Interfacial effect of Pd supported on mesoporous oxide for catalytic furfural hydrogenation

Article abstract of DOI:10.1016/j.cattod.2020.02.035

Highly dispersed Pd is loaded onto different types of mesoporous oxide supports to investigate the synergetic metal-support effect in catalytic furfural (FAL) hydrogenation. Ordered mesoporous Co₃O₄, MnO₂, NiO, CeO₂, and Fe₂O₃ are prepared by the nanocasting and the supported Pd on mesoporous oxide catalysts are obtained by the chemical reduction method. It is revealed that mesoporous oxides play an important role on Pd dispersion as well as the redox behavior of Pd, which determines the final FAL conversion. Among the catalysts used, Pd/Co₃O₄ shows the highest conversion in FAL hydrogenation and distinct product selectivity toward 2-methylfuran (MF). While FAL is converted via two distinct pathways to produce either furfuryl alcohol (FA) via aldehyde hydrogenation or MF via hydrogenolysis, MF as a secondary product is derived from FA via the hydrogenolysis of C–O over the Pd/Co₃O₄ catalyst. It is revealed that FAL is hydrogenated to FA preferentially on the Pd surface; then the secondary hydrogenolysis to MF from FA is further promoted at the interface between Pd and Co₃O₄. We confirm that the reaction pathway over Pd/Co₃O₄ is totally different from other catalysts such as Pd/MnO₂, which produces FA dominantly. The characteristics of the mesoporous oxides influence the Pd-oxide interfaces, which determine the activity and selectivity in FAL hydrogenation.

Full text of DOI:10.1016/j.cattod.2020.02.035

Journal Pre-proof
Interfacial Effect of Pd supported on Mesoporous Oxide for Catalytic
Furfural Hydrogenation
Hojeong Lee, Chinh Nguyen-Huy, Eun Jeong Jang, Jihyeon Lee,
Euiseob Yang, Man Sig Lee, Ja Hun Kwak, Kwangjin An
PII:
S0920-5861(20)30094-8
DOI:
https://doi.org/10.1016/j.cattod.2020.02.035
CATTOD 12703
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
18 November 2019
6 February 2020
24 February 2020
Please cite this article as: Lee H, Nguyen-Huy C, Jeong Jang E, Lee J, Yang E, Lee MS,
Kwak JH, An K, Interfacial Effect of Pd supported on Mesoporous Oxide for Catalytic Furfural
Hydrogenation, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.02.035
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Interfacial Effect of Pd supported on Mesoporous Oxide for
Catalytic Furfural Hydrogenation
Hojeong Lee, a Chinh Nguyen-Huy, a Eun Jeong Jang, a Jihyeon Lee, a Euiseob Yang, a Man Sig
Lee, b Ja Hun Kwak, a and Kwangjin Ana,*
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 author (E-mail: kjan@unist.ac.kr)
Table of Contents
Pd/Co3O4
H2
MF
FAL
Pd/MnO2
H2
FAL
FA
1

Highlights





Pd nanoparticles are homogeneously dispersed to mesoporous oxides.
Pd/oxide catalysts enhance catalytic conversions of furfural.
Pd/Co3O4 catalyst shows enhanced conversion in furfural hydrogenation.
Furfural is converted via either aldehyde hydrogenation or hydrogenolysis.
2-methylfuran is selectively produced over Pd/Co3O4 catalyst via hydrogenolysis.
Abstract
Highly dispersed Pd is loaded onto different types of mesoporous oxide supports to investigate
the synergetic metal-support effect in catalytic furfural (FAL) hydrogenation. Ordered
mesoporous Co3O4, MnO2, NiO, CeO2, and Fe2O3 are prepared by the nanocasting and the
supported Pd on mesoporous oxide catalysts are obtained by the chemical reduction method. It
is revealed that mesoporous oxides play an important role on Pd dispersion as well as the redox
behavior of Pd, which determines the final FAL conversion. Among the catalysts used,
Pd/Co3O4 shows the highest conversion in FAL hydrogenation and distinct product selectivity
toward 2-methylfuran (MF). While FAL is converted via two distinct pathways to produce
either furfuryl alcohol (FA) via aldehyde hydrogenation or MF via hydrogenolysis, MF as a
secondary product is derived from FA via the hydrogenolysis of C‒O over the Pd/Co3O4
catalyst. It is revealed that FAL is hydrogenated to FA preferentially on the Pd surface; then
the secondary hydrogenolysis to MF from FA is further promoted at the interface between Pd
2

