Hydrogenolysis of glycerol with in-situ produced H2 by aqueous-phase reforming of glycerol using Pt-modified Ir-ReOx/SiO2 catalyst

Article abstract of DOI:10.1016/j.cattod.2017.07.025

We found that Pt-modified Ir-ReO_x/SiO₂ acted as an effective catalyst for aqueous phase reforming (APR) and selective C-O hydrogenolysis of glycerol without external hydrogen sources at low temperature of 463 K, and that 0.5 wt% load

Full text of DOI:10.1016/j.cattod.2017.07.025

Accepted Manuscript
Title: Hydrogenolysis of glycerol with in-situ produced H2 by
aqueous-phase reforming of glycerol using Pt-modified
Ir-ReOx/SiO2 catalyst
Authors: Shiyang Liu, Masazumi Tamura, Zheng Shen, Yalei
Zhang, Yoshinao Nakagawa, Keiichi Tomishige
PII:
S0920-5861(17)30518-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cattod.2017.07.025
CATTOD 10939
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
4-7-2017
24-7-2017
29-7-2017
Please cite this article as: Shiyang Liu, Masazumi Tamura, Zheng Shen, Yalei Zhang,
Yoshinao Nakagawa, Keiichi Tomishige, Hydrogenolysis of glycerol with in-situ
produced H2 by aqueous-phase reforming of glycerol using Pt-modified Ir-ReOx/SiO2
catalyst, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.07.025
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Hydrogenolysis of glycerol with in-situ produced H2 by aqueous-
phase reforming of glycerol using Pt-modified Ir-ReOx/SiO2
catalyst
Shiyang Liua,b, Masazumi Tamuraa,*, Zheng Shenb, Yalei Zhangb, Yoshinao Nakagawaa,
Keiichi Tomishigea,*
aDepartment of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07,
Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
bDepartment of Agricultural Biological Environmental and Energy Engineering, Tongji
University, Shanghai 201804, China
Corresponding authors: mtamura@erec.che.tohoku.ac.jp, tomi@erec.che.tohoku.ac.jp
1

Graphical abstract
Highlight
 Pt-Ir-ReOx/SiO2 was an effective catalyst for Aqueous phase reforming (APR)
and selective C-O hydrogenolysis of glycerol
 Glycerol was converted to 1,2-propanediol and acetol in high total yield of 53 %
at 81% conversion without H2 at 463 K
 The initial TOF over 0.5 wt% Pt loaded Ir-ReOx/SiO2 (Pt(0.5)-Ir-ReOx/SiO2) at
463 K was estimated to be 13 h-1.
We found that Pt-modified Ir-ReOx/SiO2 acted as an effective catalyst for aqueous phase
reforming (APR) and selective C-O hydrogenolysis of glycerol without external hydrogen
sources at low temperature of 463 K, and that 5 wt% loaded Ir-ReOx/SiO2 (Pt(0.5)-Ir-
ReOx/SiO2) provided 1,2-propanediol (1,2-PrD) and acetol in high total yield of 53% at
81% conversion. The initial activity (TOFinitial) over Pt(0.5)-Ir-ReOx/SiO2 at 463 K was
estimated to be 13 h-1. Pt-Ir alloy and Ir-ReOx species in Pt-Ir-ReOx/SiO2 are main active
species for APR and selective C-O hydrogenolysis, respectively, and the high yield can
2

be achieved by balancing the APR and hydrogenolysis activities through the optimization
of Pt and Ir-ReOx molar ratio.
1. Introduction
Transformation of biomass-derived chemicals to valuable chemicals is very important for attaining
the sustainable society, and aqueous-phase reforming (APR) of polyols, which are main biomass-
derived chemicals, is a promising method because this method can directly produce H2 or
deoxyproducts with water as a sole reagent [1]. Ideally, APR can provide only CO and H2 from polyols
and water, however, actually various kinds of compounds can produced. On the other hand,
combination of APR with C-O hydrogenolysis can directly produce deoxyproducts from polyols
without external hydrogen sources, where C-O hydrogenolysis of polyols occurs by in situ generated
H2 by APR of polyols [2]. However, in general, selective synthesis of the target product is difficult,
and therefore, control of the activity for C-C and C-O dissociation is required for development of
effective catalysts.
Large amount of glycerol is produced as the main byproduct in the production of biodiesel, and
hence effective and economical transformation methods of glycerol to value-added chemicals are
desired to be developed. Among the valuable chemicals from glycerol, 1,2-propanediol (1,2-PrD) is
the main target because 1,2-PrD is a useful chemical for a wide-range of products such as polyester
resins, antifreeze liquid, food additives, cosmetics and pharmaceuticals [3]. 1,2-PrD can be produced
by selective C-O hydrogenolysis of glycerol, and many effective heterogeneous catalysts such as Ru-
based catalysts [4], Cu-based catalysts [5], Ni-based catalysts [6] and other metal catalysts [7] for
selective C-O hydrogenolysis of glycerol to 1,2-PrD by using external H2 sources have been reported.
As an alternative method, combination of APR with selective C-O hydrogenolysis can be applied to
transformation of glycerol to 1,2-PrD [8], and Pt-based [8a-g] and Ni-based [8h,i] catalysts have been
3

