Formic Acid-Assisted Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd Nanoparticles Supported on N-Doped Mesoporous Carbon

Article abstract of DOI:10.1002/anie.202012816

Biomass-derived 5-hydroxymethylfurfural (HMF) is regarded as one of the most promising platform chemicals to produce 2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd nanoparticles supported on N-containing and N-free mesoporous carbon materials were prepared, characterized, and applied in the hydrogenolysis of HMF to DMF under mild reaction conditions. Quantitative conversion of HMF to DMF was achieved in the presence of formic acid (FA) and H₂ over Pd/NMC within 2 h. The reaction mechanism, especially the multiple roles of FA, was explored through a detailed comparative study by varying hydrogen source, additive, and substrate as well as by applying in situ ATR-IR spectroscopy. The major role of FA is to shift the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via the protonation by FA at the C-OH group, lowering the activation barrier of the C?O bond cleavage and thus significantly enhancing the reaction rate. XPS results and DFT calculations revealed that Pd²⁺ species interacting with pyridine-like N atoms significantly enhance the selective hydrogenolysis of the C?OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H^?.

Full text of DOI:10.1002/anie.202012816

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Formic Acid-Assisted Selective Hydrogenolysis of 5-
Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd
Nanoparticles Supported on N-doped Mesoporous Carbon
Authors: Bin Hu, Lisa Warczinski, Xiaoyu Li, Mohong Lu, Johannes
Bitzer, Markus Heidelmann, Till Eckhard, Qi Fu, Jonas
Schulwitz, Mariia Merko, Mingshi Li, Wolfgang Kleist, Christof
Hättig, Martin Muhler, and Baoxiang Peng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202012816
Link to VoR: https://doi.org/10.1002/anie.202012816

10.1002/anie.202012816
Angewandte Chemie International Edition
RESEARCH ARTICLE
Formic Acid-Assisted Selective Hydrogenolysis of 5-
Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd
Nanoparticles Supported on N-doped Mesoporous Carbon
Bin Hu[a],[b], Lisa Warczinski[c], Xiaoyu Li[a], Mohong Lu[d], Johannes Bitzer[a], Markus Heidelmann[e], Till
Eckhard[a], Qi Fu[a], Jonas Schulwitz[a], Mariia Merko[a], Mingshi Li[d], Wolfgang Kleist[a], Christof Hättig[c],
Martin Muhler[a],[b], and Baoxiang Peng*[a],[b]
[a]
B. Hu, X. Li, J. Bitzer, T. Eckhard, Q. Fu, J. Schulwitz, M. Merko, Prof. Dr. W. Kleist, Prof. Dr. M. Muhler, Dr. B. Peng
Laboratory of Industrial Chemistry
Ruhr-University Bochum, 44780 Bochum, Germany
E-mail: baoxiang.peng@techem.rub.de
[b]
[c]
B. Hu, Prof. Dr. M. Muhler, Dr. B. Peng
Max Planck Institute for Chemical Energy Conversion,
45470 Mülheim a.d. Ruhr, Germany
L. Warczinski, Prof. Dr. C. Hättig
Chair of Theoretical Chemistry
Ruhr-University Bochum, 44780 Bochum, Germany
Prof. Dr. M. Lu, Prof. Dr. M. Li
School of Petrochemical Engineering, Changzhou University, China
Dr. M. Heidelmann
[d]
[e]
Interdisciplinary Center for Analytics on the Nanoscale
University of Duisburg-Essen, 47057 Duisburg, Germany
Supporting information for this article is given via a link at the end of the document.
Abstract: Biomass-derived 5-hydroxymethylfurfural (HMF) is
regarded as one of the most promising platform chemicals to produce
2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd
nanoparticles supported on N-containing and N-free mesoporous
carbon materials were prepared, characterized, and applied in the
hydrogenolysis of HMF to DMF under mild reaction conditions.
Quantitative conversion of HMF to DMF was achieved in the presence
of formic acid (FA) and H2 over Pd/NMC within 2 h. The reaction
mechanism, especially the multiple roles of FA, was explored through
a detailed comparative study by varying hydrogen source, additive,
and substrate as well as by applying in situ ATR-IR spectroscopy. The
major role of FA is to shift the dominant reaction pathway from the
hydrogenation of the aldehyde group to the hydrogenolysis of the
hydroxymethyl group via the protonation by FA at the C−OH group,
lowering the activation barrier of the C−O bond cleavage and thus
significantly enhancing the reaction rate. XPS results and DFT
calculations revealed that Pd2+ species interacting with pyridine-like N
atoms significantly enhance the selective hydrogenolysis of the C−OH
bond in the presence of FA due to their high ability for the activation
of FA and the stabilization of H-.
