Vapor-Phase Hydrodeoxygenation of Guaiacol to Aromatics over Pt/HBeta: Identification of the Role of Acid Sites and Metal Sites on the Reaction Pathway

Article abstract of DOI:10.1002/cctc.201701413

A comparative study of the hydrodeoxygenation of guaiacol, a phenolic compound derived from the lignin fraction of biomass with both hydroxyl and methoxyl functional groups, was performed on HBeta, Pt/SiO₂, and Pt/HBeta at 350 °C and atmospheric pressure. The reaction pathway and the roles of the acid and metal sites were studied. Acid sites catalyze transalkylation and dehydroxylation reactions to produce monohydroxyl phenolics (phenol, cresols, and xylenols) as the major final products. Pt sites catalyze demethylation to result in catechol as the primary product, which can either be deoxygenated to phenol followed by phenol to benzene, or decarbonylated to cyclopentanone and further to butane. The close proximity of Pt and acid sites in bifunctional Pt/HBeta improves transalkylation and deoxygenation (or dehydroxylation) reactions and inhibits demethylation and decarbonylation reactions significantly, which leads to aromatics as the major final products with a total yield >85 %. Both the activity and stability of bifunctional Pt/HBeta during the hydrodeoxygenation of guaiacol are improved compared to those of HBeta and Pt/SiO₂. The addition of water to the feed further improves the activity and stability through the hydrolysis of the O?CH₃ bond of guaiacol on acid sites and the removal of coke around Pt.

Full text of DOI:10.1002/cctc.201701413

Accepted Article
Title: Vapor-Phase Hydrodeoxygenation of Guaiacol to Aromatics over
Pt/HBeta: Identification of the Role of Acid Site and Metal Site on
the Reaction Pathway
Authors: Lei Nie, Bo Peng, and Xinli Zhu
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10.1002/cctc.201701413
ChemCatChem
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Vapor-Phase Hydrodeoxygenation of Guaiacol to Aromatics over
Pt/HBeta: Identification of the Role of Acid Site and Metal Site on
the Reaction Pathway
Lei Nie,[a,b] Bo Peng,[b] and Xinli Zhu*[a]
Abstract: A comparative study of hydrodeoxygenation of guaiacol, a
phenolic compound derived from lignin fraction of biomass with both
hydroxyl and methoxyl functional groups, was performed on HBeta,
Pt/SiO2 and Pt/HBeta at 350 °C and atmospheric pressure. The
reaction pathway and the role of acid site and metal site were
studied. Acid sites catalyze transalkylation and dehydroxylation
reactions, producing mono-hydroxyl phenolics (phenol, cresols and
xylenols) as the major final products. Pt sites catalyze demethylation
reaction resulting in catechol as the primary product, which can
either be deoxygenated to phenol followed by phenol to benzene, or
decarbonylated to cyclopentanone and further to butane. The close
proximity of Pt and acid site in bifunctional Pt/HBeta improves
transalkylation and deoxygenation (or dehydroxylation) reactions
while significantly inhibits demethylation and decarbonylation
reactions, producing aromatics as the major final products with a
total yield > 85%. Both activity and stability of bifunctional Pt/HBeta
during hydrodeoxygenation of guaiacol are improved compared to
HBeta and Pt/SiO2. The addition of water to the feed further
improves the activity and stability via hydrolysis of O-CH3 bond of
guaiacol on acid site and removing coke around Pt.
derivatives) derived from lignin represent an important fraction of
bio-oil. Hydroxyl (–OH) and methoxyl (–OCH3) groups connected
to the phenyl ring are the major functional groups of phenolics
(Scheme 1 in reference [3]). Compared to other groups of
molecules in bio-oil, phenolics have relatively long carbon chain
and low oxygen content.[1-4] A direct deoxygenation process of
phenolics could produce gasoline range hydrocarbon molecules.
However, the delocalization effect of the lone pair electron of O
(in both hydroxyl and methoxyl) onto the π bond of phenyl ring
strengthens the Caromatic-O bond, leading to a significantly higher
activation barrier for oxygen removal from phenolics than that for
aliphatic alcohols.[5] Hence, development of active and stable
catalyst for conversion of phenolic-rich bio-oil remains
challenge.
a
Guaiacol (2-methoxyphenol) is one of the most abundant
phenolics in bio-oil.[4,6] It has both hydroxyl and methoxyl
functional groups. Therefore, it has been used as a model
compound representing phenolics in bio-oil and been
extensively studied over various catalytic materials.[6-11] Among
those catalysts, bifunctional zeolite-supported metal catalysts
have attracted significant attention.[12-29] Most of the works on
hydrodeoxygenation (HDO) of guaiacol over metal/zeolite
catalysts were performed in liquid phase, i.e. at low temperature
(< 250 °C) and high pressure of H2 (several MPa).[14,17-20] Under
these conditions, the reaction proceeds through hydrogenation-
deoxygenation (HYD) path in a bifunctional manner: saturation
of the phenyl ring on metal sites, followed by dehydration on
acid sites.[12,13] The methoxyl group is lost as methanol either by
demethoxylation on metal site or hydrolysis on acid site.[14] The
advantage of liquid phase HDO is that it may be coupled in a
single step with liquefaction of biomass.[30-32]
On the other hand, relatively fewer works have been performed
in vapor phase at high temperatures (> 250 °C) and atmospheric
pressure on zeolite and metal/zeolite catalysts to simulate direct
upgrading of phenolics right after pyrolysis before its
condensation. Co-conversion of guaiacol with n-heptane over
acidic HZSM-5 and HY zeolites were investigated at 450 °C
under fluid catalytic cracking conditions.[33] It was found that
hydrogen transfer is the major reaction path to produce methane
and phenol. And guaiacol has a negative effect on heptane
conversion due to coke formation. Foo et al. reported HDO of
guaiacol on HBEA and Pt/HBEA at 400 °C and atmospheric
pressure.[34] Isomers of methylcatehol were found as the major
products on bare HBEA with low conversion of <5%. A wide
range of products was observed on Pt/HBEA with only 3-5%
was deoxygenated to aromatics. Similarly, on Fe-Ni/HBeta at
250-400 °C and atmospheric pressure,[29] and Ni-Fe/zeolite
catalysts at 350-450 °C and 1.5-17 bar,[35] low degree of
Introduction
Production of transportable liquid fuels from renewable and
abundant biomass may provide a sustainable energy process
that reduces both the dependency on fossil fuels and CO2
emissions. Bio-oil derived from biomass via fast pyrolysis is
composed of many fragments that contain significant amount of
oxygen.[1,2] Efficient oxygen removal and building up the carbon
chain of these oxygenates into target molecules with desired fuel
properties rely on novel catalyst with appropriate operating
conditions. The high complexity of bio-oil makes model
compound study
a crucial role on obtaining preliminary
understanding of the catalytic chemistry.
