Hydrogenolysis of glycerol to 1,3-propanediol over H-ZSM-5-supported iridium and rhenium oxide catalysts

Article abstract of DOI:10.1016/j.cattod.2021.08.014

The hydrogenolysis of glycerol to 1,3-propanediol (1,3-PrD) over Ir-ReO_x catalysts supported on H-ZSM-5 (denoted as Ir-ReO_x/H-ZSM-5) was investigated. The glycerol conversion and 1,3-PrD yield strongly depended on the catalyst composition (Re/Ir) and the amount of metal loading. The analyses of the catalysts using X-ray powder diffraction and transmission electron microscopy revealed that a higher metal dispersion of Ir and a smaller Ir particle were encouraged by the addition of Re to the catalyst. Furthermore, a strong electronic interaction between Ir and Re in the Ir-ReO_x/H-ZSM-5 catalyst was observed from X-ray photoelectron spectroscopy measurements. In the study on the effects of operating conditions, increasing the temperature and reaction time resulted in a higher glycerol conversion at the expense of 1,3-PrD selectivity due to over-hydrogenolysis, whereas increasing the pressure had a positive effect on 1,3-PrD selectivity. The highest 1,3-PrD yield observed was achieved at 2.8% with 14.9% glycerol conversion and 19.0% 1,3-PrD selectivity.

Full text of DOI:10.1016/j.cattod.2021.08.014

Catalysis Today xxx (xxxx) xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Hydrogenolysis of glycerol to 1,3-propanediol over H-ZSM-5-supported
iridium and rhenium oxide catalysts
a
a
,
b
a
a,b,c
Sarun Chanklang , Wongsaphat Mondach , Pooripong Somchuea , Thongthai Witoon
,
a,
b
,
c
d
a,b,c,*
Metta Chareonpanich
, Kajornsak Faungnawakij , Anusorn Seubsai
a
Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand
Center of Excellence on Petrochemical and Materials Technology, Kasetsart University, Bangkok 10900, Thailand
b
c
Research Network of NANOTEC–KU on NanoCatalysts and NanoMaterials for Sustainable Energy and Environment, Kasetsart University, Bangkok 10900, Thailand
Nanomaterials for Energy and Catalysis Laboratory, National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA),
d
Klong Laung, Pathum Thani 12120, Thailand
A R T I C L E I N F O
A B S T R A C T
Keywords:
The hydrogenolysis of glycerol to 1,3-propanediol (1,3-PrD) over Ir-ReOx catalysts supported on H-ZSM-5
1
,3-Propanediol
(
x
denoted as Ir-ReO /H-ZSM-5) was investigated. The glycerol conversion and 1,3-PrD yield strongly depended on
Glycerol
the catalyst composition (Re/Ir) and the amount of metal loading. The analyses of the catalysts using X-ray
powder diffraction and transmission electron microscopy revealed that a higher metal dispersion of Ir and a
smaller Ir particle were encouraged by the addition of Re to the catalyst. Furthermore, a strong electronic
Hydrogenolysis
H-ZSM-5
Iridium
x
interaction between Ir and Re in the Ir-ReO /H-ZSM-5 catalyst was observed from X-ray photoelectron spec-
Rhenium
troscopy measurements. In the study on the effects of operating conditions, increasing the temperature and
reaction time resulted in a higher glycerol conversion at the expense of 1,3-PrD selectivity due to over-
hydrogenolysis, whereas increasing the pressure had a positive effect on 1,3-PrD selectivity. The highest 1,3-
PrD yield observed was achieved at 2.8% with 14.9% glycerol conversion and 19.0% 1,3-PrD selectivity.
1
. Introduction
For past decades, many researchers have focused on the utilization of
for the sustainable utilization of glycerol. Several catalytic processes
have been established for upgrading glycerol into value-added chem-
icals and fuels, including: the selective oxidation of glycerol to glyceric
acid [3], selective dehydration of glycerol to acrolein [4,5], aqueous
biomass-derived feedstock as an alternative to conventional petroleum-
based fuel as the latter causes serious ecological damage by the emission
of greenhouse gases and depletion of natural resources. Biodiesel has
attracted considerable attention from industries world-wide since it is a
renewable and eco-friendly fuel that is produced from biomass that can
directly substitute for conventional petroleum and diesel. In the pro-
duction of biodiesel by the transesterification reaction, about 10% of
glycerol is produced as a by-product [1]. With the rising production of
biodiesel, its main co-product (crude glycerol) is expected to reach
global production of 6 million t by 2025 [2], which would cause its price
to decline substantially. Furthermore, the discarding of surplus glycerol
requires expensive procedures and is one of the main challenges faced by
the biodiesel industry. Hence, the search for a sustainable way to turn
surplus glycerol into value-added products is receiving much attention
and will contribute to the viability of the biodiesel production industry.
Heterogeneous catalysis is among the most interesting approaches
2
phase reforming of glycerol to H [6,7], and selective hydrogenolysis of
glycerol to 1,2-propanediol (1,2-PrD) [8,9] and 1,3-propanediol (1,
3-PrD) [10–15]. Among these processes, the hydrogenolysis of glycerol
to 1,3-PrD is regarded as
a promising approach for producing
value-added chemical products due to its wide range of applications in
the manufacture of polytrimethylene terephthalate (PTT), poly-
urethanes, and cyclic compounds [16,17]. The increasing use of 1,3-PrD
results in a higher demand for the products and its global market value is
expected to reach USD 776.3 million by 2022 [18]. However, the se-
lective production of 1,3-PrD that requires the removal of a sterically
hindered secondary hydroxyl group from glycerol, is far more compli-
cated than the production of 1,2-PrD, which most catalytic systems give
as the main product. While 1,2-PrD is used as a building block for
polyester resin, an antifreeze agent, and a component of paints, the
downside is that the difference in the market price between glycerol and
*
Corresponding author at: Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand.