obtaining biofuel from renewable feedstock.9 DMF is considered
as a potential liquid transportation biofuel, which has a higher
octane number than ethanol, superior energy intensity, nearly
ideal boiling point, excellent miscibility with gasoline and
immiscibility with water.10 Moreover, DMF is also a renewable
source for the production of p-xylene via the Diels-Alder
reaction.11
The common route to produce DMF is the hydrogenolysis of
carbohydrates or HMF with hydrogen, for which a large variety of
catalysts has been investigated. In 2007, Dumesic et al. reported
a two-step catalytic process, resulting in 71% yield of DMF from
fructose via HMF as intermediate over Cu/C at 220 ℃ and 6.8 bar
H2.6,12 In 2010, Bell et al. reported a two-step approach for the
conversion of glucose to DMF, achieving 15% DMF yield based
on Pd/C in ionic liquid under high pressure of H2 (62 bar).13
Recently, Abu-Omar et al. synthesized a bimetallic catalyst
containing Lewis-acidic Zn and Pd/C species and applied it in the
hydrogenolysis of HMF to DMF, leading to 99% conversion and
85% DMF selectivity, but the bimetallic catalyst suffered from
deactivation after the 4th recycle.14 Yang et al. reported that Ru-
MoOx/C led to a DMF yield of 79% at 180 °C and 15 bar H2.15
Generally, relative harsh reaction conditions, especially high
pressures of H2, were employed to obtain appropriate conversion
due to the low solubility of H2.16 Another key challenge for
upgrading HMF is to obtain high product selectivity. The chemical
transformation of HMF to DMF is a multistep process, including
the hydrogenolysis of the hydroxymethyl group and the
hydrogenation of the aldehyde group (Scheme 1). Importantly, the
main side reaction of furan-ring hydrogenation should be avoided.
Overall, the highly selective full conversion of HMF to DMF under
mild reaction conditions is still a challenge.
Introduction
The utilization of renewable biomass feedstocks for the
production of biofuels and commodity chemicals has attracted
significant attention.1,2 5-Hydroxymethylfurfural (HMF) is regarded
as one of the most important platform chemicals during biomass
transformation.3,4 Various chemicals and biofuels, such as
levulinic acid,5 2,5-dimethylfuran,6 2,5-furandicarboxylic acid,7
and ethyl levulinate,8 can be produced from HMF. The selective
hydrogenolysis of HMF to DMF is a promising utilization route for
1
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10.1002/anie.202012816
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RESEARCH ARTICLE
Scheme 1. Reaction network for the hydrogenolysis of HMF to DMF.
Molecular hydrogen is widely employed in hydrogenation
/hydrogenolysis processes, owing to its high availability and facile
activation on metal surfaces. Formic acid (FA), which can be
produced from lignocellulosic biomass or electrochemical
reduction of CO2, is being considered for reversible hydrogen
storage as well as a hydrogen donor for catalytic hydrogenation.17
Compared with H2, FA not only has a higher energy density of 6.4
MJ L-1,18 but also has excellent stability and low toxicity, which is
desirable for safe storage and transportation.19 Rauchfuss et al.
reported the conversion of HMF with FA under mild conditions
over Pd/C, yielding 95% DMF in 15 h.20,21 The key strategy is the
utilization of FA, which serves multiple roles as acidic catalyst,
hydrogen donor, catalyst moderator inhibiting decarbonylation
and ring hydrogenation, and as precursor to formate esters, which
are key intermediates that are activated toward the selective
hydrogenolysis of the C−OH bond. Tao et al. also reported a DMF
yield of 98% from HMF and FA over 1% Au^Pd0.2/t-ZrO2.22
Nevertheless, the different roles of FA have not yet been
elucidated in detail.
Pd exhibits high catalytic activity in the hydrogenolysis of HMF to
DMF using FA as the hydrogen source, and it is usually supposed
to be the optimum metal for the decomposition of FA.23 N-doped
carbon materials have been proven to be promising supports.24
Doping with electron-rich N atoms modifies the surface structure
of carbon materials and endows improved π-binding ability and
enhanced basicity,25 which enhances the catalytic activity of
supported metal catalysts.26-28 For example, isolated Pd2+ cations
supported on N-doped carbon were identified as active sites for
the H2 production from FA, which showed an enhanced reaction
rate of 2-3 times compared with Pd supported on N-free carbon.29
It was also demonstrated that the Pd2+ species are coordinated
by pyridinic N according to XPS, NEXAFS, and STEM studies.29
Arrigo et al. reported a detailed study of metal-support interactions
of N-doped carbon nanotubes with Pd2+ species.30 In our previous
study, we were able to prove the presence of bifunctional Pd
nanoparticles (NPs) supported on N-doped mesoporous carbon
(NMC), containing both Pd0 and Pd2+ active sites.31
variation of the catalyst support (N-rich vs. N-free mesoporous
carbon (CMC)) and the hydrogen donor (H2, FA, both H2 and FA).
In situ ATR-IR spectroscopy was employed to monitor the
reaction progress and to explore the reaction pathways. Density
functional theory (DFT) was applied to gain further insight into the
bifunctional electronic properties of Pd/NMC and its interaction
with FA.
Results and Discussion
Characterization. The actual Pd loadings of the catalysts
determined by AAS were found to be ca. 1.0 wt% close to their
expected values (Table S1). The isotherms can be classified as
type IV with H3 hysteresis loops for CMC and NMC representing
typical mesopores (Figure S1). CMC displays a large specific
surface area of 997 m2 g-1, while the surface area of NMC is
smaller but still exhibits a surface area of 657 m2 g-1 (Table S1).
