Catalytic transfer hydrogenation of bio-based furfural by palladium supported on nitrogen-doped porous carbon

Article abstract of DOI:10.1016/j.cattod.2018.07.033

Highly porous carbon was prepared via a zeolite hard templating route. Subsequent melamine treatment produced nitrogen-functionalized carbon (NPC), which was used as a support to anchor Pd nanoparticles (Pd/NPC) of sizes 2–4 nm using the chemical reduction method. Various analytical techniques were used to characterize the textural properties, structure, and chemical nature of the catalyst. Pd/NPC was then employed as a catalyst for the transfer hydrogenation of biomass-derived furfural (FF) to furfuryl alcohol (FFA) with alcohol as a hydrogen donor, and the effect of temperature, hydrogen donor species, reaction time, and catalysts loading were investigated. The Pd/NPC catalyst was reusable up to five times without any apparent loss of its activity and selectivity. A possible mechanistic pathway for the catalytic transfer hydrogenation of FF over Pd/NPC catalyst was proposed.

Full text of DOI:10.1016/j.cattod.2018.07.033

Accepted Manuscript
Title: Catalytic transfer hydrogenation of bio-based furfural by
palladium supported on nitrogen-doped porous carbon
Authors: Pillaiyar Puthiaraj, Kyoungsoo Kim, Wha-Seung
Ahn
PII:
S0920-5861(18)30094-4
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.07.033
CATTOD 11578
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Catalysis Today
Received date:
Revised date:
Accepted date:
19-2-2018
3-7-2018
13-7-2018
Please cite this article as: Puthiaraj P, Kim K, Ahn W-Seung, Catalytic transfer
hydrogenation of bio-based furfural by palladium supported on nitrogen-doped porous
carbon, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.07.033
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Catalytic transfer hydrogenation of bio-based furfural by palladium
supported on nitrogen-doped porous carbon
Pillaiyar Puthiaraja, Kyoungsoo Kimb, and Wha-Seung Ahna,*
a Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, South
Korea,
b Department of Chemistry, Chonbuk National University, Jeonju, Jeollabuk-do, 54896, South
Korea.
*E-mail: whasahn@inha.ac.kr
Graphical Abstract
Highlights of the present work
1. Nitrogen-doped porous carbon (NPC) was produced via zeolite templating.
2. Pd NPs with 2-4 nm sizes are incorporated on the NPC by chemical reduction.
3. Pd/NPC showed good activity in the transfer hydrogenation of furfural using 2-butanol.
1

4. Heterogeneous Pd/NPC catalyst was reusable up to five runs without decreases in activity.
Abstract
Highly porous carbon was prepared via a zeolite hard templating route. Subsequent
melamine treatment produced nitrogen-functionalized carbon (NPC), which was used as a support
to anchor Pd nanoparticles (Pd/NPC) of sizes 2-4 nm using the chemical reduction method.
Various analytical techniques were used to characterize the textural properties, structure, and
chemical nature of the catalyst. Pd/NPC was then employed as a catalyst for the transfer
hydrogenation of biomass-derived furfural (FF) to furfuryl alcohol (FFA) with alcohol as a
hydrogen donor, and the effect of temperature, hydrogen donor species, reaction time, and catalysts
loading were investigated. The Pd/NPC catalyst was reusable up to five times without any apparent
loss of its activity and selectivity. A possible mechanistic pathway for the catalytic transfer
hydrogenation of FF over Pd/NPC catalyst was proposed.
Keywords: Catalytic transfer hydrogenation; Porous carbon; Palladium nanoparticles; Biomass;
Furfural; Meerwein–Ponndorf–Verley reduction.
1. Introduction
Biomass refining is important in the context of depleting fossil fuel reserves and
environmental pollution resulting from the production of fuels, because it reduces the emission of
greenhouse gases [1, 2]. It is a sustainable process and produces high value-added chemicals.
Consequently, the high oxygen-containing hemicellulose biomass has been utilized worldwide as
a preferred carbon source, which produces fuels, FF, 5-hydroxymethyl furfural, sorbitol, and
levulinic acid [3]. Among these, FF is a major biomass-based platform chemical and enables the
2