E-mail address: fengasn@ku.ac.th (A. Seubsai).
https://doi.org/10.1016/j.cattod.2021.08.014
Received 30 March 2021; Received in revised form 7 July 2021; Accepted 7 August 2021
Available online 10 August 2021
0
920-5861/© 2021 Elsevier B.V. All rights reserved.
Please cite this article as: Sarun Chanklang, Catalysis Today, https://doi.org/10.1016/j.cattod.2021.08.014

S. Chanklang et al.
Catalysis Today xxx (xxxx) xxx
1
,2-PrD is relatively small [8]. Hence, it is necessary to find a way to
metal-support and promoter-support becomes stronger when the metal
and promoter are independently dispersed on the support. Thus, sup-
produce 1,3-PrD through sustainable and economically feasible pro-
cesses. 1,3-PrD produced from bio-glycerol through catalytic hydro-
genolysis will offer a sustainable route and provide a benefit for the
economy of the biodiesel production industry.
2
ports with a small external surface area such as H-ZSM-5 and TiO are
preferable since they would enable the Ir particles to be densely located
on the external surface of the support and Re species [32], which would
result in high activity due to the increasing number of active sites.
Generally, glycerol hydrogenolysis requires a noble metal (such as Pt
[
1,19–27] or Ir [28–30]) for the activation of H
2
molecules, coupled
Given that the investigation of Ir-ReO
to be fully investigated, the present work developed the H-ZSM-5-sup-
ported bimetallic Ir-ReO for the hydrogenolysis of glycerol to 1,3-PrD in
x
supported on H-ZSM-5 has yet
with an oxophilic promoter (such as WO or ReO ) for the activation of
3
x
the secondary hydroxyl group, they have been proven to be most effi-
cient catalytic materials for the selective production of 1,3-PrD from
glycerol [31]. Even though the highest yield of 1,3-PrD is achieved with
x
the absence of acidic additives. The synergy between Ir and Re on H-
ZSM-5 support was investigated using advanced characterization tech-
niques. Lastly, a proposed mechanism and the effects of the operating
conditions (temperature, pressure, and time) are discussed.
Pt-WO
Pt-WO
Ir-ReO
x
-based catalysts [10], the productivity of 1,3-PrD over
x
-based catalysts is noticeably lower than that of SiO -supported
x
-based catalysts, even compared at different temperatures opti-
2
mized for each catalyst [32]. Furthermore, recent reports have shown
that a higher reaction temperature (140–180 ◦C) at 1–8 MPa H
is
needed to promote 1,3-PrD production over Pt-WO catalysts [10,23,
3–35], as opposed to the lower reaction temperature (120 C) at the
typical 8 MPa H over the Ir-ReO /SiO catalyst [14,28,36,37].
The Ir-ReO /SiO
catalyst (Ir: 4 wt%, Re/Ir = 1, nominal molar ratio)
was first discovered and fully investigated by Nakagawa et al. [14,28,
6–39]. It gave a high selectivity (67 ± 3%) of 1,3-PrD at the initial stage
of glycerol hydrogenolysis. At 81% glycerol conversion, the 1,3-PrD
2. Materials and methods
2
x
2.1. Preparation of catalysts
◦
3
2
x
2
The preparation of the H-ZSM-5 support (Si/Al mole ratio = 70) has
been reported elsewhere [42]. The catalysts were prepared using a
sequential impregnation method. In the normal procedure, the H-ZSM-5
support was impregnated with an aqueous solution of H IrCl (Alfa
x
2
3
2
6
Aesar). The mixture was then stirred at room temperature for 1 h. After
that, it was heated overnight in an air oven at 100 ºC until dried. The
sample, namely Ir/H-ZSM-5, was then further impregnated with an
yield reached 38% with the addition of sulfuric acid at 8 MPa H
2
and
◦
1
20 C for 36 h. Importantly, this catalytic system was assisted by sul-
furic acid; without the acid, the activity was lowered by half. The role of
sulfuric acid in the hydrogenolysis of glycerol was acknowledged to
stabilize the catalytically active site [28]. As the complication of using
sulfuric acid has constrained the development and the application of this
system, various solid acids have been used to substitute sulfuric acid by
using a co-catalyst [36]. H-ZSM-5 has been considered an effective
aqueous solution of NH ReO4 (Alfa Aesar). After stirring at room tem-
4
perature for 1 h, the mixture was dried overnight in an air oven at 100 ºC
and finally calcined in an air furnace at 500 ºC for 3 h with a heating rate
ꢀ 1
of 5 ºC min . For the first group of the catalysts, the amounts of Ir
loaded on the H-ZSM-5 support were fixed at 4 wt percentage (wt%) and
the amount of Re was varied at 0, 2, 4, and 8 wt%. For the second group
of catalysts, the weight ratio of Re/Ir was fixed at 1:1 and the total metal
loading on the H-ZSM-5 support was varied at 2, 4, and 8 wt%. In
addition, 4 wt% of Re and Ir (weight ratio of Re/Ir = 1:1) was prepared
on TiO2 (21 nm primary particle size (transmission electron micro-
scopy), Aeroxide® P25, Aldrich) and SiO2 (amorphous fumed silica
co-catalyst for the Ir-ReO
x
/SiO
2
+ H
2 4
SO catalyst, at the expense of the
yield of 1,3-PrD. The maximum yield of 1,3-PrD when using the
co-catalyst was 33%, which was slightly lower compared with another
study where H
support for monometallic iridium nanoclusters by Wan et al. [40]. The
IrO /H-ZSM-5 catalyst showed excellent 1,3-PrD selectivity and could
2 4
SO was used [36]. H-ZSM-5 has been studied as a
2
ꢀ 1
x
powder, with a surface area of 85–115 m
ative catalysts to the H-ZSM-5 support.