As lignin is a major component (up to 30%) of biomass,
phenolic compounds (phenol, guaiacol, syringol and their
[a]
[b]
Dr L. Nie, Dr X.L. Zhu
Collaborative Innovation Center of Chemical Science and
Engineering, School of Chemical Engineering and Technology
Tianjin Univeristy
Tianjin 300072 (P.R. China)
E-mail: xinlizhu@tju.edu.cn
Dr L. Nie, Dr B. Peng
Institute for Integrated Catalysis
Pacific Northwest National Laboratory
P.O. Box 999, Richland, WA 99352 (USA)
deoxygenation to aromatics was achieved. These preliminary1
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10.1002/cctc.201701413
ChemCatChem
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works suggested guaiacol undergoes transalkylation and
demethylation followed by deoxygenation to benzene and
toluene. However, cyclopentanone was surprisingly found to be
an important product besides catechol and phenol over Pt/MgO
HBeta. Brønsted acid site (BAS) densities of HBeta and
Pt/HBeta, measured by IPA-TPD, are 695 and 674 μmol/g,
respectively.[39] These values are in good agreement with that
estimated from the Si/Al ratio of 19, indicating that the samples
mainly contain BAS of bridging Al-OH-Si. The slight reduced
BAS density originated from Pt loading suggests that Pt and
BAS sites are in close proximity. The molar ratio of BAS to
surface Pt site (estimated by CO chemisorption) is ca. 17.
[36,37]
and Pt/C [38] catalysts at 300 °C and atmospheric pressure.
It is evident that very different product distributions were
observed on metal, zeolite, and metal/zeolite catalysts at
different temperatures. Although significant efforts have been
made, the complex product distributions as a result of multiple
functional groups (-OH, -OCH3, phenyl ring) of guaiacol
conversion on different active sites at different temperatures
makes rigorous analysis of the reaction pathway on metal/zeolite
catalysts not available. Furthermore, identification of the
individual role of metal and acid site of zeolite is essential to
understand the reaction pathway and to design efficient catalytic
process.
Guaiacol conversion over HBeta
The conversion of guaiacol (Gua) as a function of W/F and the
major products evolution (plotted versus guaiacol conversion)
over zeolite HBeta are shown in Figure 1A and B, respectively.
The conversion of guaiacol increased linearly to 36% for W/F
less than 1.5 h. However, it steeply increased to 100% when
W/F increased to 2 h.
In this work, we compared the conversion of guaiacol on HBeta,
Pt/SiO2, and Pt/HBeta catalysts at identical reaction conditions
(350 °C and atmospheric pressure), to explore the reaction
pathway and to identify the role of acid site and metal site in the
bifunctional Pt/HBeta catalyst. Such optimized reaction
conditions were applied in order to avoid non-catalytic thermal
decomposition of guaiacol at higher temperatures, or complete
hydrogenation of the phenyl ring at lower temperatures.[39,40] To
investigate the reaction pathway, the conversion of possible
intermediates of catechol and cyclopentanone was also studied.
Compared to anisole (–OCH3 group only) and m-cresol (–OH
group only),[3,41,42] very different reaction chemistry was found for
guaiacol (both –OH and –OCH3 groups) over monofunctional
acidic zeolite or metallic Pt/SiO2. Besides transalkylation
100
(A)
80
60
40
20
0
reactions
observed
for
anisole
and
m-cresol,[3,41,42]
dehydroxylation of one –OH is important for guaiacol and
catechol feeds over bare HBeta. While no decarbonylation
reaction was observed for feeds of anisole and cresol on
Pt/SiO2,[3,41,42] decarbonylation reaction was a major reaction for
guaiacol. The close proximity between metal site and acid site
in bifunctional Pt/HBeta inhibits the decarbonylation reaction on
bare Pt while dramatically improves deoxygenation (or
dehydroxylation) on bare Pt site or bare acid site, resulting in
aromatics as the major final products. Aromatics of benzene,
toluene, xylenes and C9+ with yield higher than 85% were
achieved at low space time on bifunctional Pt/HBeta catalyst.
The alkyl-aromatics have high octane numbers, which can be
used as blends for gasoline.
0
1
2
3
4
W/F (h)
(B)
Ph
Cr
Xol
Cat
M-Gua
M-Cat
DM-Cat
40
30
20
10
0
Results and Discussion
Catalyst Characterizations
0
20
40
60
80
100
The detailed characterizations of the catalysts have been
reported in previous works.[3, 39] Loading of 1% Pt has little effect
on the structure, acidity and surface area of the supports. The
dispersions of Pt (i.e., the percentage of surface Pt atoms of the
total Pt atoms loaded) on SiO2 and HBeta are 20% and 71%,
corresponding to average Pt size of 5.5 and 1.6 nm,
respectively.[3] The TEM observation revealed that a large
fraction of Pt particles is located inside of the micropores of
Guaiacol conversion (%)
Figure 1. Guaiacol conversion over HBeta: (A) Effect of W/F on guaiacol
conversion; (B) major products distribution as function of guaiacol
conversion. Reaction conditions: T = 350 ºC, P = 1 atm, H2/guaiacol = 50,
a
TOS = 0.5 h for each W/F.
