Liquid-phase hydrodeoxygenation of lignin-derived phenolics on Pd/Fe: A mechanistic study

Article abstract of DOI:10.1016/j.cattod.2018.12.027

Although Pd/Fe bimetallic catalysts have been extensively studied in vapor-phase hydrodeoxygenation of phenolics (i.e., guaiacol, cresol), little is yet known about their performance in liquid-phase reactions. In this work, we present a mechanistic study on the Pd/Fe bimetallic catalysts in liquid-phase hydrodeoxygenation of phenolics. The role of tautomerization in hydrodeoxygenation of the lignin-derived phenolics, particularly for ring saturation, is systematically studied by employing two representative modeling compounds: phenol (a molecule that is keto-enol tautomeric) and diphenyl ether (a molecule that does not allow ketol-enol tautomerization). It was found that although the addition of oxyphilic Fe inhibits the direct aromatic ring saturation reaction typically occurring on Pd, tautomerization opens another reaction pathway toward ring saturation products (i.e. cyclohexanone, cyclohexanol, cyclohexane et al.), where both tautomerization and the hydrogenation of keto isomers are significantly enhanced to produce cyclohexanol followed by direct hydrogenolysis of the cyclohexanol to cyclohexane.

Full text of DOI:10.1016/j.cattod.2018.12.027

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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Liquid-phase hydrodeoxygenation of lignin-derived phenolics on Pd/Fe: A
mechanistic study
⁎
Jianghao Zhanga, Junming Suna,⁎, Berlin Suddutha, Xavier Pereira Hernandeza, Yong Wanga,b,
a The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164, United States
b Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA, 99352, United States
A R T I C L E I N F O
A B S T R A C T
Keywords:
Tautomerization
Reaction mechanism
Liquid-phase hydrodeoxygenation
Pd/Fe
Although Pd/Fe bimetallic catalysts have been extensively studied in vapor-phase hydrodeoxygenation of
phenolics (i.e., guaiacol, cresol), little is yet known about their performance in liquid-phase reactions. In this
work, we present a mechanistic study on the Pd/Fe bimetallic catalysts in liquid-phase hydrodeoxygenation of
phenolics. The role of tautomerization in hydrodeoxygenation of the lignin-derived phenolics, particularly for
ring saturation, is systematically studied by employing two representative modeling compounds: phenol (a
molecule that is keto-enol tautomeric) and diphenyl ether (a molecule that does not allow ketol-enol tauto-
merization). It was found that although the addition of oxyphilic Fe inhibits the direct aromatic ring saturation
reaction typically occurring on Pd, tautomerization opens another reaction pathway toward ring saturation
products (i.e. cyclohexanone, cyclohexanol, cyclohexane et al.), where both tautomerization and the hydro-
genation of keto isomers are significantly enhanced to produce cyclohexanol followed by direct hydrogenolysis
of the cyclohexanol to cyclohexane.
Lignin-derived phenolics
1. Introduction
have been proposed in the HDO of phenolic compounds: direct CeO
bond cleavage/hydrogenolysis, direct aromatic ring hydrogenation, and
The diminishing fossil resources initiated great efforts seeking for
the sustainable energy [1]. Among those, one promising solution is to
produce the renewable fuels and chemicals from the earth-abundant
and inexpensive lignocellulosic biomass [2–4], which mainly consists of
cellulose, hemicellulose and up to 30 wt.% lignin [5]. Pyrolysis of
biomass is a feasible process that produces the bio-oil containing hun-
dreds of oxygenates subjecting to further upgrading [6–8]. One of the
most critical processes for this upgrade is catalytic hydrodeoxygenation
(HDO) [9]. A series of HDO catalysts have been extensively studied for
the past decades [9–13]. Traditional HDO catalysts were based on metal
sulfides [7,14], which yield high arene selectivity in the conversion of
phenolics [15,16]. A common issue with sulfide catalysts is the con-
tamination of sulfur in products [17]. To this end, more and more re-
searches have been focusing on the development of other alternative
catalysts including supported noble metal catalysts such as Pt-Mo/
MWCNT [5], Pd [17–19], Pt [20,21], Pd/Nb2O5/SiO2 [22], Ru/H-Beta
[23], Pt-Zn [24], base metal catalysts such as FeMoP [25], Ni [26,27],
Fe [28,29], Ni-Fe [30], MoO3 [31,32], and metal carbides such as WCx
[33], MoCx [12,34].
