Vapor-phase hydrodeoxygenation of lignin-derived bio-oil over Al-MCM-41 supported Pd-Co and Pd-Fe catalysts

Article abstract of DOI:10.1016/j.mcat.2021.111435

Fast pyrolysis of lignocellulosic biomass is an attractive process to produce bio-oil as an alternative liquid fuel source. Upgrading of bio-oil via hydrodeoxygenation (HDO) is an important route to accomplish this renewable energy production process. Al-MCM-41 supported Pd, Co and Fe catalysts were evaluated for HDO of guaiacol and lignin-derived bio-oil at atmospheric pressure in a fixed-bed reactor. Bimetallic Pd-Co and Pd-Fe catalysts showed higher HDO yield and stability than the monometallic Co and Fe catalysts. The addition of Pd significantly enhanced the stability of Co and Fe catalysts since it helped reduce the coke formation. The lignin-derived bio-oil mainly contained phenolic compounds which had one to three oxygen atoms. The catalytic upgrading could not only eliminate significantly the oxygen of these phenolic compounds but also reduce the amount of tar and heavy components. Pd-Fe catalyst was recognized as a suitable catalyst for upgrading of lignin-derived bio-oil since it produced more deoxygenated products and less gas-phase yield than Pd-Co catalyst.

Full text of DOI:10.1016/j.mcat.2021.111435

Molecular Catalysis xxx (xxxx) xxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Vapor-phase hydrodeoxygenation of lignin-derived bio-oil over Al-MCM-41
supported Pd-Co and Pd-Fe catalysts
,
,
,
,d
Nga T.T. Trana c,*, Yoshimitsu Uemuraa **, Anita Ramlia b, Thanh H. Trinha
a Centre for Biofuel and Biochemical Research, Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak, Malaysia
b Department of Fundamental & Applied Science, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak, Malaysia
c Key Laboratory of Chemical Engineering and Petroleum Processing, Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet
Nam
d Faculty of Food Science and Technology, HCMC University of Food Industry, 140 Le Trong Tan, Ho Chi Minh City, Viet Nam
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fast pyrolysis of lignocellulosic biomass is an attractive process to produce bio-oil as an alternative liquid fuel
source. Upgrading of bio-oil via hydrodeoxygenation (HDO) is an important route to accomplish this renewable
energy production process. Al-MCM-41 supported Pd, Co and Fe catalysts were evaluated for HDO of guaiacol
and lignin-derived bio-oil at atmospheric pressure in a fixed-bed reactor. Bimetallic Pd-Co and Pd-Fe catalysts
showed higher HDO yield and stability than the monometallic Co and Fe catalysts. The addition of Pd signifi-
cantly enhanced the stability of Co and Fe catalysts since it helped reduce the coke formation. The lignin-derived
bio-oil mainly contained phenolic compounds which had one to three oxygen atoms. The catalytic upgrading
could not only eliminate significantly the oxygen of these phenolic compounds but also reduce the amount of tar
and heavy components. Pd-Fe catalyst was recognized as a suitable catalyst for upgrading of lignin-derived bio-
oil since it produced more deoxygenated products and less gas-phase yield than Pd-Co catalyst.
Hydrodeoxygenation
Bio-oil upgrading
Bimetallic catalyst
Lignin-derived phenolic
Guaiacol
1. Introduction
Ni) [9,17–19].
Lignocellulosic biomass has been recognized as the most abundant
Bio-oil from biomass pyrolysis is considered a promising second-
generation biofuel [1]. However, it is very difficult to directly utilize
the pyrolysis oil because of its high water and oxygen contents [2,3]. The
presence of oxygenated compounds (e.g. acids, esters, alcohols, ketones,
furans, phenols) makes the bio-oil low heating value, low thermal sta-
bility, high viscosity and corrosiveness [4–6]. Therefore, the upgrading
step was required to produce the useful chemical and fuel from bio-oil.
There are some upgrading processes such as catalytic cracking, hydro-
deoxygenation (HDO), supercritical fluids, esterification, emulsification,
molecular distillation, and catalytic pyrolysis [7,8]. Catalytic HDO is a
prominent process which not only eliminate the oxygen, but also pre-
serve the carbon number of the bio-oil [9,10]. To date, there have been
numerous studies on HDO of model compound and real bio-oil over
different types of catalysts, such as transition metal sulfide (CoMoS and
NiMoS) [7,11], transition metal phosphide (Ni2P and Co2P) [4,12,13],
noble metals (Ru, Pt and Pd) [3,14–16], and base metals (Cu, Fe, Co and
renewable resources to produce biochemical, biofuel and biomaterial
[20,21]. Furthermore, the potential amount of lignin is very interesting
since the development of ethanol fuel, cellulose rich fiber production
and paper industries which use cellulose and leave lignin as a residue
[22–24]. Lignin can be utilized as the renewable aromatic resources for
petrochemical production [21–23]. Guaiacol (2-methoxyphenol) is the
major phenolic compound in the lignin-derived bio-oil, and it is a
challenging component for HDO upgrading process [9,22]. Guaiacol is
usually chosen as a model compound for HDO study because it contains
both major functional groups of lignin-derived phenolic such as hy-
droxyl (ꢀ OH) and methoxy (ꢀ OCH3) groups [15]. Numerous re-
searchers studied the HDO of guaiacol since it presents as a strong
component which is more difficultly deoxygenated than other phenolic
compounds (i.e. anisole and cresol) [14–16].
In catalytic process, the continuous-flow operating is more promising
and feasible in scaling up than the batch autoclave study [25]. Table 1
* Corresponding author at: Centre for Biofuel and Biochemical Research, Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610, Seri
Iskandar, Perak, Malaysia.
