Selective hydrogenolysis of glycerol to 1,3-propanediol over egg-shell type Ir-ReOx catalysts

Article abstract of DOI:10.1039/c5ra24808f

The selective liquid-phase hydrogenolysis of glycerol to 1,3-propanediol over egg-shell catalysts in which the loadings of Ir and Re were both 2 wt% is reported. The shell thickness could be tuned by impregnating a hydrophobic silanized support with an aqueous solution of precursors containing various concentrations of ethanol and using entrapped air to prevent the impregnation solution from entering into the support pellets. The conversion of glycerol for the target reaction over egg-shell catalysts was higher than that over a uniform catalyst and was accompanied by a high selectivity for 1,3-propanediol over 1,2-propanediol. Reacting over the catalyst for which the impregnation solution ethanol concentration was 20 vol% resulted in the highest glycerol conversion (60.9%) and the highest yield of secondary hydroxyl group removal (30.2%). The properties of the egg-shell and uniform catalysts were characterized by XRD, TEM, XPS, H₂-TPR, H₂-pulse chemisorption and NH₃-TPD. An appropriate diffusion distance of the reactants for egg-shell catalysts might result in better catalytic performance because of the high viscosity of the glycerol aqueous solution.

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Selectively hydrogenolysis of glycerol to 1,3-propanediol over egg-
shell type Ir-ReO catalysts
x
Received 00th January 20xx,
Accepted 00th January 20xx
a, c
a
a
a,c
a,c
a,b
Wenting Luo , Yuan Lyu,* Leifeng Gong, Hong Du, Tao, Wang, Yunjie Ding*
DOI: 10.1039/x0xx00000x
The selective liquid-phase hydrogenolysis of glycerol to 1,3-propanediol is reported over egg-shell catalysts in which the
loadings of Ir and Re were both 2 wt.%. The shell thickness can be tuned by impregnating a hydrophobic silanized support
with an aqueous solution of precursors containing various concentrations of ethanol, using entrapped air to prevent the
impregnation solution from entering into the support pellets. The conversion of glycerol for the target reaction over egg-
shell catalysts was higher than that over a uniform catalyst, accompanied by a high selectivity for 1,3-propanediol over 1,2-
propanediol. Reacting over the catalyst for which the ethanol concentration of the impregnation solution was 20 vol.%
resulted in the best glycerol conversion (60.9%) and the highest yield of removal secondary hydroxyl group (30.2%). The
www.rsc.org/
properties of the egg-shell and uniform catalysts were characterized by XRD, TEM, XPS, H
and NH -TPD. An appropriate diffusion distance of reactants for egg-shell catalysts might result in better catalytic
performance because of the high viscosity of the glycerol aqueous solution.
2 2
-TPR, H -pulse chemisorption
3
activity in glycerol hydrogenolysis. Tomishige et al. observed high
selectivity for 1,3-propanediol on Ir-ReO /SiO with an Ir loading of
Introduction
x
2
4
wt.% when reacted with the addition of a solid acid co-catalyst in a
As a sustainable biomass-derived product, glycerol has few
environmental impacts compared with the energy- and emissions-
1
4, 15
batch reactor.
They proposed the following mechanism for Ir-
1
-3
x 2
ReO /SiO -catalyzed glycerol hydrogenolysis: Re-OH interacts with
intensive synthesis of fossil fuel-derived chemicals. Glycerol is a
byproduct of biodiesel produced from vegetable oils and animal
the OH groups of glycerol to form 2,3-dihydroxypropaneoxide;
then, hydrogen species activate the secondary carbon of 2,3-
dihydroxypropaneoxide to cleave the C-O bond; and hydrolysis
4
fat, and a large surplus glycerol has resulted from the rapid
5
, 6
expansion of the global biodiesel production capacity. From an
economic standpoint, the price decline of crude glycerol due to
increasing biodiesel production makes glycerol an attractive
11, 14, 16, 17
occurs to release the product.