keto-enol tautomerization [35], as summarized in Scheme 1 for the case
of phenol. While direct CeO bond cleavage produces benzene (black
color highlighted), direct aromatic ring hydrogenation will lead to the
formation of cyclohexane (red color highlighted). In the former
pathway, the adsorbed phenol on catalyst is activated by the dissocia-
tion on the surface via cleavage of CeO bond, being followed by hy-
drogenation of phenyl and hydroxyl radical to produce benzene and
water. In the latter case, adding the first hydrogen atom to the phenyl
ring was calculated having the highest barrier in the direct aromatic
ring hydrogenation over Ni(111) and Pt(111), [35,36] leading to the
primary formation of cyclohexanol intermediate rather than enols such
as cyclohexadienol and cyclohexenol [37]. Then, cyclohexanol can be
converted to cyclohexane by: 1) direct CeO bond cleavage or hydro-
genolysis or 2) dehydration to form cyclohexene followed by hydro-
genation of cyclohexane. It should be noted, although dehydrogenation
of cyclohexene to benzene has been shown feasible in the presence of
phenol with 2-propanol as H-transfer initiator and H donor [38], this
reaction is thermodynamically unfavorable in the presence of high
pressure H2. It is thus believed the contribution for benzene production
from cyclohexene dehydrogenation is minimum. Tautomerization (blue
Depending on the catalysts employed, several reaction pathways
⁎ Corresponding authors at: The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164,
United States.
E-mail addresses: Junming.sun@wsu.edu (J. Sun), yong.wang@pnnl.gov (Y. Wang).
https://doi.org/10.1016/j.cattod.2018.12.027
Received 5 August 2018; Received in revised form 16 November 2018; Accepted 12 December 2018
0920-5861/©2018ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Zhang,J.,CatalysisToday,https://doi.org/10.1016/j.cattod.2018.12.027

J. Zhang et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Scheme 1. The proposed reaction pathways for HDO of phenol.
color highlighted) is another important reaction pathway since it has
the lowest calculated reaction barrier (e.g. 0.78 eV for cresol [39] on Pt-
Mo catalyst and 0.52 eV for catechol [40] on Pt catalyst) among the
three proposed reaction pathways shown in Scheme 1. After the keto-
enol tautomerization, the subsequent hydrogenation of the tautomer-
ized enone could have two competitive secondary reactions: one is
hydrogenation of C]O bond of the enone to cyclohexadienol, followed
by an acid catalyzed dehydration of cyclohexadienol to form benzene as
proposed on Pt/SiO2 and Ni-Fe by Nie et al [21,30]. The other is a
consecutive hydrogenation of C]C and then C]O bond in enone to
produce cyclohexanone and cyclohexanol. The obtained cyclohexanol
could be further converted into cyclohexane by either dehydration/
hydrogenation as proposed on the Pd/Al2O3 with metal-acid bi-func-
tionality by Souza et al, [41] or direct hydrogenolysis of cyclohexanol
although it has rarely been accepted yet.
phase [46], while only ring-saturation products were detected in the
HDO of phenol at 250 °C in liquid phase [17].
Recently, our group reported the vapor-phase HDO of guaiacol over
carbon-supported Pd-Fe catalyst [54], which exhibited enhanced ac-
tivity and arene selectivity without ring saturation. The synergic effect
between Pd and Fe was further verified using Pd-promoted Fe nano-
particles for vapor-phase HDO of m-cresol, which displayed > 90% of
arene selectivity [46]. Herein, we further studied the HDO of phenolics
on the same Pd/Fe catalyst in liquid-phase reactions, and attempted to
understand the reaction mechanisms involved.