The similar pore volumes and average pore sizes of these two
carbon materials clearly indicate comparable mesoporous
structures. After the colloidal deposition of Pd, the BET surface
areas, pore sizes, and pore volumes of both supported Pd
catalysts slightly decreased compared with those of the parent
materials. The properties of active carbon (AC) and Pd/AC are
also listed in Table S1.
XRD patterns show that all samples exhibit the characteristic
reflections of graphitic carbon at ca. 26° (Figure S2). Pd/CMC
shows clearly visible Pd(111) reflections at 40.2°, suggesting the
presence of a larger number of Pd NPs above 5 nm, which can
be detected by XRD.32,33 By comparison, no significant Pd
reflections are identified in Pd/NMC, implying a higher Pd
dispersion and that most Pd NPs are smaller than the XRD
detection limit of about 5 nm. This result points to the crucial role
of N atoms in NMC as strong anchoring sites for Pd NPs. The
observation is in good agreement with previous reports, showing
that N-doped carbon and TiO2 can effectively anchor metal NPs
and enhance their dispersion.34,35
Our present work aims at developing the quantitative conversion
of HMF to DMF under mild reaction conditions, obtaining a
comprehensive understanding of the versatile roles of FA, and
revealing the influence of the bifunctionality of the Pd NPs on the
reaction. Thus, a systematic comparative study was performed by
2
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10.1002/anie.202012816
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RESEARCH ARTICLE
points to fragmentation occurring during the colloidal deposition.
The lattice fringe spacings of both catalysts are determined to be
0.22 nm representing Pd(111) lattice planes. By comparison,
Pd/AC has a larger mean Pd particle size of 4.4 nm as well as a
significantly larger number of NPs above 5 nm (Figure S4).
XPS results reveal that the surface Pd concentration of Pd/NMC
(0.61 at%) is much lower than that of Pd/CMC (1.83 at%) (Table
1, Figure S6). As both catalysts have similar bulk Pd loadings and
Pd/NMC has smaller particle size and better dispersion than
Pd/CMC according to the STEM measurements, the significant
difference in the surface Pd concentration implies that Pd is
presumably homogeneously dispersed both on the outer and
inner surface of NMC due to the incorporation of N atoms, while
Pd prefers to interact with the O-functionalized groups and
deposits on the outer CMC surface. Furthermore, the N 1s core
level XP spectra of NMC demonstrate the presence of pyridinic
(45%), pyrrolic (46%), quaternary-like (6%), and oxidized or NO-
like N atoms (3%) in NMC (Figure S7).31 For Pd/NMC, a better
fitting of the N 1s spectrum was obtained when assuming the
presence of another N species interacting with Pd NPs (N-Pd
component).
In our previous studies, we were able to achieve an in-depth
insight into the metal-support interactions of Pd/CMC and
Pd/NMC.31,36 Briefly, Pd NPs interact via directed and partially
covalent Pd–C interactions with the carbon support.36 Figure 3a
shows the most stable structure of a Pd21 cluster with a calculated
diameter of 0.85 nm on NMC determined by ground-state
geometry optimization. Two pyridinic N atoms are interacting with
one Pd atom, suggesting pyridinic N atoms as the preferential
adsorption sites for Pd clusters. The relevant part of the
corresponding electron density difference is reproduced in Figure
3b. The complicated structure can be explained with the help of
an intrinsic bond orbital analysis, showing that the pyridinic N
atoms possess a lone pair (LP), located in a sp2-orbital lying in the
Figure 1. HAADF-STEM images and Pd particle size distributions for Pd/NMC
and Pd/CMC. The yellow circles refer to sub-1 nm Pd NPs or clusters.
Figure 2. Pd 3d regions of the deconvoluted XPS results for Pd/NMC before
and after reaction in the presence of FA and H2.
Table 1. Chemical compositions of NMC, CMC, Pd/NMC, and Pd/CMC.
Sample
Pd (wt/at%)
bulka surf.b
N (wt/at%)
O (at%)
surf.b
6.3
bulkc
surf.b
10.6
–
NMC
CMC
–
–
13.4
–
–
–
11.0
7.4
Pd/NMC
Pd/CMC
0.95
0.98
0.61
1.83
13.3
–
10.9
–
11.5
Determined by [a] AAS, [b] XPS and [c] elemental analysis.
Pd NPs in the range of 2 to 5 nm can be clearly seen in the TEM
images of both Pd/NMC and Pd/CMC (Figure S3). The mean Pd
particle sizes for Pd/NMC and Pd/CMC are relatively similar
amounting to 2.8 and 3.6 nm, respectively. Nevertheless, the
HAADF-STEM images show that there are some sub-1 nm Pd
NPs or clusters in Pd/NMC, while these sub-1 nm NPs are rarely
detected in Pd/CMC (Figure 1). The presence of sub-1nm NPs
Figure 3. DFT calculations for Pd/NMC model: (a) most stable structure of Pd21
on pyridinic carbon support (two pyridinic N connected to Pd21); (b) change in
the electron density induced by the interaction between the Pd21 cluster and the
pyridinic N atoms (green: density gain; grey: density loss).