g
, Alfa Aesar) as compar-
be reused five times without changing the 1,3-PrD selectivity. The ac-
tivity strongly depended on the number of Br o¨ nsted acid sites; the 1,
3
-PrD selectivity increased with the presence of Ir-induced Br o¨ nsted
2
.2. Activity test of prepared catalysts
acid sites [40]. Furthermore, the small external surface area of the
H-ZSM-5 zeolite might promote the surface density of the Ir particles on
the external surface of H-ZSM-5 and Re species [32], as in the case of the
A teflon-lined stainless steel autoclave (Parr 4848, actual volume =
2
00 mL) was used for the activity test in the hydrogenolysis of glycerol.
high-loading Ir-ReO
H-ZSM-5 in a hot water solvent could contribute to the higher activity
36]. Liu et al. further investigated the effect of various supports and
found that rutile TiO produced high activity, even higher than the
previously reported SiO [32]. Higher 1,3-PrD productivity was ach-
x 2
/SiO catalyst [29]. Additionally, the acidity of
For the normal procedure, before the reaction, each sample (150 mg)
was reduced using an H flow in a fixed bed reactor. The sample was
packed in a quartz tube with an inner diameter of 0.5 cm and sand-
2
[
2
wiched between two layers of quartz wool. The flow of pure H
2
(UHP,
2
ꢀ 1
Air Liquide) was at 30 mL min at 300 ºC for 1 h. Then, the reduced
ieved even with a lower Re/Ir ratio (actual ratio of 0.24) without the
addition of sulfuric acid. A high Ir loading amount compared to the small
sample was transferred into the stainless-steel autoclave. Glycerol (5 mL,
9
9.5%, QReC) and deionized water (5 mL) were added to the autoclave.
After the autoclave was completely sealed, the reactor was purged three
times with 1 MPa H . Following the removal of air, the temperature was
raised to 180–240 ºC and the reactor was pressurized with H to 2–8
surface (6 wt%, on 6 m2
g
ꢀ 1
rutile TiO
activity. Their characterization results suggested that Ir metal particles
interacted with the partly oxidized ReO cluster that nearly fully covered
the surface of the rutile TiO particles, with the Ir-ReO interface being
2
) was responsible for the high
2
x
2
2
x
MPa. The stirring rate was fixed at 500 rpm to eliminate mass transfer
effects. The reaction time (1–8 h) commenced after the reactor system
had reached the set conditions. After the reaction was completed, the
liquid products were separated from the reaction mixture using a syringe
filter. A gas chromatograph (GC; Shimadzu GC14-A), equipped with a
DB-WAX column, was used to analyze the composition of the liquid
products. The components detected by the GC were glycerol, 2-propa-
nol, 1,2-PrD, 1,3-PrD, 1-propanol, among others. The activity of each
prepared catalyst was defined in terms of glycerol conversion (%),
the active site. The function of the rutile support was described as
providing a unique environment for the stabilization of the uniform and
small Ir-ReO
increased the number of active sites per Re amount.
Since the glycerol hydrogenolysis reaction occurred at the metal-
promoter (Ir and ReO ) interface on the support [29], a strong
x 2
particles with high surface density on rutile TiO , which
x
metal-promoter interaction would enhance the activity [41]. However,
such interaction may decrease, as the interaction between the
2

S. Chanklang et al.
Catalysis Today xxx (xxxx) xxx
product selectivity (%), product yield (%), and 1,3-PrD productivity (g
The electronic states of rhenium (Re 4 f) and iridium (Ir 4 f) for the
samples were analyzed using X-ray photoelectron spectrometry (XPS;
–
1
–1
g
metal
h ) [32]. The determinations for the catalyst activity are shown
in Eqs. (1)–(4):
α
Kratos Axis Ultra DLD, using Al K for the X-ray source).
Moles of all detected products
Moles of all detected products + Moles of unreacted glycerol
Glycerolconversion(%) = (
) × 100
(1)
The elemental analyses of Re and Ir in the catalysts were conducted
using an inductively coupled plasma optical emission spectrometry (ICP-
OES) technique. Before following the ICP-OES’s standard procedure, the
solid samples were dissolved in hydrochloric acid solution if the Ir
content was analyzed and the solid samples were dissolved in nitric acid
solution if the Re content was analyzed.
Moles of desired product
Productselectivity(%) = (
) × 100
(2)
(3)
Moles of all detected products
%
Glycerol conversion × %Productselectivity
00
Productyield(%) =
1
Glycerol m oles × 1, 3-PrD yield x 1, 3-PrD MW.