and Co3O4. We confirm that the reaction pathway over Pd/Co3O4 is totally different from other
catalysts such as Pd/MnO2, which produces FA dominantly. The characteristics of the
mesoporous oxides influence the Pd-oxide interfaces, which determine the activity and
selectivity in FAL hydrogenation.
Keywords:
Pd nanoparticle, mesoporous oxide, oxide–metal interface, furfural,
hydrogenation.
1. Introduction
Metal-supported oxides are the source of the most important catalysts in the chemical industry
[1–8]. The oxide support basically disperses and supports the metal evenly, and also plays a
crucial role in influencing the properties of metal. Catalytic properties are greatly affected by
oxide supports, which change the dispersion of active metals [9], increase the stability of
metals, provide sites to form a new configuration of adsorbed intermediates, and induce charge
transfer at created interfaces [10–16]. Many previous studies demonstrated the improvement
in activity as well as the change in selectivity of catalytic reaction, owning to the effect of the
oxide–metal interface [17–28]. For example, Pt nanoparticle (NP)-loaded mesoporous oxides
showed great enhancements of CO oxidation rates depending on the type of oxides (Co3O4,
NiO, MnO2, Fe2O3, and CeO2) [12]. The redox behaviors of the oxides resulted in charge
transfer at the oxide–metal interface, which amplified the activity of CO oxidation. Kijeński et
al. demonstrated not only strong metal–oxide interaction but also strong oxide–oxide
interaction, which affected both catalytic activity and selectivity using the Pt deposited on
oxides (SiO2, γ-Al2O3, MgO, TiO2), which were further covered by the transition metal oxide
monolayer (TiO2, V2O5, ZrO2) [29]. Thus, choosing an appropriate oxide support is still
considered as an important process not only to develop new catalysts but also to understand
thoroughly the catalytic phenomena.
3

While fossil fuels are still being used as the main energy source and raw material for chemicals
in the global market, the development of alternative materials based on biomass has been
steadily progressing to solve environmental problems and reduce dependence on petroleum.
Among the biomass derivatives, furfural (FAL) has been identified as one of the most
promising chemicals for sustainable production of fuels and chemicals [30–41]. However, its
undesirable storage properties requires further upgrade to more stable products in order to
incorporate them to a petroleum-derived fuel or use them as chemicals. FAL is catalytically
converted into furan (FN), 2-methylfuran (MF), and furfuryl alcohol (FA), through one of
various chemical reactions, such as decarbonylation, hydrogenolysis, or hydrogenation. FA can
be used as a potential fuel/fuel additive or solvent for the production of resins [40].
The selective deoxygenation to MF is of particular interest because this compound has been
proposed as a potential additive to gasoline owning to its suitable energy density, boiling point,
and octane number, as well as its being a good chemical intermediate [42–47]. The challenge
is to find an efficient catalyst, which possesses high activity and controllable selectivity for the
production of highly desirable products, while maintaining long-term stability. Many
heterogeneous catalysts have been investigated for the hydrogenation of FAL, with the majority
of studies focusing on Cu-, Ni- [48], Ru-[49], Pd-[50-52] or PdCu-based[53,54] systems.
Among them, Pd is an ideal catalyst for hydrogenation processes, as it readily dissociates
hydrogen under ambient conditions. On Pd catalysts, FAL binds strongly through its aromatic
ring, resulting primarily in decarbonylation to produce FN. Processes such as aldehyde
hydrogenation and hydrodeoxygenation, which are desirable for conversion of aldehydes to
fuels such as MF, do not occur readily [51].
In this study, three-dimensional ordered mesoporous oxides of Co3O4, MnO2, NiO, CeO2, and
Fe2O3 were synthesized through nanocasting from mesoporous silica (SBA-15) and loaded
4

with Pd NPs by chemical reduction to investigate the supporting effects on liquid-phase FAL
hydrogenation. The properties of Pd-supported mesoporous oxide catalysts such as oxidation
states, redox properties, and metal–support interactions are correlated with catalytic activity
and selectivity in FAL hydrogenation.
2. Experimental section
2.1. Catalyst preparation
Mesoporous silica SBA-15 with highly ordered hexagonal pore structure was used as a hard
template [55, 56]. For the preparation of SBA-15, 16 g of pluronic P123 was dissolved in 120
g of water and 480 g of 2 M HCl solution at 40 °C. Then 34 g of TEOS was added into that
solution with stirring at 40 °C for 20 h. The mixture was aged at 80 °C overnight without
stirring. The solid product was recovered, washed, and air-dried at room temperature.
Calcination was carried out by slowly increasing the temperature from room temperature to
550 °C for 8 h and maintaining at 550 °C for 6 h.
The nanocasting approach using SBA-15, which was developed by Schüth and co-workers [51,
55], was used to prepare mesoporous oxides. Co(NO3)2·6H2O, Ni(NO3)2·6H2O,
Mn(NO3)2·xH2O, Ce(NO3)3·6H2O, and Fe(NO3)3·9H2O (Sigma-Aldrich) were used without
further purification to synthesize mesoporous Co3O4, NiO, MnO2, CeO2, and Fe2O3,
respectively [12, 57–59]. In a typical synthesis, 24 mmol of metal nitrate was dissolved in 6
mL of water. Then, the solution was added to 6 g of SBA-15 in 30 mL of toluene at 65 °C with
stirring. After evaporation of toluene, the resulting sol was collected and dried at 60 °C
overnight, followed by calcination at 300 °C for 5 h. The silica templates were removed
completely using 2 M aqueous NaOH solution heated to 90 °C followed by washing steps with
water and a final drying step at 50 °C.
5

