One-Pot Defunctionalization of Lignin-Derived Compounds by Dual-Functional Pd50Ag50/Fe3O4/N-rGO Catalyst

Article abstract of DOI:10.1021/acscatal.5b01319

Generation of hydrogen from renewable sources and its safe utilization for efficient one-pot upgrading of renewable biofuels are a challenge. Bimetallic PdAg catalyst supported on Fe₃O₄/nitrogen-doped reduced graphene oxide (N-rGO) were synthesized for hydrogen generation from formic acid with high TOF (497 h^-1 at 50 °C), and the hydrogen was subsequently utilized in situ for selective defunctionalization of lignin-derived chemicals with preserved aromatic nature at ambient pressure. Hydrodeoxygenation of aromatic aldehydes and ketones gave excellent yields (99% at 130 °C) with no use of additives. Furthermore, hydrogenolysis of β-O-4 and α-O-4 C-O model compounds produced only two products with high selectivity at 120 °C, which is an efficient and versatile one-pot platform for valorization of lignin biomass.

Full text of DOI:10.1021/acscatal.5b01319

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Article
One-pot defunctionalization of lignin-derived compounds
by dual functional Pd₅₀Ag<sub>50</
sub>/Fe₃O₄/N-rGO catalyst
Ajay K. Singh, Seungwook Jang, Jae Yul Kim, Siddharth Sharma, K. C Basavaraju,
Min Gyu Kim, Kyung-Rok Kim, Jae Sung Lee, Hong H. Lee, and Dong-Pyo Kim
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01319 • Publication Date (Web): 13 Oct 2015
Downloaded from http://pubs.acs.org on October 17, 2015
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One-pot defunctionalization of lignin-derived
compounds by dual functional Pd Ag /Fe O /N-rGO
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catalyst
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Ajay K. Singh, Seungwook Jang, Jae Yul Kim, Siddharth Sharma, K.C Basavaraju, Min-Gyu Kim,
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‡
#,*
†,*
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Kyung-Rok Kim , Jae Sung Lee, Hong H. Lee and Dong-Pyo Kim
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†National Center of Applied Microfluidic Chemistry, Department of Chemical Engineering, POSTECH
(Pohang University of Science and Technology), Pohang 790-784, Korea.
‡School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology
(UNIST), 50 UNIST-gil, Ulsan 689-798, Korea
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§Department of Chemistry, U.G.C. Centre of Advance Studies in Chemistry, Guru Nanak Dev
University, Amritsar 143005, India
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Beamline Research Division, Pohang Accelerator Laboratory (PAL), POSTECH (Pohang University
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of Science and Technology), Pohang 790-784, Korea
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#School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea.
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ABSTRACT
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Generation of hydrogen from renewable source and its safe utilization for efficient one-pot upgrading of
renewable biofuels are a challenge. Bimetallic PdAg catalyst supported on Fe
3
4
O /nitrogen-doped
reduced graphene oxide (N-rGO) were synthesized for hydrogen generation from formic acid with high
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o
TOF (497 h at 50 C), and subsequently utilized the hydrogen in-situ for selective defunctionalization
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of lignin-derived chemicals with preserved aromatic nature at ambient pressure. Hydrodeoxygenation of
o
aromatic aldehydes and ketones showed excellent yield (99% at 130 C) with no use of additives.
Furthermore, hydrogenolysis of -O-4 and -O-4 C-O model compounds produced only two products
o
with high selectivity at 120 C, which is an efficient and versatile one-pot platform for valorization of
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lignin biomass.
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KEYWORDS Formic acid, bimetallic catalyst, magnetic separable, N-doped reduced graphene oxide,
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selective defunctionalization
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MANUSCRIPT TEXT
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■ Introduction
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The production of fuel and fine chemicals from biomass has received much attention due to world’s
major oil fields have been in production decline for decades.1-3 Non-edible liganocellulosic biomass is
the one of the promise source for renewable carbon feedstocks to make fuels and/or useful synthetic
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building blocks. But liganocellulosic-derived molecules pose the problem of being overfunctionalized
by virtue of having an abundance of oxygen atoms with variety of functionality including hydroxyl and
carbonyl groups.5 Lignin is only biorenewable source to make aromatic compounds.6
Defunctionalization of lignin biomass into tailor-made fuels and chemicals has been identified as one of
the key enabling technologies ultimately essential for the recognition of the “ideal biorefinery”.
However, a mixture of various phenolic monomers was mostly produced from bimetallic hydrogenolysis
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of organosolv lignin in water. Recently, the selective phenolic monomer synthesis from lignin was
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achieved only by a time-consuming three step process. Thus, the development of selective catalysts
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which are capable of dehydrating, hydrodeoxygenating and hydrogenolysis with preserved aromatic
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nature represents a promising yet challenging approach. In general, the catalytic dehydrating,
hydrogenating and hydrodeoxygenating reactions are carried out using hydrogen molecule (H ) for
upgrading the biomass and the derived chemicals. However, molecular H is dangerous to handle and
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extreme care should be taken to avoid disasters caused by even small leakage during production, storage,
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derivable from biomass, has drawn much interest. It would be highly desirable for safety that the
hydrogen is consumed in-situ while it is being produced in the same reactor.
