CoMo sulfide-catalyzed hydrodeoxygenation of lignin model compounds: An extended reaction network for the conversion of monomeric and dimeric substrates

Article abstract of DOI:10.1016/j.jcat.2011.10.006

In the present work, extensive hydrodeoxygenation (HDO) studies with a commercial sulfided CoMo/Al₂O₃ catalyst were performed on a library of lignin model compounds at 50 bar hydrogen pressure and 300 °C in dodecane, using a batch autoclave system. The catalyst was activated under hydrogen atmosphere prior to the reaction, and the spent catalyst was analyzed using thermogravimetric analysis. An extended reaction network is proposed, showing that HDO, demethylation, and hydrogenation reactions take place simultaneously. HDO of mono-oxygenated substrates proved to be difficult at the applied conditions. Starting from most positions in the network, phenol, and cresols are therefore the main final products, suggesting the possibility of convergence on a limited number of products from a mixture of substrates. HDO of dimeric model compounds mimicking typical lignin linkages revealed that coumaran alkyl ethers and β-O-4 bonds can be broken, but 5-5′ linkages not.

Full text of DOI:10.1016/j.jcat.2011.10.006

Journal of Catalysis 285 (2012) 315–323
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
CoMo sulfide-catalyzed hydrodeoxygenation of lignin model compounds: An
extended reaction network for the conversion of monomeric and dimeric substrates
⇑
Anna L. Jongerius, Robin Jastrzebski, Pieter C.A. Bruijnincx, Bert M. Weckhuysen
Inorganic Chemistry and Catalysis Group, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2011
Revised 9 October 2011
Accepted 11 October 2011
Available online 20 November 2011
In the present work, extensive hydrodeoxygenation (HDO) studies with a commercial sulfided CoMo/
Al2O3 catalyst were performed on a library of lignin model compounds at 50 bar hydrogen pressure
and 300 °C in dodecane, using a batch autoclave system. The catalyst was activated under hydrogen
atmosphere prior to the reaction, and the spent catalyst was analyzed using thermogravimetric analysis.
An extended reaction network is proposed, showing that HDO, demethylation, and hydrogenation reac-
tions take place simultaneously. HDO of mono-oxygenated substrates proved to be difficult at the applied
conditions. Starting from most positions in the network, phenol, and cresols are therefore the main final
products, suggesting the possibility of convergence on a limited number of products from a mixture of
substrates. HDO of dimeric model compounds mimicking typical lignin linkages revealed that coumaran
alkyl ethers and b-O-4 bonds can be broken, but 5–50 linkages not.
Keywords:
Catalytic biomass conversion
Lignin
Hydrodeoxygenation
CoMo/Al2O3
Ó 2011 Elsevier Inc. All rights reserved.
Reaction network
Phenols
1. Introduction
(HDO), as this allows removal of part of the extensive functional-
ities from the highly oxygenated structure resulting in the forma-
With the depletion of fossil fuels, biomass becomes an impor-
tant renewable feedstock for the production of fuels, chemicals,
and energy. One particular component of lignocellulosic biomass
that until recently has been somewhat overlooked as an important
renewable resource is lignin [1]. This three-dimensional amor-
phous cross-linked biopolymer consists of phenyl-propane units,
optionally substituted with methoxy and hydroxy groups [2]. It
is a main constituent of lignocellulosic biomass and comprises
15–30% by weight and up to 40% by energy of biomass [3]. In plant
cell walls, lignin can be found in the spaces between cellulose and
hemicellulose, holding the lignocellulosic matrix together and add-
ing rigidity to plant material [4].
Pretreatment of lignocellulosic biomass, for instance in a biore-
finery operation, will give rise to large process streams of lignin, for
which value-added applications will have to be found [5]. Catalytic
conversion to bulk chemicals or fuel components is a possible sce-
nario for future use of this lignin feedstock. To this extent, lignin or
smaller fragments derived from this biopolymer (e.g. monomers or
small oligomers) can be subjected to various chemical transforma-
tions. Several strategies can be followed, such as lignin bond
hydrolysis [6,7] and catalytic oxidation reactions [8,9], in order to
obtain value-added products. One approach that carries consider-
able potential is conversion by catalytic hydrodeoxygenation
tion of bulk platform molecules, such as phenolics and BTX. Several
studies on the catalytic hydrodeoxygenation of lignin or fragments
derived from lignin have been reported [1,10,11].
