Highly dispersed Pd catalysts supported on various carbons for furfural hydrogenation

Article abstract of DOI:10.1016/j.cattod.2019.06.032

Furfural (FAL), one of the important platform molecules derived from lignocellulosic biomass, can be converted into valuable chemicals such as furfuryl alcohol or cyclopentanone via hydrogenation. While carbon materials have been used as versatile catalyst supports for FAL hydrogenation, systematic studies on the structure of the catalytic performances are lacking. In this work, we prepare various types of carbon supports to investigate the impact of carbon structures for Pd-catalyzed FAL hydrogenation. Mesoporous carbons, including CMK-3, CMK-5, CMK-8, and MSU-F-C, as well as carbon nanotube and Vulcan XC are used as carbon supports. For the preparation of highly dispersed Pd-supported carbon (Pd/C) catalysts, chemical reduction by sodium borohydride is applied, in which trisodium citrate plays a critical role in anchoring small Pd clusters on the carbons. In the liquid-phase hydrogenation of FAL, CMK-5 with the largest surface area and hexagonal hollow tubular framework is proven to be the most efficient carbon support for Pd/C catalysts, with the highest conversion of FAL in both 2-propanol (100percent) and water (86.4percent) solvents. It is also demonstrated that the product selectivity in FAL hydrogenation over various Pd/C catalysts is changed dramatically depending on the type of solvent. The Pd/C catalysts exhibit similar fractions of product distributions containing furfuryl alcohol, cyclopentanol, tetrahydrofurfuryl alcohol, and minor products in 2-propanol. However, the production of cyclopentanone is increased when water is used as a solvent.

Full text of DOI:10.1016/j.cattod.2019.06.032

Accepted Manuscript
Title: Highly Dispersed Pd Catalysts Supported on Various
Carbons for Furfural Hydrogenation
Authors: Jihyeon Lee, Jinwoo Woo, Chinh Nguyen-Huy, Man
Sig Lee, Sang Hoon Joo, Kwangjin An
PII:
S0920-5861(19)30313-X
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.06.032
CATTOD 12279
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
1 February 2019
31 May 2019
10 June 2019
Please cite this article as: Lee J, Woo J, Nguyen-Huy C, Lee MS, Joo SH, An K, Highly
Dispersed Pd Catalysts Supported on Various Carbons for Furfural Hydrogenation,
Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.06.032
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Highly Dispersed Pd Catalysts Supported on Various Carbons for
Furfural Hydrogenation
a
a
a
b
*a
Jihyeon Lee, Jinwoo Woo, Chinh Nguyen-Huy, Man Sig Lee, Sang Hoon Joo , and
Kwangjin An *,a
aSchool of Energy and Chemical Engineering, Ulsan National Institute of Science and
Technology (UNIST), Ulsan 44919, Republic of Korea
bKorea Institute of Industrial Technology (KITECH), Ulsan 44413, Republic of Korea
*Corresponding authors at: School of Energy and Chemical Engineering, Ulsan National
Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of
Korea.
*
E-mail: shjoo@unist.ac.kr (S.H. Joo) and kjan@unist.ac.kr (K. An)
Graphical abstract
1

2

Highlights




Various carbon supports are used for highly dispersed Pd-based catalysts.
Catalytic properties are compared in Pd-catalyzed furfural (FAL) hydrogenation.
Pd/CMK-5 exhibited the highest FAL conversion.
Ordered channels of CMK-5 enable high Pd dispersion and fast adsorption/diffusion.
Abstract
Furfural (FAL), one of the important platform molecules derived from lignocellulosic
biomass, can be converted into valuable chemicals such as furfuryl alcohol or cyclopentanone
via hydrogenation. While carbon materials have been used as versatile catalyst supports for
FAL hydrogenation, systematic studies on the structure of the catalytic performances are
lacking. In this work, we prepare various types of carbon supports to investigate the impact of
carbon structures for Pd-catalyzed FAL hydrogenation. Mesoporous carbons, including
CMK-3, CMK-5, CMK-8, and MSU-F-C, as well as carbon nanotube and Vulcan XC are
used as carbon supports. For the preparation of highly dispersed Pd-supported carbon (Pd/C)
catalysts, chemical reduction by sodium borohydride is applied, in which trisodium citrate
plays a critical role in anchoring small Pd clusters on the carbons. In the liquid-phase
hydrogenation of FAL, CMK-5 with the largest surface area and hexagonal hollow tubular
framework is proven to be the most efficient carbon support for Pd/C catalysts, with the
highest conversion of FAL in both 2-propanol (100%) and water (86.4%) solvents. It is also
demonstrated that the product selectivity in FAL hydrogenation over various Pd/C catalysts is
changed dramatically depending on the type of solvent. The Pd/C catalysts exhibit similar
3