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Many homogeneous catalysts have been used for decomposition of FA, the first half of the tandem
reaction system at ambient temperatures.12,13 But the homogeneous ligand complexes are difficult to
synthesize, handle, separate, and recycle, which makes them less suitable for industrial applications.15
On the other hand, heterogeneous catalysts of metal nanoparticles (NPs) are much less active compared
to homogeneous catalyst. For formic acid decomposition, bimetallic catalysts have been shown to be
better than their single metallic catalyst due to charge redistribution between bimetals which strengthens
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the adsorption of formate through strong back donation. In fact, the bimetallic catalyst of 2.2 nm PdAg
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alloy nanoparticles turned out to be a better choice for the first part of the tandem reaction. But the
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initiation turn over frequency (TOF) was in the range of 382 h at 50 C. Improvement of TOF with
earth abundant metal is still demanded for efficient FA dehydrogenation reaction. Another hand,
heterogeneous catalysts based on metal NPs have to contend with deactivation. The deactivation via
cluster agglomeration or Oswald ripening is a serious problem.17 In addition, the metal catalysts on
supporters such as metal oxides and activated carbons could be often leached out to reaction mixture due
to weak metal-support interaction. 18,19
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Catalytic deoxygenation of lignin or lignin-model compounds with formic acid has been reported by
several groups.20-23 Lignin has a complex chemical structure that is highly stable and branched (due to
the > 90% of β-O-4, α-O-4 aryl ether bonds). Therefore, decomposition of lignin to small molecules is
quite difficult. In particular, the selective hydrodeoxygenation of lignin-derived pyrolysis oil is more
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challenging because similar bond strength of C-O and C=C leads to competition between
hydrodeoxygenation and aromatic hydrogenation reaction.24 Consider the limitation seen from the
previous biomass and chemical feedstocks upgrading, synthesis in controlling NPs size and composition
we decided to make novel dual functionalized catalyst to achieve the defunctionalization of lignin-based
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biomass with no use of H
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at mild conditions. Here we report an efficient dual functional PdAg alloy
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metal catalyst with high TOF supported on N-doped reduced graphene oxide (N-rGO) that generates H
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from formic acid and subsequently utilizes the generated hydrogen to hydrodeoxygenate selectively the
lignin biomass-derived chemicals at mild conditions without any additives in one-pot. We also loaded
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magnetite (Fe
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O ) particles for easy separation and enhanced mixing efficiency. Furthermore, the ~4
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at% basic N hetero-atom on the N-rGO facilitates binding of nanoparticles to make well distributed on
the supporter as well as reaction promotor, which prevents metal agglomeration and leaching even after
catalyst recycling process. The selective upgrading of lignin biomass-derived compounds was
successfully demonstrated by employing several model molecules including vanillin, isatin and two
types of benzyl phenyl ethers, well-known natural compounds, in an environment-friendly one-pot
manner. It enables to meet the critical needs in the production of upgraded biofuels and chemical
feedstocks from biomass resources.
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■ Results and discussion
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To evaluate efficiency and selectivity of a series of catalysts for selective hydrodeoxygenation with H
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formed in-situ from formic acid, vanillin was chosen as a model substrate for upgrading bio-fuel.
Vanillin is a common component of pyrolysis oil derived from non-edible lignin, which constitutes ~30
wt% of woody biomass. It is more challenging to deoxygenate lignin-derived pyrolysis oil than
cellulose-derived one.24 For the purpose, a series of mono-, bi-, and tri-, metallic catalysts were
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synthesized on Fe
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It is apparent from Table 1 that no reaction takes place in 10 h at 130 C (entries 1-4) when a single
component is used, as in GO, N-rGO and Fe
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separation (supporting movie S1) and enhancement of mixing properties (supporting movie S2). When
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we introduced metallic component, i.e., Pd/N-rGO (Table 1, entry 5), we observed that the aldehyde
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group in vanillin was selectively hydrodeoxygenated to methyl group with a low turnover frequency
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(TOF based on the number of active metal sites, see supporting information, S4, Table S5) of 8 h . We
further found that TOF increased significantly with Fe loading (Table 1, entry 6-10), indicating that
Fe does not simply play the physical roles of helping easy separation and enhanced mixing. As
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indicated in the Table 1, it promotes TOF of the reaction presumably by aiding the catalytic cycle of the
tandem reactions as discussed later. Among many transition metals tested, only Pd showed nearly
complete conversion. Similar results were reported with gaseous hydrogen supplied externally for Pd
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nanoparticles supported on mesoporous N-doped carbon. Hence, one may surmise that Pd could be the
main active component of the catalyst responsible for the selective hydrodeoxygenation of vanillin. Of
the bimetallic catalysts tested (entry 11-15), yield of Pd50Co50 and Pd50Ag50 pairs were better than 99%
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but the TOF was much greater for Pd50Ag50 (521 h ) than for Pd50Co50 (50 h ). Other catalysts
including a trimetallic catalyst (entry 16) did not offer any advantage over the Pd50Ag50 bimetallic
catalyst.