The structural complexity and variability of lignin have
prompted the use of several lignin model compounds to study lig-
nin hydrodeoxygenation reactions. These model compounds, typi-
cally mono-aromatics, contain linkages and functional groups that
resemble those found in the lignin polymer, and study of their
reactivity provides insight into the reactivity of the lignin polymer
itself. The models can thus be used to mimic the chemistry of typ-
ical lignin fragments and hold the additional benefit of greatly sim-
plifying product analysis. In general, depolymerization feeds
derived from lignin will likely consist of a mixture of aromatics,
and the constituents of this mixture are mimicked in the library
[12]. Moreover, reports on the hydrodeoxygenation of these lignin
model compounds also bear relevance to the production and
upgrading of bio-oils, as they can consist of up to 30–40% of
oxygen-functionalized aromatics. Chemical processing of bio-oil,
obtained for instance from a biomass pyrolysis process, is neces-
sary to convert the bio-crude into useful transportation fuels.
Stabilization of these bio-crudes can be achieved by removal of
the oxygen functionalities via hydrodeoxygenation [11,13]. The
traditional hydrotreating catalysts originally developed for the
removal of sulfur and nitrogen from oil feeds could also be de-
ployed for the removal of oxygen from biomass-derived product
streams. Indeed, many of the studies on HDO of bio-oil or
⇑
Corresponding author.
E-mail address: b.m.weckhuysen@uu.nl (B.M. Weckhuysen).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.10.006

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A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
lignin-related components employ conventional cobalt- or nickel-
promoted molybdenum catalysts [1,10]. An early study by Elliott,
screening different commercial catalysts for their activity in phenol
HDO, already showed that a sulfided CoMo catalyst gave the best
results [14]. Because of their lower intrinsic hydrogenation activ-
ity, CoMo/Al2O3 catalyst is often preferred over the nickel analogs,
as the aromaticity of the feeds should preferably be kept intact.
This is in contrast to hydrodesulfurization or hydrodenitrogenation
reactions where the heteroatom is commonly removed after ring
saturation.
Several HDO studies using CoMo/Al2O3 catalysts were per-
formed on simple aromatic model compounds, such as phenol, cre-
sol, anisole and guaiacol, which mimic parts of the lignin structure
and are important bio-oil constituents. Products originating from
deoxygenation, (de)-methylation, and hydrogenation reactions
were reported [15–18]. Hurff and Klein, for instance, observed
the formation of catechol and phenol as main products for the con-
version of guaiacol over a sulfided CoMo/Al2O3 catalyst. Formation
of products in time confirmed that catechol is a primary product,
which further reacts to phenol [19]. Several concise reaction
schemes for the conversion of guaiacol and methylguaiacol were
subsequently reported without information on selectivity [19,20].
Recently, Bui et al. reported on the promoting effect of cobalt on
molybdenum sulfide catalysts and published a more comprehen-
sive reaction scheme for guaiacol conversion also including the for-
mation of heavy products [21]. The addition of cobalt was found to
enhance the direct demethoxylation and direct deoxygenation
pathways. Bredenberg et al. observed that the carbon–oxygen
bond in phenol and cresols is more difficult to break than other
carbon–oxygen bonds and that complete deoxygenation could
therefore not be reached [16]. The role of the support material in
HDO has also been investigated. The acid sites on alumina are
thought to participate in the conversion of guaiacol, both in the for-
mation of heavier products and steps that lead to ring methylated
products [22–24]. The effect of support acidity on the performance
of the catalyst has also been studied by addition of pyridine as
selective poison [22]. Laurent and Delmon have studied the inhibi-
tion of CoMo/Al2O3 and NiMo/Al2O3 catalysts under HDO condi-
tions with water, ammonia and H2S, concluding that oxygen
elimination reactions are poisoned by sulfur- and nitrogen-con-
taining compounds [25–27]. Senol et al. reported the conversion
of phenol in the presence of H2S and also concluded that both
the formation of benzene and cyclohexene are inhibited [28].
Recently, a renewed interest in HDO reactions has resulted in an
increasing amount of literature available on the HDO conversion
of lignin-derived streams. New approaches include co-feeding aro-
matic lignin model compounds like guaiacol with straight run gas
oil for the direct addition of oxygenates to fuel streams [29,30] and
mixed HDO of aromatic and aliphatic oxygenates.
Furthermore, when using depolymerized lignin streams, it is
likely that in addition to monoaromatic compounds also various
dimers and small oligomers are present in the reaction feed, neces-
sitating the study of fragments that mimic specific structural link-
ages found in lignin under HDO conditions. Romero et al. [31] and
Edelman et al. [32], for instance, showed that benzofuran could be
hydrogenated to dihydrobenzofuran, which was further converted
to ethylphenol. Petrocelli and Klein researched the reactivity of lig-
nin structural linkages by using biphenolic model compounds in
their HDO experiments, which were shown to be stable under
HDO conditions [33].
Valuable insights were obtained from these studies, but direct
comparison of the reported data is hampered by the varying exper-
imental conditions under which they have been obtained. Indeed, a
systematic study of the conversion of a library of lignin model
compounds with one batch of catalyst under comparable reaction
conditions and in the same reactor system is still lacking.