The oxide-supported Pd catalysts were prepared by the chemical reduction method as follows
[13]: A predetermined amount of H2PdCl4 solution was added dropwise to a solution of
trisodium citrate (Na3C6H5O7, 0.526 mM, Pd/trisodium citrate ratio = 10:1, Sigma-Aldrich, >
98%), which was maintained at 5 °C under stirring. The sodium hydroxide (NaOH, 0.2 M
aqueous solution, Sigma-Aldrich, > 98%) was then added to adjust the pH of the resulting
solution to ~7. The obtained mesoporous oxides were combined with the Pd solution to give 5
wt.% of Pd. Thereafter, an ice-cold sodium borohydride solution (5 mL of a 0.3 M NaBH4
solution, Sigma-Aldrich, 98%) was added slowly, and maintained by stirring for 3 h at 5 °C.
The catalysts were collected by filtration and washed with deionized water repeatedly until the
pH of the filtrate was 6. The final catalysts were retrieved and dried at 105 °C for 5 h.
2.2. Catalyst characterization
Various techniques were used for the characterization of Pd NP supported on mesoporous oxide
catalysts. A structure of catalysts was characterized by transmission electron microscopy (TEM,
JEOL JEM-1400) and high-resolution TEM (HRTEM, JEOL JEM-2100F). Surface textural
parameters were measured by nitrogen adsorption using a BESORP-max unit. An inductively
coupled plasma optical emission spectrometry (ICP-OES, Varian 700-ES) was used for the
quantification of Pd loading. X-ray diffraction (XRD) patterns were collected on a PANalytical
X’Pert PRO diffractometer using a Cu Kα radiation source (λ = 0.154056 nm). The Pd
dispersion was measured by the CO adsorption capacity using CO chemisorption instrument
(Auto ChemⅡ2920, and Micromeritics Instruments Co., USA). Diffuse reflectance infrared
Fourier transform (DRIFT) spectroscopy by CO adsorption was carried out over Pd-supported
oxide catalysts using a spectrometer (Nicolet iS10 FTIR) equipped with a DRIFT cell and
mercury-cadmium-telluride detector. Samples were diluted with KBr and loaded into a DRIFT
cell (Harrick Scientific) with ZnSe windows. All IR spectra were collected for 128 scans with
6

4 cm–1 resolution. Samples were pretreated at 150 °C for 1 h under He (flow rate = 1 mL/s).
CO adsorption was performed using 0.2% CO/He (flow rate = 1 mL/s) at room temperature.
The collected spectrum was normalized by the number of CO adsorbed on Pd sites from CO
chemisorption, which was obtained separately.
2.3. Catalytic FAL hydrogenation
A supported Pd catalyst (10 mg) was mixed with FAL (1 g, Acros, 99%) and isopropanol (20
mL, Sigma-Aldrich) in a 100 mL stainless-steel autoclave reactor (Parr 5500) [13]. After the
reactor was sealed, air was purged by flushing four times with hydrogen and then pressurized
with hydrogen to 20 bar. If not enough hydrogen was provided, FAL conversion was very low
(Fig. S1). The solution containing the supported Pd catalyst was stirred under 600 rpm at 180
°C for 5 h. After completion of the reaction, the catalyst particles were removed from the
solution, and the product analysis was conducted by gas chromatography using a flame
ionization detector (FID; Agilent 7820A) equipped with a capillary column (DB-Wax, 30 m
length, 0.32 mm internal diameter, 0.25 μm film thickness). As a substrate chemical, FA
(Sigma-Aldrich, 98%), THFA (Sigma-Aldrich, 99%), FN (Sigma-Aldrich, 99%), THF (Alfa
Aesar, 99%), and MF (Sigma-Aldrich, 99%) were tested for the identification of products. The
product conversion and selectivity were determined as follows:
mol furfural consumed
Conversion (%) 
Selectivity (%) 
100
100
mol furfural fed
mol productformed
mol furfural consumed
3. Results and discussion
3.1. Characterization of catalysts
7

Highly dispersed Pd catalysts supported on various mesoporous oxides were prepared by
chemical reduction using sodium borohydride, in the presence of trisodium citrate as a
stabilizer [13]. The Pd content in Pd/oxide catalysts was determined by ICP-OES as presented
in Table 1. The results confirmed that approximately 5 wt.% of Pd was presented in all samples.
The Pd dispersions of the various catalysts were calculated based on CO adsorption
measurements (Table 1), with a dispersion of 29.3% and 28.9% being achieved for the
Pd/Co3O4 and Pd/MnO2 catalysts, respectively, which were significantly greater than those of
Pd-supported NiO, CeO2 and Fe2O3. N2 adsorption–desorption isotherms of the Pd/oxides
show type IV isotherms with a clear hysteresis loop at 0.4–0.9 of the relative pressure (p/p0)
(Fig. S2), revealing the presence of uniform mesopores. The texture parameters of Pd/oxide
catalysts are summarized in Table 1. All Pd/oxide catalysts show high surface areas (82‒172
m2∙g−1), in which Pd/Co3O4 (172 m2∙g−1) exhibits the highest surface area.
Table 1. Characterization of Pd/oxide catalysts.
Pd loading
(wt.%)
CO-
TPD
Chemisorbed CO
BET
Pore
(mol∙g−1)b
Surfac
Catalyst
Pd
Surface volum
Pd
Actuala dispersi
on (%)
Nom Actu
Nomi
nal
e area
(m2‧g−1
)
dispersi
area
e
inal
ala
on (%) (m2‧g−1) (cm3‧g-
1
)
Pd/Co3O4 5.0 5.8
Pd/MnO2 5.0 7.4
29.3
28.9
18.9
13.0
2.6
172
131
85
0.33
0.50
0.27
0.32
0.40
7.8
2.91
13.00
13.20
9.14
6.43
-
0.22
0.51
0.20
0.45
-
15.1 6.80
12.6 1.79
15.7 2.93
11.4 -
Pd/NiO
5.0 6.2
Pd/CeO2 5.0 7.7
Pd/Fe2O3 5.0 5.8
a Measured from ICP-AES
82
139
8

