Hydrodeoxygenation of Lignin-Derived Aromatic Oxygenates Over Pd-Fe Bimetallic Catalyst: A Mechanistic Study of Direct C–O Bond Cleavage and Direct Ring Hydrogenation

Article abstract of DOI:10.1007/s10562-020-03352-3

Hydrodeoxygenation of lignin-derived phenols could be achieved generally with three reaction pathways: tautomerization, direct ring hydrogenation and direct C–O bond cleavage. The former pathway has been extensively studied over Pd/Fe catalyst in liquid-phase reaction, however, the contribution of the latter two is yet subject to further investigations. In this report, a comparative study of direct C–O bond cleavage and direct ring hydrogenation reaction pathways is presented on Pd/Fe, Fe and Pd/C catalysts using diphenyl ether as modelling compound. Despite its much higher activation energy than direct ring hydrogenation, direct C–O bond cleavage is dominant over Pd/Fe with much higher rates than the monometallic analogues due to the synergic catalysis of Pd–Fe. Based on this study and our previous results, the detailed reaction network for HDO of diphenyl ether is proposed. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03352-3

Catalysis Letters
https://doi.org/10.1007/s10562-020-03352-3
Hydrodeoxygenation of Lignin‑Derived Aromatic Oxygenates Over
Pd‑Fe Bimetallic Catalyst: A Mechanistic Study of Direct C–O Bond
Cleavage and Direct Ring Hydrogenation
Jianghao Zhang · Berlin Sudduth · Junming Sun · Yong Wang1,2
1
1
1
Received: 8 June 2020 / Accepted: 4 August 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Hydrodeoxygenation of lignin-derived phenols could be achieved generally with three reaction pathways: tautomerization,
direct ring hydrogenation and direct C–O bond cleavage. The former pathway has been extensively studied over Pd/Fe catalyst
in liquid-phase reaction, however, the contribution of the latter two is yet subject to further investigations. In this report, a
comparative study of direct C–O bond cleavage and direct ring hydrogenation reaction pathways is presented on Pd/Fe, Fe
and Pd/C catalysts using diphenyl ether as modelling compound. Despite its much higher activation energy than direct ring
hydrogenation, direct C–O bond cleavage is dominant over Pd/Fe with much higher rates than the monometallic analogues
due to the synergic catalysis of Pd–Fe. Based on this study and our previous results, the detailed reaction network for HDO
of diphenyl ether is proposed.
Graphic Abstract
Keywords C–O bond cleavage · Direct ring hydrogenation · Pd–fe catalyst · Mechanistic study · Aromatic oxygenate
1
Introduction
Hydrotreating is one of the key process in oil reꢀning to
remove heteroatoms such as sulfur, nitrogen, and oxygen [1,
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03352-3) contains
supplementary material, which is available to authorized users.
2
]. Similar to hydrodesulfurization and hydrodenitrogena-
tion, which have been comprehensively studied in petroleum
industry due to the consideration of environment and engine
protection [3], hydrodeoxygenation (HDO) is an important
research topic since the large oxygen content in bio-oil needs
to be reduced to increase its energy density, stability and
volatility [4].
*
Junming Sun
Junming.sun@wsu.edu
*
Yong Wang
yong.wang@pnnl.gov
1
2
The Gene & Linda Voiland, School of Chemical Engineering
and Bioengineering, Washington State University, Pullman,
WA 99164, USA
The HDO of lignin-derived phenols is highly challeng-
ing in bio-oil upgrading and generally proceeds through
three reaction pathways: tautomerization, direct ring
hydrogenation and direct C–O bond cleavage [5–7]. The
Institute for Integrated Catalysis, Paciꢀc Northwest National
Laboratory, Richland, WA 99352, USA
Vol.:(0
1
123456789)
3

J. Zhang et al.
tautomerization pathway involves a fast and reversible
isomerization of phenols to form unstable ketone interme-
diates over the surface of the catalyst [8]. Accordingly, the
allows us to understand the remaining pathways, i.e. direct
C–O bond cleavage and direct ring hydrogenation.