reported to be effective for the reaction. In this reaction, acetol was also produced, and it can be
regarded as a kind of synthetic equivalent to 1,2-PrD because acetol can be easily reduced to 1,2-PrD
by hydrogenation catalysts with H2. Therefore, in this study we focus on the total yield of 1,2-PrD and
acetol. Jacobs and co-workers reported the effective catalyst of Pt/NaY zeolite and demonstrated 57%
total yield of 1,2-PrD and acetol at 503 K [8a], which is the highest yield among the reported ones.
The activity was also high (the TOF=15 h-1 and TOFinitial=26 h-1), although the reaction temperature
was high (503 K). Nichio and co-workers reported that Pt-Sn/SiO2 catalyst (Sn/Pt=0.2) was effective
for the reaction at moderate temperature of 473 K, providing 1,2-PrD and acetol in moderate total
yield (45%) [8d]. The activity of Pt-Sn/SiO2 catalyst is the highest (TOF=192 h-1) among the reported
ones of heterogeneous catalysts. However, the reaction temperature over the reported catalysts is
typically high (473 K), which means that the inherent activity of the catalysts is not high, and the
TOF over the catalysts that could achieve over 30% yield of 1,2-PrD and acetol is below 10 h-1, and
the yields are below 40% except above two examples [8]. Therefore, it is important to develop
effective catalysts that can achieve high total yield of 1,2-PrD and acetol with high activity.
Up to now, we have developed the metal oxide-modified noble metal catalysts (metal oxides:
typically ReOx, MoOx and WOx, noble metals: typically Ir, Rh and Ru) which are effective for selective
C-O hydrogenolysis [9], deep C-O hydrogenolysis [10], hydrogenation [11] and dehydrogenation [12]
reactions. Based on the kinetics and spectroscopic analyses, the following general structure for the
metal oxide-modified noble metal catalysts was proposed [13]: The noble metals are in the metallic
state, and the metal oxides are in the partially reduced oxide state (the valence is between +2~4), and
the noble metal species are partially covered with the metal oxides species. The active site of the metal
oxide-modified noble metal catalysts is the interface between noble metals and metal oxides.
Recently, we found that modification of Ir-ReOx/SiO2 with noble metal species provides positive
effects such as promotion of activity and improvement of selectivity [14,15]. Ru-modified Ir-
ReOx/SiO2 catalyst showed higher activity for selective C-O hydrogenolysis of glycerol than Ir-
ReOx/SiO2 with maintaining the selectivity to 1,3-propanediol [14a]. In hydrogenation and C-O
hydrogenolysis of furfural to 1,5-pentanediol, Pd-modified Ir-ReOx/SiO2 and Rh-modified Ir-
4

ReOx/SiO2 catalysts provided higher yield of 1,5-pentanediol than Ir-ReOx/SiO2 catalyst [14b,c],
which can be explained by the control of hydrogenation and C-O hydrogenolysis activities by addition
of noble metal species. In addition, we also found that Pt-modified Ir-ReOx/SiO2 catalyst was effective
for the combination reaction of APR and deep C-O hydrogenolysis of sorbitol to alkanes and mono-
functionalized products without external hydrogen sources [15], where Ir-ReOx and Pt mainly worked
as catalysts for C-O hydrogenolysis and APR, respectively. Herein, we applied noble metal (Pt, Ru,
Pd)-loaded Ir-ReOx/SiO2 catalysts to APR and selective C-O hydrogenolysis of glycerol without
external hydrogen sources (under Ar) at low temperature of 463 K, and found that Pt-modified Ir-
ReOx/SiO2 catalyst was effective for the reaction. 0.5 wt% Pt-loaded Ir-ReOx/SiO2 (Pt(0.5)-Ir-
ReOx/SiO2) catalyst provided 1,2-PrD and acetol in high total yield of 53% (81% conversion and 66%
selectivity to 1,2-PrD and acetol (1,2-PrD: 32%, acetol: 34%)).
5

2. Experimental
2.1 Reagents
Glycerol (99%, Wako Pure Chemical Industries, Ltd.), SiO2 (G-6, BET surface area 535 m2/g), H2IrCl6
(Furuya Metals Co., Ltd), argon (>99%, Nippon Peroxide Co., Ltd.), NH4ReO4 (Soekawa Chemical
Co., Ltd.), H2PtCl6 (Soekawa Chemical Co., Ltd.), PdCl2 (Soekawa Chemical Co., Ltd.), and RuCl3
(Strem Chemicals Co., Ltd.) are commercially available and were used as received.
2.2 Catalyst preparation
Ir-ReOx/SiO2 catalyst was prepared by sequential impregnation method as reported in our papers
[9g,15]. Ir/SiO2 was prepared by impregnating an aqueous solution of H2IrCl6 (0.1 M, 2 ml) into 0.88
g SiO2 (obtained by calcination at 973 K in air) using 50 ml beaker. After evaporating the solvent at
~353 K by the hot plate (ADVANTEC, SRS311HA) and drying at 383 K for 12 h in the drying oven
(Yamato Scientific, DX302), the obtained dried Ir/SiO2 was impregnated with an aqueous solution of
NH4ReO4 (0.14 M, 3 ml). After evaporating the solvent at ~353 K and drying at 383 K for 12 h, the
obtained dried Ir-ReOx/SiO2 was transferred to the crucible (30 ml, TGK, MM, B-1) and was calcined
in air at 773 K for 3 h by muffle furnace (Yamato Scientific, FP300), providing Ir-ReOx/SiO2 catalyst.
Pt(x)-Ir/SiO2 was prepared by the same sequential impregnation method as above-mentioned. The
obtained dried Ir/SiO2 (~0.9 g) as described above was impregnated with an aqueous solution of
H2PtCl6 (0.1 M). After evaporating the solvent at ~353 K and drying at 383 K for 12 h, the obtained
dried Pt(x)-Ir/SiO2 was calcined in air at 773 K for 3 h, providing Pt(x)-Ir/SiO2 catalyst. The loading
amount of Ir and Re is 4.0 and 7.8 wt%, respectively, and the molar ratio of Re/Ir is 2. M(x)-Ir-
ReOx/SiO2 catalysts (M=Pt, Pd, Ru) were prepared by impregnating an aqueous solution of H2PtCl6,
PdCl2, or RuCl3 into the Ir-ReOx/SiO2 catalyst. After evaporating the solvent and drying at 383 K for
12 h, the obtained dried M(x)-Ir-ReOx/SiO2 was calcined in air at 773 K for 3 h. ReOx/SiO2 and Pt(x)-
ReOx/SiO2 catalysts were prepared by the same sequential impregnation method as above-mentioned.
ReOx/SiO2 was prepared by impregnating an aqueous solution of NH4ReO4 (0.14 M, 3 ml) into 0.92
g SiO2. After evaporating the solvent at ~353 K and drying at 383 K for 12 h, the obtained dried
6