b Measured from CO-TPD and DRIFT spectroscopy (CO adsorption)
Fig. 1. XRD patterns of (a) Pd/Co3O4 and Co3O4, (b) Pd/MnO2 and MnO2, (c) Pd/NiO and NiO,
(d) Pd/CeO2 and CeO2, and (e) Pd/Fe2O3 and Fe2O3. The peaks corresponding to the oxides are
indexed.
9

Crystallographic information about the prepared oxide and Pd/oxide catalysts is obtained from
XRD measurements as shown in Fig. 1. XRD patterns of the resulting mesoporous oxides are
well-matched to Co3O4 (JCPDS 42-1467), MnO2 (JCPDS 04-779), NiO (JCPDS 47-1049), CeO2
(JCPDS 43-1002), and Fe2O3 (JCPDS 33-0664). The broadening and well-resolved diffraction
peaks are attributed to the high crystallinity of the mesoporous oxides. No diffraction peaks of Pd
species, including Pd0 or PdO, are observed on the Pd/Co3O4, Pd/MnO2, and Pd/NiO catalysts,
indicating that the Pd species are highly dispersed on the catalyst surface or Pd atoms and/or ions
are incorporated into the crystal lattice of oxides. The peaks corresponding to oxides are shifted in
the negative direction after introducing Pd into oxides except Fe2O3 and CeO2, probably due to a
crystal lattice distortion. For the Pd/Fe2O3 and Pd/CeO2 catalysts, a small peak at 40° is observed,
which is associated with Pd0 (JCPDS 65-2867), suggesting that Pd was segregated and not
incorporated into the Fe2O3 and CeO2 structure evenly.
Fig. 2 shows the TEM images of Pd/oxide catalysts. Apparently, all oxides prepared by the
nanocasting method display a well-ordered mesoporous structure. Almost all the obtained catalysts
have rod-like morphology, which is replicated from the SBA-15 template, indicating good
replication of the silica template. The Pd NPs with sizes smaller than 5 nm were highly dispersed on
the surface of mesoporous MnO2 and NiO, while there are some aggregations of Pd particles on
mesoporous CeO2 and Fe2O3. This observation is consistent with the CO-TPD and XRD results.
Fig. 3 shows the bright-field scanning transmission electron microscopy (BF-STEM), high angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy
dispersive spectroscopy (EDS) mapping images of Pd/Co3O4 and Pd/Fe2O3 catalysts. Apparently,
the mesoporous Co3O4 displayed a well-defined ordered mesoporous structure, and the loading of
Pd did not alter the mesoporous architecture (Figs. 3a–b). As shown in the high-resolution BF-
STEM images of the Pd/Co3O4 catalysts (Fig. 3b), the intraplanar spacings (d values) were measured
10

to be approximately 0.47 and 0.22 nm, which closely agree with that of the (111) crystal plane of
the standard Co3O4 (JCPDS 42-1467) and that of the (111) crystal plane of the standard Pd (JCPDS
65-2867), respectively. The HAADF-STEM and EDS mapping images (Figs. 3c–f) show a highly
dispersed Pd NPs on the surface of the mesoporous Co3O4. In contrast, the Pd NPs of the Pd/Fe2O3
catalysts (Figs. 3g–l) form large aggregates with diameters larger than 6 nm.
11

Fig. 2. TEM images of (a) Pd/MnO2, (b) Pd/NiO, (c) Pd/CeO2, and (d) Pd/Fe2O3. (e) Illustration
of the nanocasting approach for the preparation of Pd/oxide catalysts.
12