Egg-shell catalysts are pelletized catalysts in which the active phase
is located on the outer surface. Egg-shell catalysts are broadly
applied in many reactions, such as selective hydrogenation
7
material for the synthesis of fine chemicals. Many processes have
been developed to transform glycerol into different chemicals, such
as oxidation, reforming, etherification, dehydration and
1
8-21
22, 23
reactions
,
Fischer-Tropsch synthesis
, methane partial
2
4
25
8
oxidation and hydrodesulfurization . To maintain an acceptable
pressure gradient, pelletized catalysts of a certain size are usually
required for reactions in industrial fixed-bed reactors, but uniform
catalysts will sometimes limit the mass and/or heat transfer, which
negatively affect catalytic performance. Egg-shell catalysts solve
problems of gum formation, the generation of overheating spots in
the reactor bed and the undesired complete reaction of
hydrogenolysis. Among these processing approaches, glycerol
hydrogenolysis is one of the promising. The main products of
glycerol hydrogenolysis are 1,3-propanediol, 1,2-propanediol, 1-
propanol, and 2-propanol, and 1,3-propanediol has great economic
value as the monomer of polytrimethylene terephthalate (PTT).
Therefore, hydrogenolysis of glycerol to 1,3-propanediol has
8
attracted more attention in recent years.
1
8-26
reactants.
Recently, Re has been added to different noble metal catalysts to
improve their catalytic activity. Pt-Re, Rh-Re, Pd-Re, Ru-Re, and Ir-
To the best of our knowledge, few studies have investigated Ir egg-
shell catalysts for glycerol hydrogenolysis. Glycerol is strongly
adhesive, and internal diffusion probably affects the catalytic
performance in glycerol hydrogenolysis. In this study, egg-shell
catalysts were prepared by impregnating a hydrophobic silanized
support with an aqueous solution of precursors containing various
concentrations of ethanol, using entrapped air to prevent the
impregnation solution from entering the support pellets. The
properties of these egg-shell catalysts and uniform catalyst were
investigated by several characterization techniques, including
9
-13
Re have all been investigated.
Among these catalyst systems,
iridium modified with rhenium has demonstrated the best catalytic
a.
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning, China. E-
mail: dyj@dicp.ac.cn. Tel: +86 411 84379143; Fax: +86 411 84379143.
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
b.
Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning, China.
University of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing 100049,
China.
c.
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optical microscopy, X-ray powder diffraction (XRD), transmission Conversion of glycerol (%) =
electron microscopy (TEM), X-ray photoelectron spectroscopy
(
XPS),
chemisorption and temperature-programmed desorption of
ammonia (NH -TPD), and the catalytic performance of the egg-shell
2 2
H -TPR (temperature-programmed reduction), H -pulse
3
Selectivity (%) =
catalysts and uniform catalyst were all evaluated in glycerol
hydrogenolysis.
Experimental
The removal selectivity of primary (or secondary) hydroxyl group
(%) (S1-OH or S2-OH) =
Catalyst preparation
The egg-shell catalysts were prepared by the method devised by
sumꢀofꢀremovalꢀprimaryꢀorꢀsecondaryꢀhydroxylꢀgroup
2
2
ꢁ
100
2 2
Ding . TMCS-SiO was prepared by silanizing a SiO support (8-10
sumꢀofꢀremovalꢀhydroxylꢀgroup
mesh) with trimethylchlorosilane (TMCS). First, we mixed 100 ml of
toluene and 2.5 g of TMCS and stirred them uniformly. Then, 5 g of
The yield of removal secondary hydroxyl group (%) (Y2-OH) =
2
SiO was adding to this mixture, which was refluxed for 16 h. We
Conversion of glycerol ꢁ removal selectivity of secondary hydroxyl
group
filtered the above material and washed it with ethanol and
cyclohexane. Finally, the material was dried at 393 K for 8 h in air to
obtain TMCS-SiO
TMCS-SiO was impregnated in a mixture of an aqueous solution of
IrCl and ethanol, in which the ethanol concentration was 20, 25
2
.
Characterizations
2
XRD data were collected on an X’pert PRO/PANalytical
H
2
6
diffractometer with CuK
α
radiation. The X-ray patterns were
or 30 vol.%. These treated samples were heated in a rotary
evaporator at 313 K for 2 h, followed by drying at 393 K for 8 h in an
oven and calcination at 773 K for 3 h. Then, Re was deposited on
o
o
measured over a 2θ range of 10 -80 at 40 kV and 40 mA.