2
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When guaiacol conversion is close to zero, only catechol (Cat)
and methylguaiacols (M-Gua, 4 isomers) show notable yield,
accompanied with negligible product of veratrole (1,2-
dimethoxybenzene, yield < 1%). The yield of catechol is equal
to the sum of those of methylguaiacols and veratrole, indicating
that transalkylation reactions between guaiacol molecules are
the primary reaction (C-alkylation to methylguaiacols and minor
O-alkylation to veratrole) (Scheme 1). The observed significantly
lower O-alkylation than C-alkylation may be a result of the
relatively larger size of veratrole than that of methylguaiacol,
leading to low diffusion rate of veratrole inside the pore channel
of HBeta (0.67 nm). In addition, the –CH3 in –OCH3 is easily
activated in transalkylation reactions.[38,39] Therefore, once
veratrole is formed inside the pores, it quickly converts by
denoting –CH3.
the corresponding order of dehydroxylated products (Cr > Ph >
Xol).
Such difference implies that besides the direct
dehydroxylation, the transalkylation takes place simultaneously
between the reactant (Gua), intermediate products (M-Gua, M-
Cat, and Cat), and the dehydroxylated products (Ph and Cr). It
is as expected that the activated –CH3 on the surface
preferentially alkylates the relatively smaller molecules due to
shape selectivity of the zeolite. For example, phenol would be
easier to be alkylated than catechol or methlycatechol, leading
to different distributions in phenols (Ph, Cr, Xol) and catechols
(Cat, M-Cat, DM-Cat).
The dehydroxylation may evolve protonation of the O atom of a
hydroxyl of catechols over BAS, breaking the C-O bond, forming
water and hydroxyphenyl ions on the surface. Hydrogenation of
the hydroxyphenyl ions (to Ph, Cr, Xol) can take place by
hydride transfer reactions, where coke precursors (phenolic
oligomers) formed in the channel of zeolite may act as the
source of hydride donor.
OH
OH
OH
OH
OH
Small amounts (<1%) of aliphatic hydrocarbons and aromatics
(xylenes, toluene, benzene, and C9+) are produced when the
conversion is approaching 100%. Further increase in W/F (> 2
h) when guaiacol conversion is 100% leads to a slight increase
of aromatics’ yield to < 5%. The formation of aromatics may be
resulted from further dehydroxylation of phenols (Ph, Cr, Xol). In
comparison with previous studies of cresol and anisole
conversion on acid zeolite,[3,41,42] it is suggested that the
dehydroxylation of catechols (Cat, M-Cat, DM-Cat) to phenols
(Ph, Cr, Xol) has much lower activation barrier than further
dehydroxylation of phenols to aromatics.
(Cat)
(Ph)
CH
O
(Cat)
3
OH
OH
OH
OH
Dehydroxylation
Transalkylation
OH
Transalkylation
Transalkylation
CH
O
(Cr)
(M-Cat)
3
OH
(Gua)
OH
OH
(M-Gua)
(DM-Cat)
(Xol)
Scheme 1. Major reaction path of guaiacol conversion over HBeta.
At intermediate conversions, the formation of methylcatechols
(M-Cat, 2 isomers) and dimethylcatechols (DM-Cat, 3 isomers)
at the expense of methylguaiacols is evidenced. A number of
transalkylation reactions could account for such product
evolutions: (1) M-Gua and Cat to two M-Cats; (2) Gua and Cat
to another Cat and M-Cat; (3) M-Gua and M-Cat to another M-
Cat and DM-Cat. The yield of dimethylcatechols in guaiacol
conversion is significantly lower than the corresponding xylenols
(dimethylphenols) in anisole conversion.[3,41] This could be a
result of shape selectivity effect of the zeolite HBeta on products
distribution, i.e., the larger size of dimethylcatechols than
dimethylphenols makes the formation of dimethylcatechols less
favorable in the confined zeolite pore channel.[3,41] The transition
state of bimolecular transalkylation intermediate is expected to
have larger size than that of monomolecular isomerization. Here,
the product distributions are similar to bimolecular
transalkylation instead of monomolecular isomerization. Thus,
the observed transalkylation may be attributed to the
dissociation of O–CH3 to leaving –CH3 on the surface of zeolite.
The –CH3 left over the catalyst surface is preferentially reacting
with another molecular through C-alkylation.[41]
Table 1. Products distribution during catechol and cyclopentanone
conversion over HBeta, Pt/SiO2 and Pt/HBeta catalysts. [a]
Catechol
Pt/SiO2
Cyclopentanone
Pt/SiO2
Product
HBeta
Product
Pt/HBeta
Product
Yield
(%)
0.2
Product
Yield
Yield
(%)
0.05
0.1
Yield
(%)
0.1
(%)
15.8
0.2
Benezene
Phenol
C4
C1
C2
C1-3
C4
5.5
Cyclop
entane
Benzen
e
22.9
Catechol
Heavies
87.8
6.5
0.8
C3
C4
C5
0.2
3.4
1.4
C5+
2.9
Cycloh
exene
Cyclop
entano
ne
0.2
Cyclopent
ane
Cyclopent
anol
16.9
1.3
16.4
Phenol
28.2
36.4
Cyclopentan
e
C6+
5.4
3.4
Cyclopent
anone
Others
52.7
1.8
Catech
ol
Benzene
Cyclohexan
e
40.8
0.8
Cyclopentan
ol
Cyclopentan
one
1.0
0.1
Phenol
Catechol
Heavies
32.6
7.0
1.2
[a] Reaction conditions: T = 350 ºC, P = 1 atm, H2/organic feed = 50, TOS =
0.5 h, W/F = 1.75 h for catechol feed on HBeta, W/F = 0.25 h for other
catalysts and feeds.
At conversions >80%, yields of cresols (methylphenols, Cr, 3
isomers), phenol (Ph) and xylenols (Xol, 6 isomers) increase at
the expense of Cat, M-Gua, M-Cat, DM-Cat. At such high
conversion levels, the dehydroxylation reactions catalyzed by
BAS of zeolite take place: (1) Cat to Ph; (2) M-Cat to Cr; (3) DM-
Cat to Xol. However, one should note that the order of yields
before dehydroxylation (Cat > M-Cat > DM-Cat) does not follow
In summary, both transalkylation (at full W/F range) and
dehydroxylation (only at high W/F) reactions are the main
3
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10.1002/cctc.201701413
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chemistry for guaiacol conversion over the zeolite HBeta
(Scheme 1). To confirm whether the zeolite acid site is able to
catalyze the dehydroxylation of phenolics, aqueous catechol
was fed onto the zeolite HBeta at a W/F of 1.75 h. Phenol is the
major product with trace amount of benzene produced (Table 1).