2. Experimental
2.1. Catalyst preparation
It was found that the reaction pathways and the consequent pro-
ducts were affected by the adsorption configuration of phenols [42], i.e.
horizontal and vertical adsorption. The vertical position (taking place
on MoS2 et al. catalysts) is beneficial to direct CeO bond cleavage,
while the horizontal position favors hydrogenation of the aromatic ring
[43]. In the case of noble metal catalysts where horizontal adsorption is
the most favorable configuration [35,40,44], the direct CeO bond
cleavage of p-cresol over Pt(111) has a higher reaction barrier (2.45 eV)
than that of direct hydrogenation (1.11 eV) [35]. Interestingly, addition
of an oxophilic metal on a noble metal (i.e., PtZn) was also reported to
change adsorption of aromatic ring by tilting it away from Pt surface,
leading to a favorable CeO bond cleavage [24]. In addition to the ad-
sorption configuration, the reaction pathway may also be influenced by
the interaction between the functional group and catalyst surface. For
example, it has been reported that guaiacol has a stronger adsorption on
Pd(111) and Fe(110) which distorts the CeO bond in guaiacol to a
greater degree [45], and is likely the reason that Fe favors the CeO
bond cleavage while Pd favors the ring hydrogenation [46]. Other
previous studies also indicate that the dominant reaction pathway also
depends on many parameters including catalysts [47,48] and reacting
conditions such as pressure [5,49,50], temperature [25] and solvent/
additive [51,52]. For example, Nie et al. [30] reported a 47.5% se-
lectivity to ring saturation products in vapor-phase HDO of m-cresol
over 5 wt.% Ni/SiO2 at 0.1 MPa and 300 °C. With the same 5 wt.% Ni/
SiO2 catalyst, another study conducted by Mortensen et al. [53] re-
ported 100% selectivity to ring-saturation products in the liquid-phase
HDO of phenol at 10 MPa and 275 °C. Similarly, Pd/C catalyst displayed
around 75% selectivity to arene in the HDO of cresol at 300 °C in vapor-
2.1.1. Preparation of support
The Fe2O3 support was prepared with precipitation method as
previously reported [46]. Typically, the (NH4)2CO3 (Sigma-Aldrich,
99.999%, 1.5 M) solution was added dropwise to a solution of Fe(NO3)3
(Sigma-Aldrich, > 99.99%, 3 M) under stirring, forming dark crimson
slurry. The precipitate was then washed with Milli-Q water by filtration
until the pH reached to 8. After being dried at 80 °C overnight and
sieved to < 100 mesh, the sample was calcined at 400 °C for 5 h to
achieve the Fe2O3 support.
2.1.2. Preparation of supported 1 wt.% Pd catalysts
The 1 wt.% Pd/Fe2O3 was prepared using incipient wetness im-
pregnation method. Pd(NH3)4(NO3)2 solution (Sigma-Aldrich, 10 wt.%
solution with 99.99% metal based purity) was diluted by certain
amount of Milli-Q water such that the concentration and volume of the
precursor solution matched with pore volume of the support as well as
the metal loading of the obtained catalysts. The diluted solution was
then added dropwise to Fe2O3 powder, followed by drying at 80 °C
overnight in air and calcination at 350 °C for 2 h in flowing N2 (50 mL/
min).
2.2. Characterization
In situ X-ray powder diffraction (XRD) experiments were done in a
Rigaku SmartLab X-ray equipped with an XRK900 in situ reactor and a
DTex high-speed detector that allows for higher signal-to-noise ratio at
high scan rates. The diffraction spectra were collected between 10° and
80°, at 5°/min at a step of 0.01°. The radiation source was Cu K-α with a
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were sampled at 25 °C and compared with those sampled at the reaction
temperatures. No significant deviation (< 10%) in terms of both con-
version and selectivity was found after this correction.
The products were analyzed on a gas chromatograph (GC, Agilent
7890 A) equipped with a DB-FFAP column (30 m, 0.32 mm, 0.25 μm)
and flame ionization detector (FID), as well as a GC–MS (Shimadzu,
GCMS-QP2020) equipped with
a HP-5 column (30 m, 0.32 mm,
0.25 μm) connected to a FID. The conversion, selectivity and yield were
defined as follows: conversion [%] = (moles of carbon in the reacted
substrate/moles of carbon in the substrate fed) × 100; selectivity [%]
= (moles of carbon in the specific product/moles of carbon in all
product) × 100; yield [%] = (moles of carbon in the specific product/
moles of carbon in the substrate fed) ×100. The carbon balance was
usually in the range of 92%–100%.