The particle size of the samples was observed by TEM using a JEM-
2
100 microscope operated at an accelerating voltage of 200 kV. The
4 4
these materials with an aqueous solution of NH ReO . The above
catalysts were ground and placed on a copper grid after being
briefly dispersed in ethanol with ultrasonication.
material was dried at 393 K for 12 h and calcinated at 773 K for 3 h,
and the egg-shell catalysts were obtained using the impregnation
solution with different concentrations of ethanol, which were
denoted as RI20, RI25 and RI30, respectively. For comparison, a
uniform catalyst was prepared by impregnating an aqueous solution
XPS data were collected on a Thermo ESCALAB 250Xi equipped with
α
a monochromatized AlK radiation (1486.6 eV, 15 kV, 10.8 mA). The
-5
pressure in the analysis chamber was approximately 7.1×10 Pa.
TPR analyses were performed on an AMI-300 catalyst
characterization system. Approximately 100 mg of each sample was
2 6 4 4
of H IrCl and NH ReO in sequence with silica gel without any
treatments. This uniform catalyst was denoted RI. RI was broken up
and put through a series of graded sieves, and the resulting
fractions were denoted as RI-sm-n (m and n correspond to the
lower and upper mesh numbers of pellet size, respectively). RI20-
s60-80 was prepared by the same method as RI20 with silica
supports of 60-80 mesh. The contents of iridium and rhenium in all
catalysts were 2 wt.%.
2
placed into quartz reactors and reduced under a stream of H at 30
–1
ml·min (10 vol.% in Ar) from 323 to 773 K at a heating rate of 10
–1
K·min .
Optical micrographs were observed by optical microscopy, and the
images were captured by a camera. The sample was the cross-
sections of catalyst pellets prepared by cutting. The magnification
of the objective and eyepiece were both 4. The image ruler was
determined using the diameter of the catalyst pellets as a reference.
The diameters of the catalyst pellets were measured by a Vernier
calliper.
Catalytic performance test
Glycerol hydrogenolysis reactions were performed in a trickle bed
reactor (9 mm i.d.) using 1.5 g of catalyst. The hydrogen flow was 40
H
2
-chemisorption was performed on an AMI-300 catalyst
characterization system. Approximately 100 mg of the catalyst
samples were reduced under a flow of 10% H /Ar at 463 K for 2 h,
-
1
-1
ml·min , which corresponded to a space velocity of 1600 ml·(h·g) ,
-1
and the glycerol flow was 0.65 ml·h , which corresponded to a
2
-1
space velocity of 0.44 ml·(h·g) . Before reaction, all catalysts were
reduced at 463 K. The reactions were conducted at 403 K and 8
MPa. The gaseous products from the effluent were analyzed on-line
using an Agilent GC-7890. The liquid-phase products were
quantified by an internal standard method, and the internal
standard substance was n-butyl alcohol. Then, the liquid-phase
products were esterified by acetic anhydride catalyzed by pyridine
and heated to 328 K for 1.5 h. The samples were quantified on a gas
chromatograph (Agilent 7890) fitted with a flame ionization
detector and a HP-5 capillary column.
followed by purging with argon at 463 K for 0.5 h. Then, the
samples were cooled to 313 K to be used for pulse adsorption. The
adsorbed gas was 10% H
2
/Ar, and the carrier gas was argon.
NH -TPD was carried out on an AMI-300 catalyst characterization
3
system equipped with a thermal conductivity detector (TCD). A 100
mg sample was pre-treated in an Ar flow at 873 K for 0.5 h. Then,
after cooling to 373 K under a continuous flow of argon, ammonia
3
adsorption was performed by introducing a flow of 5% NH /Ar for 2
h using the static adsorption method. To remove all the physically
adsorbed ammonia, the sample was purged with a helium flow of
The conversion of glycerol and the selectivity of the products were
calculated by the following equations:
2
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[
a]
Table 1 Catalytic performance of the glycerol hydrogenolysis over the RI, RI20, RI25 and RI30 catalysts
selectivity/%
1,3-
Carbon
[h]
[c]
catalyst
Conv. /%
PD/1,2-
Y
2-OH/%
[b]
1,2-PD
[d]
[e]
[f]
1
,3-PD
1-PO
2-PO
C
3
H
8
Others
balance/%
[g]
PD
RI
48.9
60.9
56.6
53.7
37.0
31.1
31.9
32.1
12.9
7.5
35.6
45.4
42.9
33.5
9.0
9.1
9.3
8.1
4.9
6.6
0.6
0.3
0.5
0.3
100.9
94.7
2.9
4.1
3.7
3.5
24.6
30.2
27.9
25.3
RI20
RI25
RI30
8.6
6.8
101.4
95.0
9.3
16.7
-1
[
a] Reaction condition: P=8.0 MPa, T= 403 K, WHSV= 0.5 h , 1.5 g catalyst, 80 wt% glycerol aqueous solution,VH2 = 40 ml/min, TOS= 36
h. [b] 1,3-PD = 1,3-propandiol, [c] 1,2-PD = 1,2-propanediol, [d] 1-PO = 1-propanol, [e] 2-PO = 2-propanol. [f] Others include ethanol,
ethane and methane. [g] The ratio of selectivity of 1,3-propanediol to 1,2-propanediol, [h] The yield of removal secondary hydroxyl
group. Reduction condition: P=1 atm, Vꢂꢃ= 40ml/min, T=463 K, TOS=2h.