3
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10.1002/anie.202012816
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RESEARCH ARTICLE
support plane. When a Pd cluster is adsorbed on pyridinic N, two
N atoms are interacting with one Pd atom via an N-LP - Pd d휎 and
an N-LP – Pd s as well as a Pd d휋 - 휋* charge transfer. The
interaction of the N-LP orbitals with the Pd atom leads to a density
loss in the N-LP orbital, while the Pd d휋 - 휋* interaction increases
the density in the N 휋 orbital and leads to a change in the Pd
oxidation state to Pd2+. Indeed, only Pd0 and Pdδ+ are observed
by XPS for Pd/CMC (Figure S8), whereas an additional peak,
which can be attributed to a divalent Pd species stabilized by the
strong interaction with pyridine-like N atoms30, is observed for
Pd/NMC (Figure 2). Pd0 (68%) is the dominant species for
Pd/CMC, while Pd2+ (50%) is the main species for Pd/NMC (Table
S2). These results prove the presence of bifunctional Pd NPs
containing both Pd0 and Pd2+ active sites, supported by pyridinic
N atoms in NMC.31 In addition, XAS results of as-prepared
samples also strongly support the structure model of small Pd
clusters containing oxidized Pdδ+ or Pd2+ centers at their edges,
which are attached to the support via O or N atoms (see detailed
XAS and TPR analysis in Supporting Information). The higher
oxidation state of Pd in Pd/NMC compared to Pd/CMC is also
verified by TPR experiments.
Catalytic conversion of HMF to DMF. The conversion of HMF
can proceed via two pathways, depending on the sequence of
hydrogenolysis of the hydroxymethyl group and hydrogenation of
the aldehyde group (Scheme 1). These two pathways produce
distinct intermediates: 5-methylfurfural (5-MF) for path A and 2,5-
bis(hydroxymethyl)furan (BHMF) for path B. Both paths proceed
further to
a
common intermediate 2-(hydroxymethyl)-5-
methylfuran (MFA) followed by subsequent hydrogenolysis to
DMF. The hydrogenation of the furan ring leads to undesired
byproducts. The hydrogenolysis of HMF to DMF with different
hydrogen sources over Pd-based catalysts was investigated and
summarized in Table 2.
When H2 was used, the blank experiment led to only 4.6%
conversion, and DMF was not observed (Table 2, entry 1). The
experiments with NMC and CMC resulted in similar conversion as
Table 2. Product distributions of HMF hydrogenolysis over the supported Pd catalystsa.
Entry
Catalysts
Hydrogen
Source
Conversion
(%)
DMF Selectivity
(%)
Yields (%)
DMF
-
BHMF
5-MF
2.4
-
MFA
-
Otherse
-
b
1
2
3
-
H2
4.6
-
2.2
b
Pd/NMC
Pd/CMC
H2
> 99.9
78.8
35.3
49.2
35.3
38.8
-
-
42.0
18.3
22.7
15.1
b
H2
6.6
b
4
5
6
7
8
Pd/AC
H2
19.4
60.8
21.4
> 99.9
90.1
38.3
64.3
62.9
> 97.0
80.0
7.4
1.9
8.4
0.6
1.0
Pd/NMC
Pd/CMC
Pd/NMC
Pd/CMC
FAc
FAc
39.1
13.5
97.1
72.1
-
-
-
-
11.5
7.9
10.2
-
-
-
-
-
-
-
d
FA+H2
2.9
d
FA+H2
18.0
[a] Reaction conditions: 1.5 mmol HMF, 50 mg catalysts, 160 ℃, 3 h; [b] 5 bar H2; [c] 45 mmol FA (30 equiv.) at 5 bar N2 and [d] H2; [e] mainly ring-hydrogenated
products including 2,5-bis(hydroxymethyl)tetrahydrofuran, 5-methyltetrahydrofurfuryl alcohol, and 2,5-dimethyltetrahydrofuran.
Figure 4. Time course of the hydrogenolysis of HMF to DMF over Pd/NMC with (a) 5 bar H2; (b) FA and 5 bar N2; (c) FA and 5 bar H2. (d) Comparison of DMF
yields using different hydrogen donors over time.