3. Results
3.1. Characterization of the catalysts
The characterization of fresh and reduced/passivated 1 wt% Pd/
Fe2O3 samples show general consistency with our previous study [46].
For the fresh sample, Pd or PdOx is highly dispersed on the Fe2O3
support that has a uniform size distribution at ∼20 nm (Supplementary
material Fig. S1). The in situ XRD pattern of the fresh sample shows a
hematite (JCPDS: 33-0664) crystalline structure, while metallic Fe
(JCPDS: 06-0696) and magnetite (JCPDS: 19-0629) dominate on the
reduced catalyst [46]. No diffractions characteristic of Pd were ob-
served, suggesting the highly dispersed nature of Pd. In situ Raman
spectra were also studied and compared on the 1 wt% Pd/Fe2O3 cata-
lysts before and after the reduction at 350 °C, as shown in Fig. 1.
Spectrum of fresh Pd/Fe2O3 (Fig. 1b) showed the typical peaks of he-
matite [55,56], consistent with the XRD analysis. After reduction, only
a small hump at 655 cm−1, characteristic of FeO was detected, due to
dominant formation of Raman silent metallic Fe species (Fig. 1b). These
characterization results suggest that a FeOx core (incomplete reduction)
and metallic Fe shell structure was formed on the reduced catalysts. In
consistent with this deduction, our previous XPS study showed only
surface metallic Fe [46]. It is thus concluded that, the catalysts shown
in this work is representative of our previous observations that the
surface structure of the catalyst is Pd modified metallic Fe [46].
Fig. 1. In situ-XRD patterns for fresh and reduced 1 wt% Pd/Fe2O3 (a), and in
situ Raman spectra for fresh and reduced 1 wt% Pd/Fe2O3 (b). Reduction
condition: 350 °C with 50% H2 (40 mL/min, balanced by N2) for 2 h.
wavelength of 0.154056 nm. Approximately 0.1 g of catalyst was
loaded into the sample holder. A scan was then collected at RT under
flowing N2 (40 ml/min) which was set as fresh sample. After that, the
sample temperature was increased to 350 °C at a ramping rate of 10 °C/
min in flowing 50%H2/He (40 ml/min), and then held at the tem-
perature for another 2 h. Another scan was then taken before the
sample was cooled down to ambient temperature.
Raman spectra were collected on a Horiba LabRAM HR Raman/
FTIR microscope equipped with a 532 nm (Ventus LP 532) laser source
and Synapse Charge Coupled Device detector (CCD). After calibration
using the silica reference, the sample was loaded in an Linkam
CCR1000 in situ reactor, and a spectrum of the sample was then taken
at room temperature. The sample was then reduced by heating to
350 °C (ramp rate of 10 °C/min) and holding the temperature for 2 h
under flowing H2 (10 ml/min), after which it was cooled to room
temperature before collecting the spectra of the reduced samples.
3.2. Comparison of HDO of m-cresol on Pd/Fe: vapor phase vs liquid phase
2.3. Activity test
We have shown in our previous studies that the Pd/Fe catalyst ex-
hibited > 90% selectivity to arene in the vapor-phase m-cresol HDO,
and Pd played essential role in enhancing the reducibility of Fe and
maintaining the active metallic iron phase [46]. Since the liquid-phase
reaction has the advantage in minimizing the energy consumption, for
comparison, the performance of the Pd/Fe catalyst was also in-
vestigated and compared in liquid-phase reaction conditions. As such,
the N2 diluent employed in vapor phase was replaced with hexadecane
in liquid-phase reactions. Fig. 2 shows a comparison of the catalytic
performances of Pd/Fe at comparable cresol conversion in the two
phases. The high arene selectivity in vapor phase dramatically de-
creased to ∼2% in the liquid phase. This observation is not surprising
since similar observations were also reported on other catalysts re-
ported by different research groups [17,30,31,46,53,57](Supplemen-
tary material Table S1). It is should be noted, however, no evidence has
been shown to unambiguously interpret why the reaction pathway
shifted toward aromatic ring saturation in liquid phase, although it has
been reported that hydrogen pressure and temperature strongly affect
the proportion of arene in the products [5,25,49]. This motivated our
further studies on the reaction pathways aiming to understand the
factors that control the product distribution.