** Corresponding author.
E-mail addresses: Tonga.tnt@gmail.com (N.T.T. Tran), YUemura.my@gmail.com (Y. Uemura).
https://doi.org/10.1016/j.mcat.2021.111435
Received 17 September 2020; Received in revised form 5 December 2020; Accepted 24 December 2020
2468-8231/© 2021 Elsevier B.V. All rights reserved.
Please cite this article as: Nga T.T. Tran, Molecular Catalysis, https://doi.org/10.1016/j.mcat.2021.111435

N.T.T. Tran et al.
Molecular Catalysis xxx (xxxx) xxx
summarizes the HDO of guaiacol as the model compound in the
continuous fixed-bed reactor at ambient or high pressure. Previous
studies on HDO of guaiacol in the continuous flow system reveal that
high H2/feed ratio (i.e. 100–600), W/F ratio and reaction temperature
(i.e. 450 ◦C) were required to completely remove the oxygen [9,13].
When applied the low H2/guaiacol ratio (i.e. 10), the HDO of guaiacol
could not produce any oxygen-free aromatic products [26]. In addition,
high-pressure reactions (30–69 bar) produced more ring saturation
products than ambient pressure [27]. In high-pressure fixed-bed reactor,
guaiacol were diluted with organic solvents (i.e. toluene, n-hexadecane
or n-tridecane) as the feedstock [11,14,28]. This dilution step attributed
to the cost of the process and limited the reaction temperature due to the
low thermal stability of solvents.
2. Experimental
2.1. Catalyst preparation and characterization
Aluminosilicate Al-MCM-41 support (3–4 mol% Al2O3) supplied by
ACS Material was activated in air at 500 ◦C for 14 h. The bimetallic
catalysts containing 10 wt% Co or Fe metal and 2 wt% Pd metal were
prepared by an incipient wetness co-impregnation method. The support
was impregnated with aqueous solutions containing metal precursors
(palladium(II) nitrate, cobalt(II) nitrate (99.999 %) and iron(III) nitrate
(99.95 %) purchased from Sigma Aldrich) for 16 h at 33 ◦C, and then
dried at 105 ◦C for 8 h, followed by calcination at 500 ◦C for 4 h in air.
The calcined catalysts were sieved to give the particles of 35–60 mesh in
the size range.
Upgrading of actual bio-oil is a great challenge since the thermal
repolymerization reactions usually occur before HDO, forming the solid
lignin oligomers [12,29,30]. Hence, most of research works in upgrad-
ing of real bio-oil have been done with the batch autoclave reactor
instead of the continuous flow reactor [31–34]. There is very few studies
in upgrading of bio-oil using the continuous flow reactor [3,4,7,35]. In
Wang’s study, 10 wt% bio-oil in 1-methylnaphthalene was upgraded
over a sulfided NiMo/γ-Al2O3 catalyst in the fixed-bed reactor operating
at 96 bar and 330 ◦C [7]. They found that the carboxyl group was mostly
removed by decarboxylation or decarbonylation, while the methoxyl
and hydroxyl groups were eliminated by HDO. However, this study did
not give the detail of products yield or HDO yield to clarify the effect of
polymerization and gasification on bio-oil upgrading process. Sanna
et al. [3] studied the hydrogenation and HDO of the aqueous fraction of
bio-oil in a two stages continuous reactor with Ru/C and Pt/C catalysts.
The reaction temperatures were set at 125 and 275 ◦C for first and
second reactor, respectively, while the operated pressure was kept at
100 bar for both reactors. This 2-stage process could convert 45 %
carbon of the aqueous fraction of bio-oil to gasoline blend-stocks and C2
to C6 diols. Recently, Koike et al. [4] conducted fast pyrolysis and cat-
alytic upgrading successively at ambient atmosphere. The system
included a fluidized bed pyrolyzer and a fluidized bed catalytic reactor.
The Ni2P/SiO2 catalyst could reduce moderately the oxygen content of
bio-oil (ca. 40 %). They proposed some reactions occurred during cat-
alytic upgrading process such as hydrodeoxygenation, hydrogenation,
decarbonylation, and hydrolysis. The refined bio-oil mainly contained
phenolic compounds with no carbonyl or methoxy groups.
The TriStar II 3020 was used to measure the nitrogen adsorption/
desorption isotherms at ꢀ 196 ◦C. The catalyst was pretreated at 300 ◦C
for 4 h in nitrogen gas. Transmission electron microscopy (TEM) image
was obtained on a Carl Zeiss LIBRA® 200FE (operating voltage 200 kV).
The catalyst powder was dispersed ultrasonically in 2propanol for 15
min and then deposited on a carbon coated copper grid for TEM mea-
surement. The catalyst was also characterized with the scanning trans-
mission electron microscopic (STEM) images by a JEM-ARM200F
(JEOL) instrument at an accelerating voltage of 200 kV. The STEM
provides 3 types of images i.e. SE (secondary electron image), ZC (Z
contrast image) and TE (transmission electron image). The SE image
provides information for the exterior surface of the carrier, while the ZC
and TE images provide information for both interior and exterior surface
of the carrier.
Temperature programmed reduction (TPR) in hydrogen was carried
out using a Thermo Scientific TPDRO 1100 instrument. Samples were
pretreated in nitrogen at 120 ◦C for 60 min. Then, a TPR program was
performed with a heating rate of 10 ◦C/min from room temperature to
900 ◦C hold for 30 min under 5 % H2/N2 flow. The amount of hydrogen
consumption during catalyst reduction process was monitored by a
thermal conductivity detector (TCD). The acidity of Al-MCM-41 was
determined by temperature programmed desorption (TPD) of ammonia
(using Micromeritics AutoChem II 2920). Initially, the sample (~0.03 g)
was degassed at 200 ◦C at heating rate of 10 ◦C/min for 1 h under
constant helium flow (30 mL/min). Then, sample was cooled to 115 ◦C
and 10 % NH3/He was adsorbed for 30 min. After saturation, helium was
purged for 15 min at 30 mL/min to remove excess ammonia on the
catalyst surface. Finally, the sample was heated from 115 to 900 ◦C
under constant helium flow (30 mL/min). The concentration of des-
orbed ammonia was quantified by a TCD.