The result clearly demonstrates that catechol undergoes
dehydroxylation under current reaction condition. The
significantly lower yield of benzene than phenol suggests that,
again, catechol is easier to be dehydroxylated to phenol. But
sequentially phenol is difficult to be dehydroxylated to benzene.
The yield of unidentified heavy products (oligomers) shows
comparable yield as that of phenol, suggesting that
condensation reaction occurs. These heavy products could act
as the coke precursors and hydride donors. The former
deactivates the catalyst quickly, while the latter hydrogenates
the phenyl or hydroxyphenyl species formed from phenolics’
dehydroxylation.
100
80
60
40
20
0
(A)
0
1
2
3
4
W/F (h)
Ben
CPN
Ph
C1
C4
C5
C6
(B)
Guaiacol conversion over Pt/SiO2
50
40
30
20
10
0
Compared to guaiacol conversion on HBeta, the conversion
curve is much smoother on Pt/SiO2 (Figure 2A). Guaiacol was
100% converted at a W/F of 0.5 h, however, 100% oxygen
removal was not achieved even at a W/F of 4 h.
Cat
The major products evolution as a function of W/F is shown in
Figure 2B. To demonstrate the primary reactions, the major
products yield is drawn as a function of guaiacol conversion for
its conversion less than 100% (W/F < 0.5 h), as shown in Figure
2C. When the conversion is close to zero (Figure 2C), equal
amounts of catechol (Cat) and phenol are produced,
accompanied with evolution of methane (C1). This might imply
that demethylation to catechol and demethoxylation to phenol
are the parallel primary reactions over Pt/SiO2 (Scheme 2).
However, notable amount of methanol was not detected,
suggesting that direct demethoxylation can be neglected.
Furthermore, ~15% methane is produced when guaiacol
conversion is 100% (Figure 2B), which is in good agreement
with the value estimated from demethylation of guaiacol,
suggesting that the formation of methane is mainly from
demethylation (hydrogenolysis of O-CH3) of guaiacol. These
results indicate that direct removal of methoxyl from guaiacol to
phenol and methanol is a minor path. Therefore, demethylation
of guaiacol to catechol and subsequent fast deoxygenation to
0
1
2
3
4
W/F (h)
50
40
30
20
10
0
(C)
C1
C4
CPN
Ph
Cat
phenol are the primary reactions (Scheme 2).
It should be
noted that the yield of anisole (methoxybenzene) is less than 2%
under all tested conditions, suggesting that direct hydroxyl
removal from guaiacol is a minor path. Hence, direct oxygen
removal from either hydroxyl or methoxyl of guaiacol is not
favored over Pt/SiO2, as compared to hydrogenolysis of O-CH3
bond (demethylation) of guaiacol to catechol and methane.
Because of the delocalization effect between the lone pair
0
20
40
60
80
100
Guaiacol conversion (%)
electron of O and the π bond of phenyl ring, the bond of Caromatic
-
O is much stronger than that of O-CH3 bond. Thus, the initial
step of guaiacol conversion is demethylation on Pt. This result
is consistent with recent DFT calculations, showing that the
removal of –CH3 is the first step of guaiacol conversion over
metal surface, such as Pt and Ru.[43-45]
Figure 2. Guaiacol conversion over Pt/SiO2: (A) Effect of W/F on guaiacol
conversion; (B) major products distribution as a W/F; (C) Major products
distribution as a function of guaiacol conversion for guaiacol conversion <
100%. Reaction conditions: T = 350 ºC, P = 1 atm, H2/guaiacol = 50, TOS =
0.5 h for each W/F.
4
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O
CH OH
2
OH
3
consecutive decarbonylation of catechol to cyclopentanone and
butane may contain multiple steps, which is not clear so far. In
an earlier work, Sakai and Hattori reported that pyrolysis of
catechol at 500-600 °C in He or H2 atmosphere primarily results
in the formation of C4 (mainly butadiene) and CO through
Deoxygenation
Demethoxylation
(Ben)
(Ph)
H
2
O
CH3
O
OH
Deoxygenation
decarbonylation reactions.[46]
Later, Arends et al. studied
(Gua)
decomposition of anisole in hydrogen at 520-750 °C. They found
that anisole and phenol could be decarbonylated to
cyclopentadiene, and suggested several radical reactions.[47]
These observations suggest that decarbonylation of catechol is
thermodynamically and kinetically possible over a catalyst.
Recently, Nimmanwudipong et al. found that cyclopentanone is
an important product in guaiacol conversion on a Pt/MgO
catalyst at 300 °C.[36,37] They suggested that the reaction
involves sequential reactions of ring hydrogenation, ring opening
and closing, decarbonylation. And the basic support plays an
important role on the formation of cyclopentanone. Since
cyclopentanone and butane are also observed as important
intermediate and final products in the conversion of guaiacol on
inert silica supported Pt catalyst in present work, it thus can be
concluded that the decarbonylation reaction is mainly related
with metallic Pt itself instead of basic or inert support.
OH
O
Demethylation
OH
Decarbonylation
Decarbonylation
CH
4
CO
CO
(Cat)
(C4)
(CPN)
Scheme 2. Major reaction path of hydrodeoxygenation of guaiacol over
Pt/SiO2.
The yield of phenol continues increasing as guaiacol
conversion, while catechol yield starts to decrease at
intermediate conversion, accompanied by the appearances of
cyclopentanone (CPN) followed by C4 (Figure 2C). When the
yield of catechol drops to zero at W/F of 0.5 h, the yields of
cyclopentanone and phenol reach their maximum (Figure 2B).
Further increase of W/F to ~2 h leads to the yield of
cyclopentanone decreases to zero, while C4 increases to ~26%
(Figure 2B). On the other hand, benzene starts increasing at
W/F of 0.5 h at the expense of phenol. At the highest W/F
tested (4 h), phenol was not fully deoxygenated to benzene.