Scheme 1. Monomeric model compounds phenol (1), o-cresol (2) p-cresol (3),
anisole (4), 4-methylanisole (5), catechol (6), guaiacol (7), 4-methylguaiacol (8),
1,3-dimethoxybenzene (9), syringol (10), and vanillin (11).
Scheme 2. Model compounds mimicking the b-O-4 (12), 5–50 (13), and phenyl-
coumaran (14) linkages typically found in lignin.
Here, we report on the conversion of lignin model compounds
with the aim of obtaining bulk aromatics with lower oxygen
content. Several hydroxy- and methoxy-substituted monomeric
aromatic model compounds (Scheme 1) were used to conduct cat-
alytic studies with a sulfided CoMo/Al2O3 catalyst using a single set
of reaction conditions. For the first time, mono-, di-, and tri-
oxygenated compounds as well as both methylated and non-
methylated substrates are used to provide more insight in the
reaction pathways responsible for product formation. A reaction
network is presented to show the major reaction pathways and
reaction selectivities in more detail. In addition to conversion of
mono-aromatic model compounds, the effect of the catalyst sys-
tem on linkages typically present in the lignin polymer was also
studied. The model compounds shown in Scheme 2 were chosen
to represent both the b-O-4 (12) and phenylcoumaran (14) ether
linkages, as well as the 5-50 aryl–aryl linkage (13), which can be
found in the lignin structure.
2. Materials and methods
2.1. Chemicals and catalyst
All model compounds, additives, and solvents were used as
received: Phenol, o-cresol, p-cresol, anisole (Fluka, 99%), 4-methyl-
anisole (Sigma–Aldrich, 99%), catechol (Acros, 99%), guaiacol
(Sigma–Aldrich, 99%), 4-methylguaiacol (Sigma–Aldrich, 99%),
1,3-dimethylbenzene (Sigma–Aldrich, >98%), syringol (Sigma–Al-
drich, 99%), vanillin (Sigma–Aldrich, 99%), coumaran (Sigma–Al-
drich, 99%), 2,20-biphenol (Sigma–Aldrich 99%), hexadecane
(Sigma–Aldrich, 99%), pyridine (Acros, 99%), dodecane (Acros,
99%), diethylether (Biosolve, solvent grade), and methanol

A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
317
(Biosolve, solvent grade). Gases (hydrogen, 99.999% and argon,
99.999%) were obtained from Hoekloos.
of the TGA apparatus. Ion currents were recorded for m/z values
(m = molar mass of Xz+ ion, z = charge of the ion) of 28 and 32.
The b-O-4 lignin model compound 12, i.e., 1-hydroxy-1-(4-hy-
droxy-3-methoxyphenyl)-2-(2,6-dimethoxyphenoxy)ethane, was
synthesized according to a literature procedure [34]. The product
was purified using a silica gel column with a hexanes and ethyl
acetate mixture (1:3) as eluent, followed by repetitive crystalliza-
tion. The purity of the compound was confirmed by 1H NMR.
The CoMo/Al2O3 catalyst obtained from Albemarle Catalysts
was presulfided. Prior to its use, the catalyst material was crushed
3. Results and discussion
3.1. Conversion of monomeric model compounds
The conversion of the monomeric lignin model compounds
1–11 shown in Scheme 1 by a sulfided CoMo/Al2O3 catalyst was
investigated under HDO conditions. General reaction conditions
were 300 °C and 50 bar H2 pressure. The catalytic results are re-
ported in Table 1, and some general observations can be made
based on the presented data. Conversions are found to differ
widely after 4 h of reaction as only 20% of the substrate is con-
verted for mono-oxygenated aromatics, whereas conversions of
up to 90% are observed for some of the higher functionalized mol-
ecules, e.g., methylguaiacol and syringol, with mass balances that
are closed from 56% for syringol to 96% for anisole. GC analysis of
the liquid reaction product reveals complex reaction mixtures in
which the main products vary depending on the reactant. High
selectivities were obtained for benzene from phenol (1), phenol
from anisole (4), and phenol and catechol from guaiacol (7). Meth-
ylated substrates, such as cresols (2,3) and methylguaiacol (8), give
rise to methylated products. Furthermore, o- and p-cresol (2,3)
were also found in reactions with non-ring-methylated substrates,
e.g., selectivities up to 5% were obtained in the conversion of guai-
acol. Unlike in the work of Gutierrez et al. where benzene was
found to be the main product obtained from guaiacol, formation
of completely deoxygenated products is not observed in large
quantities [35].