In this study, the HDO of lignin-derived phenolic components over
Al-MCM-41 supported Pd-Co and Pd-Fe catalysts were investigated in a
fixed-bed reactor at ambient pressure. The Al-MCM-41 is an acidic and
mesoporous support which can enhance the transalkylation activity and
stability of catalyst in HDO process [36–39]. Initially, the HDO of
guaiacol was conducted to screen the activity and stability of the cata-
lysts. Moreover, the upgrading of actual lignin-derived bio-oil was
investigated with bimetallic Pd-Co and Pd-Fe/Al-MCM-41 catalysts in
the two-stage catalytic pyrolysis reactor.
Thermogravimetric analysis (TGA) under the flow of air was con-
ducted in a TA Instrument model QA50. During the analysis, tempera-
ture was increased from room temperature to 900 ◦C at a heating rate of
10 ◦C/min. Powder X-ray diffraction (XRD) pattern was performed on a
PANalytical Expert3 X-ray diffractometer using Cu K
α radiation. XRD
Table 1
Literature studied on HDO of guaiacol in continuous flow reactor.
Year/Ref.
Reactant
Catalyst
T
P
W/F
H2/Gua
XGua
Product Yield*/Selectivity (%)
◦C
bar
1
h
%
Cyclo-alkane
Aromatic
60
Mono-oxygenated
Di-oxygenated
Gas phase
NA
2011 [12]
2013 [9]
Guaiacol
Ni2P/SiO2
Pd-Fe/C
300
450
300
400
300
400
400
450
200
250
260
300
0.71
0.75
1.5
33
80
0
40
0
Guaiacol
1
100
600
50
98
0*
0*
0*
0
87*
71.9*
75*
0
0*
0*
0*
0*
8
11*
2013 [13]
2013 [17]
2014 [26]
2016 [40]
2016 [18]
2016 [41]
2012 [14]
2014 [28]
2016 [11]
2017 [27]
Guaiacol
Ni2P/SiO2
Fe/SiO2
1
99.5
100
88
2*
5.7*
17*
Guaiacol
1
7.5
8*
Guaiacol
Pt/C
1
0.3
10
70
22
Guaiacol
Co/Al-MCM-41
Fe/Ni/Hbeta
Ni@Pd SD
Pt/MZ-5
1
1.67
4.0
25
99.5
100
83
0
34.4
6.11*
52
25.5
86.5*
35
0.9
0*
0
29.7
1.79*
NA
Guaiacol
1
40
5.59*
0
Guaiacol
1
2.4
25
3.0 wt.% gua
0.4 mol% gua
3.0 wt.% gua
Guaiacol
40
69
30
30
0.5
110
17
100
62.9
92
92
25
11
83.4
0
0
0
NA
Pt/MgO
3.74
0.42
0.17
0
63.6
47.3
12.7
1.6
2.2
0.7
10
CoMoS/CCA
Ni-Fe/CNT
500
50
31.5
3.2
NA
96.8
NA
*
Indicate the product yield.
2

N.T.T. Tran et al.
Molecular Catalysis xxx (xxxx) xxx
pattern was recorded at speed of 2◦/min with step-sizes of 0.02◦ in the
bed at the bottom part for upgrading process. The experiment was kept
for 90 min until no observation of volatile in the cold trap. The products
of lignin conversion process might contain bio-char (top part of reactor),
coke deposit (catalyst bed), tar (bottom flange and reactor wall), heavy
bio-oil (first cold trap), light bio-oil (second cold trap) and bio-gas (non-
condensed gas). The product yields were calculated on a dry base yield
with the water content of lignin is 7.5 wt%.
diffraction angle range of 2–70◦.
2.2. HDO of guaiacol
Catalytic HDO reaction was conducted in a fixed-bed tubular reactor
(internal diameter of 13 mm) made of stainless steel at 400 ◦C and
ambient pressure. The detail of the experimental set-up of HDO of
guaiacol was mentioned elsewhere [42] (see Fig. S1). Before HDO re-
action, all catalysts were reduced at 450 ◦C using a hydrogen flow of 90
mL/min for 2 h. Pure guaiacol was fed at a flow rate of 1.08 mL/h using a
syringe pump and vaporized at 350 ◦C in the top glass wool bed. In HDO
of guaiacol experiments, the carbon balance was between 93 and 98 %
except the HDO over Pd catalyst (88 %). The liquid products were
quantified by gas chromatography (Shimadzu GC-2014), with a SGE
Mwater ꢀ Mbiomass × 0.075
Ywater (%) =
× 100
(4)
Mbiomass × (1 ꢀ 0.075)
Mi
Yi (%) =
× 100
(5)
Mbiomass × (1 ꢀ 0.075)
which Mi is the mass of product i and Mbiomass is the mass of lignin.
BPX–5 capillary column (30 m, ID 0.25 mm, 0.25 μm) and a flame
3. Results and discussion
ionized detector (FID). Sample was diluted in acetone, and n-dodecane
was used as an internal standard (ISTD) for all analysis. The
non-condensed gas was analyzed with GC–TCD system (Shimadzu
GC–8A) which consisted of two columns, i.e. porapak Q (for CO2 and
CH4 analysis) and molecular sieve 5A (for H2, N2, O2, CH4 and CO
analysis). Carbon-based guaiacol conversion (XGua), product yield (Yi)
and HDO yield were calculated in MolCarbon% by the following
equations.