The yield of cyclohexanone + cyclohexanol is less than 1%,
implying full hydrogenation of the aromatic ring appears to be a
minor path. C5 and C6 paraffins are produced at higher W/F
(Figure 2B), probably from hydrogenation and cracking of
benzene and cyclopentanone. Such product evolution indicates
two parallel reaction paths (Scheme 2): (1) catechol direct
deoxygenation to phenol and then to benzene in sequence, and
(2) catechol decarbonylation to cyclopentanone and further to C4
(releasing CO in each step) in sequence. The yield ratio of
(Ben+Ph)/C4 at W/F of 4 h is 9/5, which implies the ratio
between the apparent direct deoxygenation path and the
decarbonylation path. This ratio depends on the initial selectivity
of phenol and cyclopentanone during catechol conversion.
To confirm the proposed reaction path, the intermediate
products, catechol and cyclopentanone, were fed over Pt/SiO2
catalyst under identical conditions. Catechol was mainly
converted to phenol, cyclopentanone and C4 (Table 1),
indicating that two main reaction paths are in competition: direct
deoxygenation of catechol to phenol and decarbonylation of
catechol to cyclopentanone and further to C4 (Scheme 2). This
also confirms the initial fast conversion of the produced catechol
to phenol during guaiacol conversion. Note that the yield of
benzene is trace (Table 1), suggesting that deoxygenation of
phenol to benzene is less reactive than decarbonylation of
cyclopentanone to C4 under current reaction conditions.
.
OH
O
O
.
O
.
.
- CO
.
butane
OH
O
O
.
isomerization
- 2H
+ 6H
- CO
(I)
(II)
catechol
1,2-benzoquinone
catecholate
- H, - CO,
Ring closure,
+ 4H
Ring opening, + H
O
+ H, - OH, + H
cyclopentanone
cyclopentane
Scheme 3. Possible reaction mechanism of hydrodeoxygenation of catechol
over Pt/SiO2.
Scheme 3 shows possible surface steps for decarbonylation of
catechol on Pt surface. Catechol is adsorbed flatly on Pt surface
through its phenyl π bond. Similar to phenol dissociation,[48] the
dissociation of catechol leads to catecholate formation on the
surface,[43-45] which could be further isomerized to 1,2-
benzoquinone [49] with carbonyl groups away from the surface.
Cleavage of two C-C bonds leads to CO and surface species I.
Further cleavage of C-C bond results in surface butadiene
(species II), which is quickly hydrogenated to butane.
Alternatively, the species I could go through ring closure and
hydrogenation to cyclopentanone. Once cyclopentanone is
formed and desorbed from the surface, the decarbonylation
requires multiple steps including: dehydrogenation and
adsorption through the cyclopenta-carbon ring on the surface,
breaking the two C-C bonds releasing CO, and finally
hydrogenation to butane. Such reaction pathway was confirmed
by using cyclopentanone as the reaction feed. This process
must be competed by the adsorption of cyclopentanone on the
surface through its carbonyl group, followed by hydrogenation of
carbonyl group, hydrogenolysis of the C-OH bond, and forming
When cyclopentanone was fed onto Pt/SiO2, C4 and
cyclopentane are the major products (Table 1), implying
decarbonylation to C4 and hydrogenation-hydrogenolysis to
cyclopentane are the parallel major reaction paths. The
conversion of intermediates on Pt/SiO2 confirmed the proposed
major pathway of guaiacol conversion (Scheme 2). The
cyclopentane as the final product.
It is evident that
5
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10.1002/cctc.201701413
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decarbonylation of species I is easier than that for adsorbed
cyclopentanone, since the former includes cleavage of one C-C
bond while the latter requires two. This difference may result in
the different product distribution when catechol and
cyclopentanone were used as reaction feed, respectively.
Note that a recent report suggested that the decarbonylation of
catechol starts with hydrogenation of the phenyl ring over
Pt/C.[38] However, in the current work, hydrogenation is
thermodynamically constrained over group VIII metals when
temperature is > 350 °C at atmospheric pressure.[39,40] And
dehydrogenation from carbon instead of hydrogenation normally
favors C-C hydrogenolysis. Indeed, the proposed surface
reaction is consistent with high vacuum temperature
programmed desorption studies of phenol and anisole on
Pt(111) surface, which showed that surface phenoxy could be
decomposed to CO and hydrocarbon species (CxHy) in the
absence of hydrogen.[50,51]
100
80
60
40
20
0
(A)
0.00
0.00
0.00
0.25
0.50
0.75
0.75
0.75
1.00
W/F (h)
(B)
18
16
14
12
10
8
Guaiacol conversion over Pt/HBeta
Ph
Cr
Xol
In contrast to guaiacol conversion on HBeta,
a smooth
conversion curve as a function of W/F is achieved over Pt/HBeta
catalyst (Figure 3A), with 100% conversion at a W/F of 0.5 h and
complete deoxygenation at a W/F of 0.75 h. It is obvious that
Pt/HBeta is much more active for deoxygenation than either
Pt/SiO2 or HBeta, since significantly lower W/F is required for
complete deoxygenation.
Cat
M-Cat
M-Gua
6
At low W/F and conversions, the major oxygenated products
observed on Pt/HBeta are similar to those on HBeta, including:
catechol (Cat), methylguaiacol (M-Gua) isomers, methylcatechol
(M-Cat) isomers, dimethylcatechol (DM-Cat) isomers, phenol
(Ph), cresol (Cr) isomers, xylenol (Xol) isomers (Figure 3B). The
yields of primary products (Cat, M-Gua, M-Cat) from
transalkylation of guaiacol firstly increase to the maximum and
then decrease to zero. At intermediate W/F, the yields of
secondary products (Ph, Cr and Xol), produced from removal of
a hydroxyl from the primary products (Cat, M-Cat, DM-Cat),
increase to the maximum and then decrease to zero. Similar to
over HBeta, the transalkylation plays an important role on the
distribution of secondary products. For example, the yield of
DM-Cat is low, while the yield of Xol (formed from DM-Cat by
removing one hydroxyl) is high, which indicates that Xol is
generated from transalkylation between –CH3 donating
compounds (Gua, M-Gua, veratrole) and Cr.