synthesis of various other chemicals via hydrogenolysis, hydrogenation, oxidation, and aldol
condensation [4, 5]. Particularly, the selective hydrogenation of FF to FFA is regarded as a pivotal
industrial process. This is because FFA can be used to produce lysine, vitamin C, fibers, lubricants,
adhesives, and resins. [1, 6, 7]. Conventionally, copper chromite catalyst with high pressure gaseous
H2 at a high temperature has been employed in the industry for the hydrogenation of FF to FFA.
Improper disposal of the catalyst leads to severe environmental pollution owing to the high toxicity
of Cr2O3 [1]. Therefore, various Cr-free supported metal and alloy catalysts, including Pd/C [8, 9],
Pd/CNT [10], Pd/Al2O3 [11], Pt/Al2O3 [12], Ru/C [13], Cu/SBA-15 [14], Mo2C [15], Cu/ZnO [16],
Pt/CeO2 [17], Pd-Cu/SiO2 [18], Ni-Fe/SiO2 [19], Pd-Cu/MgO [20], Ru/Zr-MOFs [21], and
CuMgAl [6] have been developed for the gas/liquid phase hydrogenation reaction of FF in the
presence of high-pressure H2 gas. However, these systems suffer from several drawbacks; high-
pressure gas phase hydrogenation reaction requires considerable energy, which is a safety issue,
and yields high amounts of by-product. These limitations have encouraged the exploration for
alternative environment friendly and economically viable synthetic routes.
Catalytic transfer hydrogenation (CTH) with an alcohol and/or formic acid hydrogen
source has recently emerged as a promising greener alternative route for the hydrogenation
reactions of carbonyl compounds, which avoids the requirement of high-pressure H2 gas.
Furthermore, CTH utilizes an alcohol as the hydrogen source, which is advantageous because of
the higher selectivity in the transformation of the carbonyl group to the hydroxyl group via the
Meerwein-Ponndorf-Verley (MPV) reduction as compared to the use of corrosive formic acid [22].
Recently, several acid- and/or base-containing heterogeneous solid-supported metal complex and
nanoparticles (NPs) catalysts have been reported for the highly efficient hydrogenation of biomass-
derived FF using alcohol as a hydrogen donor. For instance, Hermans et al. [23] reported the Cu/,
3

Ni/, and Pd/Fe2O3-catalyzed hydrogenation of FF with 2-propanol (2-PrOH) at 180 °C. These
authors were able to achieve 35–45% conversion of FF and 75% selectivity to FFA using Cu and
Ni/Fe2O3 and 100% conversion of FF with 57% selectivity to FFA over Pd/Fe2O3. Yang et al. used
Ni/Fe2O4 catalyst and 2-PrOH at 180 °C for the hydrogenation of FF and obtained 94% yield of
FFA [24]. Dieguez et al. used an NHC-based Ir complex with 2-PrOH under reflux for the
hydrogenation of C=O, C=N, and C=C bonds and achieved >90% yield of the desired products
[25]. Lu et al. reported the Cu/MgO-Al2O3 catalyzed hydrogenation with 2-PrOH at 210 °C and
obtained almost full conversion with 90% FFA [26].
Several carbon-supported metal NPs have also been applied as hydrogenation catalysts.
The hydrophobic nature of carbon supports was expected to be more advantageous than other
supports as the former would adsorb the less hydrophilic organic compounds more easily and
effectively promote the reaction [27, 28]. Vlachos et al. reported Ru/RuO2/C catalyzed
hydrogenation with different alcohols to obtain 2-methylfuran as the main product [29, 30]. Fu et
al. conducted the hydrogenation of FF with 2-butanol (2-BuOH) at 120 °C over a nitrogen-doped
carbon (NPC)-supported iron catalyst, which yielded almost 80% FFA [31]. Xiao et al. used Cu-
Co/C as a catalyst and reportedly achieved 99% conversion of FF with 98% yield of FFA at 140
°C [32]. Li et al. reported the Co-Ru/C catalyzed hydrogenation of FF with 2-BuOH at 150 °C,
which resulted in complete conversion to FFA [33]. Recently, Singh et al. reported the
hydrogenation reaction of carbonyl compounds over a graphene oxide-supported Pd catalyst with
2-PrOH [34]. It is worth noting that these catalytic systems required a high reaction temperature
and suffered the loss of their stability during the recycling runs.
Recently, Kim et al. have developed a facile route to form 3D graphene-like carbon
structure selectively inside a Ca2+-exchanged zeolite template by ethylene pyrolysis [35]. The
4