3.1. Characterization of Al-MCM-41 supported Pd-Co and Pd-Fe
catalysts
Table 2 demonstrates the textural properties of Al-MCM-41 sup-
ported monometallic and bimetallic catalysts. The metal-modified Al-
MCM-41 catalysts showed a decrease in surface area and pore volume as
expected. It may be caused by the partial blockage of pores after metal
deposition [38]. The higher metal was loaded, the more difference be-
tween support and metal-modified catalysts was observed. As shown in
Table 2 and Fig. S2, all metal-modified Al-MCM-41 catalysts still
maintained same mesoporosity structure. Pore size distribution
remained the sharp peak at around 3 nm (Fig. S2). It suggests that the
metal deposition only caused the partial blockage of pores instead of
destroying the framework structure of Al-MCM-41 support. The total
acidity (Brønsted and Lewis acidity) of Al-MCM-41 measured by
NH3-TPD was 1.06 mmol/g. The acidity of Al-MCM-41 is caused by the
Al atoms incorporated in the MCM-41 framework [43,44].
Mol(gua)in ꢀ Mol(gua)out
XGua (%) =
× 100
(1)
(2)
(3)
Mol(gua)in
Moli × αi
Yi (%) =
× 100
Mol(gua)in × 7
∑
Yi × (2 ꢀ βi)
HDO yield (%) =
2
where αi and β are the carbon and oxygen numbers in product i.
i
The TEM images of the mono- and bi-metallic catalysts are displayed
in Fig. 1, including the size distribution diagrams of metal oxide parti-
cles. Although most of the particles were in a size range between 3 and 9
nm, larger particles up to 23 nm were also detected. The monometallic
Co and Fe catalysts had broader size distributions of metal oxide parti-
cles from 2 to 23 nm while the bimetallic Pd-Co and Pd-Fe catalysts had
narrower size distributions from 2 to 16 nm. This observation confirmed
the advantage of bimetallic in particle size distribution of metal oxide
comparing with monometallic [10,41,42]. To further study of the
structure of catalyst, the STEM image was done for the
Pd-Fe/Al-MCM-41 sample (Fig. 2). The SE, ZC and TE images show that
most of the small particles (~ 3 nm) existed inside the porous of
Al-MCM-41 support.
2.3. Catalytic upgrading of lignin-derived bio-oil
Alkali lignin powder (purchased from Sigma Aldrich) was used as
biomass reactant for pyrolysis and catalytic upgrading processes. The
lignin feedstock has a water content of 7.5 wt%, and the elemental
composition (wt%, dry basis): 66.5 % C, 7.1 % H, 0.8 % N, and 2.0 % S.
The tubular fixed-bed reactor was used for both pyrolysis and catalytic
upgrading processes (see Fig. S1). Lignin (3.0 g) was packed in the top
part of reactor and sandwiched with glass wool. Meanwhile, the bottom
part of reactor was packed with glass wool only in the pyrolysis process
(without catalyst). Catalyst was added to the bottom part in the
upgrading process with amount of 1.5 g (catalytic upgrading). The
reactor outlet line was heated at ~ 195 ◦C to avoid the condensation of
liquid products before they reached the 2-stage cold trap. The first cold
trap was empty and cooled by ice water (~ 5 ◦C), while the second cold
trap contained acetone and cooled by ethylene glycol (~ -10 ◦C). The
condensed liquid collected at first and second cold traps were named by
heavy and light oil, respectively. These liquid products were diluted
with acetone and analyzed with GC-FID and Karl Fisher (KF). Mean-
while, the non-condensed gas was sampled every 15 min, and analyzed
with GC-TCD. After reaction, the remained char (char residue) at the top
part of reactor was collected and weighted. In catalytic upgrading pro-
cess, the coke deposit on the catalyst was calculated from the mass loss
(from 200 to 700 ◦C) using TGA data.
The temperature-programmed reduction (TPR-H2) profiles of Al-
Table 2
Textural properties of fresh Al-MCM-41 supported mono- and bi- metallic
catalysts.
Samples
BET surface
Pore
Pore
Coke
area
sizea
volumea
formationb
(m2/g)
742
(nm)
3.21
3.33
3.74
3.53
3.72
(cm3/g)
wt%
Al-MCM-41
0.56
10Co/Al-MCM-41
10Fe/Al-MCM-41
2Pd/Al-MCM-41
2Pd10Co/Al-
MCM-41
583
0.43
16.8 ± 0.9
21.7 ± 0.2
15.6 ± 0.6
12.8 ± 0.2
573
0.44
630
0.49
The pyrolysis of lignin was conducted in hydrogen gas at 500 ◦C with
the heating rate of 80 ◦C/min. The two-stage catalytic pyrolysis process
(catalytic upgrading) was carried on in hydrogen gas at the temperatures
of lignin and catalyst bed of 500 and 400 ◦C, respectively. Hydrogen was
used in both pyrolysis and catalytic upgrading experiments in order to
clearly clarify the effect of catalyst in the upgrading process. The volatile
product of pyrolysis process at the top part was flowed to the catalyst
490
0.39
2Pd10Fe/Al-
MCM-41
507
3.69
0.42
17.5 ± 0.3
a
Calculated by BJH adsorption theory.
b
Calculated by the mass loss (from 200 to 700 ◦C) of used catalyst in TGA
analysis.
3

N.T.T. Tran et al.
Molecular Catalysis xxx (xxxx) xxx
Fig. 1. TEM image of fresh catalysts. A) Co/Al-MCM-41, B) Fe/Al-MCM-41, C) Pd-Co/Al-MCM-41, D) Pd-Fe/Al-MCM-41.