fractions of product distributions containing furfuryl alcohol, cyclopentanol,
tetrahydrofurfuryl alcohol, and minor products in 2-propanol. However, the production of
cyclopentanone is increased when water is used as a solvent.
Abbreviations: furfural (FAL); furfuryl alcohol (FA); tetrahydrofurfuryl alcohol (THFA); 2-
methylfuran (MF); furan (FN); tetrahydrofuran (THF); cyclopentanone (CPO); Pd-supported
carbon (Pd/C); ordered mesoporous carbons (OMCs); carbon nanotube (CNT); aluminum-
substituted mesoporous SBA-15 (Al-SBA-15); gas chromatograph (GC); flame ionization
detector (FID); Brunauer-Emmett-Teller (BET); transmission electron microscopy (TEM);
high-resolution TEM (HRTEM); high-angle annular dark-field imaging (HAADF); scanning
TEM (STEM); inductively coupled plasma atomic emission spectroscopy (ICP-AES);
cyclopentanol (CPL); X-ray photoelectric spectroscopy (XPS); X-ray diffraction (XRD); 4-
hydroxy-2-cyclopentenone (4-HCP); cyclopentenone (CPEO).
Keywords: Furfural Hydrogenation, Pd, Mesoporous Carbon, Selectivity, Nanocasting
4

1
. Introduction
Owing to the limited amount of fossil resources and the environmental problems associated
with their usage, the need for alternative energy resource has been continuously increasing
1-4]. Biomass as a promising renewable resource has many advantages. For example, the use
of biomass decreases greenhouse gas emissions relative to the combustion of fossil fuels,
because the CO released during the energy conversion is reused for the regrowth of biomass.
[
2
However, biomass-derived carbohydrates contain various functional groups, which require
complicated upgrading steps for utilization as fuels and chemicals. To simplify the chemical
conversion for upgrading, an efficient process involving high-performance catalysts with
high selectivity should be developed to control the functionality of the final products. [5-7].
For bio-upgrading, furfural (FAL) is considered as a significant platform molecule that is
primarily produced from the acid-catalyzed hydrolysis and dehydration of hemicellulose [8].
Through optimization of reaction conditions as well as a proper choice of catalysts, FAL is
converted into diverse small molecules depending on the reaction pathway [9-12].
Hydrogenation of FAL not only produces the hydrogenated molecules but also leads to
modified products via rearrangement, ring-opening, and decarbonylation (Scheme 1) [13, 14].
Each product derived from FAL hydrogenation has several industrial applications. Furfuryl
alcohol (FA), which is produced through the selective hydrogenation of the aldehyde group
of FAL, is used for resin production, as well as for the extraction agent of the aromatic
compounds of lubricants [15]. Tetrahydrofurfuryl alcohol (THFA), a fully hydrogenated
product from FAL, is a green solvent owing to its biodegradability [16]. 2-methylfuran (MF),
produced via the hydrodeoxygenation of FAL, serves as a potential fuel additive [17]. Furan
(FN) and tetrahydrofuran (THF) produced during the decarbonylation step in FAL
hydrogenation, are used as a fuel additive and solvent for dissolving biomass molecules,
5

respectively [18]. Cyclopentanone (CPO), produced by the rearrangement of FAL, is
extensively used as an ingredient of perfume, pesticides, and medicines [19, 20].
Numerous investigations have been performed for the selective hydrogenation of FAL over
supported metal catalysts including Ni [21, 22], Ru [23], Cu [24, 25], and Pd [26]. Based on
the previous studies, Ni favors decarbonylation and ring-opening in FAL hydrogenation [27].
On the other hand, a Cu catalyst exhibits good selectivity towards FA due to the preference
for the binding of C=O onto its surface [28]. Pd as a traditional hydrogenation catalyst
dissociates hydrogen even under ambient conditions [29-31]. To improve the activity and
selectivity, a proper choice of the support is important, because the interactions between the
active metal and support dramatically change the catalytic performance [32-34]. In a previous
study, we found that the Pd-supported carbon (Pd/C) catalyst has a higher activity in FAL
hydrogenation than the other catalysts (Pd/Al
2
O
3
and Pd/SiO ), because the carbon support
2
induced a higher dispersion of Pd than alumina or silica [35]. For example, when 5 wt% Pd
was loaded on carbon, the Pd dispersion measured by CO adsorption was 51.2%, exhibiting
1
3
00% FAL conversion, while the Pd/Al
2
O
3
and Pd/SiO catalysts showed Pd dispersions of
2
1.7% and 6.6%, with FAL conversions of 92% and 31%, respectively.
Because of its extraordinary physical, chemical, and mechanical properties along with high
thermal and electrical conductivities, carbon has been widely used as an excellent support in
many catalytic applications [36-39]. Furthermore, recent emergence of ordered mesoporous
carbons (OMCs) further provides a new opportunity for generating porous carbons with high
surface areas and tailored physicochemical properties [40-42]. As such, plenty of OMCs
including CMK (Carbon Mesostructured by KAIST) carbon series [43] have been
synthesized with high surface areas, large pore volumes, and tunable pore sizes and pore
structures via the nanocasting method [44, 45]. By filling the pores of the mesoporous silica
template with a carbon source to be cast, a negative replica of the carbon, preserving the
6