4
2
0
0.25
0.50
1.00
W/F (h)
(C)
AHC
40
30
20
10
0
Ben
Tol
Xyl
C9+
0.25
0.50
1.00
W/F (h)
Figure 3. Guaiacol conversion over Pt/HBeta: (A) Effect of W/F on guaiacol
conversion; (B) major oxygenated products distribution as a function W/F; (C)
Major hydrocarbon products distribution as
a function of W/F. Reaction
conditions: T = 350 ºC, P = 1 atm, H2/guaiacol = 50, TOS = 0.5 h.
6
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10.1002/cctc.201701413
ChemCatChem
FULL PAPER
evolution indicates the major reactions evolved on Pt/HBeta
catalyst are transalkylation over acid sites and apparent direct
deoxygenation over Pt sites, as shown in Scheme 4.
AHC
Catechol, the major intermediate product in guaiacol
conversion, was fed onto Pt/HBeta at a W/F of 0.25 h to verify
the proposed major reaction path. It is seen that the major
products are phenol and benzene (Table 1), suggesting removal
of hydroxyls from phenyl ring by direct deoxygenation to phenol
and subsequent to benzene is the major path over Pt/HBeta.
Note that the yield of benzene is higher than phenol. Moreover,
the benzene yield is also significantly higher than that on Pt/SiO2
under the same conditions, indicating that Pt/HBeta is much
more effective in removing both hydroxyls from catechol. In
contrast to Pt/SiO2, only small amounts of C4 and cyclopentane
were produced on Pt/HBeta (Table 1), which indicates that
decarbonylation of catechol is a minor path. In other words,
decarbonylation reaction on bare Pt was dramatically inhibited in
bifunctional Pt/HBeta. On the other hand, compared to HBeta,
the heavy products on Pt/HBeta were reduced significantly,
suggesting that Pt helps hydrogenating surface carbon species
and retards condensation reactions.
Ben
Tol
Xyl
30
C9+
20
10
0
0
20
40
60
80
100
Guaiacol conversion (%)
Figure 4. Effect of guaiacol conversion (< 100%) on aromatics distribution on
Pt/HBeta. Reaction conditions: T = 350 ºC, P = 1 atm, H2/guaiacol = 50, TOS
= 0.5 h.
The major fully deoxygenated products (Ben, Tol, Xyl) of
guaiacol conversion on Pt/HBeta are different from those (Ben,
C4 and C1) on Pt/SiO2. Apparently, being different from mainly
demethylation and decarbonylation on Pt/SiO2, the presence of
BAS and the close proximity between BAS and Pt on Pt/HBeta
led the reaction path to transalkylation and direct deoxygenation.
For Pt/SiO2, the Pt has an average particle size of ~5.5 nm,[3]
which contains mainly Pt(111) surface. Guaiacol is preferably
adsorbed on the Pt(111) as a flat configuration, i.e., phenyl ring
adsorption on a bridge site with –OH and –OCH3 tilted away
from the surface.[43,45] The reaction initiates with the
hydrogenolysis of the weakest bond O-CH3, resulting in the
formation of CH4 and catechol. Direct deoxygenation and
decarbonylation may occur competitively for further conversion
of surface catechol. Similar to dissociation of phenol on Pt,[48] the
dissociation of catechol to catecholate (with O atoms away from
the surface) and H atoms on the surface is readily to take place
due to the low activation barrier.[43,45,48] The conversion of
catecholate may facilitate the decarbonylation to cyclopentanone
and further to C4. In contrast, the Pt has a significantly smaller
average particle size of ~1.6 nm for Pt/HBeta, and a notable
fraction of Pt particles are located inside the micropores with
close contact with BAS. In addition, the high BAS/Pt ratio of 17
suggests BASs are in large excess. A significant fraction of
guaiacol is expected to adsorb on BAS through protonation of
the O atoms and dissociate of the –CH3 onto the surface, which
is readily to react with a phenolic molecule nearby. The
presence of Pt helps the hydrogenolysis of O–CH3 of guaiacol
and therefore accelerates the overall transalkylation reaction
rate, as evidenced by a significantly lower W/F required for
complete conversion of guaiacol on Pt/HBeta (0.5 h) than on
bare HBeta zeolite (2 h). Besides deoxygenation on Pt,
dehydroxylation of the first –OH from primary products (Cat, M-
Cat, DM-Cat) by proton of BAS may also take place at relatively
slower rates. In this case, the Pt plays a role on dissociation of
H2. And the activated H atoms diffuse to and regenerate the
OH
OH
OH
OH
OH
(Cat)
(Ph)
(Ben)
3
CH
O
(Cat)
3
CH
OH
OH
OH
OH
Deoxygenation
Transalkylation
Pt/Acid site
OH
Deoxygenation
Pt/Acid site
Transalkylation
Acid site/Pt
Transalkylation
Acid site/Pt
CH
O
(Cr)
(Tol)
(M-Cat)
3
OH
(Gua)
OH
CH
3
OH
(M-Gua)
(DM-Cat)
(Xol)
H C
3
(Xyl)
Scheme 4. Major reaction path of hydrodeoxygenation of guaiacol over
Pt/HBeta.
The fully deoxygenated products are benzene (Ben), toluene
(Tol), xylenes (Xyl), multiple methylated aromatics (C9+), as well
as aliphatic hydrocarbons (AHC, sum of C1 to C6). As shown in
Figure 3C, at low W/F, only the yield of benzene is noticeable
among aromatics. However, its yield is significantly lower than
the yields of the primary (Cat, M-Gua, M-Cat) and secondary
(Ph, Cr, Xol) products. The yields of aromatics increase slowly
with W/F before the secondary products (Ph, Cr, Xol) reach the
maximum and increase significantly after the yields of secondary
products reaching the maximum. It indicates complete
deoxygenation of secondary products to aromatics is a tertiary
reaction. Such conclusion is further verified by plotting the
yields of aromatics as a function of guaiacol conversion (Figure
4). It is clear that the yields of aromatics only increase
significantly when the guaiacol conversion is higher than 90%, at
which the primary and secondary reactions are mostly converted
to tertiary products. The complete deoxygenation is achieved at
W/F of 0.75 h, with Ben, Tol, Xyl, and C9+ yields of 42%, 29%,
12% and 5%, respectively. The AHC may be produced from
demethylation, hydrogenation, decarbonylation, ring opening,
isomerization and cracking reactions. And the C9+ aromatics
may be formed from hydrodeoxygenation of multiple-methylated
phenols and/or alkylation of C7/C8 aromatics. Such products
7
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10.1002/cctc.201701413
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FULL PAPER
BAS. On the other hand, these primary transalkylation products
(Cat, M-Cat, DM-Cat) and secondary products (Ph, Cr, Xol)
diffuse to Pt-BAS neighboring sites and adsorb on Pt through
suggesting that the BASs in micropores have been almost
inaccessible by the reactant and only those on the external
surface were available for the reaction. This could be a result of
phenyl ring and on adjacent BAS by protonation of O of hydroxyl. pore blockage due to the strong adsorption of guaiacol and its
With this configuration, one may expect the deoxygenation
products. It has been suggested that guaiacol adsorbs on BAS
through protonation of the O, and may form bidentate species on
to two adjacent acid sites.[34] Pt/SiO2 and Pt/HBeta deactivated
with a similar rate, which is noticeable slower than that on bare
HBeta. This indicates that addition of Pt to HBeta increased the
stability of the catalyst, possibly due to improved deoxygenation
activity (so that lower the concentration of phenolics in the
stream) as well as hydrogenation of coke precursors that
strongly adsorbed on BAS.