recalcitrant C
–O is transformed to a weaker C=O
aromatic
bond, and the oxygen can be readily removed with a lower
reaction barrier. Although this pathway is able to produce
arenes, as reported in vapor-phase reactions at low pressures
2 Experimental
2.1 Catalysts Preparation
[
8, 9], we have proven that tautomerization is the main con-
tributor to ring saturation in liquid phase during the HDO
of phenol over Pd/Fe catalysts [7]. The direct ring hydro-
genation pathway usually happens over noble metal catalysts
2.1.1 Preparation of Fe2O3
Fe O was synthesized by precipitation method following the
2
3
[
10] that are widely utilized in HDO reactions due to their
previous work [22, 26]. In a typical synthesis, (NH ) CO
4 2 3
negligible barriers for H dissociation. Direct ring hydro-
(Sigma-Aldrich, 99.999%, 1.5M) solution was added drop-
wise to the solution of Fe(NO ) (Sigma-Aldrich, >99.99%,
2
genation commonly causes ring saturation [11, 12] because
adding the ꢀrst hydrogen atom to the phenyl ring has the
highest barrier during aromatic ring saturation [5, 13] and
once the aromaticity is broken, the remaining C=C bonds
3
3
3M). The dark crimson precipitate was then washed and
ꢀltrated with Milli-Q water until the pH reached 8. The
obtained sample was dried at 80 °C overnight and crushed
to a ꢀne powder (sieving to <100 mesh), followed by calci-
nation at 400 °C for 5 h.
can be readily saturated, especially under high H pressures
2
where the saturation is thermodynamically favored. Direct
C–O bond cleavage of phenolics utilizes hydrogen most
eꢁciently to achieve the deoxygenation though it needs to
overcome a high reaction barrier [14]. However, this barrier
could be lowered over oxophilic metals where the activa-
2.1.2 Preparation of Supported 1 wt.% Pd Catalysts
Incipient wetness impregnation method was employed to
prepare 1 wt.% Pd/Fe O . 10 wt.% Pd(NH ) (NO ) solu-
tion of C
–O bond can be signiꢀcantly facilitated by
aromatic
2
3
3 4
3 2
elongating the bond length [15]. After cleaving the C–O
bond, the catalytic cycle is completed by hydrogenation of
the phenyl and hydroxyl species to form benzene and water.
Bimetallic catalysis employing an oxophilic metal with
tion (Sigma-Aldrich, 99.99%, metal basis) was diluted with
Milli-Q water. This solution was then mixed with the Fe O
2
3
powder by stirring. Afterwards, the sample was dried at
80 °C overnight in air before calcination at 350 °C for 2h in
a metal activating H has been investigated and proposed as
ꢃowing N (50 mL/min). Monometallic Pd, 1 wt.% Pd/C was
2
2
an eꢁcient approach to HDO [9, 16–20]. Recently, vapor-
phase HDO of guaiacol was studied over carbon-supported
Pd–Fe catalysts [21] which displayed enhanced activity
and arene selectivity without ring saturation. The synergic
eꢂect between Pd and Fe was further demonstrated using
Pd-promoted Fe nanoparticles (Pd/Fe) for vapor-phase HDO
prepared following the same procedure using active carbon
(TA70 PICATAL) as support.
2.2 Characterization
Transmission electron microscopy (TEM) analysis of the
catalysts was carried out using an FEI Technai G2 20 Twin
operated at 200 kV and equipped with a 4k Eagle CCD
camera. Before the measurement, the samples were ground
in a mortar for 5 min and suspended into ethanol with a
sonicator, then a drop of the nanoparticle suspension was
dispensed onto a 3-mm carbon-coated copper grid. Excess
ethanol was removed by an absorbent paper, and the sample
was dried at room temperature for 6 h.
of m-cresol, i.e. Pd assists H dissociation and facilitates
2
stabilization of the metallic Fe surface which can selectively
cleavage the C–O bond [22]. The following kinetic study
supports the direct C–O bond cleavage mechanism [23]. The
Pd-Fe bimetallic catalysts have also been applied in the con-
version of aryl ethers and shown high yield to the aromatics
in both hydrogenolysis [24] and transfer hydrogenolysis [25].