Fig. 2. STEM images of fresh Pd-Fe/Al-MCM-41.
MCM-41 supported metal catalysts are displayed in Fig. 3. The reduction
profile of Pd catalyst showed a small peak at 170 ◦C. It has been known
that PdO can be reduced readily below ambient temperature [9,45].
This difference indicates the formation of strong interaction of PdO with
support in our catalyst [46]. The monometallic Co catalyst had three
distinct peaks at around 360, 385 and 690 ◦C, which were associated
with the reduction of Co3O4, CoO, and exchanged Co2+ ions, respec-
tively [45–49]. In the other hand, monometallic Fe had three reduction
peaks at 445, 580 and 715 ◦C for the stepwise reductions of Fe2O3 to
Fe3O4, Fe3O4 to FeO, and FeO to Fe metal [50–53]. In general, Co oxides
might be reduced more readily than Fe oxides. The addition of Pd to Co
and Fe catalysts significantly lowered the reduction temperature. Both
bimetallic catalysts had a sharp peak at around 100 ◦C which is likely the
co-reduction of small Pd-Fe and Pd-Co alloy particles [9,52]. All the
peaks appeared in monometallic Co and Fe catalysts are shifted to lower
reduction temperature in bimetallic catalyst. These TPR results clearly
demonstrated the presence of hydrogen spillover effect where a noble
metal acts like the spilling source while a transition metal oxide is the
receptor [47,46–49]. The H2 consumption for each reduction peak was
calculated and showed in the Table S1, in which Co3O4 and Fe2O3
presented as the major oxides in the corresponding fresh catalysts. The
TEM and TPR results imply that the addition of Pd could enhance the
dispersion and reducibility of Co and Fe oxides.
Fig. 4.A and B illustrate the powder XRD patterns of the fresh and
reduced Al-MCM-41 supported metallic catalysts, respectively. The
metal-modified catalysts have a broad peak at around 23◦ which is
4

N.T.T. Tran et al.
Molecular Catalysis xxx (xxxx) xxx
Pd-Fe catalysts (Fig. 4B) implied that Pd could be formed in Pd-Co and
Pd-Fe alloys [62,65].
3.2. Hydrodeoxygenation of guaiacol
3.2.1. The synergistic effect of bimetallic in catalytic HDO
Fig. 5 compares the conversion of guaiacol and product yields over
the supported mono- and bi-metallic catalysts at the same conditions.
Monometallic Fe catalyst showed higher HDO and lower gasification
activities than monometallic Co due to its higher mono-oxygenated
products and lower gas phase (which mainly contained methane)
yield. Meanwhile, Pd was found as a poor catalyst for HDO of guaiacol
since it produced catechol and other dioxygenated products as the major
products. These heavy and viscous products might remain in the reactor
wall, caused the lowest carbon recovery of Pd catalyst (around 88 %).
Addition of Pd to Co catalyst not only increased the conversion and
deoxygenated product yields (aromatics and monooxygenated) but also
decreased the gas phase yield. However, this addition to Fe catalyst only
showed the increase of mono-oxygenated products, following by the
decrease of oxygen-free aromatic yield. In previous paper, we found that
HDO of guaiacol occurred via CArꢀ OCH3 cleavage then CArꢀ OH cleav-
age to obtain oxygen-free aromatic as the final product [40]. The com-
bination of Pd and Fe might not favor the CArꢀ OH cleavage reaction,
resulting in a slight decrease of oxygen-free aromatic yield of Pd-Fe
catalyst.
Fig. 3. Hydrogen temperature-programmed reduction (TPR-H2) of
fresh catalysts.
assigned to an amorphous silica in Al-MCM-41 support [54]. The fresh
Co-modified catalyst had five diffraction peaks at 31.3◦, 36.8◦, 44.9◦,
59.3◦, and 65.2◦ corresponding to different Co3O4 crystallites [53,55,
56]. Meanwhile, there are 7 peaks of Fe2O3 in the XRD pattern of fresh
Fe catalyst [52,53,57]. The fresh bimetallic catalysts had similar XRD
patterns comparing to the monometallic ones with the addition of PdO
peak at 33.8◦ [44,46,47]. The addition of Pd did not significantly change
the crystallographic characteristics of Fe and Co catalysts [47,57]. After
reduction, the broad peak of amorphous silica was remained as dis-
played in Fig. 4B. The reduced Co catalyst had a low intensity peak of
CoO at 42.3◦ besides the main peak of metallic Co at 44.6◦ [46,58–61].
The present of CoO peak in reduced sample might be from either
re-oxidation when the catalyst was exposed to air during the storage
period [58,60,61] or incompletely reduction at chosen condition. The
reduced Fe catalyst remained the peaks of Fe2O3 spinel, implying that
Fe2O3 was more difficult to reduce than others. This phenomenon is in
agreement with the TPR-H2 results in Fig. 3. In the bimetallic Pd-Co and
Pd-Fe catalysts, Co and Fe oxides were completely reduced, resulting in
the formation of Co◦ (at 44.6◦) [46,61] and Fe◦ (at 44.6◦ and 65.0◦) [17,
57,62]. The XRD results confirmed that Pd could promote the reduction
of Co and Fe. According to literatures, XRD pattern of reduced mono Pd
catalyst had two distinct peaks at 40.7◦ and 46.0◦ [41,62–64]. The
shifting of these diffraction peaks to the higher angle in two Pd-Co and
Fig. 6 demonstrates the variations of guaiacol conversion and HDO
yield with time on stream (TOS). The monometallic Co and Fe catalysts
showed faster deactivation than others. Although Pd catalyst had the
lowest HDO yield, this catalyst appeared very high stability during the
reaction. Therefore, the addition of Pd enhanced significantly the sta-
bility of both Co and Fe catalysts. Among all five catalysts, Pd-Fe showed
the highest stability and HDO yield. The highest HDO yield of Pd-Fe was
attributed by low gas-phase yield and high monooxygenated products
yield. In addition, Fig. 7 compares the conversion and product distri-
bution of bimetallic Pd-Co and Pd-Fe catalysts at higher W/F ratio of
1.67 h. As the W/F ratio were increased, the guaiacol conversion and
deoxygenated products yield were also increased. Although Pd-Co
catalyst showed higher oxygen free product yield, the methane yields
was much higher than Pd-Fe catalyst, resulting in lower HDO yield. The
Pd-Fe catalyst had higher ratio of phenol to methylated-phenol (C7ꢀ 10
monooxygenated) than Pd-Co catalyst due to the methyl group was
produced less. In general, the hydrogenolysis of CAr–OR and multiple
–
C
C bindings occurred simultaneously in the catalytic HDO over Pd-Co
or Co catalysts, resulting in low HDO yield. Meanwhile, Pd-Fe and Fe
Fig. 4. XRD patterns of Al-MCM-41 supported Pd-Co and Pd-Fe catalysts. A) Fresh catalyst and B) Reduced catalysts.