ReOx/SiO2 was calcined in air at 773 K for 3 h, providing ReOx/SiO2 catalyst. The loading amount of
Re is 7.8 wt%. The obtained dried ReOx/SiO2 was impregnated with an aqueous solution of H2PtCl6
(0.1 M). After evaporating the solvent at ~353 K and drying at 383 K for 12 h, the obtained dried
Pt(x)-ReOx/SiO2 was calcined in air at 773 K for 3 h, providing Pt(x)-ReOx/SiO2 catalyst. x in M(x)-
Ir-ReOx/SiO2, Pt(x)-Ir/SiO2, and Pt(x)-ReOx/SiO2 catalysts represents the weight percent of the
additives to the total catalyst.
2.3 XRD analysis
X-ray diffraction (XRD) patterns were recorded by a diffractometer (Rigaku MiniFlex 600). Averaged
metal particle size was calculated by using the Scherrer equation.
2.4 A typical procedure of transformation of glycerol with in-situ generated H2
Activity tests were performed in a 190-ml stainless steel autoclave with an inserted glass vessel.
Catalyst (0.1 g) and water solvent (18 g) was put into an autoclave together with a spinner, and the air
content in the autoclave was purged with 1 MPa H2 three times and was pressured to 8 MPa H2. The
autoclave was heated at 473 K for 1 h for the reduction pretreatment of the catalyst. The stirring rate
was 250 rpm. After the pretreatment, the autoclave was cooled down to room temperature, and the
hydrogen was removed. Glycerol (2 g) was put into the autoclave, and after sealing the reactor, the air
content was purged with 1 MPa Ar three times. After the temperature reached 463 K, the temperature
was kept for appropriate reaction time. The time when the reaction temperature reached desired one
was defined as 0 h. The stirring rate was 500 rpm. After the reaction, the reactor was cooled down by
using an ice bath, and the gases were collected in a gasbag. The autoclave contents were transferred
to a vial, and the catalyst was separated by filtration. Products in the aqueous phase were analyzed by
using a gas chromatograph (Shimadzu GC-2014) with FID by using TC-WAX capillary column
(diameter 0.25 mm, length 30 m). Alkanes in the gas phase were analyzed by a gas chromatograph
(Shimadzu GC-2014) with FID by using an Rtx-1-PONAcapillary column (diameter 0.25 mm, length
100 m). CO2 in the gas phase was analyzed by a gas chromatograph (Shimadzu GC-2014) equipped
7

with FID and a methanizer by using a Gaskuropack 54 column, and H2 was analyzed by the same GC
with TCD by using MS-13X packed column. Products were also identified using GC–MS (QP5050,
Shimadzu) and standard reagents purchased from chemical companies.
The conversion of substrate (glycerol) and the yield of products were calculated on the carbon basis
and defined as follows and carbon balance is typically between 90 and 110 %.
mol of C atoms in substrate - mol of C atoms in unreacted substrate
Conversion [%-C]=
× 100
mol of total C atoms in substrate
mol of C atoms in products
Selectivity of detected products[%-C]=
× 100
total mol of C atoms in all products
8

3. Results and discussion
3.1 Catalyst screening in APR and selective C-O hydrogenolysis of glycerol into 1,2-PrD without
external H2 sources
At first, APR and selective C-O hydrogenolysis of glycerol without external hydrogen sources was
carried out with Ir-ReOx/SiO2 and M(0.5)-Ir-ReOx/SiO2 (M=Ru, Pd, Pt, loading amount of M: 0.5
wt%) catalysts, and the results are shown in Table 1. Ir-ReOx/SiO2 showed some activity to the reaction
and moderate total selectivity to 1,2-propanediol (1,2-PrD) and acetol (Table 1, entry 1, 69%). As for
the M(0.5)-Ir-ReOx/SiO2 catalysts (Table 1, entries 2-4), Ru(0.5)-Ir-ReOx/SiO2 and Pd(0.5)-Ir-
ReOx/SiO2 catalysts provided similar conversion (5~6%) to that over Ir-ReOx/SiO2, however, the total
selectivity to 1,2-PrD and acetol was higher (~79%) than that in the case of Ir-ReOx/SiO2. On the other
hand, Pt(0.5)-Ir-ReOx/SiO2 showed higher conversion (7.9%) than the other catalysts tested, although
the selectivity to 1,2-PrD and acetol was slightly lower (64%) than those over Ru(0.5)-Ir-ReOx/SiO2
and Pd(0.5)-Ir-ReOx/SiO2 catalysts. Considering the high selectivity to CO2 and C2 products such as
ethylene glycol (EG) and ethanol (EtOH), the high activity will be derived from improved APR
activity by addition of Pt species to Ir-ReOx/SiO2. In addition, the performance of Ir-ReOx/SiO2 and
M(0.5)-Ir-ReOx/SiO2 catalysts was also compared at longer reaction time of 17 h (Table 1, entries 5-
8). The conversion increased over all the catalysts, and Pt(0.5)-Ir-ReOx/SiO2 showed the highest
conversion among the catalysts tested. On the other hand, the total selectivity to 1,2-PrD and acetol
was high over all the catalysts (75~86%). Figure 1 shows the results of XRD analyses of the catalysts.
The signals of Ru(0.5)-Ir-ReOx/SiO2 and Pd(0.5)-Ir-ReOx/SiO2 catalysts are similar to that of Ir-
ReOx/SiO2, however, the signal of Pt(0.5)-Ir-ReOx/SiO2 was slightly sharper than those of the other
catalysts. The estimated particle size of these catalysts by XRD patterns are also shown in Figure 1.
The particle sizes are not so different among these catalysts, indicating that the difference of the
activities is not derived from the difference of the particle sizes. According to the previous works on
APR of glycerol over various noble metal catalysts [16], the activity order for glycerol APR is in the
following: Pt ~ Ni > Ru > Rh ~ Pd > Ir. Therefore, the high activity of Pt(0.5)-Ir-ReOx/SiO2 can be
attributed to high APR activity of Pt metal species. In terms of the activity and selectivity, Pt loaded
9