highly ordered structures, is successfully obtained after carbonization and subsequent silica
etching [46]. The mesoporous silica templates used for the nanocasting are SBA-15 (Santa
Barbara Amorphous-15), KIT-6 (Korea Institute of Technology-6), and so on.
The OMCs can serve as model systems to study the rate of mass transfer of the reactants and
the corresponding reaction phenomena as a function of the pore size and pore structure. In
particular, the mass-transport phenomena are important factors in a liquid-phase reaction,
whereas a gas-phase reaction is mostly affected by the adsorption behavior of the gaseous
reactants on the solid metal [47]. The fast adsorption/desorption of the reactants/products, as
well as the diffusion of intermediates, should be considered together with the effect of the
solvent in liquid-phase reactions [48].
In this work, we prepare Pd/C catalysts with various carbons to investigate the supporting
effects of carbon in Pd-catalyzed FAL hydrogenation. Various OMCs with different pore
sizes and structures, including CMK-3, CMK-5, CMK-8, and MSU-F-C (MSU; Michigan
State University), are used as the carbon supports. Multiwalled carbon nanotube (CNTs) and
a commercial carbon (Vulcan XC) are also used for comparison. For the preparation of Pd/C
catalysts with high dispersions, chemical reduction by sodium borohydride is conducted in
the presence of trisodium citrate, which helps to anchor small Pd clusters on carbons. The Pd
dispersion in the Pd/C catalysts is determined by CO adsorption. Liquid-phase FAL
hydrogenation is carried out over various Pd/C catalysts in two different solvents (2-propanol
and water). Pd/CMK-5 exhibits the highest conversion of FAL in both 2-propanol and water
solvents among Pd/C catalysts, which is attributed to the largest surface area and hexagonal
hollow tubular framework of CMK-5 support. For catalytic selectivity of FAL hydrogenation,
the choice of solvent exerts a significant impact, while the type of carbon support has a
negligible influence.
7

2
. Experimental
2
.1. Synthesis of mesoporous carbons
OMCs are synthesized via the nanocasting method using mesoporous silica templates (SBA-
5, KIT-6, and MSU-F) [49-51]. The distinct pore structures of the mesoporous silicas
1
determine the final mesostructure of the OMCs. In the current study, CMK-3, CMK-5, CMK-
, and MSU-F-C were prepared by using different mesoporous silica templates (SBA-15,
8
KIT-6, and MSU-F), as well as different carbon sources. When mesoporous silicas SBA-15
with p6mm symmetry and KIT-6 with Ia3d symmetry were used as the templates, the
mesoporous carbons CMK-3 and CMK-8 were produced, respectively. In this process, 4 g of
mesoporous silica (SBA-15 or KIT-6) was completely mixed with 5.5 g of sucrose
(
C
12
H
22
O11) in 20 g of distilled water containing 0.62 g of sulfuric acid (H
2
SO ). The mixture
4
was cured in an oven at 100 ℃for 6 h and at 160 ℃for another 2 h. The above procedure was
repeated with 3.3 g of sucrose for the complete infiltration of the carbons. The dried powder
was carbonized under 1.0 L/min N (or Ar) flow at 400 ℃for 4 h and at 900 ℃for another 2
2
h in the oven. To remove the silica template from the carbon-silica composite, 10% hydrogen
fluoride (HF, J. T. Baker, 48-51 %) and ethanol (1:1) were mixed with the composite for 30
min. The composite mixture was filtrated with ethanol several times and the etching
procedure was repeated. After washing and drying samples, CMK-3 and CMK-8 were
obtained with the p6mm and Ia3d space groups, respectively.
A mesocellular carbon foam, MSU-F-C, was prepared by a similar method as that described
above. 4 g of the mesoporous silica MSU-F was dispersed in 20 g of distilled water
containing 0.62 g of sulfuric acid. Then, 5.5 g of sucrose was added to the mixture to fill the
pores of MSU-F, and the solution was placed in an oven at 100 ℃for 6 h and at 160 ℃for 2
h. The washed and dried powder after filtration was further heated for 4 h at 400 ℃and 2 h at
9
00 ℃to carbonize the sucrose inside the template.
8