-
1
3
0 ml·min at 373 K for 0.5 h. Then, the NH -TPD pattern was propanediol) would also be restricted. This is consistent with the
3
recorded by heating from 373 to 873 K with a ramp of 10 K/min in reaction results. The 1,3-propanediol/1,2-propanediol ratios of RI20
an Ar flow, and the desorbed NH was simultaneously monitored by and RI were 4.1 and 2.9, respectively, and the other egg-shell
3
online TCD analysis.
catalysts also produced a larger 1,3-propanediol/1,2-propanediol
ratio than that of the uniform catalyst RI. Table 2 shows the results
of the hydrogenolysis of 1,3-propanediol over the RI and RI20
catalysts. The conversion of 1,3-propanediol over the RI20 catalyst
was higher than over the RI catalyst primarily because this reaction
is consecutive and the active sites of glycerol hydrogenolysis and
Results and discussion
Catalytic activity measurements
The activity of glycerol hydrogenolysis over the RI, RI20, RI25 and
RI30 catalysts is summarized in Table 1. Approximately 60.9%
glycerol conversion with 31.1% selectivity for 1,3-propanediol was
obtained over the RI20 catalyst, which was impregnated in an
1
,3-propanediol hydrogenolysis both lie at the interface of the
14, 28, 29
iridium and rhenium species.
Therefore, the yields of removal
secondary hydroxyl groups (Y2-OH) given in Table 1 can express the
ability to remove secondary hydroxyl groups. This removal yield for
the RI20 catalyst was 30.2%, much larger than that of the RI catalyst
aqueous solution of H
followed by an aqueous solution of NH
of H pressure in an aqueous solution of 80% glycerol. Compared
2
IrCl
6
with a 20 vol.% ethanol concentration,
4
ReO at 403 K with 8.0 MPa
4
(
24.6%). Therefore, the removal ability of 2-OH for the RI20 sample
2
was better than the RI catalyst, which was mainly due to the high
conversion of the RI20 catalyst. To determine how egg-shell
catalysts produce high glycerol conversion, we conducted the
following catalyst characterizations and experiments.
with the uniform catalyst (RI), the glycerol conversion of the RI20
catalyst increased from 48.9 to 60.9%, and the selectivity of 1,3-
propanediol decreased from 37.0 to 31.1%. All the egg-shell
catalysts (RI20, RI25 and RI30) showed better glycerol conversion
than RI, accompanied with slightly lower selectivity for 1,3-
propanediol. For the three egg-shell catalysts, the glycerol
conversion increased from 53.7 to 60.9% with the decrease in the
ethanol concentration from 30 to 20 vol.%.
Catalysts characterizations
Table 2 Catalytic performance of the hydrogenolysis of 1,3-
[a]
propanediol over the RI and RI20 catalysts
selectivity/%
catalyst
Conv. /%
1
-PO
C3H8
8.5
Others
0.5
RI
40.6
43.0
91.0
95.8
RI20
3.7
0.5
-
1
[
a] Reaction condition: P=8.0 MPa, T= 403 K, WHSV= 0.533 h , 1.5 g
catalyst, 80 wt% 1,3-propanediol aqueous solution, VH2 = 40
ml/min, t= 36 h. Reduction condition: P=1 atm, Vꢂꢃ= 40 ml/min,
T=463 K, t=2 h.
In addition, 1,3-propanediol and 1,2-propanediol are the products
of the parallel reaction of the first step of glycerol hydrogenolysis.