Fig. 3. HR-TEM, BF-STEM, HAADF-STEM, and EDS mapping of (a–f) Pd/Co3O4, and (g–l)
Pd/Fe2O3 catalysts.
Fig. 4. DRIFT spectroscopy results by CO adsorption on Pd/oxide catalysts.
DRIFT spectroscopy after adsorption of CO as a probe molecule provides valuable information
about oxidation states and characteristic metal sites [60–62]. Fig. 4 shows the CO adsorbed DRIFT
absorption spectra of various Pd/oxide catalysts at room temperature. The bands at 1800−2300 cm−1
are associated with the adsorbed CO on metal surfaces. Pd/Co3O4, Pd/ MnO2, Pd/NiO, and Pd/CeO2
show two bands at 1971 cm−1 and 2087 cm−1 after CO adsorption which are assigned to metallic
Pd0 as a bridge mode and linear CO adsorbed Pd0 sites, respectively. The peak at 2137 cm−1 is
ascribed to CO adsorbed on Pd2+ sites [61, 62]. The relative molar ratios of PdO/Pd0 calculated by
DRIFT spectra (see Fig. S3) are summarized in Table 1. Interestingly, no adsorption bands appeared
on the Pd/Fe2O3 catalyst (Fig. 4). We further conduct DRIFT experiments on Pd/Fe2O3 at different
conditions: thermal treatment under He (60 sccm) at 120 °C for 1 h, and oxidation under 20%
13

O2/He (60 sccm) at 250 °C for 1 h. As shown in Fig. S4a, there are no peaks corresponding to Pd2+
and Pd0 on the Pd/Fe2O3 under He treatment, while the peak at 2137 cm−1 assigned to CO adsorbed
on Pd2+ sites, appears on Pd/Fe2O3 after oxidation treatment. The full range DRIFT spectra of CO
adsorbed on Pd/oxide catalysts under He treatment (Fig. S4b) show that two bands at 2839 – 2972
cm-1 and 1578 – 1718 cm-1, which are assigned to C–H bending and C=O stretching, respectively,
appear only on the Pd/Fe2O3. After oxidation at 250 °C using 20% of O2/He, these bands
disappeared (Fig. S4c). We speculate that the main reason for the absence of CO adsorption on
Pd/Fe2O3 in DRIFT spectra is due to the surfactant (trisodium citrate) which covers the Pd
surface of the Pd/Fe2O3 catalyst [63]. For the other Pd/oxide catalysts, there is no band
corresponding to C–H bending and C=O stretching, indicating that surfactants are completely
removed through the washing and drying steps.
3.2. Catalytic activity
The hydrogenation of FAL is carried out over the oxide-supported Pd catalysts (5 wt.%) and
the mesoporous oxides to examine the effect of metal dispersion and type of support on
catalytic activity and selectivity. As shown in Fig. 5, all as-synthesized mesoporous oxides
without Pd loading show a very low activity in FAL conversion. For instance, approximately
3% of FAL conversion was obtained with MnO2, NiO, CeO2, and Fe2O3 and 13% of FAL
conversion with Co3O4. When Pd NPs were loaded into the mesoporous oxides via the chemical
reduction, the catalytic activity observed was significantly higher than that of mesoporous
oxide without Pd loading. It is clearly observed that the catalytic activity of oxide-supported
Pd catalysts on FAL hydrogenation decreased in the order of Pd/Co3O4 (100%) > Pd/MnO2
(83%) > Pd/NiO (68 %) > Pd/CeO2 (22%) > Pd/Fe2O3 (18%). This catalytic behavior is
attributed to two possibilities: first, activity from Pd active sites on the support surface and
second, activity at the oxide–metal interface [12]. In Table 1, the Pd dispersion is decreased
14

similarly in the following order: Pd/Co3O4 (29.3%) > Pd/MnO2 (28.9%) > Pd/NiO (18.9%) >
Pd/CeO2 (13%) > Pd/Fe2O3 (2.6%). This indicates that a high Pd dispersion induces high
conversion in FAL hydrogenation reaction. Pd/Co3O4, Pd/MnO2, and Pd/NiO with high Pd
dispersion exhibit much higher conversions than Pd/CeO2 and Pd/Fe2O3. Surprisingly,
Pd/CeO2 exhibits a catalytic performance similar to Pd/Fe2O3, even though it showed a higher
Pd dispersion than Pd/Fe2O3.
Fig. 5. Product distribution from FAL conversion over various catalysts. Reaction conditions:
FAL = 1 g, isopropanol = 20 mL, catalyst = 10 mg, H2 = 20 bar, reaction temperature = 180 °C,
reaction time = 5 h, and stirring speed = 600 rpm.
We also compared FAL conversion over the prepared by the catalysts which prepared by the
typical impregnation method (Table S1). While overall FAL conversion of the Pd/oxide
prepared by the impregnation were much less than those of Pd/oxides prepared by the chemical
15