The HDO reaction of the modeling substrates were carried out in a
stainless steel Parr reactor (300 mL) equipped with a glass liner.
Typically, the catalyst was first reduced ex-situ with 40 ml/min 50%
H2/N2 at 350 °C for 2 h followed by a passivation with 40 ml/min 0.2%
O2/N2 at room temperature for 1 h. The passivated catalyst was then
loaded into the reactor. After 3 times purging with 4 MPa H2, the
temperature was ramped to 300 °C (15 °C/min) where it was held for
another 30 min in H2 (3 MPa) for in situ reduction. After the reduction,
the temperature was cooled down to ambient temperature where the
pressure was decreased to ambient, and 6.35 mmol modeling com-
pound (Sigma-Aldrich, ≥ 99%) in 50 mL hexadecane (Sigma-Aldrich,
99%) were then injected with syringe into the reactor. After one time
purging H2 and pressurized with H2 to 4 MPa, the mixed catalyst and
substrate solution were heated up to the target temperature at a stirring
rate of 800 rpm for the reaction. The products were sampled periodi-
cally for analysis at the reaction temperature. Given the variable
composition with temperature in the liquid phase, a linear correction
assuming a constant coefficient in the Henry’s law was made in the
concentration range studied. After the reaction, some of the analysis
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Fig. 2. Comparison of catalytic performances of Pd/Fe in vapor-phase and li-
quid-phase HDO of m-cresol. Reaction condition: 300 °C, 0.04 MPa H2 and
0.06 MPa N2 for vapor-phase reaction (data adapted from the previous study
[46]); 300 °C, hexadecane as solvent, 4 MPa H2 for liquid-phase reaction.
3.3. Reaction pathway in liquid-phase hydrodeoxygenation of phenol on
Pd/Fe
To better understand the enhanced hydrogenation of aromatic ring
of phenolics in liquid-phase reactions, the simplest phenolic, phenol,
was further studied in the HDO reaction. Fig. 3a shows the evolution of
phenol conversion and product yields as a function of time at 300 °C
and 4 MPa (at 25 °C) H2 pressure, and the product selectivity versus
phenol conversion is replotted in Fig. 3b. Similar to the m-cresol re-
actions, ring saturation is dominant in liquid phase, leading to the
formation of cyclohexane as a major product along with small amounts
of cyclohexanol, cyclohexene, and cyclohexanone. With time-on-stream
(TOS), cyclohexanol first increases and then decreases with its highest
yield being observed at around 30 min. The yield to cyclohexene ex-
hibited a similar trend as that of cyclohexanol albeit at a lower yield.
This observation implies that the cyclohexanol and cyclohexene are the
intermediates to cyclohexane during hydrodeoxygenation of phenol.
Indeed, the selectivity to cyclohexane increases at the expenses of cy-
clohexanol and cyclohexene. At extrapolated zero conversion analyzed
using SPSS 22.0 software [58], cyclohexanol selectivity surpasses the
cyclohexane and reaches up to above 50%, an indication of primary
product in HDO of phenol under liquid-phase reaction conditions. The
total selectivity to ring saturation products reaches up to 98.4%, leaving
only 1.6% selectivity to benzene via direct CeO bond cleavage. Two
reaction pathways could lead to cyclohexanol formation: one is direct
hydrogenation of aromatic ring of phenol, and another is keto-enol
tautomerization followed by hydrogenation [35], as shown in Scheme
1. Given the much higher activation barrier for direct aromatic ring
saturation than that for tautomerization calculated on phenolics
[35,40], it is believed that the primary formation of cyclohexanol
should be from the latter (i.e., tautomerization). Note that, no other
intermediate (i.e. 2, 4-cyclohexadienone) but only trace amount of
cyclohexanone was detected, and its selectivity barely shows any de-
pendence on the phenol conversion. This could be due to a high hy-
drogenation reactivity of the intermediate at a relatively high tem-
perature (i.e., 300 °C). Indeed, when the reaction temperature
decreased from 300 °C to 200 °C (Fig. 4), appreciable amount of cy-
clohexanone was produced, and its selectivity increased rapidly as
phenol conversion decreased close to zero. It should be noted that the
secondary cyclohexanol reactions are also very limited at low reaction
temperature (i.e., 200 °C), evidenced by the high selectivity above 90%
regardless of phenol conversion. To further confirm the relatively
higher reactivity of cyclohexanone than that of cyclohexanol, cyclo-
hexanone and cyclohexanol were used as substrates and tested on the
Fig. 3. (a) Phenol conversion and product yields over Pd/Fe catalyst as a
function of time-on-stream. (b) Product distribution as a function of conversion.