In addition to p- and o-cresol, di- and trimethylated phenolics
are also formed, as well as other methylated products, such as
4-methylguaiacol and toluene. Importantly, no quantifiable
amounts of m-cresol were found, not even in the conversion of
4-methylguaiacol (8). For most reactions, mono-oxygenated prod-
ucts such as phenol make up the major part of the reaction mix-
ture. In the case of syringol (10), however, also several di- and
tri-oxygenated compounds are observed. The presence of small
amounts of benzene and toluene in all reaction mixtures suggests
that full deoxygenation does take place, albeit with low overall
selectivity. The conversion of anisole and guaiacol, for instance,
only yielded minor amounts of benzene, illustrating that it is not
a major product in these reactions. The formation of mainly phen-
olics suggests that demethylation and deoxygenation are the main
reaction pathways together with ring methylation (see Scheme 3
in which guaiacol is shown as a typical example).
and sieved to particles of size between 150 and 450 lm that were
activated under 60 mL/min H2 flow at 300 °C for 3 h. The activation
method is similar to the industrially used activation procedure for
this particular catalyst. Addition of H2S or other sulfiding agents to
the H2 flow was not needed, as a sulfiding agent was already pres-
ent in the sample. To ensure that sulfur-loss was limited under
conditions of a catalytic run, elemental analysis of the fresh and
spent catalyst was performed at the Mikroanalytisches Laboratori-
um Kolbe. No loss of sulfur was found, but the elemental analysis
data rather showed a marginal increase in sulfur content of the cat-
alyst after reaction in absolute amounts (13% increase relative to
Al). NO adsorption experiments were also performed on the fresh
and spent catalyst by IR spectroscopy, the results of which can
be found in the Supporting information.
2.2. Catalytic reactions
The catalytic reactions were performed in a 100-mL stainless
steel high-pressure Parr batch autoclave reactor. The temperature
was monitored using a thermocouple, and stirring was performed
using a magnetic driver equipped with an impellor at 750 rpm. In a
typical reaction, the autoclave was loaded with 150 mg of the cat-
alyst material, 1.5 g substrate (typically guaiacol), 0.5 g hexadec-
ane as internal standard, and 30 g of the solvent dodecane. A
fresh batch of catalyst was used in every run. The reactor was
purged three times with argon, and the reaction mixture was
heated to 300 °C, unless stated otherwise, then pressurized with
H2 to 50 bar, and the catalytic reaction was carried out for 4 h.
After the reaction was stopped by cooling and release of pressure,
the reaction mixture was diluted with an equal volume of diethyl
ether in order to dissolve all of the products, the solid catalyst
was removed by filtration and washed with diethyl ether.
2.3. Analysis
The reaction products were analyzed using
a Shimadu
GC-2010A gas chromatograph equipped with a WCOT fused silica
CP-WAX 57-CB column and FID detector, and a Varian 430-GC
gas chromatograph equipped with a VF-5 ms capillary column
and a FID detector. Compounds were identified with gas chroma-
tography coupled with a mass spectrometer using a Shimadzu
QP2010 GCMS instrument equipped with a VF-5 ms capillary col-
umn and compared with retention times of pure standards.
Response factors relative to hexadecane were determined experi-
mentally when authentic samples were available. The b-O-4 lignin
model compound was silylated with N,O-bis(trimethylsilyl)acet-
amide before GC analysis. Gas-phase products were analyzed on
an Interscience compact GC equipped with a molsieve 5A column
and a TCD detector. 1H NMR analysis was performed on a Varian
300 Inova MHz spectrometer. Thermal gravimetric analysis (TGA)
was performed with a Perkin–Elmer Pyris 1 apparatus. Typically,
15 mg of impregnated silica gel was heated with a ramp of
5 °C minꢀ1 to 600 °C in a 10 mL minꢀ1 flow of air. In parallel
evolved gas, analysis was performed with a quadrupole Pfeiffer
Omnistar mass spectrometer, which was connected to the outlet
Ring hydrogenation products, such as cyclohexanol, cyclohex-
ene and methylcyclohexene, are only detected in low quantities
with the total amount adding up to no more than 5 percent. In
comparison, Hurff and Klein already found 10% cyclohexane after
4 h in the conversion of guaiacol with a sulfided CoMo/Al2O3
catalyst in the presence of sulfiding agent [19]. Bredenberg et al.
also observed only small amounts of around 1% hydrogenation
products in the conversion of guaiacol and anisole over a sulfided
NiMo/Al2O3 catalyst in a continuous flow process [16]; neverthe-
less, over 5% ring hydrogenation was observed when a sulfiding
agent was added to the feed [18]. Bui et al. reported around 4%
hydrogenation products and found that the amount of hydrogena-
tion products increases when a non-promoted MoS/Al2O3 catalyst
is used [21].
For some reactions, not all products detected by GC could be
unambiguously identified with either authentic samples or by
GC–MS analysis. These products added together, however, do not
represent more than a total selectivity of 4%. An overview of the
most common reaction products is given in Table 1.

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A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
Table 1
HDO of different lignin model compounds over sulfided CoMo/Al2O3 catalyst at 300 °C and 50 bar H2 pressure for 4 h.