Ir-ReOx/SiO2 catalyst was used in the following study.
In order to clarify the effect of Pt, Ir and Re metal species on the catalytic activity and selectivity,
the same reaction was performed by using single metal supported catalysts (Pt(0.5)/SiO2, ReOx/SiO2,
and Ir/SiO2, Table 2, entries 2-4) and combination catalysts of two metal species (Pt(0.5)-ReOx/SiO2,
Pt(0.5)-Ir/SiO2 and Ir-ReOx/SiO2, Table 2, entries 5-7) and the physical mixed catalyst of Pt(0.5)/SiO2
and Ir-ReOx/SiO2 (Table 2, entry 8). Pt(0.5)/SiO2 and ReOx/SiO2 provided almost no conversion (Table
2, entries 2 and 3), and Ir/SiO2 provided higher conversion than Pt(0.5)/SiO2 and ReOx/SiO2 catalysts
(Table 2, entry 4). As for the combination catalysts, Pt(0.5)-ReOx/SiO2 showed low conversion (0.9%),
which is similar conversion to those over Pt(0.5)/SiO2 and ReOx/SiO2 (Table 2, entry 5). According to
the previous works on H2 production by APR of polyols [17], combination of Pt and Re species is
effective for the APR of polyols, where the Pt loading amount is typically over 2 wt%. In our case, Pt
loading amount is very small (0.5 wt%), and the amount of Re species (7.8 wt%) is much larger than
that of Pt species. Therefore, low activity of the catalyst can be due to coverage of Pt surface with
large amount of Re species. In addition, formation of large Pt particles was observed over Pt(0.5)-
ReOx/SiO2 catalyst as discussed below (Figure 2), which is also one of the causes of low activity. On
the other hand, Pt(0.5)-Ir/SiO2 and Ir-ReOx/SiO2 catalysts showed higher conversion (~20%) than
single metal supported catalysts (Table 2, entries 6 and 7), while the conversion was lower than that
over Pt-Ir-ReOx/SiO2 catalyst (Table 2, entry 1). The total selectivity to 1,2-PrD and acetol was high
at both the catalysts (Table 2, entries 6 and 7). Focusing on the selectivity to each product over Pt(0.5)-
Ir/SiO2 and Ir-ReOx/SiO2, the selectivity tendency is quite different between Pt(0.5)-Ir/SiO2 and Ir-
ReOx/SiO2 catalysts, although the conversion was almost the same at both the catalysts. The selectivity
to acetol over Ir-ReOx/SiO2 was higher than that over Pt(0.5)-Ir/SiO2, and the selectivity to 1,2-PrD
over Ir-ReOx/SiO2 was lower than that over Pt(0.5)-Ir/SiO2. In addition, the selectivity to CO2 and C2
products such as EG and ethanol over Pt(0.5)-Ir/SiO2 was higher than that over Ir-ReOx/SiO2,
moreover, the residual H2 amount over Pt(0.5)-Ir/SiO2 was higher than that over Ir-ReOx/SiO2. These
results indicate that Pt(0.5)-Ir/SiO2 provided higher APR activity than Ir-ReOx/SiO2. According to our
previous work on selective C-O hydrogenolysis of polyols over Ir-ReOx/SiO2 catalyst [9g,h,l], Ir-
10

ReOx/SiO2 has low activity for C-C dissociation of polyols and high activity for dehydrogenation, C-
O hydrogenolysis and hydrogenation. In addition, Ir-ReOx/SiO2 shows acidity in water under H2.
Therefore, it is reasonable that Ir-ReOx/SiO2 provided the low selectivity to the C2 and C1 products,
and high selectivity to acetol. However, it is surprising that Pt(0.5)-Ir/SiO2 showed high APR activity
in spite that Pt(0.5)/SiO2 and Ir/SiO2 had almost no and very low activity to this reaction, respectively,
To the best of our knowledge, the phenomenon was not reported in any literatures for APR. New active
species may be formed by addition of a small amount of Pt species. On the other hand, considering
some H2 production over Ir-ReOx/SiO2, the conversion over Ir-ReOx/SiO2 can be mainly derived from
dehydrogenation+dehydration+hydrogenation of glycerol, and/or C-O hydrogenolysis with the
generated H2 (intramolecular borrowing hydrogen mechanism [10d,18]) because Ir-ReOx/SiO2
catalyst has almost no activity to APR. In the case of the physically mixed catalyst of Pt(0.5)/SiO2 and
Ir-ReOx/SiO2 (Table 2, entry 8), the conversion (24%) was similar to that over Ir-ReOx/SiO2 but is
lower than that over Pt(0.5)-Ir-ReOx/SiO2 catalyst (30%). As for the selectivity, the total selectivity to
1,2-PrD and acetol was low (57%), and the tendency of the selectivity is different from Pt(0.5)-Ir-
ReOx/SiO2 and Ir-ReOx/SiO2 catalysts, and rather similar to that Pt(0.5)-Ir/SiO2 catalyst. Therefore, it
is important that Pt species are located near Ir and ReOx species, which resulted in high activity and
selectivity.
Next, the effect of reaction conditions such as Ar pressure and reaction temperature was investigated
in the same reaction using Pt(0.5)-Ir-ReOx/SiO2 catalyst (Tables 3 and 4). In this reaction, pressurized
Ar was used for avoiding the vaporization of the substrates, products, and solvent. The tested Ar
pressures were 0.5, 1 and 2 MPa Ar (Table 3). The conversion and selectivity were similar at all Ar
pressures, indicating that Ar pressure has no influence on the performance of the catalyst. The effect
of reaction temperature was investigated by changing the temperature from 393 to 473 K (Table 4).
The conversion increased with increasing the reaction temperature, and the target products of 1,2-PrD
and acetol were observed over 423 K. The total selectivity to 1,2-PrD and acetol was almost the same
at higher temperature more than 443 K. Therefore, higher temperature more than 423 K is necessary
for obtaining the target products. In this work, 463 K was selected in the following studies.
11