However, when this Pd/Fe catalyst was applied in liquid-
phase HDO, which can minimize the energy consumption,
ring-saturated products dominated [7]. We systematically
studied the mechanism and demonstrated that tautomeriza-
tion is responsible for ring saturation during liquid-phase
HDO of phenol [7].
Temperature-programmed Reduction with H (H -TPR)
2
2
was carried out on Chemisorption Analyzer (Micromeritics
AutoChem 2920) equipped with a quadrupole mass spec-
trometer (Omnistar gas analyzer GSD 301). Approximately
50 mg of catalyst was loaded into a quartz tube and purged
with UHP Argon at a rate of 50 mL/min for 10 min. The
temperature was increased to 300 °C and held for 1 h and
then ramped to ambient. After this pretreatment, the ꢃowing
The role of tautomerization has been well understood
over Pd/Fe [7], whereas the contributions of the other two
pathways over these catalysts remains unclear. Herein, we
present a mechanistic study using a model reactant, diphenyl
ether (DPE), which structurally inhibits tautomerization and
gas was switched to UHP H at a rate of 50 mL/min and the
2
temperature was ramped to 500 °C at 2 °C/min.
1
3

Hydrodeoxygenation of Lignin-Derived Aromatic Oxygenates Over Pd-Fe Bimetallic Catalyst:…
X-Ray powder diꢂraction (XRD) was performed on a
that the reduced catalyst is stable under reaction conditions.
Rigaku Miniꢃex II X–Ray diꢂractometer with Cu radia-
The reducibility was investigated with H -TPR as shown in
κα
2
tion (λ =1.54178 Å) operating at 40 kV and 50 mA. The
Fig. 1d. The TPR proꢀle shows the typical two-step reac-
tions at 166 and 339 °C that are attributed to reduction of
Fe O to Fe O and Fe O to Fe [27]. Note that the sample
scan was conducted at the rate of 1°/min with a step-size of
0
.01° from 20 to 70°.
2
3
3
4
3
4
was pretreated at 350 °C and exposed in H at 300 °C during
2
2
.3 Activity Test
HDO, therefor the surface of catalyst should be metallic Fe
modiꢀed with Pd. Fresh Pd/Fe O , reduced and spent Pd/
2
3
Prior to the activity test, all the samples (i.e. Fe O , Pd/C
Fe were characterized with XRD as shown in Fig. 1e. The
pattern for the fresh samples shows its crystal structure type
is hematite (JCPDS: 33-0664). After reduction and HDO
reaction, the peaks for oxide disappears while the new peaks
corresponding to metallic Fe (JCPDS: 06-0696) dominate.
Though there is remaining oxide in the catalyst as suggested
by XRD, given the sample was pretreated at 350 °C and
2
3
and Pd/Fe O ) were reduced ex-situ with 40 ml/min 50%
2
3
H /N at 350 °C for 2 h followed by passivation with 40 ml/
2
2
min 0.2% O /N at room temperature for 1 h. Then each
2
2
passivated catalyst was transferred into a glass liner in the
stainless steel Parr reactor (Series 4560, 300 mL) for HDO
of the DPE. After purging the reactor with 4MPa H three
2
times, the temperature was ramped to 300 °C (15 °C/min)
exposed in H at 300 °C during HDO, the surface should be
2
and held for another 30 min in H (3 MPa) for in situ reduc-
metallic Fe modiꢀed with Pd. This is supported by in-situ
Raman [7] and X-Ray photoelectron spectroscopy [22] in
our previous study.
2
tion. Afterwards, the reactor was cooled down to ambient
temperature and the pressure was released. 6.35 mmol of
DPE in 50 mL of hexadecane was then injected via syringe
into the reactor. The reactor was then pressurized with H
3.2 The Reaction Pathways
2
and heated up to the target temperature under a stirring rate
of 800 rpm for the reaction. The products were sampled peri-
odically for analysis at the reaction temperature. Given the
variable composition with temperature in the liquid phase, a
linear correction assuming a constant coeꢁcient in Henry’s
law was made in the concentration range studied.