intrinsic properties of the zeolite-templated carbons such as the high specific surface area provided
by the 3D graphene-like carbon layers with a uniform pore network (around 1 nm pore size) and
high thermal stability can provide a useful support material for catalysis [36]. Hara et al.
synthesized the SO3H-bearing zeolite-templated microporous carbons via post-synthetic
sulfonation of carbon with chlorosulfuric acid, and the resulting acid-functionalized carbon
showed high catalytic activity for the hydrolysis of cellobiose and the Beckmann rearrangement
owing to the high surface area and uniform microporous structure [37]. Katz et al. reported the
post-synthetically acid-functionalized zeolite-templated microporous carbons for the adsorption of
long-chain glucan and their hydrolysis to glucose in up to 87% yield [38, 39]. Recently, Sazama
et al. reported that the 3D graphene-like zeolite-templated microporous carbons were used as an
efficient catalyst for the hydrogenation of alkenes, aromatic alkynes, and cycloalkenes [36].
Encouraged by these results of the zeolite-templated microporous carbon materials, herein we have
prepared nitrogen-doped porous carbon (NPC). The material was doped with nitrogen post-
synthetically by using melamine as the nitrogen source, and used as a solid support to immobilize
Pd NPs. The resultant Pd/NPC was explored as a solid catalyst for the liquid phase CTH of FF
using alcohol as a hydrogen source (Scheme 1). The catalyst showed high activity and selectivity
to FFA. A possible mechanistic pathway for the CTH of FF over Pd/NPC catalyst was also
proposed based on experimental results and previous literature reports.
2. Experimental section
2.1. Materials
Calcium chloride hexahydrate, melamine, Pd(OAc)2, hydrazine monohydrate (N2H4·H2O,
98 %), acetone, FF, KOH, triethylamine (TEA), tripotassium phosphate (K3PO4), potassium tert-
butoxide (KtOBu), and all alcohols used in this study were obtained from Sigma Aldrich (USA)
5

and used as received. Activated carbon (AC; Vulcan) was received from Cabot Corporation
(USA).
2.2. Preparation of PC
The carbon support was prepared via a hard-templating route using zeolite as the template.
The protocol was based on a previously reported procedure [35]. β-Zeolite (Si/Al = 14, Tosoh) and
ethylene gas were used as a sacrificial template and a carbon source, respectively. Prior to carbon
deposition within the β-zeolite, ion-exchange into the zeolite was performed with a 0.5 M aqueous
solution of calcium chloride hexahydrate at room temperature. Next, 20 g of Ca2+-β-zeolite was
placed in a quartz plug-flow reactor equipped with a 15-mm diameter fritted disk. The reactor was
then heated to 650 °C at a heating rate of 5 °C min−1 under dry N2 flow. After stabilizing the
temperature at 650 °C, a mixture of ethylene, N2, and water vapor (10/87/3) was passed through
the zeolite bed. The flow rate of the gas mixture was 800 cm3 min−1. The gas flow was continued
until the carbon deposition process gave a carbon yield of ca. 320 mg g−1, which indicated
complete carbon deposition into the zeolite pores. After full deposition, the ethylene and water
vapor feeds were stopped. The reactor was then heated to 900 °C under dry N2 flow. The
temperature was maintained for 2 h to rigidify the carbon frameworks formed in the β-zeolite.
After cooling the reactor, the resultant sample was treated repeatedly with a 0.3 M HF/0.15 M HCl
solution to remove the zeolite framework. The final carbon products were collected by filtration,
washed with deionized water, and dried at 100 °C.
2.3. Preparation of N-doped porous carbon (NPC)
In a typical synthesis, PC (1.0 g) and melamine (1.0 g) were suspended in ethanol (EtOH)
and the mixture was stirred at room temperature (RT) for 30 min. Subsequently, the resultant
mixture was heated to 100 °C with stirring to remove ethanol. The residual material was transferred
6