The effect of Pt loading amount in Pt-Ir-ReOx/SiO2 catalysts was investigated in the same reaction
(Table 5). The conversion was almost constant up to 0.25 wt% Pt (Table 5, entries 1-3), and increased
with increasing the Pt loading amount more than 0.25 wt% (Table 5, entries 3-6). However, the
conversion decreased with higher Pt loading amount more than 3 wt% (Table 5, entry 7). On the other
hand, the total selectivity to 1,2-PrD and acetol was almost constant up to 0.25 wt% (Table 5, entries
1-3). The selectivity decreased with higher Pt loading amount more than 0.25 wt% (Table 5, entries 3-
7), in contrast, the selectivity to CO2 and C2 products such as EG and EtOH increased, which means
that APR activity increased with increasing the Pt loading amount to result in increase of the
conversion.
In order to compare the performance between Pt(0.5)-Ir-ReOx/SiO2 and Pt(3)-Ir-ReOx/SiO2, the
time-courses of APR and selective C-O hydrogenolysis of glycerol without addition of external
hydrogen sources were studied with these catalysts. Table 6 shows the results of the time-course over
Pt(0.5)-Ir-ReOx/SiO2 catalyst. The conversion increased with the reaction time up to 72 h, and leveled
off over 72 h. As for the selectivity, the total selectivity to 1,2-PrD and acetol was almost constant
from 4 to 42 h, and decreased with the reaction time more than 42 h. The yield of 1,2-PrD and acetol
reached 53% at 72 h, and the selectivity to 1,2-PrD and acetol was 32% and 34%, respectively. As is
known, the turnover frequency (TOF) should be estimated on the basis of the number of the
catalytically active sites. In the present case, the conversion of glycerol to 1,2-PrD and acetol with in-
situ produced hydrogen by aqueous phase reforming of glycerol consists of multi-steps including
dehydrogenation, decarbonylation, dehydration, hydrogenolysis, etc. as discussed below, and the
active sites for these steps may not be identical. Therefore, it is difficult to estimate the actual number
of the active sites. First, as indicator of productivity based on noble metal amount, TOF was calculated
by the following equation: TOF (h-1) = (total amount of produced 1,2-PrD and acetol (mmol))/(total
amount of Ir and Pt (mmol))/(reaction time (h-1)). The TOF was calculated to be 7 h-1 at 0-72 h and
the TOFinitial was also calculated to be 13 h-1 at 4 h at 463 K. In addition, the TOFinitial based on the
surface metal (TOFs, initial) was also calculated to be 29 h-1 based on the dispersion of metal species
determined by the particle size in XRD analysis (Figure 1), assuming that the number of the
12

catalytically active site is the same as that of the number of surface metal (Irs0+Pts0) atoms. Here, the
dispersion is calculated to be 0.46, which is determined by the established method by using the atomic
diameter of Ir (0.277 nm) and Pt (0.278 nm) metal and by assuming that the Pt-Ir alloy has a cubo-
octahedral shape [19]. Considering that the reaction temperature is lower than those of the previous
reports [8] , the activity of Pt(0.5)-Ir-ReOx/SiO2 will be high compared with the previous reports.
Focusing on the individual selectivity to 1,2-PrD and acetol, the selectivity to 1,2-PrD increased with
the reaction time and leveled off at more than 72 h. On the other hand, the selectivity to acetol was
high at the initial stage (62%) and decreased with the reaction time. These results indicate that acetol
was produced mainly from glycerol, and then 1,2-PrD was produced by hydrogenation of acetol with
in situ produced H2. On the other hand, focusing on the other products, the selectivity to CO2 increased
with the reaction, indicating that APR also proceeded with the reaction time. Table 7 shows the results
of the time-course over Pt(3)-Ir-ReOx/SiO2 catalyst. The conversion increased with the reaction time
up to 21 h, and leveled off over 21 h. As for the selectivity, the total selectivity to 1,2-PrD and acetol
increased with the reaction time up to 78 h, and decreased with the reaction time. The yield reached
44% at 17 h, which is lower than that over Pt(0.5)-Ir-ReOx/SiO2 catalyst. The total selectivity to 1,2-
PrD and acetol was lower than that over Pt(0.5)-Ir-ReOx/SiO2 catalyst at any time, indicating that a
large amount of Pt decreases the selectivity by increase of APR activity. Therefore, a low loading
amount of Pt was effective for obtaining the C3 target products of 1,2-PrD and acetol. Therefore, it
can be concluded that Pt(0.5)-Ir-ReOx/SiO2 catalyst is superior to Pt(3)-Ir-ReOx/SiO2 catalyst in APR
and selective C-O hydrogenolysis of glycerol.
The reusability of Pt-Ir-ReOx/SiO2 was already investigated in the conversion of sorbitol to
gasoline-ranged products with in-situ produced hydrogen by aqueous phase reforming of sorbitol [15].
The activity for C-O hydrogenolysis and the yield of the gasoline-ranged products were decreased
with the reuse time, although the APR activity was almost maintained. Based on ICP analyses, no
leaching of noble metals (Ir and Pt) was observed, while slight leaching of Re species (1.0-2.7%) was
observed. Judging from these previous results, it is expected that the reusability of the present catalyst
is not easy. The development of stable and reusable catalysts in this reaction system is very important
13