Reaction condition: 300 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.15 g cat-
alyst, 4 MPa H2.
Fig. 4. Production distribution as a function of conversion over Pd/Fe catalyst.
Reaction condition: 200 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.3 g cata-
lyst, 4 MPa H2.
Pd/Fe catalyst under the same reaction conditions (Table 1). It is clear
that the conversion rate for cyclohexanone is much faster than that of
cyclohexanol. It is thus hypothesized that, over the Pd/Fe bimetallic
catalysts, majority of the aromatic ring saturation should be from
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Table 1
Catalytic performances of Pd/Fe catalyst in converting possible intermediates. Reaction condition: 200 °C, 6.35 mmol substrates, 50 mL hexadecane, 0.3 g catalyst,
4 MPa H2.
Substrate
Time
conversion
Selectivity
C6H12
C6H10
Benzene
C6H12
O
Phenol
C6H10O
Benzene
Cyclohexanol
Cyclohexanone
4h
4h
10min
10.0
9.3
67.0
99.4
64.2
0.2
0.6
31.5
trace
–
0.0
0.0
0.0
–
99.8
0.0
0.0
0.0
0.0
0.0
–
tautomerization reaction pathway.
Diphenyl ether (DPE) is a lignin-derived compound with 4-O-5
linkage, which is the strongest ether bond in lignin [59]. As one of the
lignin-derived model compounds, DPE has also been widely studied in
the HDO reactions [60]. Note that the structure of DPE with two
symmetric Caromatic-O bonds is unique, making the keto-enol tauto-
merization impossible within this molecule [61]. Based on the proposed
reaction pathways [62], it is anticipated that, unlike phenol, DPE
should only proceed with two primary reactions on the Pd/Fe catalysts
in organic phase: direct ring hydrogenation and direct CaromaticeO bond
cleavage [63]. A comparison of HDO of phenol and DPE could help
differentiate the contribution of tautomerization to the ring-saturation
products. It should be mentioned that although anisole could be con-
sidered as an alternative model compounds for this purpose, the pre-
sence and easy hydrogenolysis of Caliphatic-O [11,54,64] could lead to
the facile formation of phenol and thus complicate this study. To con-
firm our hypothesis that tautomerization rather than direct aromatic
ring hydrogenation is dominating toward ring saturation products on
Pd/Fe catalyst, we further studied the HDO of DPE (Supplementary
material Fig. S2), which was performed under the same reaction con-
ditions as that of phenol. In this particular case, if direct ring hydro-
genation occurs, cyclohexyl phenyl ether (CPE) and dicyclohexyl ether
(DCE) primary products are expected. Whereas, direct CeO bond
cleavage (i.e., hydrogenolysis) will result in benzene and phenol pri-
mary products. Fig. 5 depicts the product distribution as a function of
conversions. Benzene and cyclohexane/cyclohexanol (secondary pro-
ducts from phenol) from direct CeO bond cleavage are always domi-
nant in the products. At extrapolated zero conversion where secondary
reaction is minimized, ∼94% selectivity to direct CeO bond cleavage
was observed. In contrast, selectivity to CPE is only ∼6% and no DCE is
observed, suggesting the low aromatic ring hydrogenation activity on
Pd/Fe. This result confirms that aromatic ring saturation products in
the phenol conversion over Pd/Fe catalyst were mainly from tauto-
merization/hydrogenation rather than direct aromatic ring hydro-
genation. It is worth mentioning that, as one of the primary CeO
cleavage product, phenol was not detected in the diphenyl ether con-
version, indicating that the CeO bond cleavage during the DPE con-
version is the kinetic relevant step, matching well with the high acti-
vation energy of the CaromaticeO bond [39]. Moreover, our results
(Fig. 3) showed that cyclohexane is always dominant in the final pro-
ducts for the HDO of phenol, especially at high phenol conversion. It is
thus expected that cyclohexane is the other major products during the
HDO of DPE (Fig. 5 and Fig. S2) for the relatively facile secondary
phenol conversion. In addition, as DPE conversion increases (longer
reaction time), cyclohexane increases at the expense of benzene. It
suggests secondary benzene hydrogenation occurs on the Pd/Fe cata-
lysts, which is also confirmed at low reaction temperature (Table 1).
