
silica shell nanoparticles. J. Taiwan Inst. Chem. Eng. 2015,52, 79–84. [CrossRef]
19.
Xu, Y.; Ma, J.; Xu, Y.; Xu, L.; Li, H. Palladium nanoparticles encapsulated in porous silica shells: An efficient
and highly stable catalyst for CO oxidation. RSC Adv. 2013,3, 851–858. [CrossRef]
20.
Bonet, F.; Delmas, V.; Grugeon, S.; Herrara Urbina, R.; Silvert, P.Y.; Tekaia-Elhsissen, K. Synthesis of
monodisperse Au, Pt, Ru and Ir nanoparticles in ethylene glycerol. Nanostruct. Mater.
1999
,11, 1277–1284.
[CrossRef]
Nanomaterials 2018,8, 153 14 of 14
21.
Bachiller-Baeza, B.; Guerero-Ruiz, A.; Rodriguez-Ramos, I. Ruhtenium-supported catalysts for the
selectoselective hydrogenation of paracetamol to 4-trans-acetamidocyclohexanol: Effect of support, metal
precursor, and solvent. J. Catal. 2005,229, 439–445. [CrossRef]
22.
Shozi, M.L.; Dasireddy, V.D.B.C.; Singh, S.; Mohlala, P.; Morgan, D.J.; Iqbal, S.; Friedrich, H.B. An
investigation of Cu-Re-ZnO catalysts for the hydrogenolysis of glycerol under continuous flow conditions.
Sustain. Energy Fuels 2017,1, 1437–1445. [CrossRef]
23.
Koopman, P.G.J.; Kieboom, A.P.G.; van Bekkum, H. Activation of ruthenium on silica hydrogenation catalysts.
React. Kinet. Catal. Lett. 1978,8, 389–393. [CrossRef]
24. Satterfield, C.N. Heterogeneous Catalysis in Industrial Practice; McGraw-Hill: New York, NY, USA, 1991.
25.
Reinikainen, M.; Niemela, M.K.; Kakuta, N.; Suhonen, S. Characterisation and activity evaluation of silica
supported cobalt and ruthenium catalysts. Appl. Catal. A Gen. 1998,174, 61–75. [CrossRef]
26.
Chia, M.; O’Neill, B.J.; Dietrich, P.J.; Riberio, F.H.; Mille, J.T.; Dumesic, J.A. Bimetallic RhRe/C catalysts for
the production of biomass-derived chemicals. J. Catal. 2013,308, 226–236.
27.
Chia, M.; Pagan-Torres, Y.J.; Hibbits, D.; Tan, Q.; Pham, H.N.; Datye, A.K.; Neurock, M.; Davies, R.J.;
Dumesic, J.A. Selective hydrogenolysis of polyols and cyclic ethers over bifunctional surface sites on
rhodium-rhenium catalyst. J. Am. Chem. Soc. 2011,133, 12675–12689. [CrossRef] [PubMed]
28. Amada, Y.; Koso, S.; Nakagawa, Y.; Tomishige, K. Hydrogenolysis of 1,2-propanediol for the production of
biopropanols from glycerol. ChemSusChem 2010,3, 728–736. [CrossRef] [PubMed]
29.
Mauriello, F.; Vinci, A.; Espro, C.; Gumina, B.; Musolino, M.G.; Pietropaolo, R. Hydrogenolysis vs. aqueous
phase reforming (APR) of glycerol promoted by a heterogeneous Pd/Fe catalyst. Catal. Sci. Technol.
2015
,5,
4466–4473.
30.
Peng, B.; Zhao, C.; Mejía-Centeno, I.; Fuentes, G.A.; Jentys, A.; Lercher, J.A. Comparison of kinetics and
reaction pathways for hydrodeoxygenation of C
3
alcohols on Pt/Al
2
O
3
.Catal. Today
2012
,183, 3–9.
[CrossRef]
31.
Wawrzetz, A.; Peng, B.; Hrabar, A.; Jentys, A.; Lemonidou, A.A.; Lercher, J.A. Towards understanding the
bifunctional hydrodeoxygenation and aqueous phase reforming of glycerol. J. Catal.
2010
,269, 411–420.
[CrossRef]
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