The reaction pathway was ꢀrst studied over a Pd/Fe bime-
tallic catalyst at 300 °C and 4 MPa H . Fig. 2a shows the
2
yield of each product as a function of reaction time. With
increasing time-on-stream, benzene reaches a vertex and
then gradually decreases likely due to slow hydrogenation
over the Pd/Fe surface. The amount of cyclohexane continu-
ously increases as a result of the slow hydrogenation of ben-
zene and the conversion of phenol (the primary product from
C–O bond cleavage of DPE) by tautomerization pathway [7].
The product selectivity versus DPE conversion is replotted
and shown in Fig. 2b and c. The monomers (cyclohexane
and benzene) which are derived from direct C–O bond cleav-
age are the major products during the entire reaction. By
extrapolating the selectivity curve to 0% conversion using
SPSS 22.0 software [28], the contribution of each primary
reaction pathway in the reaction can be determined [29].
The direct C–O bond cleavage dominates with 94% selec-
tivity versus 6% for direct ring hydrogenation. The reaction
pathways over monometallic catalysts were also studied
to gain insight into synergistic eꢂects and how diꢂerent
metals inꢃuence the reacting mechanism. Over monome-
tallic Pd (i.e., 1 wt.%Pd/C, detailed structures have been
presented in previous work [22]), a comparably high activ-
ity was observed but it is mainly through direct aromatic
ring hydrogenation as evidenced by the dominant formation
of cyclohexyl phenyl ether (CPE) and dicyclohexyl ether
(DCE), especially at low DPE conversion. As DPE conver-
sion increases, the selectivity to cyclohexane increases at the
expenses of CPE and DCE, suggesting the further hydrog-
enolysis of the dimers to cyclohexane. This result also sug-
gests that the mechanism of cyclohexane formation on Pd is
Product analyses was conducted with a gas chromato-
graph (GC, Agilent 7890A) equipped with DB-FFAP col-
umn (30m, 0.32mm, 0.25μm) and ꢃame ionization detec-
tor (FID), as well as GC-MS (Shimadzu, GCMS-QP2020)
equipped with a HP-5 column (30m, 0.32mm, 0.25μm) con-
nected to an FID. The conversion, selectivity and yield were
calculated as follows: conversion [%] = (moles of carbon
in the converted substrate/moles of carbon in the substrate
fed) × 100; selectivity [%] = (moles of carbon in the speciꢀc
product/moles of carbon in all product) × 100; yield [%] =
(
moles of carbon in the speciꢀc product/moles of carbon in
the substrate fed) ×100. The carbon balance was above 92%.
The diꢂerence of conversion between 1h and the ꢀrst data
point was used to calculate the speciꢀc reaction rate.
3
Results and Discussions
3.1 Characterization of the Catalysts
Figure 1a, b and c display the TEM images of fresh, reduced
and spent samples. The fresh sample (1 wt.% Pd/ Fe O )
2
3
has a uniform size around 20 nm, being consistent with our
previous study [22]. After a reduction at 350 °C with 50%
H /N for 2 h, the particles of reduced Pd/Fe grow larger and
2
2
are similar in size compared to the spent Pd/Fe, indicating
1
3

J. Zhang et al.
Fig. 1 Representative a–c TEM images of fresh 1 wt.% Pd/ Fe O ,
Pd/Fe catalysts (the patterns are scaled to produce an approximate
match in intensity)
2
3
reduced Pd/Fe and spent Pd/Fe samples; d H -TPR proꢀle of 1 wt.%
2
Pd/Fe O ; e XRD patterns of 1 wt.% Pd/ Fe O , reduced and spent
2
3
2
3
diꢂerent from that on Pd/Fe and Fe in liquid-phase reactions.
Using the same extrapolation method, the selectivities at
high C–O bond cleavage selectivity observed on Fe. On the
other hand, Fe also modiꢀed the Pd [30], leading to inhibited
activity for aromatic ring saturation (0.17 mmol/h for Pd/Fe
and 1.02 mmol/h for Pd/C) though the two catalysts expose
same amount of Pd. This synergy between Pd and Fe has
also been found in the vapor-phase HDO of phenols (i.e.,
guaiacol [21], cresol [22]).