However, it does not affect our conclusion that during the HDO of
phenol, the facile keto-enol tautomerization opened a pathway to
readily form enone from phenol and played the major role in the aro-
matic ring saturation reaction under liquid-phase reaction conditions.
Fig. 5. (a) Product distribution as a function of conversion over Pd/Fe. (b)
Distribution of classified products as a function of conversion. Reaction con-
dition: 300 °C, 6.35 mmol diphenyl ether (1 mL), 50 mL hexadecane, 0.15 g
catalyst, 4 MPa H2.
liquid-phase HDO reaction, particularly for inhibiting the direct aro-
matic ring hydrogenation reactions, the reaction mechanism is sig-
nificantly different from that in vapor-phase reaction. In the vapor-
phase reaction at atmospheric pressure (H2 partial pressure 0.04 MPa),
majority of the final products are arene whereas ring-saturation pro-
ducts (mainly cyclohexane for phenol) dominate in the liquid-phase
reaction at ∼6 MPa H2 (at 300 °C). More importantly, our experimental
results explicitly revealed ring-saturation products in the HDO of
phenol on PdFe catalysts were mainly from tautomerization pathway.
The enhancement of tautomerization and thus ring saturation in the
liquid-phase HDO of phenol likely results from two factors: hydrogen
partial pressure and solvent effect. In the former case, the H supply and
thus further hydrogenation of tautomerized enone could be limited by
the low hydrogen pressure in the vapor-phase conditions. In liquid
phase, although H2 solubility could be low in hexadecane, the high
4. Discussions
Although the synergistic effect on Pd/Fe still plays key roles in the
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Fig. 6. The influence of H2 pressure to the reaction rate of benzene production
and ring saturation products. Reaction condition: 300 °C, 6.35 mmol phenol,
50 mL hexadecane, 0.15 g monometallic Fe as catalyst. Note that this experi-
ment was done on monometallic Fe to minimize the direct ring hydrogenation
observed on Pd/Fe albeit low, which can complicate this verification.
Fig. 7. Product distribution as a function of conversion over Pd/Fe. Reaction
condition: 300 °C, 6.35 mmol cyclohexanol, 50 mL hexadecane, 0.15 g catalyst,
4 MPa H2.
on the less or non-acidic Pd/Fe catalyst, which is different with the acid
catalysts showing dehydration as the main pathway [17].
hydrogen pressure could offset this [66] and thus ascertain an efficient
H supply on the catalyst surface [50]. Therefore, the enone hydro-
genation could be enhanced in liquid-phase reaction conditions. Indeed,
Yohe et al. [5] reported a significant influence of H2 partial pressure on
the product distribution. The aromatic hydrocarbon reaches up to
93.2% at 0.1 MPa H2, whereas this selectivity dramatically decreased to
be less than 3% at 2.3 MPa H2. Note that the Caromatic-O bond cleavage
has been identified as the rate-determining step in the HDO of phenol to
produce benzene [67], which means, if this is the case, the improved H
supply should have negligible effect on the formation rate of benzene.