Total m etal am ount (g) × Tim e (h)
1
, 3 ꢀ PrDproductivity(ggm etal–1h–1) =
(4)
3
. Result and discussion
Note that i) a standard calibration curve with four data points and R2
>
0.999 for each pure standard chemical was used to determine the
3.1. Effect of the catalyst composition on glycerol hydrogenolysis
amount of each product. ii) The glycerol conversion (%) and the product
selectivity are expressed in terms of average value ± standard deviation.
iii) For the catalytic activity data, “others” stands for 2-propanol,
ethylene glycol, and ethanol. The gaseous products including propane,
ethane, and methane are excluded from the determinations. The total
moles for “others” and the gaseous products of every experiment were
below 5%, calculated based on the carbon balance.
x
The Ir-ReO /H-ZSM-5 catalyst with a fixed Ir amount of 4 wt% was
loaded with different amounts of Re at varying Re/Ir ratios of 0.5, 1.0,
and 2.0 to observe their activities in glycerol hydrogenolysis. The results
suggested that the Re/Ir ratio substantially affected the activity of the Ir-
x
ReO /H-ZSM-5 catalyst. The highest activity in terms of glycerol con-
version (5.6%) and 1,3-PrD yield (1.7%) was achieved with the same
Re/Ir ratio (Re/Ir = 1), as shown in Table 1. When the Re/Ir ratio was
changed to 0.5 or 2.0, the activity (both glycerol conversion and 1,3-PrD
yield) was lower. Next, the effect of the amount of Ir loaded was studied
by fixing the ratio of Re/Ir at 1.0 and the amounts of Ir loading varied
with 2, 4, and 8 wt%. The results are shown in Table 1 and indicate that
the 4 wt% catalyst produced the highest activity (5.6% glycerol con-
version and 1.7% 1,3-PrD yield), while the catalysts with 2, 6, and 8 wt
2
.3. Characterization of prepared catalysts
Before the measurement of each catalyst for each characterization
technique, the catalyst was reduced using the H
2
flow in a fixed bed
◦
reactor at 300 C for 1 h, a process similar to that described in Section
2
.2.
The powder X-ray diffraction (XRD) patterns of the samples were
%
-Ir were clearly less effective. The reason behind the good activity will
′
be explained in the section about catalyst characterization (Section 3.2).
determined using an X-ray diffractometer (PANalytical X Pert PRO,
◦
For the comparative study of the bimetallic Ir-ReOx (Re/Ir = 1 at
using Cu-K radiation, 45 kV and 40 mA, a step size of 0.02 , and a step
α
4
wt%) on the different supports, the use of H-ZSM-5 as the support
time of 0.5 s).
provided a much higher activity in the glycerol hydrogenolysis to 1,3-
PrD relative to the TiO and SiO supports, respectively. According to
The surface area, pore-volume, and pore size of the samples were
2
determined using an N -physisorption analyzer (BET: 3Flex Phys-
2
2
the measurements of the BET surface area, pore-volume, and pore size of
the 4 wt% Ir-catalysts supported on the different supports, the H-ZSM-5
isorption Micromeritics). Before collecting the data, each sample was
◦
treated with He flow at 200 C for 2 h and then cooled down to
2
◦
support provided the highest BET surface at 333 m /g (pore size of
ꢀ
196 C. The average specific BET surface area of the samples was
3
0
2.38 nm and pore volume of 0.19 cm /s), followed by the SiO support
determined in a relative pressure (P/P ) range of 0.05–0.30 and the total
pore volume was calculated at P/P0 = 0.995. The Barrett-Joyner-
2
2
3
at 75 m /g (pore size of 30.22 nm and pore volume of 0.57 cm /s), and
2
2
the TiO support at 40 m /g (pore size of 32.51 nm and pore volume of
Halenda method was used to calculate the pore size of the samples.
The particle size distribution and the elemental composition of the
samples were analyzed using transmission electron microscopy with
energy-dispersive X-ray spectroscopy (TEM with EDX: JEM-2100). The
3
0
.32 cm /s), respectively. This implies that the high BET surface area is
not strongly related to the high performance of the catalysts. Nonethe-
less, as suggested by Liu et al., H-ZSM-5 zeolite is appropriate for
enhancing the activity of the Ir-ReO
x
/H-ZSM-5 catalyst because that H-
par-
2
samples after reduction with H were suspended in ethanol solvent,
ZSM-5 zeolite’s small external surface area allows the Ir and ReO
ticles to densely pack that surface, and the reactants can interact with
the active sites easily [32]. As such, the reactants can interact with the
active sites easily. Moreover, the Ir-containing catalysts supported on
x
dropped on Cu grids, and dried in air at room temperature before the
analysis. The particle size of Ir determined by TEM (dTEM) and the Ir
dispersion in the catalysts were also determined using a formula re-
ported elsewhere [43].
3

S. Chanklang et al.
Catalysis Today xxx (xxxx) xxx
Table 1
Effect of loading Ir amount and Re/Ir ratio on glycerol hydrogenolysis of Ir-ReO
x
on H-ZSM-5, TiO
2
, and SiO
2
supports. Reaction conditions: 200 ºC, 4 MPa, and 4 h.