4
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10.1002/anie.202012816
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RESEARCH ARTICLE
the blank experiment. The degrees of HMF conversion over the
Pd-based catalysts were 99.9%, 78.8%, and 19.4% for Pd/NMC,
Pd/CMC, and Pd/AC (Table 2, entries 2-4), respectively. The
yields of DMF were found to be similar for the mesoporous
carbon-supported Pd catalysts, amounting to 35.3% for Pd/NMC
and 38.8% for Pd/CMC, while the yield was significantly lower for
Pd/AC. The poor catalytic activity of Pd/AC is due to the larger Pd
particle size and the lower Pd dispersion. Besides, mainly furan-
ring-hydrogenation products were obtained as byproducts. The
hydrogenolysis of HMF over Pd/NMC as a function of time was
further studied (Figure 4a). The conversion of HMF proceeded
very fast, obtaining full conversion within 2 h. BHMF and 5-MF
were the primary intermediates, which reached a maximum yield
within 1 h and then gradually decreased to zero after 4 h. By
comparison, the maximum yield of BHMF was much higher than
that of 5-MF, suggesting that the BHMF reaction pathway (path
B) excels the 5-MF pathway (path A) for HMF hydrogenolysis in
H2. MFA was found to be the secondary intermediate, which
reached a maximum yield of 42% at 3 h and then gradually
decreased to 17.8% accompanied by an increase in DMF yield to
59.6% after 5 h. Additionally, the ring-hydrogenation products
continuously increased to 20%, which is commonly observed in
H2.37,38 By comparison, the product evolution profiles over
Pd/CMC are rather similar to that over Pd/NMC, but with slightly
lower activity (Figure S13).
When FA was used as a hydrogen donor, the degrees of
conversion and product distributions changed significantly for
both Pd/NMC and Pd/CMC (Table 2, entries 5, 6). The degrees of
conversion decreased strongly, and the selectivities to DMF
increased by 13-29%. Ultimately, it resulted in a similar DMF yield
(39% vs. 35%) over Pd/NMC after 3 h compared to that in H2
atmosphere. Notably, the hydrogenation of the furan ring, which
commonly occurs and is difficult to avoid with H2, is completely
suppressed in the presence of FA. This can be explained by the
sterically hindered adsorption of the furan ring, because the active
Pd sites are preferably covered by the more strongly bound
formate anions. In contrast to the reaction in H2, the reaction
progress in FA over Pd/NMC shows that BHMF was not detected,
while only 5-MF with similar concentration (10%) was observed
(Figure 4b). This result suggests that the hydrogenolysis of the
hydroxymethyl group (path A) precedes the hydrogenation of the
aldehyde group (path B) in FA. After 5 h, a conversion of 78.1%
and a DMF yield of 60% were obtained. By extending the reaction
time to 12 h, the DMF yield increased to almost 90%. It is worth
noting that the yield of the primary intermediate 5-MF slowly but
continuously increased, while the secondary intermediate MFA
first reached a plateau and then gradually decreased after 3 h.
This observation suggests that the hydrogenation of 5-MF to MFA
is the rate-determining step (rds) because of the relatively poor
hydrogenation ability of Pd/NMC in FA.
Figure 5. Reusability of Pd/NMC in the hydrogenolysis of HMF to DMF in the
presence of FA and H2 (a), and XRD patterns of Pd/NMC before and after
reaction (b).
increased strongly to 97.5% and 73.9% (Table 2, entries 7, 8),
respectively. Similar to the reaction progress in FA, Figure 4c
shows that BHMF was not detected and path A (5-MF) is the main
reaction pathway in the presence of external H2. The difference is
that the change of MFA yield occurred concurrently with that of 5-
MF, indicating that the reaction rate of the hydrogenation of 5-MF
to MFA was significantly enhanced. Finally, the quantitative
conversion of HMF to DMF is achieved within 2 h over Pd/NMC
(Scheme 2). Compared with Pd/CMC, Pd/NMC exhibits superior
catalytic activity with doubled TOF values both in FA and in H2+FA
(Table S4), suggesting strong promoting effects of FA and the N
species in Pd/NMC. Furthermore, Pd/NMC outperforms other
reported catalysts in terms of TOF (Table S4), which can be
attributed to smaller Pd NPs, strong metal-support interactions
and modified electronic properties by the N species.
By comparison, the combination of FA and H2 strongly enhances
the hydrogenolysis of HMF to DMF, obtaining excellent HMF
conversion and DMF yield in short reaction time (Figure 4d). The
addition of FA changes the reaction pathway from path B to path
A, enhances the hydrogenolysis ability, and inhibits the ring-
hydrogenation, whereas the presence of H2 ensures the fast
hydrogenation step.
The stability and the reusability of Pd/NMC were also evaluated.
The used catalyst kept its high catalytic activity after three runs,
and the yield of DMF only slightly decreased from 97.5% to 95.1%
(Figure 5a). The low degree of Pd leaching, which is only 0.06 %
(0.9 ppm) of the total Pd, indicates an efficient stabilization of the
Pd NPs by the NMC support. Furthermore, the recycled catalyst
was characterized by XRD, TEM, and XPS (Figures 5b, S5, and
2). The Pd(111) reflection in the used catalyst is still hardly visible
without significant changes as compared to the fresh catalyst. The
mean Pd particle size increased slightly from 2.8 to 2.9 nm and
the sub-1 nm Pd NPs are still visible. After reaction, the relative
abundance of Pd0 in Pd/NMC increased from 36 to 59%, while
Pd2+ decreased from 50 to 30% (Table S2). For comparison,
Pd/CMC was almost completely reduced to Pd0 (91%) after
reaction. The moderate reduction of Pd/NMC even in the
presence of FA and H2 is ascribed to the strong anchoring by
pyridinic N species. Overall, Pd/NMC is highly stable and exhibits
excellent reusability.