For the preparation of CMK-5, aluminum-substituted mesoporous SBA-15 (Al-SBA-15)
was used as the template. In this process, 3.0 g of SBA-15 were treated with 0.33 g of AlCl
3
(
Sigma Aldrich, 98 %) in 59.20 g of ethanol. The resulting Al-SBA-15 was obtained after
removing the solvent in a rotary evaporator at 60 ℃and subsequent calcination at 550 ℃for 5
h. The as-prepared Al-SBA-15 (3.05 g) was mixed with 2.75 g of FA (C , Sigma
Aldrich, 98%) as a carbon source and dried at 90 ℃for 24 h. After carbonization at 350 ℃for
5
H
6
O
2
3
5
h and at 900 ℃for 6 h, and subsequent silica etching with 10 % HF in ethanol (1:1), CMK-
with hexagonally ordered tubular structures was obtained.
2
.2. Preparation of Pd/C catalysts
Pd/C catalysts with various carbon supports were prepared through the chemical reduction
method by using sodium borohydride as a reducing agent (Scheme 2) [52]. H
PdCl , Sigma Aldrich 90 % in 0.1 M HCl) with the calculated amount of Pd depending on the
loading, was dissolved in 100 ml of a trisodium citrate (2.5 mM, Na , Sigma Aldrich,
8+ %) solution under vigorous stirring (~500 rpm). After slowing the stirring rate (350 rpm),
g of the carbon support was added to the Pd precursor solution, in which the carbon-metal
2
PdCl
4
(0.1 M
2
3
C
6
H
5
O
7
9
1
precursor mixture became a black slurry. After stirring for 2 h, 75 ml of a freshly prepared
ice-cold sodium borohydride (0.06 M, NaBH , Sigma Aldrich 98%) solution was added
4
dropwise to the slurry and stirred for 5 h at room temperature. The solution was filtered and
washed with deionized water, and dried at 105 ℃overnight in an oven. Through these
procedures, Pd supported on mesoporous carbons, Pd/CMK-3, Pd/CMK-5, Pd/CMK-8, and
Pd/MSU-F-C were prepared. For comparisons, CNTs (MR99, Carbon Nano-material
Technology Co.) and commercial carbon (Vulcan, Cabot Co.) were used for the Pd/C
catalysts (Pd/CNT and Pd/Vulcan). The prepared Pd/C catalysts were reduced under a flow of
H
2
(4 % H in Ar) at 250 ℃for 2 h.
2
9

2
.3. Catalytic FAL hydrogenation over Pd/C catalysts
Catalytic hydrogenation of FAL was conducted in a 100 ml stainless steel autoclave reactor
equipped with a mechanical stirrer (Parr reactor). In a typical procedure, 0.01 g of the Pd/C
catalyst was added to the reactor containing 1.0 g of FAL (C
0 ml of 2-propanol ((CH CHOH, Alfa Aesar, anhydrous, 99.5+%). The closed reactor was
then purged with H gas to remove the dissolved air, and pressurized to 2 MPa. The reaction
5
H
4
2
O , Sigma Aldrich, 99%) and
2
)
3 2
2
temperature was increased to 180 ℃and maintained there for 5 h under stirring at 600 rpm.
After the completion of the reaction, the resulting solutions were filtrated with a syringe filter
with an average channel size of 0.2 µm.
Aqueous FAL hydrogenation was performed using the same procedure in water, instead of
2
-propanol. In this reaction, 0.02 g of the catalyst was used with 1.0 g of FAL in 20 ml of
deionized water in a 100 ml batch reactor. After flushing three times with pure hydrogen, the
sample was heated to 160 ℃under 3 MPa of H gas pressure and 600 rpm stirring for 5 h. For
2
analysis of the products in water, 2-propanol was added as the external standard.
The products were identified and quantified using a gas chromatograph (GC, Agilent
7
820A) equipped with an Agilent DB-WAX column and a flame ionization detector (FID).
The conversion, selectivity, and yield were calculated using the equations shown below. The
carbon balance was generally greater than 94% in all FAL reactions.
;
and
.
2
.4. Characterization
1
0