In order to investigate the relative efficiency of the bimetallic Pd50Ag50/Fe
3
O
4
/N-rGO catalyst with
BET surface area 251 m /g , control experiments of Pd/C, Pd on N doped activated carbon (Pd/N-AC),
Pd/N-rGO, core-shell Pd@Ag/C, Pd/Fe /N-rGO and Pd50Ag50/N-rGO were carried out by comparing
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the conversion and yield of 2-methoxy-4-methylphenol (MMP) as a main product of
hydrodeoxygenation in Table 2. Commercial Pd/C with high surface area produced 63% conversion
only with 32% yield of MMP by forming considerable by-products (entry 1, Figure S1). The Pd/N-AC
produced 64% conversion but slightly improved selectivity (40% of MMP, entry 2).26 Upon
immobilizing the Pd nanoparticles with identical size on N-rGO (as measured by TEM and XRD), the
similar conversion (66%) but highly improved yield (51%) of MMP were achieved (entry 3),
presumably due to reaction promoting effect of well distributed Pd nanoparticles on basic nitrogen sites
as well as - interaction between graphene and aromatic reactant to activate the -carbon position.27
And the core-shell Ag@Pd/C catalyst produced 69% MMP yield with conversion 100% (entry 4, Figure
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S2). However, Pd/Fe
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80% (entry 5, Figure S3). Both Pd50Ag50/N-rGO and Pd50Ag50/Fe
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O /N-rGO catalysts showed excellent
conversion (100%) and yield (~99%) in water solvent under mild conditions (entry 6&7, Figure S4&S5).
Note that the decreased Ag composition lowered the efficiency (entry 8&9, Figure S6&S7). Furthermore,
the excellent catalytic efficiency was not affected by the loaded amount of formic acid (entry 10) and a
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gram scale of MMP production (entry 11). In addition, catalytic superiority of the Pd50Ag50/Fe
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rGO catalyst in the selective hydrodeoxygenation of vanillin was clearly illustrated in comparison to
various reported catalysts in the literature (Table 3). The time profile of the reaction revealed that
vanillin was converted to MMP by two pathway; (i) two step-wise hydrogenation/hydrogenolysis via 4-
(hydroxymethyl)-2-methoxyphenol intermediate and (ii) direct hydrogenolysis, similar to the previous
report.24
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To understand the behaviour of Pd50Ag50/Fe
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O /N-rGO catalyst in the tandem reaction of formic acid
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decomposition-hydrodeoxygenation sequence, decomposition of formic acid alone was first studied at
50 °C (supplementary section S5, TOF calculation for formic acid decomposition). The total volume of
CO
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and H collected for 1 h was 20.3 ml with the ratio of 1.06 (Figure S9). The corresponding TOF for
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formic acid dehydrogenation was found to be in the range of 497 h (see Table S6), using H
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chemisorption measurement methods for active surface Pd atoms, which comparable to the reported
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values for active formic acid dehydrogenation catalysts.11,29,30 Possible by-product of CO by formic acid
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dehydroxylation was not detected even at a higher temperature (90 C) and the catalyst showed
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prolonged stability. This finding together with the hydrodeoxygenation of vanillin shows that our
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Pd50Ag50/Fe
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reactions, without any additives and additional pressure.
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To look into the structure and nature of the catalyst, we resorted to various instrumental methods.
First, X-ray photoelectron spectroscopy (XPS) was utilized to find surface compositions (Table S4,
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Figure S10&S11) for C, O, and N in N-rGO and Pd50Ag50/Fe
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O /N-rGO catalyst. Binding energies C1s,
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O1s, and N1s of Pd50Ag50/Fe
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/N-rGO (Figure S11a-c) were identical with those of the as-prepared N-
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rGO, indicating that the structure was unaffected by the loading of catalyst NPs.27,31 The absence of the
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satellite peak at 719 eV characteristic of Fe in Fe
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(Figure S11d) clearly indicates the absence of
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Fe
2
O
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. As given in Table S4, N-rGO has large amounts of oxygen (~10 at %) and nitrogen (~4 at %).