In order to enhance the reaction rate of the rds, the conversion of
HMF with FA was also conducted in the presence of additional
external H2. The yields of DMF over Pd/NMC and Pd/CMC
Reaction mechanism. In situ ATR-IR spectroscopy was
employed to investigate the reaction pathway and product
distributions with different hydrogen donors. Figure 6a and 6b
depict the time-resolved ATR-IR spectra collected during the
hydrogenolysis of HMF to DMF over Pd/NMC at 160 °C by H2 and
by H2+FA, respectively. The reference spectra of the main
compounds are shown in Figure S14.
Scheme 2. Quantitative conversion of HMF to DMF with FA and H2 over
Pd/NMC.
5
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Figure 6. In situ ATR-IR spectra and qualitative trends of peak heights during HMF conversion over Pd/NMC in (a) (c) 10 bar H2 after subtraction of the background
of THF; (b) (d) 15 mL FA at 10 bar H2 after subtraction of the background of THF and FA. Reaction conditions: 45 mmol HMF, 500 mg catalysts, 120 mL THF, 10
bar H2, 160 ℃, 500 rpm.
The assignment of the characteristic bands to the reactant and
intermediates is summarized in Table S5. Due to the similarity of
the intermediates and the reactant, the differentiation of C=O and
C=C stretching vibrations is impossible, and therefore, the bands
of out-of-plane C−H bending were compared in detail, using the
characteristic C−H bending vibrations for HMF (805, 779, 767 and
756 cm-1), DMF (781 cm-1), 5-MF (796 and 765 cm-1), and BHMF
(792 cm-1).
Using H2 as the hydrogen donor, the intensity of the IR bands at
805 and 756 cm-1 (HMF) continuously declined, whereas the
intensity of the IR band at 781 cm-1 (DMF) gradually increased.
The IR peak of 5-MF was not detected probably due to the low
concentration, while the peak at 792 cm-1 (BHMF) first formed and
then declined. The time-resolved evolution of these peaks is
presented in Figure 6c, which once again verifies the conclusion
that Pd/NMC tends to catalyze the hydrogenation of the aldehyde
group (path B) compared with the hydrogenolysis of the
hydroxymethyl group (path A) in H2.
In the case of FA and H2 as hydrogen donors, the consumption of
HMF and the formation of DMF were still observed (Figure 6b).
Nevertheless, the intensities of the IR bands at 765 and 796 cm-1
(5-MF) increased significantly without the formation of BHMF in
good agreement with the GC results (Table 2, Figure 4). These
bands were deconvoluted and plotted in Figure 6d. The decrease
of HMF as well as the increase of DMF and the primary
intermediate 5-MF (path A) can be clearly observed in the
presence of FA+H2.
With respect to the mechanism, FA not only changes the
dominant reaction pathway from path B to path A, but also
significantly improves the conversion of HMF to DMF over
Pd/NMC, which is proposed to proceed via the enhancement of
the hydrogenolysis of the C−OH bond (path A). On the contrary,
path B was identified as the main reaction pathway over Pd/C in
the presence of FA and H2 in a previous report,21 implying that
Pd/NMC may also play a role in the shift of reaction pathway.
FA is considered to serve multiple roles: it acts as a mild hydrogen
source, a catalyst moderator inhibiting ring hydrogenation, and a
precursor to formate esters as key intermediates that are
activated toward selective hydrogenolysis of C−OH bonds.20,21
The role of FA as a hydrogen source in the absence of external
H2 is rather clear. The suppression of ring-hydrogenation resulting
in improved selectivity has been clearly observed in the present
study, but the differentiation or clarification of other roles affecting
catalytic activity has not been investigated yet. Therefore, further
experiments were conducted to probe the influence of FA on the
hydrogenolysis and hydrogenation of furanic substrates catalyzed
by Pd/NMC with additional external H2 (Table 3). When strongly
decreasing the molar ratio of FA to HMF from 30 via 3 to 0.3, only
a slight decrease of HMF conversion and DMF yield was
observed (Table 3, entries 3-5), implying that FA only has a limited
role as a hydrogen source in the presence of external H2.
Furthermore, by addition of only 0.3 equiv. FA, the yield of DMF
increased significantly from 35.3% to 76.0%. Although ester
formation was restricted using such a low concentration of FA,
excellent DMF yield was still obtained. This result suggests that
the formate ester pathway might not be the main route for HMF
hydrogenolysis under the employed reaction conditions. In order
to further verify the role of the formate ester, the conversion of 5-
[(formyloxy)methyl]furfural (FMF) to DMF was investigated (Table
3, entries 8-10). Compared with the conversion of HMF, lower
degrees of conversion and DMF yields were obtained for all cases.
Thus, it can be concluded that the reaction pathway of formate
ester as an intermediate is a minor route for the hydrogenolysis of
HMF to DMF. Note that FMF was not detected in all experiments,
probably because the conversion of FMF to DMF is much faster
than the formation of FMF under the present reaction conditions.