into an alumina boat for carbonization. The boat was placed in a horizontal quartz tube furnace
under dry N2 flow for 30 min and then heated to 800 °C for 2 h with a ramp rate of 2 °C min–1.
After carbonization, the system was cooled to RT naturally to obtain NPC. For comparison,
nitrogen-doped AC (NAC) was also prepared by the same procedure.
2.4. Preparation of Pd/NPC catalyst
Pd NPs were anchored on NPC via an impregnation method. NPC (1.0 g) was suspended
in acetone (40 mL) and Pd(OAc)2 (40 mg) was added at RT under stirring. After 4 h, the mixture
was filtered and washed with acetone to remove the unbound Pd(OAc)2 and dried at RT for 1 h.
The resultant solid was immersed in methanol (40 mL) and then N2H4·H2O (2 mL) was added
slowly under vigorous stirring. After 2 h, the final black solid was isolated by filtration, washed
with methanol (20 mL), and dried at 80 °C for 3 h in vacuum to obtain the Pd/NPC solid catalyst.
For comparison of the catalytic performance, Pd/PC (without N-doping), Pd/AC, and Pd/NAC
were also prepared by the aforementioned procedure.
2.5. Catalysts characterization
Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku diffractometer with
Cu Kα radiation (λ = 1.54 Å) at a scan rate of 2° min−1 over 2θ = 5-90°. Raman spectra were
measured on a MonoRa 750i/ELT10000 spectrometer with λex = 1064 nm using a Nd:YAG laser
source. X-ray photoelectron spectroscopy (XPS) was conducted using an X-ray photoelectron
spectrometer (Thermo Scientific, USA) with AlKα radiation. The nitrogen adsorption
measurements were performed with a BELsorp-max analyzer (BEL, Japan) at 77 K to obtain the
adsorption-desorption isotherms. Specific surface area and pore size distribution were calculated
from Brunauer–Emmett–Teller (BET) and non-local density functional theory with slit pore
model. Elemental analysis (EA) for carbon, hydrogen, and nitrogen was performed using an
7

Elementar Vario EL-III analyzer. Inductively coupled plasma-optical emission spectroscopy (ICP-
OES, PerkinElmer Optima 7300 DV, USA) was performed to determine the Pd loading. The
morphologies, particle sizes, and dispersions of Pd particles deposited onto the PC and/or NPC
support materials were determined by transmission electron microscopy (TEM, Philips, CM200
UT).
2.6. Catalytic reactions
CTH of FF was performed in a 15 mL ACE pressure tube (Sigma Aldrich, USA) equipped
with a magnetic stirrer. In a typical procedure, FF (1.0 mmol), base (0.5 mmol), Pd/NPC catalyst
(60 mg; 1.05 mol%), and 2-BuOH (5 mL) were charged into the pressure tube, which was then
sealed. The reaction tube was heated to a desired temperature in a temperature-controlled silicone
oil bath during the reaction. After the reaction was complete, the tube was cooled to RT and the
catalyst was separated using a 0.2 μm membrane filter. The products in the liquid phase were
quantitatively determined by gas chromatography (GC). The gas chromatograph was equipped
with a HP-5 capillary column (HP6890 model, Agilent Technologies, USA) and a flame ionization
detector. The side product was identified by GC together with mass spectroscopy (GC-MS, Varian
1200 L single quadrupole with 3800GC model, Agilent Technologies, USA). The conversion of
FF was estimated from the amount of FF consumed after the reaction divided by the initial amount
of FF. Similarly, the selectivity to FFA was determined from the amount of FFA product divided
by the amount of all products in the liquid mixture.
3. Results and discussion
3.1. Characterization of the catalyst
Highly porous nitrogen-doped carbon was successfully synthesized, and Pd NPs were
immobilized via a chemical reduction method using Pd(OAc)2 as the precursor. The structures of
8

the as-synthesized carbon materials and carbon-supported Pd NPs were characterized by powder
XRD, Raman spectroscopy, EA, XPS, N2-adsorption isotherms, and TEM. The XRD patterns of
the prepared PC, NPC, Pd/PC, and Pd/NPC materials are shown in Fig. 1(a). The highly ordered
microporous structure of PC was confirmed by a characteristic peak at ~7.5° [35]. In addition, all
samples showed two broad diffraction peaks at ~24 and 43°, which corresponded to the (002) and
(100) diffraction planes, respectively. The high intensity peak of the (002) plane could be ascribed
to a long-range order of the stacking of the aromatic planes of carbon and the XRD peak of the
(100) plane was attributed to the highly disordered graphitic structures [40, 41]. The diffraction
peaks of Pd NPs were not observed in the XRD pattern of Pd/NPC as the Pd particles were highly
dispersed on the nitrogen-doped porous carbon support and were of small sizes [42, 43]. The
chemical and structural features, degree of hybridization, and disorder properties of the prepared
carbon samples were examined using Raman spectroscopy. The Raman spectra of PC, NPC, and
Pd/NPC materials in the range from 1000 to 2000 cm–1 (Fig. 1(b)) showed two characteristic G
and D bands: the G band at ~1590 cm–1 was assigned to the primary in-plane vibrational mode sp2-
hydridized carbon atoms in a hexagonal graphitic ring and the D band at ~1300 cm–1 corresponded
to the structural defects in the graphitic sp2 network [40]. These structural defects were caused by
the impregnation of nitrogen, oxygen, metals, and other impurities that dissociated the sp2 bonds
to produce sp3 C–C bonds [44]. The degree of graphitization and the interaction between the nano-
metals and carbon support was determined by the ratio ID/IG. The ID/IG ratio of the prepared
catalysts increased from 0.98 for PC to 1.04 for NPC and 1.38 for Pd/NPC. The ID/IG ratio of Pd-
incorporated NPC was higher than that of pure PC and N-doped PC, indicating that the introduction
of Pd metal on the NPC reduced the degree of graphitization. This result was attributed to the
interaction between the π-electrons of the carbon material and the metallic Pd particles. The C, H,
9