Lower temperature would thermodynamically favour the selectivity
2
7
for 1,3-propanediol over 1,2-propaendiol. The short diffusion
distance would also lead to better heat diffusion from the catalyst
surface. Therefore, overheating on the catalytic active sites would
be prevented, and the generation of undesired by-products (1,2-
Figure 1 Optical micrographs of cross sections of catalysts. (a) RI20,
(
b) RI25, (c) RI30 and (d) RI.
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Optical micrographs of catalyst cross-sections are shown in Figure Although Ir was mainly distributed in the shell layer of the egg-shell
. Black part of the circle section was where Ir species loaded on, catalysts, the silyl groups on the supports were wiped out after
and the white center was the part without supported Ir species. The calcination at 773 K. Therefore, ReO was impregnated in the
active component of the uniform catalyst spread to every part of catalyst pellets using a common impregnation method without the
the pellet. The RI20 catalyst had the thinnest active component existence of silyl groups. In other words, ReO should be distributed
1
x
x
uniformly. Therefore, the mass ratio of Re and Ir (Re/Ir) on the
outer surface of the catalysts was different. Thin shells would be
accompanied by a low value of Re/Ir, and the Re/Ir ratio of the RI20
catalyst with the thinnest shell was 0.75. The catalytic activities of
catalysts with a Re/Ir ratio of 0.75, which were prepared by the
same method as the RI sample, are listed in Table 3. Reducing the
value of Re/Ir from 1 to 0.75 will decrease the conversion of
layer of approximately 0.4 mm. The thickness of shell for the RI25
and RI30 catalysts were approximately 0.7 and 1.0 mm. Combined
with the results in Table 1, when a higher concentration of ethanol
was used to prepare the catalysts, a thicker layer resulted, and the
catalyst conversion was lower. The mass fraction of glycerol in the
aqueous solution used in the reaction was 80%, which was highly
viscous (85.00 Pa·s, 300K). Therefore, this hydrogenolysis reaction
might be influenced by internal diffusion. To verify the influence of
internal diffusion, the catalytic activities of the RI catalysts of
different sizes were tested, and the results are shown in Table 3.
The conversion of glycerol increased as the mesh of the catalysts
decreased, and the conversion remained almost unchanged when
the mesh of the catalysts decreased below 60. Egg-shell catalysts
concentrate the active Ir component on their outer surface, which
weakens the effect of internal diffusion. The conversion was higher
when the shell was thinner, which might be due to difficult diffusion
into the centre of the catalyst pellets. Glycerol may also react with
hydrogen mostly on the outer surface of the catalyst pellets.
16
glycerol. These results are in agreement with Tomishige’s work .
Therefore, the high activity of the RI20 catalyst was not due to the
decrease in Re/Ir.
Table 4 The average particle size of Ir species calculated from XRD
results
-6
The average particle size of Ir species (×10
mm)
Dispersion
of
After
After
catalyst
Before
After
reaction reaction
reduced
[a]
reaction reduction
for
for
500h
3.25
3.20
-
catalysts
150h
3.12
3.06
RI
6.18
4.53
5.34
5.84
2.24
2.11
2.13
2.19
49.2
52.2
51.9
50.3
Table 3 Catalytic performance of the glycerol hydrogenolysis over
RI20
RI25
RI30
[
a]
the RI and RI20 catalysts in smaller mesh
-
Conv.
selectivity/%
[a] calculated with the Ir particles obtained from XRD
catalyst
/%
1,3-PD
34.2
33.5
31.5
32.9
32
1,2-PD
9.8
1-PO
2-PO
10.9
11.0
9.1
The XRD analysis was also performed on fresh catalysts (Figure 2a).
The diffraction peak at 2θ=21.4° was attributed to amorphous silica
gel. Several peaks were observed in the patterns of the fresh
catalysts at approximately 26.7°, 34.2° and 53.1°, indexed to
RI-s10-20
RI-s20-40
RI-s40-60
RI-s60-80
RI20-s60-80
RI0.75-s20-
52.7
54.5
57.4
56.4
58.3
41.9
42.2
41.8
37.6
43.6
9.5
8.7
IrO
of the samples reduced at 463 K contain new reflections at
θ=40.5° that are characteristic of metallic iridium (PDF 01-087-
2 2 2
(110), IrO (101) and IrO (211) (PDF 01-088-0288). XRD patterns
9.7
10.7
11.2
9.8
2
[b]
0
51.4
35.5
16.4
34.2
11.3
0715) (Figure 2b). For the catalysts reacted for 150 h or 500 h, a
diffusive diffraction peak of metallic iridium was also observed
4
-1
[
a] Reaction condition: P=8.0 MPa, T= 403 K, WHSV= 0.5 h , 1.5 g
(
Figure 2c). Whether the catalysts were used/not used or
catalyst, 80 wt% glycerol aqueous solution,VH2 = 40 ml/min, t= 36 h.