and further studies are necessary.
3.2 Structure of Pt-Ir-ReOx/SiO2 catalysts
In our previous works on characterization of Pt(3)-Ir-ReOx/SiO2 catalyst in catalytic conversion of
sorbitol without external hydrogen sources [15], the following structure of Pt(3)-Ir-ReOx/SiO2 catalyst
was proposed based on XRD, TEM, TPR, CO-adsorption and XAS analyses: Pt-Ir alloy particles are
formed by reduction and the size of the particles is about 3~4 nm. Pt-Ir alloy particles are partially
covered with ReOx species, and the average valence of ReOx species is +3~+4. In order to confirm
that the structure of Pt(3)-Ir-ReOx/SiO2 in this reaction is similar to the above structure, XRD analyses
were carried out with a series of Pt- and/or Ir-loaded catalysts after the reaction (Figure 2), and the
estimated particle sizes are also shown in Figure 2, although the particle size of alloy particles may be
underestimated because of heterogeneity of alloy compositions. The signal of Pt(3)-Ir-ReOx/SiO2 is
almost the same as that in the previous work, although a small signal due to Pt metal can be detected.
The signal position of the strongest peak (around 40o) was located between the signals due to the Pt
(2=39.7, 46.2, 67.4) and Ir (2=41.0, 47.3, 69.1) metals, suggesting the formation of Pt-Ir alloy. Since
the reaction conditions such as temperature (463 K) and Ar pressure are similar to those of the previous
work [15], the structure of Pt(3)-Ir-ReOx/SiO2 in this reaction will be the same as the above one.
Focusing on the XRD patterns of Pt(x)-Ir-ReOx/SiO2 catalysts (x=0, 0.1, 0.25, 0.5, 1, 3, 5), the
estimated particle sizes were similar and between 1.9~3.0 nm, and the top of signals around 40o is
shifted to lower angle with increase of the Pt loading amount. These results indicate that Pt-Ir alloy
particles with different compositions were formed depending on the Pt loading amount. On the other
hand, Pt(3)/SiO2, Pt(0.5)/SiO2, and Pt(0.5)-ReOx/SiO2 showed large Pt particles (≥15 nm), and Ir/SiO2
and Pt(0.5)-Ir/SiO2 showed similar metal particle size of ~3.5 nm. The low activity over Pt(0.5)/SiO2
and Pt(0.5)-ReOx/SiO2 (Table 2, entries 2 and 9) are related to the formation of the large Pt particles.
The moderate activity over Pt(0.5)-Ir/SiO2 will be due to formation of Pt-Ir alloy particles. These
results suggest that in the case of Pt(x)-Ir-ReOx/SiO2 catalysts, increase of the metal particle size was
drastically suppressed by the coverage of ReOx species [13c] or alloy formation of Pt and Ir [15]. As
14

above, Pt-Ir alloy particles were formed also on Pt(0.5)-Ir-ReOx/SiO2 and the Pt-Ir alloy particles can
be partially covered with ReOx species.
3.3 Proposed reaction mechanism
A typical reaction route of APR and hydrogenolysis of glycerol is shown in Scheme 1. The reaction
can start with dehydrogenation or dehydration of glycerol to glyceraldehyde and acetol, respectively.
From the results of the effect of reaction temperature (Table 4), H2 production was observed even at
low temperature at 393 and 423 K, while production of acetol was not observed. These results indicate
that dehydrogenation of glycerol can occur easier than dehydration of glycerol. On the other hand,
dehydration of secondary OH group in glycerol can produce 1,3-propanediol (1,3-PrD). However,
production of 1,3-PrD was not observed over Pt-Ir-ReOx/SiO2 catalyst, in spite that Ir-ReOx/SiO2
catalyst is well-known to be effective for selective C-O hydrogenolysis of glycerol with H2 to 1,3-PrD
with high selectivity [9g,h,l]. This would be due to high reaction temperature and low H2 pressure.
Glyceraldehyde can undergo dehydration, decarbonylation and retro-aldol reactions. The retro-aldol
reaction can produce ethylene glycol and methanol, which consumes the produced H2 by
hydrogenation of the unsaturated compounds of glycolaldehyde and formaldehyde because APR of
methanol is difficult at such a low temperature of 463 K. In general, retro-aldol can be accelerated by
base catalysts more than acid catalysts. The low selectivity to methanol over Pt-Ir-ReOx/SiO2 catalysts
will due to the acidic nature of Pt-Ir-ReOx/SiO2 catalysts, which is reported in our previous works [9l].
On the other hand, glyceraldehyde can be easily decarbonylated to EG and CO over Pt catalyst at room
temperature [2a], and CO can also be easily converted to CO2 and H2 by water-gas shift reaction
because of low reaction temperature. This route is the main one for H2 production by APR of glycerol.
Dehydration of glyceraldehyde can easily take place to produce pyruvaldehyde because the -proton
of the aldehyde group in glyceraldehyde has higher acidity than -proton of OH group of polyols, and
hydrogenation of the aldehyde group in pyruvaldehyde provides acetol. However, the rearrangement
of acetol can produce acids such as propanoic acid. According to the our previous work on
hydrogenation of aldehyde over Ir-ReOx/SiO2 catalyst, Ir-ReOx/SiO2 shows high activity to
15

hydrogenation of aldehyde groups even at low temperature (~30 oC) and low H2 pressure. In addition,
it is known that the rearrangement reaction can be promoted by base catalysts. Therefore, the low
selectivity to acidic products such as propanoic acid and acetic acid will be due to the acidic nature of
the catalyst and high hydrogenation activity. In addition, hydrogenation of acetol with the generated
H2 provides 1,2-PrD. In order to obtain high selectivity to 1,2-PrD and acetol, the activity balance
between dehydration and decarbonylation of glyceraldehyde is very important. As shown in the time-
courses (Tables 6 and 7), Pt(3)-Ir-ReOx/SiO2 catalyst provided higher selectivity to CO2 and EG than
Pt(0.5)-Ir-ReOx/SiO2 catalyst at longer reaction time, which indicates that Pt-Ir alloy species mainly
acted as a catalyst for APR. Therefore, high Pt loading is not suitable for the reaction because of high
APR activity. The activity for dehydration and decarbonylation of glyceraldehyde can be balanced
over Pt(0.5)-Ir-ReOx/SiO2, leading to high total yield of 1,2-PrD and acetol.
16

keto-enol
tautomerism
H2
O
OH
HO
OH
HO
OH
1,3-PrD
H2
-H2O
Direct C-O hydrogenolysis
keto-enol
tautomerism
OH
O
OH
OH
OH
dehydration
-H2O
H2
OH
OH
HO
OH
OH
Acetol
1,2-PrD
r
e
a
r
r
a
n
g
e
m
e
n
t
H2
H2
OH
keto-enol
tautomerism
dehydration
-H2O
O
OH
O
O
O
HO
O
OH
Decarbonylation
HO
+
CO
H2O
CO2
+
H2
H2
O
Decarbonylation
CH3OH
+
CO
HO
H2O
CO2
+
H2
OH
retro-aldol
+
HCHO
HO
H2
CH3OH
Scheme 1. Typical reaction route for APR and hydrogenation of glycerol. The reaction route in the
dotted closing line is the target reaction. Blue colored H2 means H2 production, and red colored H2
means H2 consumption.
17