The surface area, pore volume, and average pore diameter of the Pd/C catalysts were
characterized by N physisorption in a BEL SORP-max unit, and the physical values were
2
converted by the Brunauer-Emmett-Teller (BET) method. The size distribution of the
mesopores was calculated using the Barrett-Joyner-Halenda method from the adsorption
isotherms. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM)
images of the Pd/C catalysts were obtained using JEOL JEM-1400 and JEOL JEM-2100F,
respectively. The morphology and Pd metal dispersion of the Pd/C catalysts were confirmed
by TEM and HRTEM, along with the dark-field image of scanning TEM (STEM), high-angle
annular dark-field imaging (HAADF), and EDS mapping.
The content of Pd in the catalyst was analyzed using inductively coupled plasma atomic
emission spectroscopy (ICP-AES, Varian 700-ES). Before the ICP-AES measurement, the
concentration of Pd was calibrated using a standard Pd solution (Sigma Aldrich,
TraceCERT®), and the catalyst dissolved in aqua regia was measured after dilution in water.
The Pd dispersion was evaluated on the basis of the CO adsorption capacity using a pulse
technique involving CO chemisorption (AutoChem II 2920; Micromeritics Instruments Co.).
Through ICP-AES and CO chemisorption, the metallic surfaces of the Pd catalysts were
confirmed. X-ray photoelectric spectroscopy (XPS) analysis was carried out on a K-alpha
(ThermoFisher) equipped with an Al Kα X-ray radiation source. All the catalysts were
reduced by hydrogen (4% H in Ar) immediately before the measurements. The binding
2
energy of adventitious carbon contamination, C (1s), was used as the reference for the
binding energies of Pd (3d). The X-ray diffraction (XRD) patterns were recorded on a
PANalytical X’Pert PRO using a Cu Kα X-ray radiation source operating at 40 kV and 40
mA.
3
. Result and Discussion
1
1

3
.1. Characterization of OMCs and Pd/C catalysts
OMCs are synthesized through the nanocasting method using various mesoporous silicas as
the hard templates (Scheme 2). After filling a carbon source into the mesoporous silica
template in solution, the infiltrated carbon precursor is carbonized, resulting in the OMC
replica after silica etching by a HF solution. When the mesoporous silicas SBA-15, KIT-6,
and MSU-F are used as templates, with sucrose as the carbon source, CMK-3, CMK-8, and
MSU-F-C are produced, respectively. When Al-SBA-15 is filled by FA instead of sucrose as
the carbon source, CMK-5 with hexagonally ordered tubular structures is obtained. The
morphologies, pore structures, and space groups of the OMCs are summarized in Scheme 3.
Pd/C catalysts are prepared through the chemical reduction of aqueous H
borohydride in the presence of OMCs. The physicochemical properties of the OMCs and
corresponding Pd/C catalysts are determined by XRD, N physisorption, TEM, HR-TEM, CO
2
PdCl
4
by sodium
2
chemisorption, and XPS analyses. Low-angle XRD patterns reveal the structural order of the
mesoporous carbons CMK-3, CMK-5, and CMK-8 (Fig. 1). CMK-3 and CMK-5 show three
distinct peaks, indexed as (100), (110), and (200) in the XRD patterns (Fig. 1a),
demonstrating that CMK-3 and CMK-5 are replica of the SBA-15 template with hexagonal
structure of p6mm space group. The (100) XRD peak of CMK-5 has lower intensity than
those of SBA-15 and CMK-3 because of the interference of the diffraction originating from
the hollow structure of CMK-5. CMK-8, a replica of KIT-6, has a three-dimensional
interconnected cubic structure with Ia3d space group, which is confirmed by XRD (Fig. 1b).
However, MSU-F-C exhibits a featureless low-angle XRD pattern due to its poorly ordered
mesostructure, which is in good agreement with the mesocellular carbon foam structure of the
MCF-17 template (Fig. 1c).
The structural properties of the OMCs after Pd loading are investigated by N
2
adsorption-
desorption isotherms and TEM. For comparison, multiwalled CNTs and Vulcan XC, are also
1
2