Reduction condition: P=1 atm, Vꢂꢃ= 40ml/min, T=463 K, t=2h. [b]
Ir%=2wt%, Re%=1.5wt%, 20-40 mesh.
reduced/not reduced, all showed no evidence of Re species due to
its small particle size. Table 4 shows the average particle size of
Figure 2 XRD patterns of the RI, RI20, RI25 and RI30 catalysts. (a) before reaction, (b) after reduction at 463K, (c) after reaction.
4
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Figure 3 TEM images of catalysts. (A) before reduction, (B) after reduction. (a) RI, (b) RI20, (c) RI25 and (d) RI30..
iridium species of the catalysts that were fresh, reduced and K. The signal, which belonged to the reduction of the iridium
component, was observed at 493 K in the TPR profile of Ir/SiO . The
reacted for 150 or 500 h. The average IrO
RI, RI20, RI25 and RI30 catalysts were 6.184×10 , 4.534×10 ,
2
particle sizes of the fresh
2
-6
-6
reduction peaks of all Ir catalysts modified by Re have obvious shifts
in their reduction temperatures towards lower values, implying an
interaction between the Ir and Re species. Contrary to the uniform
catalyst, the iridium oxide species were reduced at a much lower
temperature for all egg-shell catalysts. Only one peak was observed
on the RI and RI30 catalysts at 472 K and 467 K, respectively. This
peak might be assigned to the co-reduction of Ir and Re species.
However, the TPR profile of the RI20 and RI25 samples contained
-6
-6
5
.344×10 and 5.84×10 mm, respectively. After reduction, the
-6
average Ir particle size of the RI catalyst was 2.24×10 mm, which
was also larger than the particle size of other samples. As the
reaction proceeded, the average particle size of Ir increased. The
-
6
average Ir particle size of the RI catalyst increased from 2.24×10
-6
-6
mm to 3.12×10 mm after reaction for 150 h and to 3.25×10 mm
-6
after 500 h, and that of the RI20 catalyst grew from 2.11×10 mm two reduction peaks. The first peak at 426 K likely resulted from the
to 3.06×10 mm and then to 3.20×10 mm, respectively. Therefore, reduction of small Ir oxide species deposited on the support
the Ir species were the same, and the average particle size of Ir was surface, whereas the second peak at 459 K may have been due to
-6
-6
3
4-36
the co-reduction of Ir and Re oxide species.
The existence of
similar for all of these catalysts.
smaller IrO , which was revealed by H -TPR, is coincident with the
results of XRD and TEM.
2
2
Figures 3A and 3B show the TEM images of the fresh and reduced
catalysts, and the corresponding average Ir particle size statistically
2
calculated from TEM are shown in Table 5. The average IrO particle
size on the fresh egg-shell catalysts and the average particle size of
Ir species on reduced egg-shell catalysts exhibited smaller mean
particle diameters than those of the uniform catalyst, which is
consistent with the XRD results. According to the results reported in
30-33
the literature
, superficial silyl or alkoxyl groups hinder the
aggregation of the active metal precursors during drying and
reducing processes. The egg-shell catalyst supports were treated in
trimethylchlorosilane, causing the trimethylchlorosilane to
chemically bond to the surface of the support. The existence of
these groups might be the reason that particle sizes on the egg-shell
catalysts were smaller than on the uniform catalyst. However,
similar dimensions were observed for the reduced catalysts due to
the re-dispersion of active components during reduction.
Table 5 The average particle size of Ir species calculated from TEM
-6
The average particle size of Ir species (×10 mm)
Figure 4 Temperature programmed reduction (TPR) profiles of
catalysts.
catalyst
Fresh catalysts
6.08
Reduced catalysts
2.14
RI
Information on the metal state was obtained from XPS analysis.