0
% conversion for direct C–O bond cleavage and direct ring
saturation were determined to be 22% and 78%, respectively.
Over monometallic Fe catalyst (the structure has been stud-
ied in previous work [22]), despite its low activity, no direct
aromatic ring hydrogenation and only direct C–O cleavage
was observed (Fig. 2g–i). Table 1 summarizes the reaction
rates for direct C–O bond cleavage and direct ring hydro-
genation over these three catalysts. Pd/C displayed a C–O
cleavage (22% selectivity) rate of 0.29 mmol/h at 300 °C,
while Fe only showed a rate of 0.27 mmol/h with 100%
selectivity. In contrast, Pd/Fe catalyst not only maintained
high selectivity (94%), but also exhibited the highest rate
for direct C–O cleavage (2.64 mmol/h). The above results
indicate that the dominant reaction pathway diꢂers over
these three catalysts. Pd primarily hydrogenates the aromatic
ring with high reaction rates while Fe primarily cleaves the
C–O bond with lower rates. The synergy between Pd and
Fe signiꢀcantly enhanced activity while maintaining the
3.3 Activation Energies for the Two Pathways
To understand the energy proꢀle of the two pathways, Arrhe-
nius plot for HDO of DPE was derived as shown in Fig. 3.
The two parallel reactions displayed distinct apparent activa-
tion energies (E ): 46 kJ/mol for direct hydrogenation and
a
86 kJ/mol for direct C–O cleavage of DPE. Wang et al. [31]
studied cresol HDO over Mo-based catalysts and measured
the E for direct deoxygenation through C–O bond cleavage
a
as 125 kJ/mol and hydrogenation followed by deoxygenation
as 89 kJ/mol. These results display the same trend that the
E for direct C–O bond cleavage is much higher than that
a
1
3

Hydrodeoxygenation of Lignin-Derived Aromatic Oxygenates Over Pd-Fe Bimetallic Catalyst:…
Fig. 2 The DPE conversion over Pd/Fe (a–c, data taken from ref[7]),
Pd/C (d–f) and Fe (g–i) catalysts. a, d, g, Conversion and yield of
each products as a function of reacting time; b, e, h, Product distribu-
tion as a function of DPE conversion; c, f, i, Selectivity of monomers
of 1%Pd/C were determined to expose same amount of Pd as Pd/Fe
according to the CO-TPD results (Fig. S1). Catalysts amounts: 150
mg for Pd/Fe, 20 mg for Pd/C and 150 mg for Fe. Reaction condition:
300 °C, 6.35 mmol diphenyl ether (1 mL), 50 mL hexadecane, 6MPa
(
from direct C–O bond cleavage) and ring-saturated dimers (from
H
2
direct ring hydrogenation) as a function of conversion. The amount
for ring hydrogenation. The E for DPE HDO is lower than
as shown in Scheme 1. Overall, there are two parallel pri-
mary routes under the studied reaction conditions: direct
C–O bond cleavage, which mainly exists over Pd/Fe and
Fe catalysts (Fig. 2c and i), and direct ring hydrogena-
tion, which mainly exists over Pd/C catalyst (Fig. 2f). In
the direct C–O bond cleavage route, the primary product,
phenol, is converted via tautomerization and hydrogena-
tion to form cyclohexanone and cyclohexadienol which
are subsequently converted to cyclohexanol, followed by
C–O bond cleavage to form cyclohexane [7]. However,
over Pd/C catalyst, DPE is primarily hydrogenated to form
CPE. The formed CPE is then converted via C–O bond
cleavage, as evidenced in Fig. 2d–e and Fig. S2. Given
a
cresol, likely due to the much lower C–O bond dissociation
energy of DPE [32]. Note that although the direct hydrogen-
ation has a much smaller activation barrier, its reaction rate
is still lower than direct C–O cleavage in the tested range of
temperature. This suggests that the pre-exponential factor
in the Arrhenius rate law and the amounts of active sites for
direct ring-hydrogenation should be much smaller than that
of C–O cleavage.