In other words, given the unaltered rate for benzene formation, the
selectivity to ring saturation products in liquid phase reaction condi-
tions should be improved by an enhanced kinetics for the parallel
tautomerization pathway. To verify this, the HDO of phenol was tested
at different H2 pressures. For a better comparison of kinetics, the con-
version was controlled below 15%. As shown in Fig. 6, the rate to
produce benzene remains almost constant (∼0.13 mmol/g/h) when the
H2 pressure increases from 1 MPa to 4 MPa, whereas the rate to ring-
saturated products increases dramatically from 0.38 mmol/g/h to
1.11 mmol/g/h, leading to decreased benzene selectivity from 26.6% to
9.8%. This result confirms that the enhanced tautomerization pathway
is indeed facilitated by hydrogen supply in liquid-phase conditions. In
addition, a recent DFT study also showed that, although the barrier for
tautomerization in vacuum condition is higher than direct Ce–O bond
cleavage, this barrier significantly decreases in the presence of water
solvent due to the nearly barrierless proton transfer [65]. In the current
case with batch reaction, the generated water molecule may adsorb/re-
adsorb on the catalyst surface, which in turn facilitated the tautomer-
ization. It can be concluded that, under liquid-phase conditions, the
improved formation of ring-saturation products is mainly due to a
combination of enhanced tautomerization by water solvent and hy-
drogenation of tautomerized enone by H supply.
Based on the above results, the reaction pathway for liquid-phase
HDO of phenolics can be proposed over Pd/Fe catalysts. For the case of
phenol, as shown in Scheme 2, two parallel routes exist: direct CeO
bond cleavage and tautomerization followed by ring saturation. Dif-
ferent from the vapor-phase HDO of phenolics where direct hydro-
genolysis of CaromaticeO bond prevails, ring saturation becomes the
major reaction pathway in liquid phase. The fact that ring saturation
pathway is dominant in liquid phase is mainly due to the enhanced
tautomerization reaction pathway toward cylcohexanol, followed by a
direct hydrogenolysis of the cyclohexanol to cyclohexane.
5. Conclusions
By using phenol and DPE as the model compounds, HDO of phe-
nolics has been carefully studied on Pd/Fe catalyst in liquid-phase, in
an attempt to elucidate the distinct product distribution from those in
vapor-phase conditions. Our results suggest that, although the direct
aromatic ring saturation observed on Pd is inhibited by Fe (i.e., on Pd/
Fe bimetallic catalyst), the tautomerization of phenol opened another
pathway toward ring saturation products under liquid-phase reaction
conditions. The favorable tautomerization reaction pathway in liquid
phase against vapor phase is likely due to a combination of enhanced
tautomerization and hydrogenation of tautomerized enone. Due to the
lack of acidity, direct hydrogenolysis of cyclohexanol rather than the
dehydration followed hydrogenation proposed on the catalyst with
acidity, is dominant pathway for the formation of cyclohexane on the
Pd/Fe catalysts.
Conflict of interest
To complete the whole pathway, conversion of cyclohexanol was
also studied under the same reaction condition as that of phenol
(Fig. 7). As expected, both cyclohexane and cyclohexene are observed.
As cyclohexanol conversion decreases, the cyclohexene selectivity in-
creases slowly at the expense of cyclohexane. Regardless, selectivity to
cyclohexane are always above > 90% selectivity. Even at extrapolated
zero conversion, the cyclohexane selectivity is still much higher than
cyclohexene. This result indicates that two parallel reactions occur in
the cyclohexanol reaction on the Pd/Fe catalysts, namely the direct C–O
bond cleavage (hydrogenolysis) to produce cyclohexane and the de-
hydration of cyclohexanol to form cyclohexene followed by hydro-
genation. The former should be the major pathway (> 90% selectivity)
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the U.S. Department of Energy (DOE),
Office of Science, Basic Energy Sciences (BES) and Division of Chemical
Sciences (grant DE-FG02-05ER15712). The authors thank the
Franceschi Microscopy and Imaging Center (FMIC) in Washington State
University for the access to TEM. We would also like to acknowledge
Prof. Hongfei Lin and Dr. Teng Li in Washington State University for
their GC–MS analysis of the products.
6

J. Zhang et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Scheme 2. Proposed reaction pathway for the conversion of phenol over Pd/Fe catalyst.
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