used as carbon supports for the Pd/C catalysts. In N sorption analyses (Supplementary
2
Material Fig. S1), the Pd-supported OMCs show the type-IV isotherms with pronounced
capillary condensation steps, indicating that the mesopore structure of the OMCs is preserved
after Pd loading. The surface areas determined by the BET method indicate that the Pd-
2
-1
supported OMCs (>1,000 m ‧ g ) have higher surface areas than those of the commercial
2
-1
CNT and Vulcan catalysts (<260 m ‧ g ). As listed in Table 1, CMK-5 has the highest
2
-1
surface area (1,424 m ‧ g ) owing to its distinct hollow two-dimensional hexagonal
structure. Pd/OMC catalysts show sharp and narrow pore size distributions because of their
unique pore structures (Fig. S2). The peak pore widths of Pd/CMK-3, Pd/CMK-5, and
Pd/CMK-8 measured by the BJH distribution curve are 5.5, 4.9, and 3.8 nm, respectively.
TEM images further confirm the textural properties of the Pd/C catalysts (Fig. 2). Fig. 2a–c
show that CMK-3, CMK-5, and CMK-8 have highly ordered mesoporous structures with
two-dimensional hexagonal channels, hollow hexagonal tubes, and three-dimensional cubic
structures, respectively. As shown in Fig. 2d, MSU-F-C has a foam-like amorphous
mesoporous structure. Fig. 2e and 2f show CNTs with a representative tubular shape and
oval-shaped Vulcan carbons, as illustrated in Scheme 3. Pd/CNT and Pd/MSU-F-C show
broad pore distribution curves, indicating that CNT and MSU-F-C carbons have random pore
distributions (Fig. S2).
The amount of Pd metal content in the Pd/C catalysts is measured by ICP-AES. The
resulting value is ca. 1 wt% Pd on carbon in all the Pd/C catalysts (Table 1). To determine the
Pd dispersion in Pd/C catalysts, CO chemisorption is used. The Pd surface area was
calculated by combining Pd contents determined by ICP-AES and CO chemisorption results.
As indicated in Table 1, the calculated Pd dispersion of Pd/C catalysts is 32‒42 %, in which
OMC supported catalysts show similar dispersion. Hence, the chemical reduction method
provides an efficient way for the preparation of Pd/C catalysts with high dispersion of Pd,
1
3

regardless of the effect of carbon surface area. In the chemical reduction step, trisodium
citrate plays a critical role in anchoring small Pd clusters on carbons [53]. The specific
interactions between H
2
PdCl and trisodium citrate facilitate the high dispersion of Pd on the
4
carbons. As shown in TEM images (Fig. 2), the Pd nanoparticles formed on the carbons are
too small to be identified. The size of Pd nanoparticles was also accessed by XRD patterns.
As shown in Fig. S3, none of the Pd/C catalysts reveals distinct Pd peaks, demonstrating that
Pd nanoparticles are homogeneously deposited on the carbons and that the size of Pd is very
extremely small. The HR-TEM and STEM-HAADF images of the Pd-supported OMC
catalysts (Pd/OMCs) prepared by using CMK-3, CMK-5, and CMK-8 also confirm that small
Pd nanoparticles (<2 nm) are homogeneously deposited over the OMCs (Fig. 3), regardless of
their morphologies.
The chemical states of the Pd nanoparticles in the Pd/C catalysts are investigated by XPS.
0
2+
As shown in the Pd 3d spectra (Fig. 4), the Pd species has two chemical states: Pd and Pd .
0
The peaks at 335.6 eV and 340.8 eV are assigned to the metallic Pd state, while the higher
2
+
peaks at 337.6 eV and 342.4 eV belong to Pd . The presence of PdO species in the catalyst is
attributed to the small size of Pd nanoparticles (~ 5 nm), which are easily oxidized due to the
high surface to volume ratio [54-56]. Based on the XPS result, all the Pd/C catalysts have a
0
2+
Pd /Pd ratio higher than 1.0, indicating that the Pd nanoparticles have a predominantly
0
metallic (Pd ) state.
3
.2. FAL hydrogenation over Pd/C catalysts
Liquid-phase hydrogenation of FAL is carried out over Pd/C catalysts to investigate the
effect of the morphology of carbons. Because the Pd/C catalysts are prepared with high Pd
dispersion regardless of the effect of carbon surface area, they are useful to investigate the
support effect of carbons with different morphologies. For FAL hydrogenation, 0.01 g of
1
4