5

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Molecular Catalysis xxx (xxxx) xxx
Fig. 5. HDO of guaiacol over mono and bimetallic catalysts. Reaction conditions: T = 400 ◦C, P =1 bar, W/F =0.83 h, H2/Gua = 25, TOS = 30 min.
Fig. 6. Guaiacol conversion and HDO yield of HDO reactions over mono- and bi-metallic catalysts. Reaction conditions: T = 400 ◦C, P = 1 bar, W/F = 0.83 h, H2/Gua
= 25.
Fig. 7. Guaiacol conversion and products yield of HDO reactions over bimetallic catalysts. A) Pd-Co, B) Pd-Fe. Reaction conditions: T = 400 ◦C, P = 1 bar, W/F =
1.67 h, H2/Gua = 25.
6

N.T.T. Tran et al.
Molecular Catalysis xxx (xxxx) xxx
–
favored the CArꢀ OCH3 hydrogenolysis and inhibited the successive C
C
products of guaiacol over Pd-Fe catalyst, but no methanol was observed
in that over Pd-Co catalyst. In summary, the major reaction pathway of
guaiacol is as following: guaiacol → catechol → phenol → benzene for
Pd-Co catalyst, and guaiacol → phenol → benzene for Pd-Fe catalyst.
Besides the metallic sites, support also plays an important role in the
reaction network. The presence of acid sites in Al-MCM-41 catalyzed the
transalkylation of the methyl groups (blue arrows in Scheme 1) which
help to minimize the carbon loss via methanization. The combination of
HDO by metal sites and transalkylation by acid sites could produce the
C6-12 aromatics (from benzene to hexamethylbenzene) in the products.
hydrogenolysis, yielding higher HDO yield.
To understand the contribution of Pd to the stability of Co and Fe
catalysts, the TGA of used catalysts was applied to understand the coke
formation in catalysts as shown in Fig. S3. The main peaks in DTG curves
were observed from 200 to 700 ◦C which associated with coke removal
by oxidation. As shown in Table 2, the bimetallic Pd-Co and Pd-Fe cat-
alysts have lower coke formation than the corresponding monometallic
catalysts. The addition of Pd reduced the coke formation during HDO
reactions and made the catalyst more stable. Additionally, the used Fe
and Pd-Fe catalysts had higher coke deposit than the used Co and Pd-Co
catalysts. Nevertheless, the stability of the Fe and Pd-Fe catalysts was
higher than that of the Co and Pd-Co catalysts (see Fig. 6). This
contradiction could be explained by the DTG results (Fig. S3), in which
the used Fe and Pd-Fe catalysts had lower temperature degradation
peaks (at 350 ◦C) than Co and Pd-Co catalysts (at 500 ◦C). Therefore, the
coke deposit during catalytic HDO over Fe and Pd-Fe catalysts was more
easily degraded than the one over Co and Pd-Co catalysts.
3.3. Catalytic upgrading of lignin-derived bio-oil
The upgrading of lignin-derived bio-oil was conducted in the two-
stage catalytic pyrolysis reactor at ambient pressure. The products
consisted of char residue, coke deposit, tar, heavy and light oils, and gas
phase, and their yields were compared in Fig. 8A and B. In our experi-
ment, the yield of tar and others was estimated by difference. This dark
brown tar-like material was observed at the wall and the bottom flange
of the stainless-steel reactor. As shown in Fig. 8A, the bio-char yields of
all experiments had no significant difference since the bio-char was only
produced from pyrolysis process at the top part of reactor. The per-
centage of biochar after pyrolysis is consistent with the mass loss of
lignin which recorded from TGA analysis in nitrogen atmosphere
(Fig. S4). The amount of fixed-carbon in lignin is high [70], hence the
yield of bio-char from lignin pyrolysis was significant. Comparing with
the pyrolysis of cedar chips in Koike et al. research [4], the pyrolysis of
lignin in this study had higher bio-char and lower gas-phase yields. The
hemicellulose and cellulose in cedar chips in their study could be more
easily degraded to form the light component in the gas phase than lignin
in this reserach. In catalytic upgrading process, the coke deposit on the
Pd-Fe catalyst was higher than that on the Pd-Co catalyst. This trend is
similar to the coke deposit during HDO of guaiacol as in Table 2.
Additionally, the dark brown tar yield in pyrolysis experiment was ob-
tained more significantly than in catalytic upgrading process. The acid
sites of Al-MCM-41 support might catalyzed the cracking reaction of the
heavy compounds, resulting in the decrease of tar yield. The gas prod-
uct, consisted of CH4, CO and CO2, was produced more significantly in
catalytic upgrading than without catalyst.