Figures S1a and S1b show the Ir 4f XPS spectra of the catalysts
before and after reaction for 150 h. For all these fresh catalysts, the
RI20
RI25
RI30
4.65
5.01
5.08
1.85
1.88
1.89
4
+ 37
binding energy of Ir 4f7/2 at 61.3-61.7 eV was assigned to Ir . After
The TPR profiles of the RI, RI20, RI25, RI30 and mono-component reaction, the XPS spectra showed a peak at approximately 60.7 eV,
0
38
catalysts are shown in Figure 4. The TPR profile of the Re/SiO
catalyst contained a single reduction peak with a maximum at 583 species is consistent with the results of XRD. Figures S1c and S1d
2
corresponding to the Ir 4f7/2 level of Ir species. The valence of Ir
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present the Re 4f XPS spectra for the fresh catalysts and spent types of acidity on these catalysts. The acidity might be ascribed to
4
2
catalysts. The XPS spectra for these catalysts contained a peak at
Re-OH, and the weak acidity might be assigned to Re-O-Re . The
7
+
39, 40
4
4.9-45.7 eV. The binding energy of Re 4f7/2 of Re is 46.9 eV.
However, the binding energy of Re 4f7/2 shown in Figure 5 is below
6.9 eV. According to the literature, NH ReO is pyrolyzed wholly
into Re in air; therefore, the Re species here should be +7. The
lower binding energies observed for Re 4f7/2 compared with that of
desorption amounts of NH
reacted from 150h to 500h.
3
was reduced by about 4.5% when
4
4
4
4
1
2
O
7
Table 7 Uptake of NH3 of catalysts
-1
7
+
Catalyst
RI
Acidity ( µmol·g )
312.1
Re species may have been caused by Re species interacting with
3
9
the support or Ir species.
binding energies of Re 4f7/2 were similar to those for the fresh
catalysts, which might have been caused by the oxidation of ReO
when the spent catalysts were exposed to air.
chemisorption was carried out to determine the number of
For the catalysts after reaction, the
RI20
368.9
x
4
1
RI25
363.3
RI30
373.2
H
2
iridium active sites, and the results are shown in Table 6. The
uptake of hydrogen on all the egg-shell catalysts was greater than
Re/SiO2
239.6
2
that of the uniform catalyst. The H uptake of the RI20 sample
-1
which was 1.303 µmol·g was the highest among these catalysts.
The dispersion of Ir shown in Tables 6 and 4 was calculated from
2
the H -chemisorption and XRD results, respectively. The dispersion
calculated by the former analysis was much lower than by the latter,
implying that only approximately 4% of Ir atoms were exposed on
the surface and the other Ir atoms on the surface of the Ir particles
might have been covered with Re species. The results obtained
from H -chemisorption demonstrate that more superficial Ir atoms
2
were present on the egg-shell catalysts than on the uniform
catalyst. The uptake of the RI20 catalyst was almost 7 times greater
than that of the RI sample. The uptake of the RI25 and RI30 samples
were also lager than that of the RI sample. This suggests that the Ir
particle size might be smaller on the egg-shell catalysts than on the
uniform catalyst, and this result is consistent with the TEM and XRD
results.
Table 6 The H uptake and dispersion
2
Dispersion (%)
Figure 5 NH -TPD profiles of the RI, RI20, RI25, RI30, TMCS-SiO ,
3
2
Catalyst
H
2
uptake (μmol/g)
Ir/SiO
2
2
and Re/SiO catalysts.
H
2
chemisorption
RI
RI20
0.19
1.303
0.884
0.774
0.499
0.4
2.5
1.7
1.5
1.0
RI25
RI30
RI20-s60-80
Figure 5 shows the NH
SiO , no NH desorption could be detected in the NH
experiment. This indicates that TMCS-SiO had no acidity. A very
weak NH desorption peak at a wide temperature range near 573 K
was detected over the Ir/SiO catalyst. However, the Re/SiO
sample showed one desorption peak at 490 K, and all the catalysts
containing Re showed a NH desorption peak at nearly 490 K,
3
-TPD profiles of fresh catalysts. For TMCS-
2
3
3
-TPD
2
3
2
2
3
suggesting that the Re species constituted the main source of
acidity. The supply of acidic sites by Re species was also investigated
1
3
in the work of He et al. Table 7 shows the acidity of all the
samples calculated by integrating the NH desorption curves. All the
3
Figure 6 NH
3
-TPD profiles of the RI, RI20 catalysts reacting after
egg-shell catalysts had more acidity than the uniform catalyst, and
their acidity might be proportional to the amount of superficial Re
species. Therefore, Re species might be better dispersed on the
1
50h and 500h.