and N contents of the PC and NPC materials were determined by EA. The O content was calculated
using EA results by subtracting the sum of C, N, and H contents from 100. The C, H, and O
contents of PC were 91.41, 2.47, and 6.12%, respectively. Likewise, the C, H, N, and O contents
of NPC were determined to be 79.68, 2.04, 11.30, and 6.98%, respectively. The Pd contents of
Pd/PC, Pd/NPC, Pd/AC, and Pd/NAC, were measured by ICP-OES and found to be 1.73, 1.85,
1.52, and 1.68 wt%, respectively.
The nature of chemical bonding and presence of nitrogen and oxygen atoms in the chemical
structure of the carbon samples were assessed by XPS. The C 1s high resolution XPS of PC (Fig.
2(a)) exhibited six deconvoluted binding energy peaks: the strong peak at 284.4 eV was ascribed
to the graphitic C=C structure, the peak at 285.6 eV corresponded to the sp3 hybridized C–C
species (sp3 hybridization was due to the presence of graphitic structural defects, which was
consistent with the XRD and Raman spectroscopy results), the peak at 286.5 eV was ascribed to
the presence of the C–O bond, and the peaks at 287.7, 289.2, and 291.1 eV were attributed to the
presence of C=O, COO bonds (–COOH group), and π–π* transition [45, 46]. The O 1s
deconvoluted spectrum of PC (Fig. 2(b)) showed three different environmental binding energy
peaks at 531.4, 533.0, and 534.7 eV, which were assigned to the C–O, C=O, and COO groups,
respectively. These results indicated that the prepared PC was distorted graphitic in character with
C–C, C–O, C=O, and –COOH functional groups.
Figure 3 shows the C, O, and N 1s XPS of NPC, in which C and O 1s revealed almost a
similar set of binding energy peaks as PC. This result implied that the same functional groups as
PC were present in NPC. While untreated PC did not show any noticeable peak in the N 1s XPS
region (Fig. 2(c)), the nitrogen-treated PC displayed a strong N 1s peak at ~400 eV (Fig. 3(c))
corresponding to the incorporation of nitrogen on the carbon surface. The N 1s XPS of NPC
10

exhibited four deconvoluted binding energy peaks at 398.8, 400.0, 401.1, and 402.6 eV for
pyridinic-N, pyrrolic-N, graphitic-N, and oxygenated-N, respectively [47], These species were
involved in the stabilization of the metal particles and improved the catalytic activity in the
hydrogenation reaction.
The N 1s high resolution XPS of Pd/NPC is shown in Fig. 4(a). The deconvolution of the
N 1s spectrum of Pd/NPC exhibited four different binding energy peaks associated with the
different nitrogen states of pyridinic-N (397.8 eV), pyrrolic-N (399.7 eV), graphitic-N (400.8 eV),
and oxygenated-N (402.7 eV), in which the pyridinic-N binding energy was strongly shifted from
the original value of 398.8 eV (without Pd). This result clearly demonstrated that most of the Pd
particles were strongly bound to pyridinic-N in Pd/NPC. The level of reduction and electronic
states of Pd particles were also examined using XPS and these results are shown in Fig. 4(b). The
two strong Pd 3d binding energy peaks located at 340.7 and 335.4 eV were attributed to metallic
Pd 3d3/2 and 3d5/2 states, whereas the other very weak doublet located at about 335.7 and 341.4 eV
was assignable to Pd2+. These results indicated that the most of the Pd2+ has been reduced to Pd0
and only a small portion of Pd2+ has been bound on the Lewis-base sites of pyridinic nitrogen
species [48, 49].
The N2 adsorption-desorption isotherms were measured at 77 K to determine the porous
properties of the prepared carbon and Pd-incorporated samples (Fig. 5). It can be seen that all the
samples exhibited type I isotherms with steep uptakes at low pressure (Fig. 5(a)), corresponding
to the presence of micropores. The specific surface areas calculated from the BET model for PC,
NPC, Pd/PC, and Pd/NPC were 3004, 1492, 2198, and 1010 m2 g-1, respectively. A substantial
reduction in the surface area was evident after melamine treatment and introduction of Pd NPs.
The Pd NPs were bigger in size (see later) than the average pore diameter of PC (1.04 nm) and
11