4. Conclusion
Noble metal (Pt, Ru, Pd)-loaded Ir-ReOx/SiO2 catalysts were applied to APR and selective C-O
hydrogenolysis of glycerol without external hydrogen sources (under Ar) at low temperature of 463
K. Pt was the best noble metal for Ir-ReOx/SiO2, and 0.5 wt% Pt-loaded Ir-ReOx/SiO2 (Pt(0.5)-Ir-
ReOx/SiO2) catalyst provided high total yield of 1,2-PrD and acetol, 53%, and high activity of TOF=7
h-1 (at 0-72 h) and TOFinitial=13 h-1 (at 4 h) at low temperature of 463 K. Based on the kinetic studies,
Pt-Ir alloy species mainly worked for APR reaction of glycerol and Ir-ReOx species mainly worked
for selective C-O hydrogenolysis of glycerol. High total yield of 1,2-PrD and acetol can be attributed
to the suitable balance of APR and C-O hydrogenolysis, which can be attained by optimization of the
molar ratio of Pt to Ir-ReOx species.
18

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Manyar, C. Hardacre, T. Salmi, J.-P. Mikkola, D.Y. Murzin, Aqueous phase reforming of xylitol over
Pt-Re bimetallic catalyst: Effect of the Re addition, Catal. Today 223 (2014) 97-107. (d) H.-D. Kim,
H.J. Park, T.-W. Kim, K.-E. Jeong, H.-J. Chae, S.-Y. Jeong, C.-H. Lee, C.-U. Kim, The effect of
support and reaction conditions on aqueous phase reforming of polyol over supported Pt–Re bimetallic
catalysts, Catal. Today 185 (2012) 73-80. (e) L. Zhang, A.M. Karim, M.H. Engelhard, Z. Wei, D.L.
King, Y. Wang, Correlation of Pt–Re surface properties with reaction pathways for the aqueous-phase
reforming of glycerol, J. Catal. 287 (2012) 37-43. (f) Z. Wei, A. Karim, Y. Li, Y. Wang, Elucidation of
the Roles of Re in Aqueous-Phase Reforming of Glycerol over Pt–Re/C Catalysts, ACS Catal. 5 (2015)
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Surf. Sci. 15 (1969) 189-230.
25

Table 1. APR and C-O hydrogenolysis of glycerol without external hydrogen sources (under Ar) using
Ir-ReOx/SiO2 and M-Ir-ReOx/SiO2 catalysts
Total
selecti
vity of
1,2-
PrD
and
Selectivity /%
t
/
h
H2
(mm
ol)
1,2
-
Pr tol
D
Ent
ry
Conv.
/%
1- 2-
Pr Pr
OH OH
Propa
noic
Ace
tic
acid
Catalyst
Ace
Acet
one
C3
H8
E
G
Et C2
OH H6
C
C
CH3
O2 H4 OH
acid
acetol
/%
69
<0.
1
<0. 3. 1.
1
2
Ir-ReOx/SiO2
4.7
5.2
67 3.9
1.7
0.3
0.4
10 1.
0
4.3
3.9
<0.1 0.51
<0.1 0.69
4
4
2.2
1
6
7
79
Ru(0.5)-Ir-
ReOx/SiO2
<0.
1
<0.
3.1
1
<0. 4. 1.
73 1.8
<0.1
7.0 1.
2
5.8
1
8
7
Pd(0.5)-Ir-
ReOx/SiO2
Pt(0.5)-Ir-
ReOx/SiO2
79
71
86
<0.
1
<0.
1
<0 <0.
<0. 6. 1.
3
4
5.7
7.9
71 2.5
62 5.5
2.2 <0.1 7.4
1.1 <0.1 10
2.5
4.6
<0.1 0.88
<0.1 0.11
4
4
7.8
9.2
.1
1
1
7
2
<0 <0.
<0. 6. 1.
.1
1
1
7
8
1
7
<0.
1
2. 0.
0.7
5
6
7
8
Ir-ReOx/SiO2
20
25
19
30
76 1.0
1.9
0.6
3.3 0.
5
1.2
1.5
1.6
2.9
<0.1
1
9.6
1.9
0.6
6
4
79
84
75
Ru(0.5)-Ir-
ReOx/SiO2
1
7
<0.
1
7. 0.
17 62 2.1
<0.1 <0.1 3.6 1.
8
<0.1 1.4
1.5
0.5
2.4
1.2
0
7
Pd(0.5)-Ir-
ReOx/SiO2
1
7
<0.
1
<0. 3. 0.
0.9
74 1.7
9.2
3.7 <0.1 4.3 1.
2
<0.1
9
1
7
7
Pt(0.5)-Ir-
ReOx/SiO2
1
7
<0.
1
<0. 5. 1.
19 56 4.1
2.5
1.5
5.9 0.
9
<0.1 0.84
1
6
1
Reaction Conditions: catalyst 0.1 g (Ir: 4wt%, Re/Ir=2), glycerol 2 g, H2O 18 g, 463 K, Ar 2 MPa. 1,2-PrD: 1,2-
propanediol, PrOH: propanol, EG: ethylene glycol, EtOH: ethanol.
26