These hetero-atoms in graphene structure can coordinate metals, resulting in highly distributed and
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doped carbon. And the metallic nature of Pd and Ag in the Pd50Ag50/Fe
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doublet XPS peaks at 368 and 374 eV designated as Ag 3d, 336.1 and 341.2 eV as Pd 3d, respectively
(Figure S11e, f), which is consistent with the literature.33
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Detailed SEM results of our bifunctional Pd50Ag50/Fe
3
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O /N-rGO catalyst shown in Figure 1a&b
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reveal that the catalyst takes on a shape of nanosheets resembling tiny rose petals. High resolution
transmission electron microscopy (HR-TEM) shows that the catalytic nanoparticles are reasonably
dispersed with no agglomeration (Figure 1c&d). Ordered crystalline structure of N-rGO supporter was
demonstrated by selected area electron diffraction pattern (SAED) with two bright rings corresponding
to (1100) plane and (1100) reflections patterns (Figure 1e), in addition to interlayer distance 0.37 nm of
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brighter part (red circle) corresponds to PdAg atoms (Figure 1i) and the less bright part (yellow
rectangle) to Fe atoms (Figure 1h) because the intensity is directly proportional to the square of the
3
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atomic number of the elements. Importantly, selected area EDX line scans across the PdAg/Fe
nanoparticles indicated that three metals were co-located in the identical NP with different sizes of 5-8
nm for PdAg and 8-15 nm for Fe . Conversely, the PdAg alloy NP display uniform lattice patterns
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with no intraparticular boundaries, indicative of a single alloyed phase with a (111) lattice fringe
distance of 0.23 nm (Figure 1i) between the (111) lattice of face-centered cubic (fcc) Ag (0.24 nm) and
fcc Pd (0.22 nm) NPs. These findings indicate that PdAg is formed as an alloy and not as a core/shell
structure.14 Alternatively, it is additional evidence of alloy structure that the ratio of Pd/Ag (2.47
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wt%/2.53 wt% = 0.97, Table S3) measured by inductively coupled plasma atomic emission spectroscopy
(ICP-AES) was nearly similar to the surface compositional Pd/Ag ratio (0.32 at%/0.34 at% = 0.94)
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measured by XPS. Fe
3
O
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nanoparticles displayed 0.25 nm (311) lattice fringe distance indicating that co-
(Figure 1h)35 and its superparamagnetic
presence with PdAg did not affect crystallization of Fe
3
O
4
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property (see supplementary S6, Figure S12, Table S7). The XRD patterns of the PdAg NP showed a
very weak peak intensity because of their small sizes and the distance of (111) planes of 2.3 Å (Figure
S13&S14), further indicating alloy type of the NP. From the (111) diffraction peak and Scherrer’s
formula, the average size of the crystallites was calculated to be 6.2±0.5 nm, in agreement with the value
observed by TEM. Based on these results of physical characterization, a structural model of the
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Pd50Ag50/Fe
3
O
4
/N-rGO catalyst could be proposed as shown in Figure 1j-m. Over a 2D surface of N-
(8-15 nm in size) particles are uniformly distributed with PdAg (5-8 nm in size) alloy
rGO sheet, Fe
3
O
4
NP in close proximity.
In order to understand the evolution of catalyst structure during the reaction, X-ray absorption near edge
structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses have been carried
out in-situ under reaction conditions. (Figure 2, Figure S15, see the supplementary S7). From XANES
data of the pristine Pd50Ag50/Fe
3
4
O /N-rGO catalyst, it is clear that both Pd and Ag show typical feature
8 17
of metallic phases (Figure S15a&b). In the radial distribution functions (RDF) of Fourier-transformed
(FT) Pd and Ag K-edge EXAFS spectra, the pristine catalyst also shows intense singlet peaks at around
2.5 and 2.6 Å, corresponding to the Pd-Pd/Ag and Ag-Ag/Pd nearest metal-metal bonding, respectively
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(Figure 2a&b). The data indicate that PdAg exists in a metallic phase in the pristine Pd50Ag50/Fe
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O /N-
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rGO catalyst. In addition, Pd atom in the pristine catalyst shows a slight peak shift to higher energy in
XANES and an additional weak FT peak in the lower r region at ~2.0 Å. The result could be interpreted
as the chemical bonding between surface Pd atoms on the metallic phase and hetero-atoms (N or O) on
N-rGO layer. These spectral features indicate partial oxidation of Pd atom by a metal-to-ligand charge
transfer, and anchoring of PdAg nanoparticles onto N-rGO layer. Under reaction conditions the first
metallic FT peak in Pd K-edge EXAFS is slightly shifted to a longer interatomic distance, while the Ag
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K-edge spectral behaviour is clearly invariant before and after the catalytic reaction (Figure 2b). These
results indicate that the Ag does not actively participate in the reaction. An earlier report for
decomposition of formic acid with an Ag-Pd core/shell nanoparticles suggested that Ag electronically
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promotes Pd metal for formate ion adsorption by a strong back donation. In this work, both Fourier-
transformed (FT) magnitudes of Pd K-edge and Ag K-edge EXAFS decreased at similar ratio, compared
to those of each bulk metallic phase (Figure S15d). It indicated that the averaged coordination numbers
around each atom are similar to each other under the face-centered cubic structure, as additional
evidence of alloy PdAg NPs.
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Finally, Fe-K edge XANES spectra of pristine catalyst show peak shifting into higher energy due to
binding hetero-atoms (O/N) of graphene (Figure S15c, black line). In RDF of Figure 2c, a clear peak at
~2 Å denoted by arrow would be assigned to weak chemical bond between Fe elements and hetero-
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atoms (N or O) in the N-rGO layer, which can be expected to drive the Fe
distributed through N-rGO layer. Note that detailed structural arrangement (Pd-Ag-Fe
Fe ) of catalyst is still difficult due to the similar atomic number of Ag and Pd. The in-situ
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nanoparticles uniformly
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decomposition of formic acid leads to peak broadening of the XANES and abrupt collapse of Fe-O-Fe
long-range ordering around 2 Å in the RDF. On the other hand, the phase change of iron oxide is not
observed in the in-situ vanillin hydrodeoxygenation reaction. Hence, hydrogen formed by formic acid
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decomposition on the Pd50Ag50/Fe
When the vanillin is present in the medium, however, the hydrogen preferentially reacts with vanillin
rather than Fe . From these in-situ XANES and EXAFS spectral analyses, we could confirm that the
O
4
/N-rGO catalyst can effectively reduce Fe
4
O in N-rGO layer.