NPC (0.96 nm) as shown in Fig. 5(b), and were likely formed primarily on the external surface of
NPC via electrostatic interactions with the nitrogen [50]. Similar types of metal nanoparticles
formed on external surface of the microporous solid materials for various liquid phase organic
transformations in heterogeneous catalysis have been reported frequently [50-54]. Figure 6 shows
the TEM images of Pd/PC and Pd/NPC catalysts. The Pd particles supported on NPC (Fig. 6(a, b))
were highly dispersed with uniform particle sizes of 2-4 nm, whereas the Pd particles supported
on PC (Fig. 6(c)) were agglomerated and had an average size of ~7 nm. Uniformly dispersed Pd
particles of small sizes were formed on NPC stabilized by nitrogen atoms [55]. On the other hand,
the average Pd particle sizes are ca. 9 and 6 nm for Pd/AC (Fig. 6(d)) and Pd/NAC (Fig. 6(e)),
respectively.
3.2. Catalytic transfer hydrogenation of FF over Pd/NPC
The catalytic performance of the synthesized Pd catalysts was investigated for the transfer
hydrogenation of FF to FFA using an alcohol as a hydrogen donor and solvents. The results of
these experiments are summarized in Table 1. The reaction was conducted for 10 h with 2-PrOH
and the conversion of FF and the selectivity to FFA was determined by GC analysis. Almost no
FF conversion was observed when the reaction was conducted without a catalyst (Table 1, entry
1). The reaction performed either with PC and/or NPC alone led to ~10% conversion of FF (Table
1, entries 2 and 3), and only acetal product was formed (acetalization reaction between FF and 2-
PrOH), as confirmed by GC-MS analysis, which may be due to the presence of the small portion
of acid sites (–COOH group) in the catalysts [31, 56]. The Pd/PC catalyst achieved 42% conversion
of FF with 71% selectivity to FFA, and acetal and tetrahydrofurfuryl alcohol (THFFA; over
hydrogenation of FFA) were formed as by-products at 120 °C after 10 h (Table 1, entry 4),
indicating that the Pd catalyst promoted the CTH reaction. On the other hand, when the reaction
12

was carried out over Pd/NPC under identical conditions, FF conversion increased to 68% with
82% selectivity to FFA (Table 1, entry 5), and a small amount of acetal and THFFA was formed
as by-products, in which the high nitrogen contents on the support as well as the microporosity
inhibited the acid-catalyzed bulky acetal by-product. These results clearly indicated that Pd/NPC
possessed higher catalytic activity than Pd/PC, which suggested that specific Pd and nitrogen sites
in the catalyst likely had a synergistic effect in the transfer hydrogenation of FF to FFA via the
Meerwein–Ponndorf–Verley reduction. We also studied the importance and nature of a base in the
CTH of FF. It was found that a base was essential when an alcohol was used as a hydrogen donor
for extracting the proton from the alcohol, as the conversion of FF decreased in the absence of the
base (Table 1, entry 6). Among the various inorganic (KOH, activated K3PO4 (at 200 °C), and
KtOBu) and organic (TEA) bases (Table 1, entries 5, 7–9), KtOBu gave the highest conversion of
FF (92%) with 84% selectivity to FFA. When we reduced the concentration of the base from 0.5
mmol to 0.3 mmol, the conversion of FF was also reduced to 74% (Table 1, entry 10), indicating
that 0.5 mmol of KtOBu was the optimum concentration for this reaction. The influence of
o
temperature on the CTH of FF with 2-PrOH was examined in the range of 90-120 C over the
Pd/NPC catalyst. After 10 h, FF conversion improved from 32 to 92% when the reaction
o
temperature was increased from 90 to 120 C (Table 1, entries 9 and 11). At the same time, the
FFA selectivity was also slightly affected by the reaction temperature.
The influence of various hydrogen donors such as primary and secondary alcohols on the
catalytic performance in the hydrogenation of FF with KtOBu was also studied at 120 oC and the
results are shown in Fig. 7. When primary alcohols were used as hydrogen donors, ~30%
conversion of FF with mainly acetal products was observed. On the other hand, linear secondary
alcohols, especially 2-BuOH, led to 97% conversion of FF with 92% selectivity to FFA. In
13