Catalysts
Ir amount (wt%)
Re/Ir ratio
Conversion (%)
Selectivity (%)
1,3-PrD yield (%)
1
,3-PrD
1,2-PrD
1-PO
Others
Ir-ReO
x
/H-ZSM-5
4
0.5
1
1.9 ± 0
17 ± 0
34 ± 0
47 ± 1.3
56 ± 0.1
47 ± 0.2
54 ± 0.5
34 ± 0.4
24 ± 0.7
25 ± 0.7
2 ± <0.1
1 ± <0.1
1 ± <0.1
2 ± <0.1
2 ± <0.1
1 ± <0.1
2 ± <0.1
0.30
1.60
0.50
0.90
0.50
0.70
0.40
5.5 ± 0.1
2.6 ± 0
30 ± 0.1
21 ± 0
13 ± 0.1
31 ± 0.4
10 ± 1.9
40 ± 1.1
57 ± 0.5
60 ± 0.7
2
2
8
4
4
1
2.8 ± 0.1
2.3 ± 0
34 ± 0.1
24 ± 0
1
Ir-ReO
Ir-ReO
x
/TiO
/SiO
2
1
3.9 ± 0.1
2.8 ± 0
18 ± 0.2
13 ± 0.1
x
2
1
H-ZSM-5 are beneficial for enhancing the activity in the glycerol
hydrogenolysis to 1,3-PrD activity, due to the interfacial synergistic
interaction between IrO
x
and H-ZSM-5 [40]. In any case, the Ir-ReO
was not of further interest in this work.
x
supported on TiO and SiO
2
2
3
3
.2. Catalyst characterization
.2.1. XRD analysis
The XRD patterns of Ir/H-ZSM-5, Ir-ReO
x
/H-ZSM-5 (Re/Ir = 0.5, 1,
and 2), Re/H-ZSM-5, and H-ZSM-5 are shown in Fig. 1. The diffraction
◦
peaks of H-ZSM-5 with the MFI-structure were observed at 2θ = 23.0 ,
◦ ◦
2
3.8 , and 24.3 [44] in every catalyst. Furthermore, all the peak posi-
tions were almost the same for every catalyst. This suggested that the
crystalline structure of H-ZSM-5 was not affected by the loading of Ir and
x
Re metal. In the XRD profiles of Ir/H-ZSM-5 and Ir-ReO /H-ZSM-5
(
Re/Ir = 0.5, 1.0, and 2.0), the diffraction peak of metallic Ir at 2θ
◦
40.7 [29] was observed. The decline in the H-ZSM-5 peak intensity
=
Fig. 1. XRD patterns of Ir/H-ZSM-5, Ir-ReO
x
/H-ZSM-5 (Re/Ir = 0.5, 1, and 2),
was recognized after the loading of Ir. Conversely, increasing the ratio of
Re/Ir increased the H-ZSM-5 peak intensity, in contrast to the decline in
the intensity of the peak of the metallic Ir. The Re/H-ZSM-5 diffraction
peak was almost identical to the peak of H-ZSM-5 and no diffraction
peak of Re-related species was observed, indicating that the Re species
were well dispersed on the support. In summary, the XRD results sug-
gested that the introduction of Re to Ir/H-ZSM-5 encouraged the
dispersion of Ir particles over the H-ZSM-5 surface in the Ir-Re-
Re/H-ZSM-5, and H-ZSM-5.
x
O /H-ZSM-5 catalyst.
3
.2.2. N
The N
2
adsorption-desorption
2
adsorption-desorption isotherms of the synthesized catalysts
are shown in Fig. 2. All of the samples showed type IV isotherms ac-
cording to the International Union of Pure and Applied Chemistry
0
(
IUPAC) classification with H4 hysteresis loop at a relative pressure P/P
of 0.45–1.0, indicating the presence of both micropores and mesopores
in the samples [45]. The BET surface area, pore size, and pore volume of
the catalysts are presented in Table 2. The surface area of the samples
with metal loading slightly decreased, while the pore size and pore
volume were unaffected by the addition of the active metal components.
This might be because the impregnated components were not located
inside the H-ZSM-5 support’s pores; they were located mostly on its
external surface. Thus, it was suspected that the pore size and pore
volume of the catalysts had no substantial effect on the catalytic activity.
Fig. 2. N
2
adsorption-desorption isotherms of various catalysts on H-ZSM-
5
support.
3
.2.3. Morphological analysis using TEM
Table 2
The particle size distribution and the morphological images of H-
Physical properties of various catalysts on H-ZSM-5 support.
ZSM-5, Ir/H-ZSM-5, Re/H-ZSM-5, and Ir-ReO
x
/H-ZSM-5 (Re/Ir = 1) are
2
3
Catalysts
Surface area (m
Pore size
(nm)
Pore volume (cm
shown in Fig. 3. Fig. 3a shows the TEM image of the H-ZSM-5 support.
No dark spots were visible on the surface. For the Ir/H-ZSM-5 catalyst
ꢀ
1
ꢀ 1
g
)
g
)
H-ZSM-5
349
330
333
2.35
2.36
2.38
0.2054
0.1980
0.1946
(
Fig. 3b), numerous rod-shaped particles (61 nm long and 11 nm wide)
Ir/H-ZSM-5
were seen all over the H-ZSM-5 surface. It was suspected that these rod-
shaped particles were the agglomeration of the Ir-particles because some
small particles (dTEM = 3.2 ± 0.2) can also be observed and the
dispersion of the Ir particles was 29%. For the Re/H-ZSM-5 catalyst
Ir-ReO
Ir = 1)
ReO /H-ZSM-5
x
/H-ZSM-5 (Re/
x
331
2.36
0.1961
(
Fig. 3c), the relatively small particles of the Re species (average particle
4

S. Chanklang et al.
Catalysis Today xxx (xxxx) xxx
x
Fig. 3. TEM images of a) H-ZSM-5, b) Ir/H-ZSM-5, c) Re/H-ZSM-5, and d) Ir-ReO /H-ZSM-5.
size of 34 nm) in the Re/H-ZSM-5 catalyst were well dispersed on the H-
ZSM-5 support (Fig. 3c). This confirmed the analysis from the XRD re-
sults. After the addition of Re (Fig. 3d), the rod-shaped particles of Ir had
disappeared and the average particle size of Ir-Re was 1.9 ± 0.2 nm,
which was much smaller compared to the particle sizes of Ir (see
Fig. 3b). More importantly, the Ir-Re particles were well dispersed on the
H-ZSM-5 support. These results suggested that the addition of Re sub-
stantially affected the Ir particle size and encouraged the dispersion of
the Ir species and, thus enhanced the hydrogenolysis of glycerol.