3.2.2. Reaction network
HDO experiments were conducted over Pd-Co and Pd-Fe catalysts by
varying the W/F ratio for reaction network study (detail can be found in
our prior work [66]). Based on the detail analysis of species in the
product stream, the possible pathways were proposed as seen in Scheme
1.
This
reaction
network
comprised
HDO,
decarbon-
ylation/decarboxylation, demethylation, cracking, and methyl transfer.
The kinetic results were summarized in Table S2. In the case of Pd-Co,
the demethylation of guaiacol to form catechol is the major pathway
while the formation of phenol through demethoxylation of guaiacol is
very slow (k”10 > > k”1, Table S2). The formation of phenol through
demethylation of catechol is very fast, it explained the low concentra-
tion of catechol in the products. The formation of anisole is the slowest
reaction among three intermediates and following the rule of bond
dissociation energy (BDE) which ranked as CArOꢀ CH3 < CArꢀ OCH3
<
CArꢀ OH [67]. In contrast, the formation of phenol from guaiacol is
higher than that of catechol in Pd-Fe catalyst (k”1 > > k”10, Table S2). It
suggests that phenol was directly formed through demethoxylation of
guaiacol rather than from catechol in the present of Pd-Fe catalyst, and
the similar results were also observed in the literatures [9,68,69]. This
hypothesis was further confirmed by the presence of methanol in HDO
The liquid products were mostly collected at first cold trap and
Scheme 1. Possible reaction pathways for HDO of guaiacol over Pd-Fe and Pd-Co catalysts.
7

N.T.T. Tran et al.
Molecular Catalysis xxx (xxxx) xxx
could not be determined with our GC-FID analysis and appeared more in
without catalyst sample. Moreover, the Pd-Fe catalyst showed higher
HDO activity and lower gasification activity than the Pd-Co catalyst,
yielding higher signal intensity of mono-oxygenated peaks (Fig. 9A).
Zhang et al. studied the HDO of lignin-derived bio-oil over Ni/SiO2-
ZrO2 at 300 ◦C and 50 bar for 8 h in an autoclave reactor [22]. Their
feedstock was prepared separately by degradation of lignin in a stirred
autoclave at 280 ◦C for 1 h with the present of 5 wt.% ZnCl2 and
methanol. The major component in their lignin-derived bio-oil were
guaiacol, xylenol, trimethylphenol, propylanisole and cresol which were
similar with our results. However, their high-pressure process produced
more cyclic saturated hydrocarbons (yield of 55.0 wt%) than aromatic
hydrocarbons (yield of 7.8 wt%) in the upgraded products. Our exper-
iments were conducted at atmospheric pressure hence no saturation
product was observed.
The lignin-derived pyrolysis oil mainly contained phenolic com-
pounds which had one to three oxygen atoms. The catalytic upgrading
can eliminate significantly the oxygen in phenolic molecules to produce
the lower oxygen content pyrolysis oil. In addition, the increase of water
(liquid phase) and CO + CO2 (gas phase) in catalytic upgrading indi-
cated that the oxygen could be eliminated by both HDO and decarbox-
ylation/decarbonylation [4,7]. In general, the HDO was catalyzed by
metallic sites while the decarboxylation/decarbonylation was catalyzed
by acid sites of Al-MCM-41 support. The Pd-Co catalyzed the multiple
Cꢀ C, CArOꢀ CH3 and CArꢀ OH hydrogenolysis. These reactions helped to
reduce the oxygen and tar content but raise the carbon loss to the gas
phase. The Pd-Fe favored the CArꢀ OCH3 hydrogenolysis and some
cracking reaction which could reduce the oxygen and tar yield and
preserve the carbon in the bio-oil. In summary, Pd-Fe presented better
performance since it produced not only more mono-oxygenated
phenolic but also less dioxygenated, trioxygenated and gas-phase
products than Pd-Co catalyst.
Fig. 8. Upgrading of lignin-derived bio-oil over Pd-Fe and Pd-Co catalysts, A)
Product yields, B) Liquid-phase (light and heavy oil) yields. Pyrolysis condition:
T = 500 ◦C, mLignin = 3.0 g. Upgrading condition: T = 400 ◦C, P =1 bar, mCat
1.5 g, H2 flow rate = 90 mL/min.
=
considered as heavy oil, while the light oil was collected in a relatively
small amount (less than 0.9 wt% yield). The visual image of three heavy
oil samples is shown at the top of Fig. 9A. The oil was separated to three
layers, i.e. heavy-organic phase, aqueous phase and light-organic phase.
The heavy oil obtained via catalytic upgrading had less heavy-organic
phase and more light-organic phase than that via without catalyst. It
implied that the catalyst could catalyze not only deoxygenation reaction
but also cracking reaction of the heavy compounds, yielding more light-
organic phase. These reactions attributed to the decrease of heavy oil
and tar yields in catalytic upgrading process comparing to the pyrolysis
process (Fig. 8A). Comparing to without catalyst, the catalytic upgrad-
ing had lower methanol, dioxygenated and trioxygenated phenolics
yield and higher water yield (Fig. 8B). The catalytic HDO helped remove
the oxygen in methanol, di- and tri-oxygenated phenolics to produce the
deoxygenated products and water, resulting in higher water yield of the
catalytic upgrading. In addition, the liquid-phase product of the Pd-Co
contained more water, dioxygenated and less monooxygenated com-
pounds than the Pd-Fe as shown in Fig. 8B. Similar to HDO of guaiacol
4. Conclusion
Hydrodeoxygenation of guaiacol and lignin-derived bio-oil over Al-
MCM-41 supported Pd-Co and Pd-Fe catalysts have been studied at
400 ◦C and 1 atm. The Fe catalyst gave higher HDO yield and lower gas
phase yield compared with the Co catalyst in HDO of guaiacol. The
bimetallic Pd-Co catalyst presented a higher HDO activity than the
monometallic Co while the Pd-Fe showed an insignificant improvement
in HDO activity. Interestingly, the addition of Pd significantly improved
the stability of catalysts since it could reduce the coke deposition on the
catalysts.