Conversion Balance
Methylated
productsb
Hydrogenated
productsc
Xa
Phenol
Anisole
27
58
80
84
78
86
96
56
76
60
37
10
1
<1
<1
x
–
x
–
3
2
–
–
x
11
–
–
–
–
x
–
–
4
3
4
3
1
6
–
4
–
40
64
36
34
3
10
<1
19
2
Catechol
Guaiacol
1,3-Dimethoxy
benzene
Syringol
83
57
<1
1
<1
3
5
6
1
26
o-Cresol
p-Cresol
4-Methylanisole
4-Methylguaiacol
Vanillin
23
22
85
89
53
95
91
85
91
75
65
60
13
2
–
–
–
x
–
–
–
–
–
–
–
–
x
–
3
6
2
–
–
12
–
2
4
17
<1
47
42
3
1
13
23
–
13d
9
–
23
Selectivities are displayed as a percentage of the conversion in moles.
a
X indicates the total sum of all other peaks in the chromatogram including dimethoxyphenol, methoxycatechol, resorcinol, dimethoxybenzene, methylmethoxyphenol,
trimethoxybenzene, and 1,4-benzenediol.
b
Methylated products include toluene, p- and o-cresol, di- and trimethylated phenols, 3- and 4-methylguaiacol, 4-methylcatechol and methylmethoxybenzene. For
methylated substrates, only those products with more than one methyl functionality are included.
c
Hydrogenated products include cyclohexene, cyclohexanol, methylcyclohexane, and methylcyclohexene.
Both 4-methylguaiacol and 5-methylguaiacol are found in a 1:1 ratio.
d
Scheme 3. Reaction pathways in the conversion of guaiacol: (a) formation of phenol by HDO (i) followed by methylation to o-cresol (iii); (b) formation of catechol by
demethylation (ii) followed by formation of phenol by HDO (iv) and methylation to o-cresol (iii).
3.2. Formation of heavy products
some peaks of higher molecular weight in the product mixtures,
but these products could not be identified. As blank reactions
for guaiacol and anisole, run without catalyst, show a closed mass
balance, it is clear that the side reactions that cause oligomeriza-
tion of the substrate are also catalyzed by the sulfided CoMo/Al2O3
catalyst. Coke formation on the catalyst to such extent that it
explains the observed weight loss, on the other hand, can be ex-
cluded. The catalyst sample shows no significant change in weight
after reaction, whereas the missing weight should have increased
the catalyst mass by 300%.
Thermogravimetric analysis of the spent catalyst was carried
out to estimate the coke deposit after the reaction with guaiacol.
The thermograms show a small increase in the formation of CO2
with respect to the fresh catalyst and a total weight loss of 17%.
Judging on the major weight loss at 440 °C, most of the coke
formed on the catalyst is so-called hard coke, which consists of
the more bulky and highly aromatic compounds [37].
Intermolecular coupling between guaiacol or catechol-like
molecules at higher temperatures can cause oligomerization, lead-
ing to higher molecular weight products [20,21]. When formed,
these products cannot be detected by our GC analysis, resulting
in mass balances that can be closed for 80–90% in most reactions.
In general, we observe that substrates with more oxygen func-
tional groups give a lower mass balance (e.g., syringol (5) 57%
compared with guaiacol (7) 76%). Carbon balances of around
80% for di-oxygenated compounds such as guaiacol and around
90% for phenol and anisole, and 95% for methylated phenols are
comparable to those previously reported for similar substrates
[18–20,27]. The remainder being equal to the amount of heavy
products observed by Bui et al. by GC–MS after gas-phase HDO
of guaiacol [21,24]. In some cases, only the product distribution
of the isolated products is published, leaving the not isolated
products out of the mass balance [15,36]. Petrocelli and Klein
found that the mass balance decreased with increasing reaction
time [33] this is also indicative of an increase in formation of hea-
vy products that could not be identified by GC methods. In our
case, ESI-MS analysis of the reaction mixture indeed revealed
3.3. Reaction network
The formation of several different products in each reaction sug-
gests that HDO and O-demethylation reaction pathways, indicated

A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
319
Fig. 1. Conversion, mass balance, demethylation, and HDO selectivity for ring methylated model compounds 2, 3, 5, and 8 compared with non-methylated analogs 1, 4, and 7.
in Scheme 3, as well as the hydrogenation reaction pathway are tak-
ing place simultaneously. Depending on the substrate used, selec-
tivity for one route may decline in favor of another. Nonetheless,
the conversion of the different model compounds leads
toward the formation of phenolic products via a combination of
demethylation/dehydroxygenation steps. As previously observed,
demethylation and methylation reactions are prone to take place
on the alumina support [22–24].
alternative reactions, such as oligomerization and hydrogenation.