The elemental distribution of the prepared catalysts was analyzed
using TEM with EDX-mapping, as shown in Fig. 4 and the elemental
composition of each catalyst in terms of weight and atomic percentages
was also analyzed using ICP-OES, as summarized in Table 3. The bright
red and green spots on the TEM with EDX-mapping images corre-
sponded to the iridium and rhenium species on the support, respectively.
Those color spots were well-correlated with the TEM images in which
the Ir or Re alone showed poor dispersion on the H-ZSM-5 support
surface of the catalyst. For Ir/H-ZSM-5 (Fig. 5a), the binding energy (BE)
0
corresponding to Ir in the orbital 4 f7/2 was 60.1 eV and the deconvo-
luted peak that indicated the presence of Ir3 in the orbital 4 f7/2 had a
+
BE of 61.7 eV. After adding Re to Ir/H-ZSM-5 catalyst (Fig. 5b), there
was a positive shift of the BE on Ir-ReO
tronic interaction between Ir and ReO
x
/H-ZSM-5, suggesting an elec-
x
on the support surface. This
n+
implied that the electrons could transfer from the Ir species to Re to
stabilize ReO
in the catalytic activity has resulted from this stronger interaction be-
tween Ir and ReO
The existence of ReO
x
at intermediate valence [32]. Therefore, the improvement
x
.
x
in the Ir-ReO
x
/H-ZSM-5 (Re/Ir = 1) catalyst
was confirmed by the XPS measurement in the Re 4 f region, as shown in
Fig. 5c. three deconvoluted peaks were found at 40.3, 41.8, and 46.0 eV,
0
3+
7+
corresponding to the Re 4 f7/2 BE values for Re , Re , and Re ,
0
3+
7+
respectively. The peak area ratio of Re /Re /Re was 0.81/0.27/1.00.
The results indicated that some amounts of the Re species were not fully
reduced due to the strong oxophilicity of Re [46] and considering the Re
7
+
(
Fig. 4a and b, respectively), but the distributions of both Ir and Re
species were substantially improved (more well dispersed) for the
bimetallic Ir-ReO catalyst (Fig. 4c). This was confirmed by the deter-
mination of the Ir dispersion in the Ir-ReO /H-ZSM-5 catalyst, 48%
TEM = 1.9 ± 0.2 nm), compared to the Ir dispersion in the Ir/H-ZSM-5
catalyst at 29% (dTEM = 3.2 ± 0.2). In Table 3, a smaller atomic ratio of
species in the catalyst, the amount of Re species was the greatest,
followed by the Re0 and Re species, respectively. It should be noted
that the catalysts might be exposed to the air during the sample prepa-
ration before the XPS measurement and thus, the metal components
(especially for the iridium species) might be partially oxidized. There
seems to be no appreciable difference, however, between the chemical
3+
x
x
(
d
Re/Ir = 0.62 was obtained from the Ir-ReO
x
/H-ZSM-5 (Re/Ir = 1)
properties of those air-exposed catalysts and the catalysts after
0
catalyst relative to the expected atomic ratio (Re/Ir = 1). This was
possible that there was a loss of Re species during the catalyst prepa-
ration, especially when the particle size of Re was small (see Fig. 3d),
H
2
-reduction. The Ir form can be observed in both, and the rhenium
species are normally in the form of oxides under the reduced conditions
[32,37,46–49].
which could have been because of the sublimation of ReO
x
at the
◦
calcination temperature (500 C) [29].
3
3
.3. Effect of operating conditions
3
.2.4. XPS analysis
The oxidation state of Ir and the interaction between Ir and ReO
were investigated using XPS, as shown in Fig. 5. The deconvolution of
the Ir 4 f peaks of the Ir/H-ZSM-5 and Ir-ReO /H-ZSM-5 catalysts
.3.1. Effect of reaction temperature
x
The effect of temperature on glycerol hydrogenolysis was investi-
gated and is summarized in Table 4. Almost no activity was found at
x
◦
◦
◦
1
80 C, but after increasing the temperature from 200 to 240 C, the
0
3+
revealed the presence of two iridium species—Ir and Ir —on the
glycerol conversion of the catalysts increased from 5.5% to 9.0% with
5

S. Chanklang et al.
Catalysis Today xxx (xxxx) xxx
x x
Fig. 4. TEM with EDX of a) Ir/H-ZSM-5, b) ReO /H-ZSM-5, and c) Ir-ReO /H-
ZSM-5 (Re/Ir = 1), where inserts show the elemental distribution of Ir or Re.
Table 3
Elemental composition of prepared catalysts obtained from ICP-OES.