3.4 Reaction Mechanism
Based on the above results and our previous study [7],
the pathway of catalytic conversion of DPE is proposed
1
3

J. Zhang et al.
Table 1 Catalytic performances of the Pd/C, Fe and Pd/Fe catalysts
of CPE also occurs leading to the formation of DCE. Note
that, due to its high bond energy, the C–O bond in DCE
could hardly be cleaved on Pd at low temperature (e.g.
200 °C [34]). Herein, at 300 °C and 6MPa pressure, it is
found that hydrogenolysis of DCE slowly occurs to form
cyclohexanol, cyclohexene and cyclohexane, as evidenced
in Fig. 2d–e and Fig. S2.
for the HDO of DPE. Reaction condition: 300 °C, 6.35 mmol diphe-
nyl ether (1 mL), 50 mL hexadecane, 6 MPa H
2
Catalyst Speciꢀc rate of
C-O cleavage
Speciꢀc rate of
ring hydrogena-
tion (mmol/h)
Selectivity at 0%
a
conversion
(mmol/h)
C-O
Hydro-
cleavage genation
(
%)
(%)
3
.5 Stability of Pd/Fe
Pd/Fe
Fe
2.64
0.27
0.29
0.17
0
1.02
94
100
22
6
0
78
Pd/Cb
The stability of Pd/Fe catalyst was further tested using
HDO of diphenyl ether at 300 °C and 6 MPa H , as shown
2
a
Selectivity was derived by extrapolating the curve in Fig. 2c, f, i to
conversion = 0.
in Fig. 4. After one cycle of reaction, the catalyst was
washed with ethanol, followed by drying at 50 °C over-
night. In four tested cycles, the conversion and deoxygena-
tion selectivity are maintained at around 55% and 97%,
respectively. The good stability of catalytic performance is
in agreement with the TEM images that show no obvious
change of the particle size when comparing the fresh and
spent catalysts. More importantly, the high stability indi-
cates that there is no surface reconstruction nor change in
active site during the reaction. Thus, the change of product
concentration during HDO in the above study is exclu-
sively attributed to intrinsic catalytic processes over stable
active sites.
b
The amount of catalysts was determined to expose same amount of
Pd according to the CO-TPD results (Fig. S1). Catalysts amounts: 150
mg for Pd/Fe, 20 mg for Pd/C and 150 mg for Fe.
4
Conclusion
A comparative study of direct C–O bond cleavage and
direct ring hydrogenation reaction pathways has been con-
ducted over Pd/Fe, Fe and Pd/C catalysts with a modelling
compound: diphenyl ether (a molecule that is exclusively
converted via two pathways during HDO). Although direct
C–O bond cleavage has higher apparent activation energy
than direct ring hydrogenation, Pd/Fe catalysts display a
synergic eꢂect resulting in enhanced reaction rate and high
direct C–O cleavage selectivity. This is likely due to the
surface of this catalyst is dominated with the active sites
selective for direct C–O bond cleavage. A detailed reaction
network is proposed where Pd/Fe and Fe mainly catalyze
the direct C–O bond cleavage of DPE and the produced
phenol is then converted through the tautomerization path-
way; in contrast, monometallic Pd mainly catalyzes direct
ring hydrogenation.
Fig. 3 Arrhenius plot for direct ring hydrogenation and C–O cleav-
age of diphenyl ether (data was derived at 150 °C, 200 °C, 250 °C,
3
00 °C). Reaction condition: 6.35 mmol diphenyl ether (1 mL), 50
mL hexadecane, 6MPa H
2
the lower bond dissociation energy of C
–O com-
–O is
aliphatic
–O in CPE [33], C
aliphatic
paring with that of C
aromatic
cleaved forming cyclohexane and phenol. The phenol is
further converted to cyclohexanol by direct ring hydro-
genation or tautomerization pathways, and subsequently
to cyclohexane/cyclohexane. In parallel, the hydrogenation
1
3

Hydrodeoxygenation of Lignin-Derived Aromatic Oxygenates Over Pd-Fe Bimetallic Catalyst:…
Scheme 1 Proposed reaction pathway for the HDO of DPE
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conꢃict of
interest.
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