The results of H
showed that the Ir sites in RI20 catalyst were 7-times higher than in
2 3
-chemisorption and NH -TPD characterizations
3
egg-shell catalysts than on the RI sample. Figure 6 shows the NH -
TPD profiles of RI and RI20 after reacting for 150 h and 500 h. The
existence of two desorption peaks suggests that there are two
x
RI catalyst, and ReO sites in RI20 catalyst were 18% higher than
6
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and ReO
Therefore, we think that the activity of the catalyst supplying more
Ir and ReO sites might be higher than others, and it was
x
, which might result in more active sites formation.
x
corresponded to this that the glycerol conversion over RI20-s60-80
catalysts was a little higher than the RI-s60-80 catalyst. However,
only a little difference existed between the glycerol conversion over
RI20-s60-80 catalyst and over RI-s60-80 catalyst. Therefore, the
main reason for higher conversion over RI20 was shorter diffusion
distance which could reduce the effect of internal transfer
limitation.
5
00h catalytic performances measurements of catalysts
Figure 7 shows the plot of catalytic performances of RI and RI20
catalysts vs. time on-stream. The conversion of the RI and RI20
catalysts increased to a maximum after 36 h on-stream and then
decreased gradually. The selectivities of the RI and RI20 catalysts
increased with time on-stream, and the selectivity of the RI20
catalyst increased more quickly. Therefore, the yield of the RI
catalyst declined more after the induction period. After reaction for
5
00 h, the 1,3-propanediol yield of the RI catalyst declined to 48%
of its maximum value, whereas that of the RI20 catalyst only
declined to 70%. In addition, it took 348 h for the glycerol
conversion of the RI catalyst to reduce by half, but 468 h for the
RI20 catalyst. Therefore, to some extent, the RI20 catalyst was
more stable than the RI catalyst. According to the XRD (Table 4) and
NH -TPD (Figure 6) results, the Ir particles grew larger and the
3
acidity of RI and RI20 decreased as the reaction progressed, which
may have caused the interfacial sites of Ir and ReO to decrease.
x
However, the active sites were the interface of Ir and ReOx.
Therefore, fewer interfacial sites of Ir and ReOx support fewer
active sites, which may have caused the deactivation.
Conclusions
x
Egg-shell Ir-ReO catalysts were prepared with silanized support
using entrapped air to prevent the impregnation solution from
entering into the support pellets. The ethanol concentration of the
impregnation solution controlled the thickness of the shells. When
a 20% ethanol impregnation solution was used, the best glycerol
conversion and 1,3-propanediol yield were obtained. The
decreasing diffusion distance in the egg-shell catalyst structure was
the primary cause of its high conversion of glycerol. Better heat
diffusion hindered the hydrogenolysis of glycerol to 1,2-propanediol,
resulting in a higher ratio of 1,3-propandiol to 1,2-propanediol. The
2
generation of smaller particles of IrO on the fresh egg-shell
catalysts resulted from the superficial trimethylchlorosilane groups
hindering the aggregation of the active metal precursors, but the
average particle size of Ir trend to accord after reduction. Catalysts
with high conversion lead to greater consecutive hydrogenolysis of
Figure 7 Catalytic performances of RI and RI20 catalysts vs. time on-
stream
1
,3-propanediol. Therefore, there was higher conversion along with
lower 1,3-propandiol selectivity when using the egg-shell catalysts.
However, the yields of removal secondary hydroxyl groups (Y2-OH
those in RI catalyst. It could be found in Table 1 that the conversion
of glycerol over RI20 catalyst was 24.5% higher than that over RI
)
catalyst. The activity was not proportion to Ir sites or ReO
x
sites,
on the egg-shell catalysts were higher than on the uniform catalyst.
Our results also suggest that the RI20 catalyst was more stable than
the RI catalyst. The egg-shell catalysts could promote the
conversion of glycerol while maintaining an acceptable pressure
which resulted from the fact that the active sites were the interface
4
3
of Ir and ReO
x x
, not separate Ir or ReO sites . Nonetheless, more Ir
and ReO sites support more opportunities for the interaction of Ir
x
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The egg-shell catalysts could promote the conversion of glycerol while maintaining an acceptable
pressure gradient.