reduction, the impregnated Pd/NiO catalyst exhibited the best FAL conversion (50 %) (Fig.
S5). In our previous report, chemical reduction by sodium borohydride has already proven to
be effective for the preparation of highly dispersed and ultrafine Pd catalyst on various oxides,
compared to conventional impregnated Pd/oxide catalysts. When the mesoporous oxide
supports were replaced with a commercially avaialble oxide support, the FAL conversion was
significantly reduced. For example, when preparing Pd/Co3O4, and Pd/MnO2 catalysts
composed of commerial Co3O4 (Sigma-Aldrich, powder <10 μm) and MnO2 (Sigma-Aldrich,
powder <10 μm), the FAL conversions were 80 and 16 %, respectively, due to the low surface
area and poor Pd dispersion. From these results, it was revealed that Pd/oxides catalysts
prepared by the chemical redcution were higly active catalysts for FAL hydrogenation as
compared the the impregnated Pd/oxide catalysts. The mesoporous oxides further improved Pd
dispersion than commeral oxides with low surface area. In our previous study, mesoporous
carbons, including CMK-3, CMK-5, CMK-8, and MSU-F-C, were used as carbon supports to
produce Pd/C catalysts to investigate the effect of pore size and structure of carbons [64]. It
was revealed that CMK-5 with the largest surface area and hexagonal hollow tubular
framework was the most efficient carbon support for Pd/C catalysts, with the highest
conversion of FAL in isopropanol. The FAL conversion was greatly influenced by the surface
area of carbons and the corresponding Pd dispersion. It is demonstrated that the activity in FAL
hydrogenation was determined by the surface area of carbons. In the present study, the effect
of mesopores is not prevalent, because the average size and structure of mesoporous Co3O4,
MnO2, NiO, CeO2, and Fe2O3 are the same.
Depending on the type of mesoprous oxide, the Pd dispersion was varied due to the specific
interfacial effect. The DRIFT spectroscopy results during CO adsorption on Pd/oxide catalysts
prepared by the chemical reduction after H2 reduction under reaction conditions (180 °C)
16

provide the degree of reduction of surface Pd (Fig. S6). Interestingly, Pd/CeO2 exhibited a
lower Pd reducibility than other catalysts. After 1 h pretreatment under a flow of 10% H2/He
at 180 °C, Pd/Co3O4, Pd/MnO2 and Pd/NiO were totally reduced, while there is a peak
corresponding to PdO on Pd/CeO2. The intensity of Pd0 peak was decreased over Pd/Co3O4,
Pd/MnO2, and Pd/NiO catalysts after reduction, indicating that there was a strong interfacial
effect between Pd and oxides (Co3O4, MnO2, and NiO). Therefore, we speculate that the Pd
species were fully reduced under the reaction condition except for the Pd/CeO2 catalyst, which
induces a low catalytic activity of Pd/CeO2. In catalytic selectivity, the main product obtained
by the FAL hydrogenation over the Pd/MnO2, Pd/NiO, Pd/CeO2, and Pd/Fe2O3 catalysts was
FA, while MF was the dominant product over the Pd/Co3O4 catalyst. Although Pd/MnO2 and
Pd/Co3O4 have the same Pd dispersion (approximately 29%), Pd/Co3O4 exhibits a much higher
FAL conversion with preferred selectivity toward MF. While the mesoporous Co3O4 without
Pd shows low activity, it is revealed that not only the Pd active site but also the oxide–metal
interface contribute to the activity of Pd/Co3O4. As such, the binding energy (BE) shift in XPS
spectra of Co3O4 surface before and after Pd loading (Fig. S7) demonstrates the creation of a
new interface between Pd and Co3O4. The Co 2p3/2 signal at BE = 779.4 eV indicates the
presence of surface Co3+, whereas the Co 2p3/2 signal at BE = 780.9 eV refers to the surface
Co2+. It is observed that after deposition of Pd NPs on a mesoporous Co3O4, the surface
Co2+/Co3+ molar ratio increases from 0.76 to 2.16, indicating that the surface Co2+
concentration increases significantly via the reaction of Pd0 + Co3+ → Pd2+ + Co2+. The
change in oxidation state of Co3O4 is due to charge transfer from Pd NPs to Co3O4, in which
the oxidized Pd2+ species in the Pd/Co3O4 further promotes high distribution of Pd on Co3O4
(Fig. S8). The existence of oxidized Pd2+ species (Fig. S9) and the BE shift in XPS spectra of
oxides surface before and after Pd addition on Pd/MnO2, Pd/NiO, Pd/CeO2, and Pd/Fe2O3
catalysts (Fig. S10) indicate that new interfaces are generated between Pd and the oxide.
17

Although the original surface area of mesoporous oxides determines the Pd dispersion of the
supported catalysts (Table 1), the relative ratio of Pd0 to PdO in the Pd/oxide catalysts further
influences the overall catalytic behaviors.
In order to confirm the reaction behaviors on Pd/oxide catalysts, FAL hydrogenation was
conducted at different reaction times. Fig. 6a illustrates the FAL conversion as a function of
reaction time over Pd/oxide catalysts at 180 °C. Based on the data shown in Fig. 6a, the FAL
hydrogenation activities have the following order: Pd/Co3O4 > Pd/MnO2 > Pd/NiO > Pd/CeO2
> Pd/Fe2O3. The first-order dependence of FAL hydrogenation reaction using Pd/oxide
catalysts were supported by the linearity of -ln(1 - X) versus time plots in Fig. 6b, where X is
the FAL conversion. As the first-order dependence of the reaction was validated, the apparent
reaction rate constants of FAL hydrogenation (kapp) were determined from the slope of the plot
of -ln(1 - X) = kapp × t and summarized in Table 2. The apparent rate constant of Pd/Co3O4 was
much greater than those of other catalysts. The kapp values were 13 × 10−3, 5.6 × 10−3, 2.4 × 10-
3, 1.1 × 10-3, and 0.9 × 10-3 min−1 for Pd/Co3O4, Pd/MnO2, Pd/NiO, Pd/CeO2, and Pd/Fe2O3,
respectively. The results also confirmed that Pd/Co3O4 possessed a much higher activity in
FAL hydrogenation, compared to Pd/MnO2, Pd/NiO, Pd/CeO2, and Pd/Fe2O3 catalysts.
18