occurs in a bifunctional way: Pt hydrogenates the phenyl ring
[52]
partially to reduce the deoxygenation barrier
and concerted
dehydration readily happens on vicinal BAS,[3] forming aromatics
directly or through further dehydrogenation. Since few large Pt
particles exists in Pt/HBeta catalyst, the probability for catechol
adsorption only on Pt(111) surface is low. Therefore, the
decarbonylation to cyclopentanone and further to C4 takes place
to a minor extent. As a result, the decarbonylation reaction path
was significantly reduced and not observable on Pt/HBeta.
Due to different reactivities of the two hydroxyl groups of
catechol toward direct deoxygenation, and different reaction
pathways (direct deoxygenation and decarbonylation) are
involved during hydrodeoxygenation of guaiacol, it is not straight
forward to direct compare the turnover frequency (TOF) of
hydrodeoxygenation over Pt/SiO2 and Pt/HBeta. Considering
the facts the W/F for complete deoxygenation of guaiacol (~6 h
and 0.75 h for Pt/SiO2 and Pt/HBeta, respectively) and the
dispersion of Pt in Pt/SiO2 and Pt/HBeta, the roughly estimated
TOF of hydrodeoxygenation of guaiacol is about 3 times higher
for Pt/HBeta than for Pt/SiO2. This improved deoxygenation
activity could be ascribed to concerted participation of Pt and
BAS in the reactions.
(A) CO2
(B) H2O
485
564
534
473
553
645
447
214
214
HBeta
HBeta
553
Pt/HBeta
Pt/SiO2
Pt/HBeta
Pt/SiO2
0
200
400
600
800
0
200
400
600
800
Temperature (oC)
Temperature (oC)
HBeta (W/F=1.75 h)
Pt/SiO (W/F=0.25 h)
2
80
70
60
50
40
30
20
10
0
Figure 6. Temperature programmed oxidation profiles of (A) CO2 and (B) H2O
evolution of spent HBeta, Pt/SiO2 and Pt/HBeta catalysts. The spent catalyst
samples were taken after stability test with reaction conditions shown in Figure
5.
Pt/HBeta (W/F=0.25 h)
The coke deposition on the spent catalysts was evaluated by
temperature programmed oxidation. Note that both CO2 and CO
were produced on bare HBeta, while only CO2 was produced on
Pt/SiO2 and Pt/HBeta. CO signal was calibrated and lumped with
CO2 signal for better comparison. As shown in Figure 6A, an
intense CO2 evolution peak was centered at 564 °C for bare
HBeta, corresponding to 14.6 wt.% carbon. Its H2O evolution
profile (Figure 6B) showed a major peak at 485 °C and a
shoulder peak at 645 °C. Comparison of CO2 and H2O profiles
may indicate that two types of carbon species were deposited on
the catalyst: reactant and products adsorbed strongly on the
BAS with richer H for peaks at lower temperatures, while
graphite carbon deeply inside of micropores with leaner H for
peaks at higher temperatures. Only a small CO2 evolution peak
(2.6 wt.% carbon) was observed for Pt/SiO2, accompanied with
H2O evolution at the same temperature. This suggests that only
graphite type carbon was deposited around Pt, with very few
reactant and products adsorbed on the inert SiO2 support. For
Pt/HBeta, the CO2 profile was split into three peaks at 214, 473,
0
1
2
3
4
5
Time on stream (h)
Figure 5. Stability of HBeta, Pt/SiO2 and Pt/HBeta catalyst during
hydrodeoxygenation of guaiacol. Reaction conditions: T = 350 ºC, P = 1 atm,
H2/guaiacol = 50, W/F is 1.75, 0.25 and 0.25 h for HBeta, Pt/SiO2, and
Pt/HBeta catalysts, respectively, to achieve a similar initial conversion.
Stability test and coke analysis
Figure 5 compares the stability of different catalysts as a
function of time-on-stream. The W/F for each catalyst was
adjusted to achieve a similar initial conversion of guaiacol. The
deactivation of HBeta catalyst was fast in the first hour, and
maintained a stable and low conversion after 3 h reaction,
8
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10.1002/cctc.201701413
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FULL PAPER
and 534 °C, with corresponding H2O peaks at 214, 447 and
553 °C, respectively. The former two peaks may be assigned to
combustion of reactant and products adsorbed on the external
surface and in the micropores, while the latter one may be
related to graphite carbon inside of the pores. Importantly, the
total amount of carbon decreased significantly to 9.1 wt.%, with
respect to bare HBeta. This indicates that Pt plays an important
role on cleaning the catalyst surface by hydrogenating the coke
precursors.
hydrogenolysis of guaiacol to catechol, which could be either
directly deoxygenated in sequence to phenol and benzene or
decarbonylated sequentially to cyclopentanone and butane. The
combination of Pt site and Brønsted acid site with close
proximity in Pt/HBeta enables both transalkylation and
deoxygenation reactions with significantly improved activity
toward direct deoxygenation (or dehydroxylation). Moreover, the
demethylation and decarbonylation reactions observed on bare
Pt/SiO2 were dramatically inhibited on Pt/HBeta, due to the
adsorption of catechols on Pt/HBeta through both phenyl ring on
Pt and oxygen on Brønsted acid site.