Table 2. APR and C-O hydrogenolysis of glycerol without external hydrogen sources (under Ar) using
various Pt and/or Ir-supported catalysts
Total
selecti
vity of
1,2-
PrD
and
Selectivity /%
Con
v.
/%
H2
(mm
ol)
1,2
-
Pr
D
Ent
ry
1-
2-
Propan
oic
Acet
ic
acid
Catalyst
Acet
ol
Aceto
ne
C3
H8
E
G
EtO C2
C
C
CH3
PrO PrO
H
H
H6
O2 H4 OH
H
acid
acetol
/%
Pt(0.5)-Ir-
ReOx/SiO2
3
0
<0.
1
<0.1
<0.1
<0.1
1
2
75
19 56
2.5
1.5 5.9
2.9
1.3
1.1
5.6
0.84
0.53
4.1
0.9 2.4
<0.
<0.1
Pt(0.5)/SiO
2
<0. <0.
<0.
0.3
1
88
<0. 88
1
<0.1
<0.1 3.3
0.6
1
1
1
7.2
<0.1
<0.1
<0. <0.
<0.
0.4
1
3
4
5
ReOx/SiO2
43
89
86
<0. 43
1
<0.1
1.0
11
<0.1 2.6
<0.1 1.5
<0.1 2.6
54
0.7
0.8
0.4
1.2
<0.1
0.66
0.27
1.5
0.5
5.1
1
1
0.1
<0. <0.
<0.1
<0.1
<0.1
Ir/SiO2
88
0.4
2.0
1.4
1
1
0.2 4.5
<0. <0.
<0.1
Pt(0.5)-
ReOx/SiO2 0.9
<0. <0.
<0.
0.2
1
<0. 86
1
1
1
1
1
Pt(0.5)-
Ir/SiO2
Ir-
ReOx/SiO2
Pt(0.5)/SiO
2
<0.1
6
7
73
86
20 53
1.1
1.9
0.2 1.5
0.6 3.3
11 0.7 0.2
1
0
0.6 0.6
<0.
7.1 2.8
0.5 1.9
2
2
<0.1
0.7
1
76
1.2 0.6
0.4
2.6
9.6
1.0
1
<0.1
8a 2+Ir-
57
15 43
3.5
1.2 5.9
3.2 <0.1 12 1.5
1.4
4
4.3 0.5
7.4 3.5
ReOx/SiO2
Reaction Conditions: catalyst 0.1 g (Ir: 4wt%, Re: 7.8 wt%, Re/Ir=2), aPt(0.5)/SiO2 0.1 g and Ir-ReOx/SiO2 (Ir: 4wt%,
Re/Ir=2) 0.1 g, glycerol 2 g, H2O 18 g, 463 K, Ar 2 MPa, 17 h. 1,2-PrD: 1,2-propanediol, PrOH: propanol, EG: ethylene
glycol, EtOH: ethanol.
27

Table 3. Effect of Ar pressure on APR and C-O hydrogenolysis of glycerol over Pt(0.5)-Ir-ReOx/SiO2
catalyst without external hydrogen sources (under Ar)
Total
selectiv
ity of
1,2-
PrD
and
Selectivity /%
Ar
pressu v.
re
Con
H2
(mm
ol)
1,
Entr
y
1-
2-
Propan
oic
acid
Acet
ic
acid
2- Acet
Pr
D
Aceto
ne
C3
H8
EtO C2
C
C
CH3O
H
PrO PrO
H
EG
/%
ol
H
H6
O2 H4
H
acetol
/%
6.
2
9.
2
9.
3
<0.
1
<0.
1
<0.
1
<0.1
<0.1
<0.1
<0. <0.
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
1
2
3
0.5
1
6.2
7.8
7.9
73
67
62
62
4.0
5.4
5.5
1.1
<0.1
1.1
3.7
4.7
4.6
8.4 1.4
6.7 1.9
6.7 1.8
0.87
1.0
8.4
10
1
1
<0. <0.
71
71
1
1
<0. <0.
2
10
1.1
1
1
Reaction Conditions: Pt(0.5)-Ir-ReOx/SiO2 0.1 g (Ir: 4wt%, Re/Ir=2), glycerol 2 g, H2O 18 g, 463 K, 4 h. 1,2-PrD: 1,2-
propanediol, PrOH: propanol, EG: ethylene glycol, EtOH: ethanol.
28

Table 4. Effect of reaction temperature on APR and C-O hydrogenolysis of glycerol over Pt(0.5)-Ir-
ReOx/SiO2 catalyst without external hydrogen sources (under Ar)
Total
selecti
vity of
1,2-
PrD
and
Selectivity /%
Tempera Con
H2
(mm
ol)
1,2
-
Pr
D
Ent
ry
1-
2-
Propan
oic
acid
Acet
ic
acid
ture
/K
v.
/%
Acet
ol
Aceto
ne
C3
H8
EtO C2
C
C
CH3
PrO PrO
H
EG
H
H6
O2 H4 OH
H
acetol
/%
<0.
1
<0.
1
<0.
1
<0. <0.
<0. <0.
1
2
3
4
5
6
393
423
443
453
463
473
0.1
0.3
2.4
3.4
7.9
11
<1
<1
<0.1
<0.1
61
<0.1
<0.1
1.7
<0.1 61
<0.1 41
<0.1 14
<0.1 13
<0.1 10
<0.1 9.0
19 <0.1 14 6.7 <0.1
14 <0.1 39 6.2 <0.1
0.13
0.16
0.45
0.62
1.1
1
1
1
1
<0. <0.
<0. <0.
1
1
1
1
<0. <0.
<0. <0.
61
67
<0.1 11 2.1 <0.1
6.1
1
1
<0.
1
1
1
<0. <0.
7.0 60 6.1
9.3 62 5.5
9.9 67 3.8
<0.1
1.1
<0.1 6.7 1.8 <0.1
5.2
1
1
71
<0.
1
<0.
1
<0. <0.
<0.1 6.7 1.8 <0.1
4.6
1
1
<0. <0.
77
<0.1
<0.1 3.7 1.7 <0.1
4.3
1.1
1
1
Reaction Conditions: Pt(0.5)-Ir-ReOx/SiO2 0.1 g (Ir: 4wt%, Re/Ir=2), glycerol 2 g, H2O 18 g, Ar 2 MPa, 4 h. 1,2-PrD:
1,2-propanediol, PrOH: propanol, EG: ethylene glycol, EtOH: ethanol.
29