Fig. 6. (a) FAL conversion as a function of reaction time over various supported Pd catalysts.
(b) Time dependent FAL conversion. Reaction conditions: FAL =1 g, isopropanol = 20 mL,
catalyst = 10 mg, H2 = 20 bar, reaction temperature = 180 °C, reaction time = 0.1–5 h, and
stirring speed = 600 rpm.
Table 2. Hydrogenation of FAL over mesoporous oxides and Pd/oxidesa
19

Yield (%)
kapp
Catalysts Conversi
on (%)
(min−1)
(×10−3)b
Cyclope
ntanol
FA
MF
FN
THF
THFA
Others
Co3O4
MnO2
NiO
13
3
-
9.44
1.52
1.34
0.81
1.40
8.00
59.76
28.56
13.20
9.72
3.72
-
3.73
1.49
1.61
0.94
1.73
22
-
-
-
-
-
-
-
-
-
-
-
-
2
-
-
-
-
-
-
1
-
-
-
-
3
-
-
-
CeO2
Fe2O3
2
-
-
-
3
-
-
-
Pd/Co3O4 100
Pd/MnO2 83
13
5.6
2.4
1.1
0.9
-
54
-
13
8.3
14.11 0.83
Pd/NiO
68
-
2.72
3.74
8.28
1.04
5.44
12.24 14.28 4.76
Pd/CeO2 22
Pd/Fe2O3 18
-
-
2.64
-
-
2.42
-
-
-
-
Pd/SiO2
6
0.51
0.49
0.45
a
Reaction conditions: FAL = 1 g, isopropanol = 20 mL, catalyst = 10 mg, H2 = 20 bar, reaction
temperature = 180 °C, reaction time = 5 h, stirring speed = 600 rpm.
b Apparent first-order reaction rate constant evaluated from the slope of the plot of ln(1 - X) = -kapp × t.
To investigate the reaction pathways, Pd/Co3O4 and Pd/MnO2, which have totally different
selectivity results were selected and compared for the FAL hydrogenation. The FAL
conversion and product selectivity as a function of reaction time are shown in Fig. 7. As the
reaction proceeded, 55% of FAL was converted along with 40 % of FA selectivity after 1 h
reaction over the Pd/Co3O4 catalyst (Fig. 7a). FA reached a maximum yield of 50% at 2 h and
then decreased gradually afterward. The MF yield was very small at the initial time and rapidly
increased following the decrease in FA yield. After 10 h, the MF reached a maximum yield of
50%, while the fraction of FA was very low. The yields of FN and THFA were increased over
20

the entire reaction time until they reached 22% and 10% yield, respectively. From these results,
it was proven that FAL was converted quickly to FA as a main intermediate with a maximum
selectivity at 2 h, and then MF was produced via hydrogenolysis at the expense of FA after 2
h. In contrast, Pd/MnO2 had a significant different selectivity behavior from the Pd/Co3O4
catalyst (Fig. 7b). It was observed that on Pd/MnO2 catalyst, FA was produced as a major
product from FAL with a small portion of FN and THFA, while MF was hardly obtained. The
yield of FA was increased over the entire reaction time until it reached 60 % yield over
Pd/MnO2. These results demonstrate that the FAL hydrogenation over Pd/Co3O4 catalyst
occurred by the following pathway: aldehyde hydrogenation toward FA and secondary
hydrogenolysis to MF. Meanwhile, Pd/MnO2 promoted only the aldehyde hydrogenation
toward FA. To further confirm the reaction pathways over Pd/Co3O4 and Pd/MnO2 catalysts,
the hydrogenation of FA was conducted in the same reaction conditions as the FAL
hydrogenation. Fig. 8 shows the degradation of FA and product distribution over Pd/Co3O4 and
Pd/MnO2 catalysts. Fig. 8a clearly shows that the Pd/Co3O4 catalyst converted all of the FA
within 3 h, while approximately 5% of FA was degraded simultaneously in the presence of
Pd/MnO2 afterward. It indicates that Pd/MnO2 exhibits no activity for the reaction of FA. Fig.
8b shows the evolution of product distribution as a function of time over Pd/Co3O4 catalyst in
the FA hydrogenation. It confirms that on the Pd/Co3O4 catalyst, the yields toward MF, FN,
and THFA were increased over the entire reaction until it reached 45, 30, and 15%,
respectively. In the previous study, Pd/C, Pd/SiO2, and Pd/Al2O3 catalysts were used to
investigate the contribution of Pd to catalytic activity and selectivity, while carbon, SiO2, and
Al2O3 are known as chemically inert supports [13]. Depending on the type of support, the Pd
dispersion changed and the resulting FAL conversion also changed. While Pd/C catalyst
showed higher FAL conversion than Pd/SiO2 and Pd/Al2O3, the FAL conversion and THFA
selectivity increased with smaller Pd size or Pd dispersion, reaction temperature and time
21

increased [13]. From these results, it was confirmed that Pd catalyzed FAL hydrogenation is
advantageous for producing THFA. However, current research shows that the activity and
selectivity changes in various mesoporous oxides supported Pd catalysts due to the specific
interfacial effect. For example, the main products of Pd/Co3O4 and Pd/MnO2 are MF and FA,
respectively.
22