contrast, with 2-PrOH as the hydrogen donor, a slightly lower conversion (92%) and selectivity
(84%) was achieved, which may be due to the differences in alcohol dehydrogenation. However,
in the case of cyclic secondary alcohols such as cyclohexanol, a very poor conversion of FF was
detected. This was likely due to the poor interaction between the catalyst and cyclohexanol because
of the steric hindrance, high viscosity, and high boiling point of cyclohexanol. The above results
clearly showed that the FF conversion and selectivity were strongly dependent on the nature of the
alcohol, and an increased FF conversion and high selectivity to FFA with different alcohols was
achieved in the following order: cyclohexanol < EtOH ≤ 1-PrOH < 2-PrOH < 2-BuOH. We also
compared the catalytic performance of Pd/NPC with that by Pd/AC and Pd/NAC catalysts in the
CTH of FF. The Pd/AC catalyst showed only moderate catalytic activity (59% conversion of FF)
with 51% yield of FFA (Table 1, entry 12) and acetal as a by-product. When the reaction was
carried out with Pd/NAC, 81% conversion of FF with 77% yield of FFA (Table 1, entry 13) and
13% yield of THFFA was achieved. These results indicated that the Pd/NPC catalyst has better
catalytic activity than Pd/AC and Pd/NAC, owing to the small particle size, high nitrogen contents,
and 3D graphene-like porous structure with connectivity to form a porous network [36].
The effect of Pd/NPC catalyst loading on the conversion of FF was also investigated using
2-BuOH and KtOBu at 120 oC for 10 h. As shown in Fig. 8(a), 28% conversion of FF was detected
using 30 mg of Pd/NPC, and it increased to 97% when the catalyst loading was increased to 60
mg. Interestingly, while the selectivity to FFA was unaffected by the increase in catalyst loading
up to 60 mg, a further increase in the catalyst amount to 70 mg led to decrease in the selectivity to
FFA and was accompanied by the formation of THFFA by-product because of the reaction of FFA
with 2-BuOH.
14

To obtain a better understanding of the hydrogenation of FF, a time course plot was made
for the reaction conducted over Pd/NPC catalyst (60 mg; 1.05 mol%) using FF (1 mmol), 2-BuOH
o
(5 mL), and KtOBu (0.5 mmol) at 120 C (Fig. 8(b)). The conversion of FF reached up to 30%
with 90% FFA selectivity after 2 h and then increased gradually with increasing reaction time. The
maximum conversion of FF (97%) was detected after 10 h reaction with 92% selectivity to FFA.
After 10 h, the selectivity of FFA dropped to 85% because of the formation of THFFA by-product.
3.3. Hot-filtration and recycling runs
A hot filtration experiment was performed to confirm the heterogeneous nature of the
Pd/NPC catalyst in the CTH of FF in the presence of 2-BuOH and KtOBu at 120 °C (Fig. 8(b)).
The Pd/NPC catalyst was separated from the reaction mixture by centrifugation after 4 h of
reaction and the reaction was continued with the filtrate at 120 °C for another 6 h. Conversion of
FF stopped completely after the catalyst was removed at 4 h of reaction time, which proved the
heterogeneity of the catalyst during the reaction.
In addition, the stability of the Pd/NPC catalyst was examined by conducting recycling
experiments under optimized conditions (i.e., KtOBu, 120 °C, 10 h). After each hydrogenation run,
the Pd/NPC catalyst was recovered by centrifugation, washed with water (2×10 mL, to remove the
base) and acetone (2×5 mL), dried at 50 °C for 1 h, and then directly reused for the next run. Figure
9 shows that the FF conversion and FFA selectivity remained nearly constant after five consecutive
runs, demonstrating the high stability of the Pd/NPC catalyst. Subsequently, the spent catalyst
(after fifth use) was further characterized by XRD, ICP-OES, XPS, TEM, and N2-adsorption
isotherm analyses. The XRD pattern of the spent Pd/NPC catalyst (Fig. 1) exhibited the same
pattern as the fresh catalyst, indicating that the structure of the catalyst was maintained after the
reaction at 120 °C. The Pd content in the spent Pd/NPC catalyst was found to be 1.81 wt% by ICP-
15