Pd/C catalysts is used with 1.0 g of FAL in 20 ml of 2-propanol, and the reaction proceeds
under 2 MPa H at 180 ℃for 5 h. Under these conditions, FAL is converted via three major
2
reaction pathways to produce FN by decarbonylation, CPO via rearrangement, and FA via
aldehyde hydrogenation. FN, CPO, and FA are further hydrogenated to THF, cyclopentanol
(CPL), and THFA, respectively. MF is also produced as a final product by the hydration of
FAL (Scheme 1). The FAL conversion and product selectivity are summarized in Table 2 and
Fig. 5. The major products in FAL hydrogenation are FA and THFA. It was reported that an
alcohol solvent led to the reduction of the C=O group and/or the furan ring in FAL
hydrogenation [19]. While most of the reaction results show product selectivity similar to the
previous results [19], the overall FAL conversion reveals obvious differences among the
Pd/C catalysts. When OMCs are used as the support, Pd-catalyzed FAL hydrogenation
exhibits higher conversions (<60%) than those involving the Pd/CNT and Pd/Vulcan
catalysts (<50%). In particular, Pd/CMK-5 shows the highest conversion, up to 100%, while
considerable differences in product selectivity among the catalysts are not observed. The
major products involving the Pd/CMK-5 catalyst are FA, THFA, MF, and CPL, with
selectivities of 20.3, 31.5, 13.4, and 16.3%, respectively. FAL conversion over Pd/C catalysts
increases in the order: Pd/CMK-5 > Pd/CMK-3 > Pd/CMK-8 > Pd/MSU-F-C > Pd/Vulcan >
Pd/CNT. The major reason of high activity of Pd/CMK-5 is high dispersion and
corresponding the highly exposed Pd surface area (See Table 1). In our previous study on
FAL hydrogenation over Pd/C catalysts, the conversion further increased with increasing
reaction time and temperature [35]. With the same reaction condition, the conversion was
mainly determined by the degree of the Pd dispersion. In the current study, homogeneously
distributed Pd nanoparticles on various carbons have the same structure of fcc Pd with a
(111) facet (Fig. S4). HR-TEM and XPS reveal that the crystal structure and chemical states
of the Pd nanoparticles are not significantly different between Pd/C catalysts.
1
5

The morphological effect of the carbons can only be considered in terms of the
configuration of the carbon frameworks and the structure of the mesopores, because the Pd
has no structural and chemical difference. Comparing to the commercial carbons (CNTs and
Vulcan), Pd/OMCs (CMK-3, CMK-5, and CMK-8) have high FAL conversions, which are
ascribed to the well-ordered pore structures, having either p6mm or Ia3d space group and
2
-1
high surface areas. Although MSU-F-C has a high surface area (1163 m ‧ g ) and high Pd
dispersion (40.9 %), its carbon mesocellular foam-like structure results in less FAL
conversion compared to those of Pd/OMCs. The ordered carbon framework is responsible for
the activity of the Pd/C catalysts in FAL hydrogenation, based on the fact that faster mass
transport and absorption/diffusion processes are possible with the ordered mesoporous
structure having a narrow pore distribution [57, 58].
Before the comparison of the Pd/OMCs, it is significant to estimate the limitation of mass
transport in the OMCs during the reaction [57]. When the stirring speed is increased, there is
no significant change in the FAL hydrogenation reaction over Pd/OMCs, indicating that no
limitation on interphase transfer exists during the reaction [59]. In the previous report [59],
the computed intraparticle transfer number (i.e., Weisz-Prater number) was much less than
0
.3, indicating the absence of intraparticle diffusion limitations. Therefore, in the current
study, only internal transport limitations should be considered as a structural factor that affect
the different FAL conversions over Pd/OMCs. The lowest conversion of Pd/CMK-8 among
the three Pd/OMCs (Fig. 5) is attributed to the three-dimensional cubic mesoporous structure
(
Ia3d). While the two-dimensional hexagonally ordered mesoporous channels (p6mm) of the
Pd/CMK-5 and Pd/CMK-3 catalysts induce high mass transports owing to their
straightforward internal channels, the interconnected cubic pores (Ia3d) of Pd/CMK-8 permit
less internal diffusion compared to those of p6mm. Eventually, Pd/CMK-5 consisted of
1
6

hexagonal hollow tubular frameworks results in the highest FAL conversion (Table 2 and
Fig. 5).
3
.3. Selective FAL hydrogenation towards CPO
CPO is produced through FAL hydrogenation when water is used as a solvent; water
molecules facilitate the rearrangement of FAL [60]. Based on the knowledge of activity
enhancement as a function of the morphology of carbons, both Pd/CMK-3 and Pd/CMK-5
catalysts are selected for the selective FAL hydrogenation towards CPO in water under
modified reaction conditions. In accordance with the previous trends of FAL hydrogenation
in 2-propanol, Pd/CMK-5 (86.4%) shows a higher FAL conversion than Pd/CMK-3 (80.8%)
in water (Table 3 and Fig. 6a). However, the product distribution of aqueous FAL
hydrogenation is significantly different from the corresponding result for 2-propanol. In Fig.
6
b, CPO is a major product for both the catalysts, at the expense of FA. According to the
reported mechanism [61], the produced FA by FAL hydrogenation rearranged to produce 4-
hydroxy-2-cyclopentenone (4-HCP). The 4-HCP intermediate was further hydrogenated into
cyclopentenone (CPEO), and CPO is finally produced from hydrogenation of CPEO (Fig.
6
b). From these results, it is found that the product selectivity in FAL hydrogenation over
various Pd/C catalysts is affected strongly by the type of solvent, and not by the choice of
carbon. In the previous study, when Pd was replaced by Co and Cu, or when the oxide
support was changed instead of carbon, the product selectivity was significantly changed in
FAL hydrogenation, because the specific interface changed the reaction pathway [62]. Of the
various supports including alumina, silica, and other transition metal oxides, carbon induced
the highest dispersion of Pd. From these results, it is concluded that the main role of carbon is
to generate high dispersion of Pd catalyst, due to its large surface area. The product
1
7