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Pd atom actively participates in both formic acid decomposition and vanillin hydrodeoxygenation
reactions.
Well defined PdAg nanoalloys or the core-shell nanopaticles have been investigated in the past for
1
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formic acid decomposition, which suggested that Ag could play the role of electronic promoters. It has
also been reported that vanillin is hydrodeoxygenated by Pd nanoparticle catalysts with hydrogen gas.24
We found that magnetite Fe
3
O
4
particle itself or Fe
3
4
O /N-rGO have no catalytic activity for vanillin
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hydrodeoxygenation with formic acid (Table 1, entry 3, 4). Yet, it is interesting that absence or separate
addition of the magnetite particles showed only a low TOF of 99 and 100 (Table 1, entry 17, 19), while
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Pd50Ag50/Fe
3
O
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/N-rGO catalyst had a much higher TOF of 521 (Table 1, entry 14). To elucidate the role
for the notable synergistic effect, formic acid adsorption over Pd50Ag50/Fe /N-rGO catalyst
was investigated by ATR-IR spectroscopy. We observed characteristic peaks of monodentate formate at
of Fe
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O
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O
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-1
-1
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1620 cm arising from νas(OCO) stretching, bridging formate at 1362 cm and 1585 cm from
symmetrical and asymmetrical modes of O-C-O stretching (Figure S16c) on both Pd50Ag50/Fe /N-
rGO and Fe alone. The peak positions are consistent with the literature reports on formate species
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binding to magnetite center in a variety of ways through formic acid adsorption,37,38 consistent with the
molecular scale investigation.39 It could be assumed here that the formate species could migrate to
nearby PdAg particle and is decomposed to facilitate hydrodeoxygenation of vanillin on the PdAg
surface. Hence, PdAg alloy surface is the main active sites catalyzing both formic acid decomposition
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and vanillin reduction. Magnetic Fe
3
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O particles promote the reaction by chemisorbing formic acid and
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transferring the resulting formate and hydrogen to PdAg surface.
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Based on these mechanistic ideas derived from literature and our own experimental results for our
bifunctional Pd50Ag50/Fe
3
O
4
/N-rGO catalyst, the reaction pathways and the related hypothetical
is further
9 17
structures are proposed as shown in Figure 3 and Figure S8. The promoting role of Fe
O
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supported by the importance of the close proximity of magnetite particle to the PdAg alloy particles,
which is necessary for the facile migration of the formate species. Furthermore, in-situ formic acid
decomposition in the absence of vanillin revealed collapse of long range Fe-O-Fe order as observed in
XANES spectra. Thus, rapid consumption of the hydrogen by the vanillin preserves structural integrity
of the magnetite phase. It is important for practical applications that the catalyst is highly stable, easily
separable, and readily recyclable without loss of activity. With the model reaction involving vanillin
(Table S8), the catalyst was recycled up to eleven times with negligible loss of activity. Yield to 2-
methoxy-4-methylphenol remained constant all throughout the eleven cycles. Moreover, the catalyst
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could easily be separated from the reaction solution by simply using either magnet (supporting movies
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S1) or filtration/centrifugation (in filtration process 100% catalyst recovery was not possible). After each
reaction cycle the composite catalyst was checked by XRD (Figure S17&S18), which showed negligible
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change in PdAg but slight oxidation of Fe
3
4
O due to phase changing behaviour of iron catalyst during
biomass upgradation (details in supplementary S9). To check the possible leaching of metal NPs during
reaction, the reaction mixture was analyzed by inductive coupled plasma emission spectroscopy (ICP-
ES). Less than 0.1 ppm of Ag and Pd was detected, indicating negligible leaching of Ag and Pd into the
reaction mixture. This fact clearly shows that the hetero-atoms (N, O) of N-rGO bind strongly metal
NPs, making them well distributed throughout the surface. The surface of the catalyst recycled eleven
times was analyzed in detail by XPS. It was found that the catalyst Pd and Ag surface was almost the
same with little alteration in elemental atomic percentage (Table S4, Figure S19) and that the catalyst
showed only slightly decreased TOF (Table 1, Entry 20). On the other hand, securing mass balance is
the most important consideration for industrial scale biofuel upgrading. During mass balance study, we
observed that catalyst was fully recovered without any coke deposition over catalyst surface and also
total mass balance during the reaction was almost completely closed (>99 %, Table S6).