It was previously noted that in order for hydrogenation to take
place, the molecule must adsorb flat on the catalyst surface,
whereas deoxygenation takes place by
g1 coordination of the oxy-
gen on the edge of the MoS2 phase [18,38]. Furthermore, in order
for ring methylation reactions to take place, the aromatic ring must
also approach the catalyst surface and forced non-planar coordina-
tion might hamper ring methylation reactions.
Hydrogenation products are observed, but no oxygenated cyclo-
hexanes are found. This implies that the demethylation and HDO
reactions take place in an earlier stage of the reaction and that
the reaction can be stopped at the point where HDO is completed
before large amounts of hydrogenation products start to form.
Notable is the difference in reactivity of the compounds
guaiacol (7), anisole (4), and phenol (1) compared with their ring-
methylated derivates (8, 5, 2 and 3). As depicted in Fig. 1, conver-
sion, mass balance, and selectivity toward deoxygenation products
of the methylated substrates are slightly higher than for the non-
methylated model compounds. The difference in HDO selectivity
between phenol and both cresols is even more than 20%. Also, less
ring methylation products are found for the methylated substrates.
The presence of a methyl group on the ring, therefore, most likely
makes addition of a second one sterically less favorable. The rela-
tive increase in HDO activity of the methylated compounds can
also probably be attributed to steric factors, as a methyl group
makes the aromatic ring more difficult to approach thus limiting
As most aromatic rings in the original lignin structure as well as
in lignin-derived depolymerization feeds carry alkyl functional-
ities, the increased selectivity toward HDO observed for alkyl-
substituted model compounds is useful when applied to lignin
itself.
Several routes can be envisaged for the conversion of guaiacol
to phenol and ultimately benzene. (i) The direct deoxygenation
pathway (DDO) where phenol is formed via loss of the methoxy
functionality; (ii) demethylation of the methoxy group followed
by HDO. The phenol then further reacted toward benzene. Impor-
tantly, catechol (6) is observed as the main reaction intermediate
in the conversion of guaiacol (vide infra), suggesting (ii) to be the
dominant pathway. Experiments with catechol as the substrate
also show that it readily reacts further via HDO to phenol (see
Table 1). It is not clear, however, whether the reaction from guai-
acol to phenol proceeds exclusively via catechol by means of con-
secutive demethylation and HDO (ii), or whether DDO (i) also takes
place. Although minor amounts of methanol are clearly observed
Scheme 4. Formation of o- and p-cresol in the sulfided CoMo/Al2O3-catalyzed conversion of ring-methylated and non-methylated monoaromatic model compounds. No m-
cresol was observed for any of the substrates.

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A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
by GC for some of the reactions, it was not possible to reliably
quantify the amount of methanol formed by demethoxylation.
Overall, these results thus indicate that the demethylation path-
way (ii) is the major pathway in the conversion of guaiacol, with
demethoxylation (i) most likely being only a minor route. This
might be expected as a result of the higher bond strength of the
MeO–Ar compared with the ArO–Me bond. Indeed, the bond disso-
ciation energy for RO–Ar bonds (422 kJ/mol) is higher than for
RO–R bonds (339 kJ/mol) [10]. The absence of large quantities of
anisole combined with the lower reactivity of anisole compared
with catechol under HDO conditions further supports that the
major reaction pathway from guaiacol to phenol goes via catechol.
There is no evidence, however, to fully exclude demethoxylation
(ii) as one of the many reaction routes that take place simulta-
neously. These results are in agreement with Laurent and Delmon
[20] and Bredenberg et al. [18], who also concluded that demethyl-
ation to form catechol is the first step in guaiacol deoxygenation. In
contrast, in a general reaction network proposed by Petrocelli and
Klein [33], it was concluded that demethylation was only a minor
pathway.
Similarly, it is difficult to distinguish between the two possible
routes for catechol formation from guaiacol, as both demethylation
Scheme 5. Reaction pathways in HDO of lignin model compounds. Hydrogenation is very limited, and these pathways are not included; arrow size contains no information
on conversion.

A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
321
as well as hydrogenolysis of the O–Me bond can be envisaged. The
observation of various ring methylated structures in addition to
some activity observed for the support itself (vide infra) points to
the former route. Nonetheless, gas-phase analysis of a guaiacol
conversion reaction shows some formation of methane, showing
that hydrogenolysis also takes place, a least to a minor extent.
Interestingly, conversion of 4-methylguaiacol (8) gives methyl-
ated catechol (17) and p-cresol (3) as the main reaction products.