6
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10.1002/anie.202012816
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RESEARCH ARTICLE
Table 3. Effect of additives on the conversion of HMF and FMF to DMFa.
Entry
Catalysts
Reactant
Additiveb
Gas Phase
Conversion (%)
DMF Yield (%)
1
Pd/NMC
Pd/NMC
Pd/NMC
Pd/NMC
Pd/NMC
Pd/NMC
Pd/NMC
Pd/NMC
Pd/NMC
Pd/NMC
HMF
HMF
HMF
HMF
HMF
HMF
HMF
FMF
FMF
FMF
-
H2
N2
H2
H2
H2
N2
H2
H2
N2
H2
> 99.9
60.8
> 99.9
92.0
82.1
10.6
95.1
66.1
34.2
83.1
35.3
39.1
> 97.0
89.3
76.0
Trace
85.5
46.0
20.5
67.3
2
FA
3
FA
4
FA (3 equiv.)
5
FA (0.3 equiv.)
6
AcOHc
AcOHc
-
7
8
9
FA
10
FA
[a] Reaction conditions: 1.5 mmol HMF or FMF, 50 mg catalysts, 160 ℃, 5 bar H2 or N2, 3 h; [b] 45 mmol FA (30 equiv.); [c] 45 mmol acetic acid (30 equiv.).
Furthermore, the addition of acetic acid (pKa 4.7 vs 3.8 for FA)
instead of FA led to only 10.6% HMF conversion and traces of
DMF (Table 3, entry 6), demonstrating that acetic acid cannot be
used as the hydrogen donor. Nevertheless, excellent HMF
conversion and DMF yield of 85.5% were obtained in the
presence of acetic acid and additional external H2 (Table 3, entry
7). In addition, 5-MF was detected as primary intermediate and
BHMF was not detected in good agreement with the result using
FA. These results suggest that the acids probably enhance the
hydrogenolysis of the RCH2-OH bond via an acid-catalyzed route.
Assary et al.39 proposed the reaction mechanism of fructose
dehydration to HMF using high-level quantum chemical methods.
It was demonstrated that the acidity of the solution is essential to
lower the activation energy barrier by involving initial protonation.
Moreover, Lercher et al.40 reported a catalytic pathway for the
cleavage of the C−O bond in benzyl phenyl ether. DFT
calculations suggested that the ether is initially protonated rather
than the direct cleavage of the ether bond in the proposed C−O
bond cleavage mechanisms due to the lower activation energy.
Combining our experimental observations and these literature
reports, we propose that the hydrogenolysis of RCH2-OH in the
presence of acids also proceeds via a similar protonation process
initially, which can lower the activation barrier of the C-O bond
cleavage.
hydrogenolysis path A in the presence of FA or FA+H2, the higher
catalytic activity of Pd/NMC can be attributed to the improved
activity of the C−OH bond hydrogenolysis. Since Pd/NMC and
Pd/CMC exhibit similar Pd particle sizes and mesoporous
structures, the nature of the N atoms and different oxidation states
of Pd species caused by strong metal-support interactions should
provide a clue.
Figure 7. Comparison of Pd/NMC and Pd/CMC with respect to the conversion
of HMF to DMF using different hydrogen sources after 3 h.
Overall, FA serves multiple roles in the hydrogenolysis of HMF to
DMF: 1) suppressing ring-hydrogenation to improve the DMF
selectivity, 2) acting as a mild hydrogen source, 3) producing the
To further identify the effect of FA and of the N-doping of the
support on the hydrogenolysis of the C−O bond, a DFT study of
H2 and FA on Pd/CMC and Pd/NMC was performed (see detailed
procedures and results in Supporting Information). The DFT
calculations support the hypothesis that FA does not only act as
a mild hydrogen source (function 2), but also can form protonated
intermediates (function 4), as it interacts with carbon-supported
Pd NPs in two ways: a) FA binds via the C atom (Figure 8a),
subsequently decomposing into H2 and CO2; b) FA binds via the
C=O group (Figure 8b), subsequently decomposing into the
formate anion and H+.
At the Pd21 cluster bound to NMC, we find that at the Pd2+ centers
FA binds exclusively via the C=O bond and decomposes with a
small activation energy (∆E = 5.3 kJ mol-1, neglecting solvation
effects) into the formate anion and H+. The adsorption via the C
atom as well as the reaction pathway to the formation of H2 and
CO2 are unfavorable (Table S6). Accordingly, the bond to the
positively charged center enhances the acidity of FA, which
facilitates the hydrogenolysis of the C−OH bond of HMF. This is
formate esters as intermediates, 4) forming
a protonated
intermediate and thus lowering the activation barrier of the C-O
bond cleavage. Functions 2-4 contribute together to the activity
improvement, among which the protonation mechanism results in
the reaction pathway shift from path B to path A, being the main
reason for the significant catalytic activity enhancement.