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Now that the suitability of the catalyst for practical applications is assured, we went into examination
of the efficiency of our Pd50Ag50/Fe
3
4
O /N-rGO catalyst in defunctionalizing a number of compounds
8 17
derived from carbonyl groups, including vanillin and isatin as a well-known natural compounds. Of the
hydrodeoxygenation reactions, which are considered the most effective method for upgrading oxygen-
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rich bio-fuels, the most challenging reaction involves direct hydrodeoxygenation of carbon-oxygen
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bonds in aromatic aldehydes. The usual direct hydrodeoxygenation leads to significant amounts of
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undesired products such as aromatic alcohol, methyl cyclohexane and benzene. The usual approach,
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therefore, is to reduce aldehyde/ketones to alkanes in the presence of aniline, or H S gas at high
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2 23
temperature and pressure. Direct hydrodeoxygenation of aromatic aldehydes and ketones with hydrogen
produced in-situ from formic acid was attempted with our catalyst to produce a variety of industrially
applicable synthetic feedstocks, and the results are given in Figure 4. Aldehydes with various
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functionalities were converted selectively to methyl substituted aromatic compounds (1-9) at 130 C,
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and the yield was better than 90% for all the molecules tested whenever the conversion took place.
Aromatic ketones, such as acetophenone, benzophenone, and fluorenone, were also selectively
hydrodeoxygenated to aromatic alkanes (10-12) without arene hydrogenation.
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To our delight, even substrates containing amide and aliphatic ester substituted aldehyde or ketone
group were also converted smoothly and selectively to the corresponding alkanes (13-16). The
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Pd50Ag50/Fe
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O /N-rGO catalyst, however, was only reactive to aromatic carbonyl groups; it did not have
any activity toward the C=O bonds in amides, carboxylic acids, esters and aliphatic carbonyls (Figure
S20).
Selective catalytic hydrogenolysis of C-O bonds with its preserved aromatic nature is a challenging
chemistry for lignin biomass utilization. In particular, hydrogenolysis of -O-4 and -O-4 C-O model
compounds, taking up to 90% monomeric units of the alkyl aryl ether in lignin heterogeneous structure,
led to unsatisfactory level of selective hydrogenolysis with various by-products.7,44 Recently selective
depolymerisation of lignin model compounds with prevented benzene ring was achieved by a complex
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multiple step process that aerobic oxidation of the -O-4 linkage model under high O
2
gas pressure was
CN mediated
followed by zinc metal mediated cleavage of aryl C-O bond, and subsequent NaBH
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reduction. However, a simple one-pot single step of selective hydrogenolysis at atmospheric pressure
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was demonstrated by guaiacylglycerol--guaiacyl ether with -O-4 C-O bond as a model compound that
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was selectively hydrogenated and deoxygenated with no use of additives except formic acid, over
o
Pd50Ag50/Fe
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O
4
/N-rGO bifunctional catalyst at 120 C (Figure 5). The reaction resulted in complete
conversion, yielding two products: 3-(3-methoxy-4-hydroxyphenyl)-1-propanol (17a) (till to date the
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production of 17a as a major product has rarely been reported) and guaiacol (17b) with excellent yields
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(Figure S21&S22). Another reaction of interest is hydrogenolysis reaction of 2-phenoxy-1-
phenylethanol. It has been reported that the reaction with bimetallic catalyst under high hydrogen
pressure led to more than 12 products with poorly preserved aromatic nature of product.44,45 In contrast,
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/N-rGO catalyst for hydrogenolysis of 2-phenoxy-1-phenylethanol in the
o
presence of formic acid at comparatively low temperature of 120 C produced only two products (18a,
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18b) with high selectivity via -O-4 C-O model bond cleavage. It is noted in this regard that the
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absence/presence of substituted functional groups did not affect hydrogenolysis of benzyl phenyl ether,
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and excellent conversion and yield were generally obtained at 120 C (Figure 6, 19-21). Comparison to
previously reported heterogeneous catalysts for hydrogenolysis of -O-4 and -O-4 C-O bonds reveals
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superior performance of Pd50Ag50/Fe
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conditions (Table S9).
The PdAg bimetallic catalyst promoted by Fe
3
4
O introduced here produces hydrogen from biomass-
derived formic acid and then utilizes the generated hydrogen in situ to upgrade biofuels and chemical
feedstocks. The efficiency of this dual functional catalyst is exemplified by more than an order of
magnitude increase in TOF and yield when compared with only Pd supported on N-rGO (Table 1). The
versatility and effectiveness of the catalyst advanced here is demonstrated by 99% yield in almost all
cases involving selective hydrodeoxygenation of aromatic aldehydes and ketones with various functional
groups, and selective hydrogenolysis of -O-4 and -O-4 C-O model compounds. This new catalyst
could be an efficient and versatile platform for valorisation of biomass.
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■ Conclusions
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We have introduced a novel strategy for the preparation of a multicomponent and multi-functional
transition metal solid catalyst Pd50Ag50/Fe
3
4
O /N-rGO for the selective upgrading of biomass-derived
2 18
vanillin, isatin and two types of benzyl phenyl ethers as lignin biomass-derived model compounds, and
synthetic feedstocks derived from aromatic carbonyl by simple one-pot reaction under mild conditions.