Fig. 7. Effect of reaction time on selectivity and FAL conversion over (a) Pd/Co3O4 and (b)
Pd/MnO2 catalyst. Reaction conditions: FAL = 1 g, isopropanol = 20 mL, catalyst = 10 mg, H2
= 20 bar, reaction temperature = 180 °C, and stirring speed = 600 rpm.
23

a
100
80
60
40
20
0
Pd/MnO2
Pd/Co3O4
0
100
200
300
Reaction time (min)
b
100
80
60
40
20
0
FA conversion-Pd/Co3O4
F
2-MF
THFA
others
0
100
200
300
Reaction time (min)
Fig. 8. (a) Change in concentration of FA on Pd/Co3O4 and Pd/MnO2 catalysts. (b) Effect of
reaction time on selectivity and FA conversion over Pd/Co3O4 catalysts. Reaction conditions:
FA = 0.5 g, isopropanol = 20 mL, catalyst = 10 mg, H2 = 20 bar, reaction temperature = 180 °C,
and stirring speed = 600 rpm.
24

Based on these results, we found that Pd/Co3O4 catalysts selectively produced MF as a major
secondary product via the hydrogenolysis of the C‒O bond of FA, which was generated from
FAL. It was confirmed that the yield of FA was dominant at an initial stage of the FAL
hydrogenation over the Pd/Co3O4 catalyst. When the production of FA was maximized, it then
decreased at the expense of MF production. It was obvious that a much higher yield of MF was
observed, when FA was used as a feed over Pd/Co3O4 instead of FAL. However, over the
Pd/MnO2 catalyst, FA was the main reaction product, at which aldehyde hydrogenation toward
FA was the preferred reaction pathway for the conversion of FAL. Therefore, when FA was
used as a feed, very low activity was observed on the Pd/MnO2 catalyst, indicating that the
secondary hydrogenolysis to MF from FA did not occur (Fig. 8a). The extraordinary
enhancement of the reaction rate and the changes in selectivity over Pd/Co3O4 compared to
other Pd/oxides are illustrated in Scheme 1. FAL was hydrogenated to FA on the Pd surface,
followed by the secondary hydrogenolysis to MF from FA at the interface between Pd and
Co3O4 oxide. For the other Pd/oxide catalysts such as Pd/MnO2, only FAL hydrogenation to
FA occurs dominantly on the Pd surface.
MF
Pd/Co3O4
FAL
FAL
MF
Pd/MnO2
FA
25

Scheme 1. Illustration showing potential reaction sites and reaction pathways over Pd/Co3O4
and Pd/MnO2 catalysts during FAL hydrogenation.
4. Conclusion
The highly dispersed Pd NPs deposited on mesoporous oxides of Co3O4, MnO2, NiO, CeO2,
and Fe2O3 were prepared via the nanocasting and chemical reduction method for the
development of efficient catalysts for liquid-phase FAL hydrogenation. While mesoporous
oxides show a trace amount of catalytic activity in FAL hydrogenation, Pd/oxide catalysts
greatly enhance the catalytic conversions of FAL depending on the type of mesoporous oxides.
The change in support affects the Pd dispersion as well as the redox behavior of Pd species
which strongly influence the catalytic activity and selectivity in the FAL hydrogenation.
Pd/Co3O4, Pd/MnO2, and Pd/NiO possess a high Pd dispersion, resulting in a high FAL
conversion, while low conversions are observed in Pd/CeO2 and Pd/Fe2O3 catalysts owing to
their poor Pd dispersion. In the product selectivity, the Pd/Co3O4 catalyst induces predominant
production of MF via hydrogenolysis, while other Pd/oxide catalysts prefer the reaction
pathway via the aldehyde hydrogenation, yielding FA as the major product. In the
hydrogenation of FA instead of FAL, it is revealed that MF as a secondary product is derived
from FA via the hydrogenolysis of C‒O over the Pd/Co3O4 catalyst, rather than direct formation
from FAL. As a result, FAL is hydrogenated to FA preferentially on the Pd surface, and then
the secondary hydrogenolysis to MF from FA is further promoted at the interface between Pd
and Co3O4. This reaction pathway is totally different from that of Pd/MnO2, which produces
FA dominantly. It is also demonstrated that Pd/Fe2O3 shows unusual interfacial behaviors due
to the remaining surfactants, which cover the active site of Pd, resulting in a low conversion of
FAL. Because the characteristics of the mesoporous oxides determine the Pd-oxide interfaces
26

in the current study, a proper selection of the support facilitates a high catalytic conversion of
desired products in many biomass conversions.
Associated content
Supplementary Materials
Author Information
Declaration of interests
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.
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
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 (1.180082.01).
27

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