Catalysts
Weight (%)
Ir
Re/Ir (actual)
Re/Ir (nominal)
Re
Ir/H-ZSM-5
5.07
5.40
–
–
3.36
5.40
–
–
1
–
Ir-Re/H-ZSM-5
0.62
ReO
x
/H-ZSM-5
–
the decrease of 1,3-PrD selectivity from 30% to 14%. However, the 1,3-
PrD yield increased with the increase in the reaction temperature and
Fig. 5. Ir 4 f XPS spectra of a) Ir/H-ZSM-5, b) Ir-ReO
and c) Re 4 f spectra of Ir-ReO
/H-ZSM-5 (Re/Ir = 1).
x
/H-ZSM-5 (Re/Ir = 1),
◦
◦
x
reached its peak at 220 C. Further increasing the temperature to 240 C
resulted in a decline in the 1,3-PrD yield due to over-hydrogenolysis, as
can be seen from the higher selectivity of 1-PO.
6

S. Chanklang et al.
Catalysis Today xxx (xxxx) xxx
Table 4
Glycerol hydrogenolysis over Ir-ReO
x
/H-ZSM-5 catalyst at different temperatures. Reaction conditions: Re/Ir = 1, 4 MPa, and 4 h.
◦
Temperature ( C)
Conversion (%)
Selectivity (%)
1,3-PrD Yield (%)
1
,3-PrD
1,2-PrD
1-PO
Others
1
2
2
2
80
00
20
40
0.5 ± 0.0
5.5 ± 0.1
8.9 ± 0.3
9.0 ± 0.0
41 ± 0.6
30 ± 0.1
23 ± 0.6
14 ± 0.3
48 ± 0.3
13 ± 0.1
13 ± 0.2
16 ± 0.3
9 ± 0.3
2 ± <0.1
1 ± <0.1
1 ± <0.1
1 ± <0.1
0.2
1.6
2.0
1.2
56 ± 0.3
63 ± 0.9
69 ± 0.7
Table 5
Glycerol hydrogenolysis over Ir-ReO
x
/H-ZSM-5 catalyst at different pressures. Reaction conditions: Re/Ir = 1, 220 ºC, and 4 h.
Pressure (MPa)
Conversion (%)
Selectivity (%)
1,3-PrD Yield (%)
1
,3-PrD
1,2-PrD
1-PO
Others
2
4
6
8
8.3 ± 0.1
8.9 ± 0.2
9.4 ± 0.3
9.6 ± 0.0
14 ± 0.2
23 ± 0.6
27 ± 0.1
35 ± 0.2
20 ± 0.5
13 ± 0.2
11 ± 0.0
9 ± 0.0
65 ± 0.4
63 ± 0.9
61 ± 0.1
54 ± 0.3
2 ± <0.1
1 ± <0.1
2 ± <0.1
2 ± <0.1
1.6
2.0
2.2
2.7
increased the 1,3-PrD selectivity from 14% to 35%, while the selectiv-
ities of 1,2-PrD and 1-PO decreased. The 1,3-PrD yield benefited from
the increase in pressure and produced the highest 1,3-PrD yield
observed (2.7%) at 8 MPa. This resulted from the increasing pressure
Table 6
Glycerol hydrogenolysis over Ir-ReO
x
/H-ZSM-5 catalyst for different reaction
times. Reaction conditions: 220 ºC, 4 MPa, and Re/Ir = 1.
Time
Conversion
Selectivity (%)
1,3-
PrD
2
resulting in higher solubility of the H gas into the liquid media and thus
(
h)
(%)
the hydrogenolysis of glycerol was enhanced [50].
Yield
(
%)
1
,3-PrD
1,2-PrD
1-PO
Others
3.3.3. Effect of reaction time
The effect of the reaction time in the glycerol hydrogenolysis reac-
tion was investigated for 1–8 h, as shown in Table 6. The results sug-
gested that the glycerol conversion and the 1,3-PrD yield increased with
the extended reaction time from 1 to 8 h. Nevertheless, increasing the
reaction time over 4 h the 1,3-PrD selectivity did not increase because of
the over-hydrogenolysis of 1,3-PrD to 1-PO [29].
1
2
4
8
4.9 ± 0.1
8.5 ± 0.2
9.5 ± 0.0
14.9 ± 0.5
26 ± 0.4
23 ± 0.2
21 ± 0.5
19 ± 0.5
18 ± 0.2
14 ± 0.4
12 ± 0.1
11 ± 0.3
55 ± 0.7
62 ± 0.6
66 ± 0.5
68 ± 0.8
2 ± <0.1
1 ± <0.1
1 ± <0.1
2 ± <0.1
1.0
1.7
2.0
2.8
3
.3.2. Effect of reaction pressure
The effect of pressure on glycerol hydrogenolysis was also examined,
as shown in Table 5. Increasing the reaction pressure from 2 to 8 MPa
not only increased the glycerol conversion from 8.3% to 9.6% but also
Scheme 1. Proposed reaction mechanism of Ir-ReO
x
/H-ZSM-5 catalyst in hydrogenolysis of glycerol to 1,3-propanediol.
7

S. Chanklang et al.
Catalysis Today xxx (xxxx) xxx
Table 7
glycerol. As also shown in Table 7, mild reaction temperatures
◦
Ir-based or Re-based glycerol hydrogenolysis catalysts.
(120–180 C) are commonly used since these conditions can prevent the
over-hydrogenolysis of products. Furthermore, high pressures
a
Catalysts
Composition
Conditions
P (H
P
1,3-PrD
Ref.