This represents a significant advance toward the efficient valorisation of biomass by production of bio-
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oil and useful chemicals. Our novel catalyst is effective in dehydrogenating formic acid to form H and
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subsequently utilizing the hydrogen generated in-situ for upgrading bio-oil and chemical feedstocks. The
catalyst is easy to synthesize, stable, easily separable, and readily recyclable without loss of activity, and
the reactions can be carried out in water, making the process environment-friendly. It could play a
critical role in upgrading biomass-derived chemicals to fill future gap in the supply of transportation
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fuels and chemical feedstocks.
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ASSOCIATED CONTENT
Supporting Information
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The following file is available free of charge on the ACS Publications website at DOI: Experimental
details, magnetic separable catalyst video, catalyst characterization data, formic acid dehydrogenation
graph.
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AUTHOR INFORMATION
Corresponding Author
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*E-mail: honghlee@snu.ac.kr
*E-mail: dpkim@postech.ac.kr
Author Contributions
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A.K.S. and S.J. contributed equally to this work.
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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We gratefully acknowledge the support from the National Research Foundation (NRF) of Korea grant
funded by the Korean government (MEST) (No.2008-0061983). AKS thanks the National Institute of
Nanomaterial Technology (NINT), Material Science Departments, Chemistry Departments, PAL,
POSTECH, Pohang, Korea Basic Science Institute (KBSI) Daejeon, for the analytical support. The
Chemical Engineering, POSTECH, has filed a patent on the catalysts reported herein.
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(46) Zhang, B.; Wen, Z. H.; Ci, S. Q.; Mao, S.; Chen, J. H.; He, Z. ACS Appl. Mater. Inter. 2014, 6,
7464-7470.
(47) Crossley, S.; Faria, J.; Shen, M.; Resasco, D. E. Science 2010, 327, 68-72.
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Table 1. Comparative TOF of vanillin hydrodeoxygenation reaction with various synthesized catalysts.
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2
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5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
-
1
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Entry
Catalyst
Temperature BET area (m /g)
TOF (h )
1
2
3
4
5
6
7
8
9
GO
130
130
130
130
130
130
130
130
130
130
130
130
130
130
80
558
230
-
N/A
N/A
N/A
N/A
8
N-rGO
Fe
/N-rGO
Pd/N-rGO
Pd/Fe /N-rGO
3
O
4
Fe
3
O
4
220
202
202
214
222
226
210
152
260
201
251
251
206
222
251
NA
-
3
O
4
32
Ag/Fe
3
3
O
4
/N-rGO
/N-rGO
N/A
N/A
N/A
N/A
50
Au/Fe
O
4
Pt/Fe
3
O
4
/N-rGO
/N-rGO
/N-rGO
1
0
Co/Fe
3
O
4
11
Pd50Co50/Fe
3
O
4
1
1
1
2
3
4
Pd50Au50/Fe
3
O
4
/N-rGO
15
Pd50Pt50/Fe
Pd50Ag50/Fe
Pd50Ag50/Fe
3
O
4
/N-rGO
N/A
521
5
3
3
O
4
/N-rGO
/N-rGO
15
O
4
1
1
6
7
Pd33Co33Au33/Fe
Pd50Ag50/N-rGO
Pd50Ag50/Fe
Pd50Ag50/N-rGO+Fe
Pd50Ag50/Fe /N-rGO
3
O
4
/N-rGO
130
130
150
130
130
63
99
1
8a
3
O
4
/N-rGO
2937
100
482
19b
20c
3
O
4
3
O
4
Reaction condition: vanillin (2 mmol), formic acid (2.5 equiv.), catalyst (20 mg), water (1 ml), specified
reaction time (2 h), conversion based on GC analysis, Turnover frequency (TOF, average of three run,
error limit 1%) = mole of 2-methoxy-4-methylphenol production per hour/mole of active site, (a) with
o
hydrogen gas, water 5 ml, pressure 1.0 MPa, reaction time 2 h, catalyst 20 mg, temperature 150 C, (b)
addition 20 mg Fe
3
O
4
powder, (c) eleven times reused catalyst.
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ACS Catalysis
Table 2. Control selectivity experiments of hydrodeoxygenation of vanillin over various Pd-based
catalysts.
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3
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4
4
4
4
4
4
4
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5
5
5
5
5
5
5
5
5
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0
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4
5
6
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9
0
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Yield (%)a
Conversion
Entry
Catalyst
S/C
a
(
%)
2-methoxy-4-
2-methoxy
Other
methylphenol
phenol
1b
2c
3
Pd/C
10
63
64
66
32
40
51
69
70
98
99
43
24
99
99
14
10
17
13
11
14
10
1
Pd/N-AC
Pd/N-rGO
Pd@Ag/C
10
10
4
4d
106
106
106
106
106
106
106
303
100
80
17
5
Pd/Fe
3
O
4
/N-rGO
NA
NA
NA
NA
NA
NA
NA
6
Pd50Ag50/N-rGO
100
100
98
7
Pd50Ag50/Fe
Pd70Ag30/Fe
Pd80Ag20/Fe
Pd50Ag50/Fe
Pd50Ag50/Fe
3
3
3
3
3
O
O
O
O
O
4
4
4
4
4
/N-rGO
/N-rGO
/N-rGO
/N-rGO
/N-rGO
NA
54
63
NA
NA
8
9
88
1
0e
1f
100
100
1
Reaction condition: vanillin (0.5 mmol), formic acid (2.5 equiv.), S/C = 106 (substrate mmol/Pd mmol);
o
water (1 ml), specified reaction time (6 h); temperature 130 C; (a) conversion and yield based on GC
analysis and anisole as a standard, average of two measurements data errors <1% ; (b) commercial
2
available Pd/C (BET area 822 m /g) was used as such; (c) Pd/N-AC was synthesized by simply modified
method of the reported46 (d) core-shell Pd@Ag/C catalyst was synthesized by reported paper11; (e)
vanillin (0.5 mmol), catalyst amount 30 mg, formic acid (5 equiv.), water (1 ml); (f) gram-scale reaction
=
vanillin (10 mmol, 1.52 g), formic acid (2.5 equiv.), catalyst 140 mg, specified reaction time (6 h);
o
temperature 130 C, other by-product were unidentified.