Such adsorption
configuration facilitates the apparent direct deoxygenation.
Being different from the carbon loss due to demethylation and
decarbonylation over Pt/SiO2, bifunctional Pt/HBeta retains most
carbon in liquid products. The major final products are benzene,
toluene, xylenes and C9+ aromatics with yields >85%. Addition of
Pt to HBeta significantly improved the stability of catalyst
through hydrogenating coke precursors. The presence of water
in the feed improves the activity of Pt/HBeta by enhancing
hydrolysis of O-CH3 bond of guaiacol and moderately improves
its stability in hydrodeoxygenation of guaiacol.
(C) Pt/HBeta
90
80
70
60
50
40
30
20
10
0
(A) HBeta
(B) Pt/SiO2
No water
Water
No water
Water
No water
Water
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
Time on stream (h)
Time on stream (h)
Time on stream (h)
Figure 7. Effect of water addition on the conversion of guaiacol over (A) HBeta,
(B) Pt/SiO2 and (C) Pt/HBeta catalysts. Reaction conditions: T = 350 ºC, P = 1
atm, H2/guaiacol = 50, W/F is 1.75, 0.25 and 0.25 h for HBeta, Pt/SiO2, and
Pt/HBeta catalysts, respectively, to achieve a similar initial conversion. The
mass ratio of guaiacol/H2O is 4/1.
Experimental Section
Catalyst preparation and characterization
The catalysts used in this study were HBeta zeolite (Si/Al=19, Zeolyst,
SBET=710 m2/g), 1 wt.% Pt/SiO2, and 1 wt.% Pt/HBeta. The supported Pt
catalysts were prepared using incipient wetness impregnation of HBeta
or SiO2 (HiSil 210, SBET=135 m2/g) with an aqueous solution of
H2PtCl4·6H2O (Aldrich) overnight, followed by drying at 110 °C for 12 h
and calcination at 400 °C for 4 h. The samples were characterized by
scanning electron microscopy (SEM), transmission electron microscopy
(TEM), X-ray diffraction (XRD), CO chemisorption, temperature
programmed desorption of NH3 (NH3-TPD) and isopropylamine (IPA-
TPD), which have been reported in previous work.[3,39] The spent
catalyst samples were characterized by temperature programmed
oxidation.[3]
Water effect
The effect of water on the activity and stability of the catalysts
during hydrodeoxygenation of guaiacol was reported in Figure 7.
Addition of water to the feed improved the reactivity of guaiacol
on HBeta (Figure 7A), which can be attributed to that water
helps hydrolysis of the ether bond of O-CH3 on BAS and,
therefore, improves the overall reaction rate of transalkylation.[38]
H2O addition seems to have little effect on the stability of HBeta.
In contrast, the addition of H2O has little effect on the activity of
Pt/SiO2 but improves its stability remarkably (Figure 7B). This
suggests that Pt does not catalyze the hydrolysis of O-CH3 bond,
while H2O plays a role on cleaning the surface of Pt during
hydrodeoxygenation. Interestingly, addition of H2O improves
both the activity of Pt/HBeta and its stability moderately (Figure
7C). Obviously, this improvement is due to a combination of H2O
enhancing the BAS catalyzed hydrolysis of O-CH3 bond of
guaiacol and cleaning the Pt surface.
Catalytic performance
Catalytic conversion of guaiacol was investigated in a fixed-bed quartz
tube (6 mm o.d.) reactor at atmospheric pressure. The catalyst sample
(40-60 mesh) was packed in the reactor between two layers of quartz
wool, and was reduced in H2 at 300 °C for 1 h prior to reaction. The
reaction was carried out at 350 ºC to avoid non-catalytic thermal
reactions at higher temperatures and saturation of phenyl ring at lower
temperatures. H2 flow was controlled by mass flow controllers (Porter).
Guaiacol was fed using
a syringe pump (Kd scientific), and was
Conclusions
vaporized before entering the reactor. Ultra-high pure hydrogen was
used as the reactant and carrier gas. The H2/guaiacol molar ratio was
kept at 50 in all runs. All lines were heated to avoid any condensation.
The products were quantified online by a gas chromatograph (GC 6890,
Agilent), equipped with a DB-5 capillary column (30 m) and a flame
ionized detector (FID). The effluent was trapped in methanol using an
ice-water bath, and qualified by an offline GC-MS (Shimadzu QP2010s).
Standard samples were injected to GC for isomer identification.
Through comparative study of hydrodeoxygenation of guaiacol
and the intermediate products (catechol and cyclopentanone)
over HBeta, Pt/SiO2 and Pt/HBeta catalysts at 350 °C under
atmospheric hydrogen pressure, the roles of Brønsted acid sites
and Pt sites were identified. The Brønsted acid sites of HBeta
catalyze transalkylation and dehydroxylation reactions, resulting
in phenol, cresols, and xylenols as the major final products for
hydrodeoxygenation of guaiacol. The Pt sites catalyze
9
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10.1002/cctc.201701413
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FULL PAPER
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10.1002/cctc.201701413
ChemCatChem
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Role of acid and metal site: The
Lei Nie, Bo Peng, Xinli Zhu*
HBeta
Tansal kyl ati on
Dehydr oxyl ati on
OH
OH
OH
close proximity of Pt and acid site in
bifunctional Pt/HBeta improves
transalkylation and deoxygenation (or
dehydroxylation) reactions while
greatly inhibits demethylation and
decarbonylation reactions.
Page No. – Page No.
De met hyl ati on
Decar bonyl ati on
Deoxygenati on
Pt/SiO2
CH3
O
OH
O
CH
4
Vapor-Phase Hydrodeoxygenation of
Guaiacol to Aromatics over Pt/HBeta:
Identification of the Role of Acid Site
and Metal Site on the Reaction
Pathway
Pt/HBeta
Tansal kyl ati on
Deoxygenati on
CH3
CH3
H3
C
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Author(s), Corresponding Author(s)*
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Title
Text for Table of Contents
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