1
Ir wt
Re/Ir
T
◦
2
)
(g g
metal–
(4–8 MPa) of H
dispersion of H
2
2
gas are preferably used because they produce a higher
and more H dissolved in the liquid phase can be ob-
b
–1
%
molar
ratio
( C)
(MPa)
h
)
2
tained, which is beneficial for the hydrogenolysis of glycerol. Comparing
our results with the others, the performance of the present catalyst is
relatively low, possibly because of the lower-than-desired amount of
Ir-ReO
x
/H-ZSM-5
4
1
220
160
4
8
3.1
This
work
[52]
Ru-Re/SiO
2
(Ru)
4.54
(Re/
0.7
1.0
0.4
0.5
0.9
2.8
x x x
ReO in the Ir-ReO /H-ZSM-5 catalyst due to the sublimation of ReO at
Ru-Re/ZrO
2
Ru) 1
high calcination temperature [29,32]. Thus, further studies to improve
the catalytic activity with the use of mild operating conditions are still
challenging.
Ru-Re/Hß
Ru-Re/H-ZSM-5
Ru-Re/TiO
Ir-ReO /SiO
SO
Rh-ReO /SiO
2
x
2
4
1
120
120
8
8
[28]
[15]
+
H
2
4
4
. Conclusion
The Ir-ReO
x
2
(Rh)
(Re/
8.1
4
2
4
Rh) 0.5
IrO
Ir-ReO
ZSM-5
Ir-Re/KIT-6-R
Amberlyst-15
x
/H-ZSM-5_100
–
1
180
120
8
8
4.0
3.0
[40]
[36]
/H-ZSM-5 catalyst with Re/Ir = 1 and with a 4 wt%
x
x
/SiO
2
+ H-
metal loading was the most active in glycerol hydrogenolysis. The XRD
and TEM results suggested that the Ir species were more dispersed on the
H-ZSM-5 surface after the addition of Re. The electronic interaction
4
1
120
120
8
4
5.1
[46]
[53]
+
Pt-Re/SiO
Pt-Re/SiO
Pt-Re/C
2
(Pt) 8
(Re/Pt)
1
0.7
0.6
0.2
4.9
x
between Ir and Re over the bimetallic Ir-ReO catalyst was observed over
2
-W
the XPS spectra, resulting in the enhancement of the glycerol hydro-
genolysis to 1,3-PrD. In the study on the effects of operating conditions,
increasing the reaction temperature and time promoted the formation of
1,3-PrD, whereas increasing the pressure slightly increased the 1,3-PrD
yield.
Ir-Re/D-ASA-
4
1
120
8
[54]
2
1
.0 + Amberlyst-
5
Ru-Ir-ReO
x
/SiO
2
4
2
200
120
150
8
8
4
5.4
[51]
[32]
[23]
Ir-ReO /Rutile
x
4
0.25
(W/Pt)
13.2
1.5
Pt/W–SBA-15
(Pt) 3
CRediT authorship contribution statement
0
.16
a
3
-PrD productivity calculated using Eq. (4);
Sarun Chanklang: Methodology, Investigation, Validation, Formal
analysis, Data curation, Writing – original draft. Wongsaphat Mon-
dach: Methodology, Investigation, Formal analysis, Data curation.
Pooripong Somchuea: Methodology, Investigation. Thongthai
Witoon: Resources, Supervision. Metta Chareonpanich: Resources,
Supervision. Kajornsak Faungnawakij: Resources, Supervision. Anu-
sorn Seubsai: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Resources, Data curation, Writing – original
draft, Writing – review & editing, Supervision, Project administration.
b
Nominal weight loadings.
3
.4. Proposed reaction mechanism
The analyses of catalyst characterization in Section 3.2 identified
several important findings that can be summarized as follows. 1) The Ir-
ReO . 2) The Ir and ReO
/H-ZSM-5 catalyst consisted of Ir0 and ReO
particles of the Ir-ReO /H-ZSM-5 catalyst were well mixed, highly
x
x
x
x
dispersed on the support, and relatively much smaller compared to the
single-metal catalysts of its components. 3) There was an electronic
Declaration of Competing Interest
x
interaction between Ir and ReO that enhanced the reaction. From these
findings, together with the proposed mechanisms for the hydrogenolysis
of glycerol to propanediols reported in the literature [28–32,36–38,51],
a proposed mechanism for the hydrogenolysis of glycerol to 1,3-PrD can
be made as shown in Scheme 1. As illustrated, glycerol is first adsorbed
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
on the surface of an ReO
form 2,3-dihydroxtpropoxide. Simultaneously, H
metallic Ir to form hydride species. Subsequently, the C—O bond at the
-position of the 2,3-dihydroxtpropoxide is attacked by hydride species
x
cluster, preferentially at a terminal position, to
Acknowledgments
2
is activated on the
This research work was funded by the Kasetsart University Research
and Development Institute (KURDI), Bangkok, Thailand; the Faculty of
Engineering at Kasetsart University; the National Nanotechnology
Center (NANOTEC), NSTDA, Ministry of Science and Technology,
Thailand, through its Research Network program NANOTEC (RNN); and
the Center of Excellence on Petrochemical and Materials Technology,
Thailand.
2
to produce 3-hydroxypropoxide. Finally, the hydrolysis of 3-hydroxy-
propoxide releases 1,3-PrD as a product. Note that 1,2-PrD can also be
produced if the hydride attacks the 3-position of 2,3-dihydroxypropox-
ide [29,30,32,37]. However, the formation of 1,2-PrD is less favored
in this catalytic system because of the unfavorable geometric structure
during the hydride attack [28].
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