As p-cresol is formed selectively and no m-cresol is observed, the
HDO step must be selective for the oxygen in the 2 position. Reac-
tions with m-cresol show no formation of p-cresol, so fast isomer-
ization of this product can be excluded and p-cresol is the only
cresol formed via HDO of methylated aromatics (Scheme 4). This
is not in line with the results obtained by Bui et al. who reported
the formation of m-cresol in the conversion of guaiacol over pure
Conversion of catechol has a selectivity of 36% to mono-
oxygenated products, whereas dimethoxybenzene produces
single oxygen products with a selectivity of around only 5%
(note that these can also be formed by subsequent demethyla-
tion and deoxygenation). Substrates containing only one oxygen
atom such as phenol and cresols (conversion ꢂ20%) appear to
be much more stable than higher functionalized aromatics such
as catechol and guaiacol (conversion ꢂ80%).
ꢁ Hydrogenation activity has been commonly observed with HDO
catalysts. Only very small amounts of hydrogenation products
are observed after 4 h, however, under our present conditions
(<5%). Hydrogenated products mainly consist of cyclohexene
and cyclohexanol, but in some cases also small amounts of
methylated hydrogenation products are found. The low
amounts of these products in the reaction mixture indicate that
hydrogenation is the least favorable pathway for this system.
c-alumina [24]. Huuska and Rintala [22] have previously reported
a preference for ring methylation at the o-position in the conver-
sion of anisole over a NiMoS/Al2O3 catalyst in a continuous flow
system, claiming this as evidence for a non-planar orientation of
the phenoxide ion. However, in the conversion of non-methylated
substrates, we observe a slight excess in p-cresol over o-cresol,
again no m-cresol is found (Scheme 4).
No evidence was found for ring demethylation reactions at
equal or comparable rates to O-demethylation reactions. Catalytic
tests on methylated model compounds, 4-methylguaiacol, 4-meth-
ylanisole and the cresols, show no formation of ring demethylated
products. Small amounts of phenol that are found in cresol reaction
mixtures can be attributed to impurities in the starting material.
Reactions with 4-methylguaiacol, however, show evidence for
reversible O-methylation, as equal amounts of 4-methylguaiacol
and 5-methylguaiacol are found.
Although reactions starting from phenol and cresol show a high
selectivity toward HDO products benzene and toluene, activity is
rather low, as total phenol and cresol conversions are under 30%.
This indicates that phenol itself is quite stable under the applied
reaction conditions. From a lignin valorization point of view, phe-
nol and cresols constitute attractive targets for this type of conver-
sions. It is therefore important to note that starting from every
position in the reaction network, phenol and cresols can indeed
be obtained as the main reaction products under these process
conditions.
3.4. Conversion of dimeric model compounds
During plant growth, the lignin polymer is generally believed to
be formed via radical polymerization of three phenylpropane
monomeric units, i.e., p-coumaryl alcohol, coniferyl alcohol and
sinapyl alcohol [1]. As a result of the radical nature of lignin
synthesis, many different linkages can be found in the resulting
biopolymer. The most common of these linkages are the so-called
b-O-4, 5–50 and b-5 ones, which are present in different quantities
in native lignin depending on the plant species and in processed
lignin depending on the pretreatment method. Scheme 6 shows a
typical softwood lignin fragment with some examples of different
linkages highlighted. The b-O-4 linkage is by far the most
abundant, and model compound 12 (Scheme 2) was synthesized
to investigate the reactivity of this linkage under HDO conditions.
The compounds 2,20-biphenol (13) and coumaran (14) represent
some key features of the 5–50 and the phenylcoumaran-type link-
ages encountered in lignin and are commercially available.
The results of the HDO reaction of the b-O-4 lignin model com-
pound 12 are shown in Table 2 and Scheme 7. 1H NMR analysis of
the reaction mixture showed complete conversion of the substrate
12. Only mono-aromatic products could be identified by GC analy-
sis with a total amount of 33% of aromatics recovered after 4 h of
reaction; the presence of these monoaromatic compounds
confirms cleavage of the b-O-4 bond under HDO conditions. Phe-
nol, guaiacol, and syringol-like products are mainly obtained. No
ethylbenzene derivatives could, however, be identified which indi-
cates that all observed products likely originate from the syringol
part of 12.
The use of both methylated and non-methylated substrates
provided more insight in the mechanisms responsible for the for-
mation of the observed products. Scheme 5 shows a reaction net-
work for the reductive conversion of methoxy- and hydroxy-
functionalized mono-aromatics over sulfided CoMo/Al2O3. The
reaction pathways presented in this network are based on
the experiments discussed above and those listed in Table 1. For
the sake of simplicity, not all minor direct demethoxylation routes
are shown in the proposed reaction network. However, when the
demethylated intermediate was not identified, only direct demeth-
oxylation routes are included, for example, in going from syringol
(10) to guaiacol (7).
In summary, distinction is made in the reaction network
between three types of reactions that occur simultaneously. The
arrow size indicates reaction selectivity for this path, but does
not contain information on conversion. All tested mono-aromatic
model compounds follow the same three pathways that eventually
lead to the formation of phenol and benzene or cresols and toluene.