OES, which was almost close to that of the fresh catalyst (1.85 wt%). This indicated a negligible
loss of Pd particles. The Pd XPS analysis of the spent catalyst also showed almost the same pattern
of binding energy peaks, (Fig. 4(b)), indicating that the most of the Pd particles maintained the Pd0
state on the NPC support. The N2-adsorption isotherm of the spent Pd/NPC catalyst (Fig. 5) also
almost exactly matched with that of the fresh catalyst and the BET surface area was found to be
1007 m2 g–1. In addition, the TEM image of the spent catalyst (Fig. 6(f)) also showed an identical
morphology of metallic Pd and no agglomeration with a slightly increased size of ~4 nm. These
characterized results reflect that the Pd/NPC catalyst showed excellent stability and reusability
without loss of its activity.
3.4. Reaction pathway
Recently, Zhao et al. reported the catalytic FF hydrogenation using Ni supported on Lewis-
base nitrogen (from pyridinic nitrogen)-containing carbon. In this catalytic system, the nitrogen on
the catalyst increased the adsorption of hydrogen molecules and metallic Ni, and also assisted in
improving the interaction between the reactant and the catalytic sites of metallic Ni [57]. Similarly,
NPC materials can reportedly improve the catalytic activity in many hydrogenation and oxidation
reactions [58-60]. Based on the previous reports [57-60] and our experimental results, a possible
reaction pathway for the CTH of FF was proposed and is shown in Scheme 2. The N species in the
NPC support played two important roles in the hydrogenation reaction; the lone pair of electrons
on nitrogen could abstract the proton from the alcohol and it could also increase the electron
density of Pd NPs. In the first step, the base promoted the deprotonation of 2-BuOH, which led to
the anchoring of the generated butanoxide ion on the metallic Pd surface and the proton was
adsorbed on the pyridinic-N present in the catalyst. Next, the carbonyl group of FF was weakly
adsorbed on Pd surface and formed intermediate (I), followed by the elimination of 2-butanone
16

yielded intermediate (III) via an attack of hydride ion to the carbonyl group of FF. Finally, furfuryl
alkoxide abstracted a proton from the adjacent pyridinium ion and formed FFA. Thus, the catalytic
results and mechanism proved that the presence of both nitrogen and Pd species in a single catalyst
was synergistic for the hydrogenation of FF to FFA.
Finally, the catalytic efficiency of the present Pd/NPC system in the CTH of FF to FFA
was compared with the efficiencies achieved with previously reported heterogeneous catalytic
systems (Table 2). The reported catalytic systems often required high-pressure H2 gas, expensive
Ir and Ru precursors, relatively higher metal loading, longer reaction time, and higher reaction
temperature. In contrast, the CTH system described in this work has shown a higher turnover
frequency compared to others except for the high-cost Ir-carbene-complex.
4. Conclusions
Pd NPs supported on N-doped porous carbon were prepared by the chemical reduction of
the Pd2+ ions immobilized on the support. The Pd/NPC catalyst was comprised of highly dispersed
uniform particles with sizes of 2–4 nm, which were anchored on the pyridinic nitrogen atom in the
carbon support. The Pd/NPC catalyst exhibited high activity and selectivity toward FFA in the
catalytic transfer hydrogenation of biomass-derived FF using 2-BuOH as a hydrogen donor. The
promising catalytic performance of Pd/NPC in comparison with other systems was likely a result
of the synergistic effect offered by the nitrogen species and the Pd particles on the carbon network.
The Pd/NPC catalyst could be recycled for up to five successive runs without any noticeable
decrease in its activity and selectivity, and the hot-filtration experiments confirmed the
heterogeneity of the catalyst. The high stability of the recovered catalyst was confirmed by various
characterization tools.
17

Acknowledgments
This work was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF-
2015R1A4A1042434). PP acknowledges the support by the Korea Research Fellowship through
the NRF of Korea funded by the Ministry of Science and ICT (Grant number:
2017H1D3A1A02013620).
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24

Scheme 1. Reaction scheme for the CTH of FF.
25

Scheme 2. Possible mechanistic pathway for the CTH of FF over Pd/NPC catalyst.
26

Fig. 1. (a) Powder XRD patterns of PC, NPC, Pd/PC, Pd/NPC, and reused Pd/NPC and (b) Raman
spectra of PC, NPC, and Pd/NPC.
27

Fig. 2. (a) C 1s and (b) O 1s high resolution XPS of PC.
28

Fig. 3. (a) C 1s, (b) O 1s, and (c) N 1s high resolution XPS of NPC.
29