The upgrading of actual lignin-derived bio-oil was conducted suc-
cessfully in a fi xed-bed reactor. The Pd-Co and Pd-Fe catalysts could
eliminate significantly the oxygen content in lignin-derived bio-oil. The
Pd-Co and Pd-Fe catalyzed not only deoxygenation reaction to reduce
the oxygen content but also cracking reaction to reduce the tar yield. In
catalytic upgrading process, oxygen was removed by both HDO and
decarboxylation/decarbonylation. Pd-Co catalyzed simultaneously the
discussion, the Pd-Co catalyzed simultaneously the multiple Cꢀ C, CAr
-
Oꢀ CH3 and CArꢀ OH hydrogenolysis while the Pd-Fe showed as an
effective catalyst in CArꢀ OCH3 hydrogenolysis. Hence, the Pd-Co
upgrading of lignin-derived bio-oil produced not only more gas phase
and dioxygenated compounds but also less monooxygenated than the
Pd-Fe upgrading.
C
ArOꢀ CH3, CArꢀ OH and multiple Cꢀ C hydrogenolysis while Pd-Fe
catalyzed principally the CArꢀ OCH3 hydrogenolysis. In summary, the
Pd-Fe/Al-MCM-41 presented as a suitable catalyst for upgrading of
lignin-derived bio-oil since its higher deoxygenated products and lower
gas-phase yields than Pd-Co/Al-MCM-41.
Fig. 9A and B show the GC-FID chromatography of heavy and light
oil products. The lignin-derived bio-oil of without catalyst had much
more dioxygenated and trioxygenated phenolic compounds i.e. guaiacol
(7), methyl-guaiacol (9), catechol (10) and dimethoxy-phenol (12) than
that of catalytic upgrading samples. These products were further deox-
ygenated in catalytic upgrading process to produce the mono-
oxygenated and oxygen-free products. The oxygen-free aromatic prod-
ucts, which contained benzene and toluene as the major compound,
were mostly collected at the second cold trap (Fig. 9B). Toluene, which
formed by HDO and transalkylation reactions, was found as the major
aromatic in bio-oil product. Some heavier components (Other products)
CRediT authorship contribution statement
Nga T.T. Tran: Conceptualization, Investigation, Validation, Writing
- original draft, Visualization. Yoshimitsu Uemura: Resources, Project
administration, Funding acquisition, Supervision, Writing - review &
editing. Anita Ramli: Methodology, Supervision, Writing - review &
editing. Thanh H. Trinh: Software, Methodology, Writing - review &
editing.
8

N.T.T. Tran et al.
Molecular Catalysis xxx (xxxx) xxx
Fig. 9. Chromatography of heavy oil (A) and light oil (B) products in upgrading of lignin-derived bio-oil on Al-MCM-41 supported Pd-Fe and Pd-Co catalysts.
[2] S.R. Naqvi, Y. Uemura, S.B. Yusup, Catalytic pyrolysis of paddy husk in a drop type
Declaration of Competing Interest
pyrolyzer for bio-oil production: the role of temperature and catalyst, J. Anal. Appl.
Pyrolysis 106 (2014) 57–62, https://doi.org/10.1016/j.jaap.2013.12.009.
The authors report no declarations of interest.
[3] A. Sanna, T.P. Vispute, G.W. Huber, Hydrodeoxygenation of the aqueous fraction
of bio-oil with Ru/C and Pt/C catalysts, Appl. Catal. B 165 (2015) 446–456,
https://doi.org/10.1016/j.apcatb.2014.10.013.
Acknowledgment
[4] N. Koike, S. Hosokai, A. Takagaki, S. Nishimura, R. Kikuchi, K. Ebitani, Y. Suzuki,
S.T. Oyama, Upgrading of pyrolysis bio-oil using nickel phosphide catalysts,
J. Catal. 333 (2016) 115–126, https://doi.org/10.1016/j.jcat.2015.10.022.
[5] Yoshimitsu UEMURA, Wissam N. OMAR, Suhair RAZLAN, Hafizah AFIF,
Suzana Yusup, Kaoru Onoe, Mass and energy yields of bio-oil obtained by
microwave-induced pyrolysis of oil palm kernel shell, J. Jpn. Inst. Energy 91
(2012) 954–959.
The authors gratefully acknowledge the financial support from Mit-
subishi Corporation Educational Trust Fund. We wish to thank Uni-
versiti Teknologi PETRONAS for providing
a congenial work
environment and state-of-the-art research facilities. We acknowledge
the support of time and financial from Ho Chi Minh City University of
Technology (HCMUT). We also thank Prof. Masaharu Komiyama for his
valuable supports, comments and suggestions.
[6] M.V. Olarte, A.H. Zacher, A.B. Padmaperuma, S.D. Burton, H.M. Job, T.L. Lemmon,
M.S. Swita, L.J. Rotness, G.N. Neuenschwander, J.G. Frye, D.C. Elliott, Stabilization
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Appendix A. Supplementary data
[8] H.A. Baloch, S. Nizamuddin, M.T.H. Siddiqui, S. Riaz, A.S. Jatoi, D.K. Dumbre, N.
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[9] J. Sun, A.M. Karim, H. Zhang, L. Kovarik, X.S. Li, A.J. Hensley, J.-S. McEwen,
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