The catalytic performance of the hydrogenolysis of HMF over
Pd/NMC and Pd/CMC with different hydrogen donors is
presented in Figure 7. A significant difference in the reaction rate
of the hydrogenolysis of the C−O bond was observed for N-rich
and N-free carbon-supported Pd catalysts. When H2 was
employed, Pd/NMC and Pd/CMC led to similar DMF yields,
implying their comparable catalytic activities for the
hydrogenolysis of HMF in H2. Using FA as the hydrogen source,
Pd/NMC resulted in both higher activity and DMF yield compared
to Pd/CMC. In FA and H2, much higher conversion and yield over
Pd/NMC were also obtained. As the main reaction pathway is the
7
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10.1002/anie.202012816
Angewandte Chemie International Edition
RESEARCH ARTICLE
similar process like the hydrogenolysis of HMF to 5-MF. In the
case of FA and H2 as hydrogen donors, H2 is activated by the Pd0
NPs supported on NMC, which are more active for hydrogenation
steps such as 5-MF to MFA and HMF to BHMF. H2 may also
directly contribute to the hydrogenolysis reactions, but probably
plays a minor role in the presence of FA.
Figure 8. Two different binding configurations of FA on carbon supported Pd21
(model for Pd/CMC): (a) FA binds via the C atom; (b) FA binds via the C=O
group.
Scheme 3. Proposed reaction pathway for the conversion of HMF to DMF over
Pd/NMC in FA.
in agreement with previous studies, in which Pd2+ species were
identified as the active sites for the selective decomposition of
FA.29 For example, Pd2+ species supported on N-rich carbon
showed 3 times higher catalytic activity in FA decomposition
compared with the activity of Pd supported on N-free carbon.29
The isolated Pd2+ species stabilized by the pyridinic N atoms were
identified as the active sites.
Furthermore, H- formed by heterolytic H2 dissociation on the Pd
nanoparticle binds significantly stronger to Pd/NMC compared
with Pd/CMC, preferring positions close to the Pd2+ center (Table
S7). This can be explained by the higher positive charge of Pd
NPs on NMC, especially close to the Pd2+ center. The stabilization
of H- on Pd/NMC increases the nucleophilicity, and therefore, also
the selectivity of H2 as a reducing agent.
In summary, the remarkable catalytic activity of Pd/NMC for HMF
conversion in FA can be attributed to the shift to reaction path A
and to the presence of Pd2+ species, showing higher catalytic
activity in the FA dissociation to the formate anion and H+ as well
as in the H- stabilization, which jointly improve the hydrogenolysis
of the C−OH bond significantly.
Based on the kinetic, spectroscopic and computational results,
the following reaction pathway over the bifunctional Pd/NMC in
FA is proposed (Scheme 3). First, FA is activated via Pd2+ species
and Pd NPs. The reversible reaction between HMF and FMF
exists in the presence of FA, but the HMF pathway is the main
route instead of the formate ester pathway. Then, Pd2+ species
bind the oxygen atom of the C−OH group, which is protonated by
FA. The hydrogenolysis of the hydroxymethyl group occurs via
the cleavage of the C-OH bond forming 5-MF. During the process,
Pd2+ species and FA cooperate synergistically in the
hydrogenolysis of the C−OH bond. Furthermore, Pd0 NPs bind the
C=O double bond leading to the fast hydrogenation of 5-MF to
MFA. Subsequently, MFA is directly converted to DMF via a
Conclusion
Pd NPs supported on N-doped and N-free mesoporous carbon
materials were successfully synthesized via the immobilization
method and systematically characterized. These catalysts exhibit
similar mean Pd particle sizes, but sub-1 nm Pd clusters were only
observed in Pd/NMC. DFT calculations and XPS measurements
revealed the strong metal-support interactions between Pd2+
species and pyridine-like
N
atoms for Pd/NMC. The
hydrogenolysis of HMF to DMF over the Pd-based catalysts was
investigated by varying different hydrogen sources. A favorable
HMF full conversion, DMF yield of > 97% and a higher TOF of 150
h-1 were obtained over Pd/NMC in the presence of FA and H2 after
2 h.
Detailed comparative studies mainly by varying hydrogen source,
additive, and reactant as well as by applying in situ ATR-IR
spectroscopy and DFT calculations were performed to explore the
reaction mechanism. FA tunes not only the reactivity of HMF but
also its reaction pathways, serving multiple roles, such as
suppressing ring-hydrogenation, acting as a mild reducing agent,
producing the formate esters as intermediates, and forming a
protonated intermediate, thus lowering the activation barrier of the
C-OH bond cleavage. Protonation results in a reaction pathway
shift from the hydrogenation of the C=O bond (path B) to the
hydrogenolysis of the C–OH bond (path A) and is the main reason
for the significant catalytic activity enhancement. Furthermore, a
synergistic effect between FA and the Pd2+ species of Pd/NMC
was observed for the hydrogenolysis of the C−OH bond, with Pd2+
exhibiting higher catalytic activity for the activation of FA and
stabilization of H-.
8
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RESEARCH ARTICLE
Entry for the Table of Contents
Quantitative conversion of HMF to DMF was achieved in the presence of formic acid and H2 over the bifunctional Pd/NMC catalyst
within 2 h. Experimental investigation and DFT calculation revealed that formic acid significantly enhances the reaction rate and shifts
the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via its
protonation.
10
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