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Table 3. Comparative selective hydrogenation of vanillin by various reported catalysts.
Temp.
H
2
gas
Additive/
Solvent
Conv.
(%)
MMP
(%)
Catalyst
Ref.
o
Time (h)
(
C)
pressure
6
Zn/Pd/C
Pd/Ca
150
130
150
150
120
150
150
300
130
150
20.6 bar
No
NA/Methanol
HCOOH/Water
Water
8
6
6
6
6
6
6
2
6
2
>99
63
81
32
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
5
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0
1
2
3
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8
9
0
This study
2
2
4
4
Pd@CN0.132
Pd@C
10 bar
10 bar
10 bar
10 bar
10 bar
No
98
100
74
Water
98
24
Pd@CeO
Pd@γ-Al
Pd@SWNT-SiO
2
Water
88
14
2
4
4
7
2
O
3
Water
95
69
2
Water/decalin
Water
85
47
Pd-Nafion SAC-13b
20
100
100
100
3
/N-rGOa
/N-rGOa
No
HCOOH/Water
Water
>99
100
This study
This study
Pd50Ag50/Fe
3
3
O
4
4
Pd50Ag50/Fe
O
10 bar
Reaction condition: (a) vanillin (0.5 mmol), formic acid (2.5 equiv.), S/C = 106 (substrate mmol/Pd
mmol (wt%), water (1 ml).; (b) vanillin (1.31 mmol), S/C = 70, formic acid (10.9 equiv.)
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FIGURE CAPTIONS
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Figure 1. Microscopic analysis and proposed structure of Pd50Ag50/Fe
3
O /N-rGO catalyst. (a&b) SEM images; (c) low resolution TEM analysis; (d)
4
high resolution TEM analysis; (e) selected area electron diffraction pattern of N-rGO layer; (f) d-spacing from stacked N-rGO layers; (g) Dark field
HAADF images with elemental line profile (yellow line Fe, green line Pd, and red line Ag); (h&i) CS-corrected TEM image of magnetite and PdAg
nanoalloy; (j&k) proposed model for Fe
3
O and PdAg nanoalloy; (l) 2D scratch art model (yellow: magnetite, blue background: N-rGO, and green/red
4
cluster: PdAg alloy); (m) proposed 3D structural model.
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(
c)
(
a)
(b)
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Figure 2. X-ray absorption spectroscopy. (a) Pd, (b) Ag and (c) Fe radial distribution functions of Fourier-transformed EXAFS spectra under
different catalytic reactions.
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Figure 3. Proposed pathways for tandem hydrodeoxygenation of vanillin with formic acid.
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Figure 4. Selective hydrodeoxygenation of aromatic aldehyde/ketone and functionalized aromatic
aldehyde/ketone to aromatic methylene compounds catalyzed by Pd50Ag50/Fe /N-rGO. Reaction
3
O
4
conditions: reactant (0.5 mmol), formic acid (2.5 equiv), 30 mg catalyst (~0.32 at% Pd), water (1 mL).
Conversion and yield are based on gas chromatography (GC) analysis using anisole (0.5 mmol) as an
internal standard. At least three measurements were taken to obtain an average yield. Yield in
parenthesis refers to an isolated yield.
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Figure 5. Selective hydrogenolysis reactions of -O-4 linkage model aryl C-O group with formic acid
over Pd50Ag50/Fe
3
O
4
/N-rGO catalyst. Reactant (0.1 mmol), formic acid (3.5 equiv), catalyst (50 mg),
o
time 3h, temperature 120 C; Conversion and yield are based on NMR analysis using decane (0.1 mmol)
as an internal standard. At least two measurements were taken for an average yield. In all cases complete
conversion of -O-4 linkage aryl C-O group was achieved.
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Figure 6. Selective hydrogenolysis reactions of -O-4 linkage model aryl C-O group with formic acid
over Pd50Ag50/Fe
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4
O /N-rGO catalyst. Reactant (0.5 mmol), formic acid (1.5 equiv), catalyst (50 mg),
o
temperature 120 C; Conversion and yield are based on GC analysis using decane (0.5 mmol) as an
internal standard. At least two measurements were taken for an average yield. In all cases complete
conversion of -O-4 linkage model aryl C-O group was achieved.
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5
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