selectivity is determined by a number of factors, including the type of metal and support, the
solvent used, and the degree of reaction conditions
4
. Conclusion
Various OMCs, including CMK-3, CMK-5, CMK-8, and MSU-F-C, are used as carbon
supports for Pd/C catalysts. The catalytic properties are compared by using the different
carbon supports in Pd-catalyzed FAL hydrogenation. While FAL conversion is greatly
enhanced by using OMCs with Pd, the Pd/CMK-5 catalyst exhibited the highest conversion.
The two-dimensional hexagonally ordered mesoporous channels (p6mm) of CMK-5 enable
fast adsorption/diffusion during the reaction during Pd-catalyzed FAL hydrogenation. The
product selectivity in FAL hydrogenation over various Pd/C catalysts is affected
predominantly by the type of solvent, and not by the carbon supports. For example, the Pd/C
catalysts show similar product distributions in 2-propanol, however a high selectivity towards
CPO is observed when water is used as the solvent. We expect that the clever choice of a
carbon support composed of OMCs will contribute to the development of high-performance
catalysts, as well as help understand the underlying catalytic phenomena.
Acknowledgements
This research was supported by Basic Science Research Program (2018R1A1A1A05079555)
and Technology Development Program to Solve Climate Changes (2017M1A2A2087630) of
the National Research Foundation of Korea (NRF) funded by the Ministry of Science and
ICT, and MOTIE (KIAT_N0001754) and UNIST (1.190013.01).
Appendix A. Supplementary data
1
8

Supplementary Material related to this article can be found, in the online version, at doi:
https://doi.org./
1
9

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2

Fig. 1. Low-angle XRD patterns of Pd/C catalysts; (a) SBA-15 template, CMK-3, and CMK-
, (b) KIT-6 and CMK-8, (c) MSU-F-C, CNT, and Vulcan.
5
2
3

(a)
(b)
50nm
50nm
(c)
(d)
50nm
50nm
(e)
(f)
50nm
50nm
Fig. 2. TEM images of synthesized Pd/various C catalysts; (a) Pd/CMK-3, (b) Pd/CMK-5, (c)
Pd/CMK-8, (d) Pd/MSU-F-C, (e) Pd/CNT, (f) Pd/Vulcan.
2
4

Fig. 3. HR-TEM (left) and STEM-HADDF images (right) of Pd/OMC catalysts: (a-b),
Pd/CMK-3, (c-d), Pd/CMK-5, (e-f), Pd/CMK-8.
2
5

Pd0
Pd2+
Pd 3d5/2
Pd 3d3/2
Pd/CMK-3
Pd/CMK-5
Pd/CMK-8
Pd/MSU-F-C
Pd/CNT
Pd/Vulcan
3
47
343
340
336
333
329
Binding Energy (eV)
Fig. 4. XPS spectra of Pd 3d for the Pd/C catalysts reduced by H at 250 ℃.
2
2
6

Conversion
Others
THF
100
FN
CPL
MF
THFA
FA
80
60
40
20
0
can
/
CNT
/
CMK-3
/CMK-5
Pd
/CMK-8
Pd
Pd
/Vul
Pd
Pd
/MSU-F-C
Pd
Fig. 5. Conversions and selectivity yields of FAL hydrogenation in 2-propanol.
Fig. 6. (a) Conversions and selectivity yields of aqueous FAL hydrogenation and (b)
Reaction mechanism of aqueous FAL hydrogenation to 4-hydroxy-2-cyclopentenone (HCP),
cyclopentenone (CPEO), and CPO.
2
7

Scheme 1. Reaction pathways of FAL hydrogenation.
Nanocasting
Chemical reduction
H PdCl6
2
Infiltration
Etching
SiO2
NaBH4
Carbon-Silica
Composite
Ordered m-Carbon
(OMC)
Pd/OMC
m-Silica template
Scheme 2. A synthetic procedure of Pd/C catalysts with ordered mesoporous carbons
prepared by the nanocasting and chemical reduction method.
2
8

Scheme 3. Various carbon support with different morphologies.
2
9