ꢁ O-demethylation is the fastest pathway; the removal of methyl
groups from methoxy functionalities is more favorable than
direct demethoxylation. In the conversion of anisole, for
instance, phenol is observed as the main product, whereas only
small amounts of benzene were formed. O-demethylation goes
hand in hand with methylation of the aromatic ring. Indeed,
aromatic alkylations are observed in most reaction mixtures
for which the p- and o-positions are strongly favored over the
m-position. It is likely that also transfer of the methyl groups
between hydroxy functional groups occurs, as is indicated by
the isomerization of 4-methylguaiacol to 5-methylguaiacol.
Methylation and demethylation activity are often attributed to
support acidity [23,24].
2,20-Biphenol (13) was used as a simple analog of the 5–50lignin
carbon–carbon bond. The results are shown in Table 3 and
Scheme 8. 2,20-Biphenol is not soluble in the reaction mixture at
room temperature, leading to some losses of the remaining starting
material during workup. This can in part account for the relatively
low mass balance found in this reaction. Nonetheless, all detected
products still contain the aryl–aryl linkage. The recalcitrance of the
5–50 bond and its insensitivity toward chemical reactions is not
unexpected, as previous research on treatments for lignin-derived
ꢁ Hydrodeoxygenation is occurring at
a slower rate than
O-demethylation. Both methoxy and hydroxy groups can be
removed via HDO although demethoxylation is much slower.

322
A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
Scheme 6. Schematic representation of a softwood lignin structure, highlighting examples of the b-O-4, phenylcoumaran, and 5–50 linkages.
Table 2
to the catalyst surface so that the other can react. Interestingly,
large amounts of dibenzofuran are found in the reaction mixture.
HDO conversion and selectivities in mol% of b-O-4 lignin
model compound 12 over sulfided CoMo/Al2O3 catalyst
at 300 °C and 50 bar H2 pressure for 4 h.
Dibenzofuran can be formed via an acid-catalyzed dehydration
reaction of the 2,20-biphenol in which the ring closing step is an
Conversion (%)
Mass balance (%)
Phenol (%)
o-Oresol (%)
p-Cresol (%)
Dimethylphenol (%)
Catechol (%)
Resorcinol (%)
Guaiacol (%)
Methoxyphenol (%)
Syringol (%)
100
33
9
2
2
5
1
2
6
intramolecular nucleophilic attack of one of the hydroxy groups
[39,40]. This indicates that dibenzofuran and 2-hydroxybiphenyl
are formed from 2,20-biphenol via separate pathways and that
dibenzofuran formation competes with HDO.
Dihydrobenzofuran or coumaran (14) is a common probe mol-
ecule for HDO reactions [31] and can serve as a model compound
that resembles phenylcoumaran linkages in lignin. After reacting
over a sulfided CoMo/Al2O3 catalyst for 4 h at 300 °C and 50 bar
hydrogen pressure, 18% of the substrate was converted. As can
be seen in Table 4 and Scheme 9, high selectivity toward 2-ethyl-
phenol is obtained. This is in line with the results obtained by
Romero et al. [31], who found 2-ethylphenol to be the main prod-
uct and ethylbenzene the main deoxygenated product after HDO
over a NiMoP/Al2O3 catalyst using a fixed-bed reactor. They con-
tributed the low conversions to an inhibitive effect of coumaran it-
self on the HDO activity of the catalyst. The obtained results do
2
4
molecules always report the 5–50 link to remain intact [33]. In line
with the observations made for catechol and other bis-oxygen-
functionalized substrates, only one hydroxy group is removed by
HDO activity leading to a mono-oxygenated product. Possibly, at
least one oxygen atom is needed for positioning of the molecule
Scheme 7. Conversion of b-O-4 dimer 12.

A.L. Jongerius et al. / Journal of Catalysis 285 (2012) 315–323
323
Table 3
b-5 ether linkages can be broken under HDO conditions. The rigid-
ity of the 5–50 linkage prevents cleavage of the 5–50 model
compound 2,20-biphenol.
HDO conversion and selectivities in mol% of 2,20-
biphenol (13) over sulfided CoMo/Al2O3 catalyst
at 300 °C and 50 bar H2 pressure for 4 h.
Conversion (%)
Mass balance (%)
Biphenyl (%)
55
79
2
Acknowledgments
The authors gratefully thank the Smart Mix Program of the
Netherlands Ministry of Economic Affairs and the Netherlands
Ministry of Education, Culture and Science within the framework
of the CatchBio Program. Albemarle Catalysts is kindly
acknowledged for providing the catalyst.
2-Hydroxybiphenyl (%)
Dibenzofuran (%)
Tetrahydrodibenzofuran (%)
22
34
4
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2011.10.006.
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