Role of transalkylation reactions in the conversion of anisole over HZSM-5

Article abstract of DOI:10.1016/j.apcata.2010.03.018

Conversion of anisole, a typical component of bio-oil, was studied over an HZSM-5 zeolite at varying space times (W/F), reaction temperatures, type of carrier gas, and concentration of water in the feed. Several bimolecular and unimolecular reactions are proposed to explain the evolution of products observed. The bimolecular reactions include the following transalkylation reactions: (a) anisoles to phenol and methylanisole; (b) phenol and methylanisole to cresols; (c) phenol and anisole to cresol and phenol; (d) methylanisole and cresol to phenol and xylenol. A pseudo first-order kinetic model based on these bimolecular reactions was found to describe well the observed product distribution as a function of W/F. It is observed that shape selectivity effects prevail over electrophilic substitution and thermodynamic equilibrium effects in the formation of methylanisole isomers. However, the opposite is true for the distribution of cresol isomers. The kinetic analysis indicates that the contribution of unimolecular reactions such as isomerization is much lower than that of bimolecular reactions. The carrier gas composition was found to have a moderate effect on catalyst activity. When H₂ was used as a carrier, catalyst stability showed a moderate improvement in comparison to the runs under He. However, a remarkable increase in catalytic activity was observed upon the addition of water in the feed.

Full text of DOI:10.1016/j.apcata.2010.03.018

Applied Catalysis A: General 379 (2010) 172–181
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Role of transalkylation reactions in the conversion of
anisole over HZSM-5
∗
Xinli Zhu, Richard G. Mallinson, Daniel E. Resasco
Center for Biomass Refining, School of Chemical, Biological, and Materials Engineering, The University of Oklahoma, 100 East Boyd St., Norman, OK 73019, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Conversion of anisole, a typical component of bio-oil, was studied over an HZSM-5 zeolite at varying
space times (W/F), reaction temperatures, type of carrier gas, and concentration of water in the feed. Sev-
eral bimolecular and unimolecular reactions are proposed to explain the evolution of products observed.
The bimolecular reactions include the following transalkylation reactions: (a) anisoles to phenol and
methylanisole; (b) phenol and methylanisole to cresols; (c) phenol and anisole to cresol and phenol;
Received 28 January 2010
Received in revised form 6 March 2010
Accepted 9 March 2010
Available online 15 March 2010
(d) methylanisole and cresol to phenol and xylenol. A pseudo first-order kinetic model based on these
Keywords:
bimolecular reactions was found to describe well the observed product distribution as a function of W/F.
It is observed that shape selectivity effects prevail over electrophilic substitution and thermodynamic
equilibrium effects in the formation of methylanisole isomers. However, the opposite is true for the dis-
tribution of cresol isomers. The kinetic analysis indicates that the contribution of unimolecular reactions
such as isomerization is much lower than that of bimolecular reactions. The carrier gas composition was
found to have a moderate effect on catalyst activity. When H2 was used as a carrier, catalyst stability
showed a moderate improvement in comparison to the runs under He. However, a remarkable increase
in catalytic activity was observed upon the addition of water in the feed.
Anisole
Phenolic compounds
HZSM-5
Methoxy group
Biomass conversion
Bio-oil upgrading
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
of lignin fractions in biomass. Depending on the biomass source
and the pyrolysis process conditions these compounds can repre-
sent a significant fraction of the total bio-oil [2]. Among them, the
methoxy phenols (guaiacol, syringol and their derivatives) are par-
ticularlyabundant. Therefore, it is important to identifythe reaction
pathways that methoxy phenols may undergo on different zeolites
Bio-oil derived from lignocellulosic biomass (via fast pyrol-
ysis, high pressure liquefaction, or solvolysis) contains a large
variety of oxygenated compounds including acids, aldehydes,
ketones, furans, phenolics, sugars, and dehydrosugars, along with
large amounts of water [1]. Upgrading this chemically unsta-
ble, highly viscous, corrosive, and low-heating value oxygenated
mixture to liquid hydrocarbon fuels is an important step in the
development of sustainable fuel production [2,3]. Compared to
conventional hydrotreating [4–9], bio-oil upgrading via conver-
sion on acid zeolites appears highly attractive as it does not
require hydrogen, which increases production costs and may
not be readily available in small and distributed plants. Previ-
ous studies have demonstrated the effectiveness of zeolites in
bio-oil upgrading either during or after pyrolysis [10–18]. How-
ever, the highly complex nature of bio-oil requires further studies
that can shed light on the possible reaction pathways that each
kind of compound and oxygen functionality may undergo in the
zeolite.
[
17–21].
In this contribution, we have focused on the reaction pathways
that can occur on acid zeolites when the starting molecule con-
tains the methoxy group (–OCH ). Anisole (or methoxybenzene)
3
is an attractive model molecule to investigate the relative reactiv-
ity of the methoxy group, since this is the only functionality in the
molecule.
At the same time, anisole is an important primary product from
the alkylation of phenol with methanol over acidic catalysts, which
further transforms to cresols and xylenols [22–35]. It is generally
accepted [26,27,29] that the first step in the conversion of anisole
over acid zeolites is the disproportionation to phenol and methy-
lanisole. However, there are some discrepancies on the proposed
subsequent transformations to cresols and xylenols; some authors
have proposed an intramolecular rearrangement path [29,35] while
others support a bimolecular reaction [26,27].
Phenolic compounds present in bio-oil (phenol, catechol, gua-
iacol, syringol and their derivatives) arise from the decomposition
The conversion of anisole over HZSM-5 was systematically stud-
ied with varying space time, temperature, feed composition, carrier
gas, as well as water addition with the purpose of understanding
the reaction pathways and the influence of these variables.
∗ Corresponding author. Tel.: +1 405 325 4370; fax: +1 405 325 5813.
E-mail address: resasco@ou.edu (D.E. Resasco).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.03.018

X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
173
2
. Experimental
firmed the MFI structure. SEM was used to assess the crystallite size
and shape. Crystallite morphology is important because it deter-
mines the diffusion path length of reactants and products inside
the zeolite and thus may influence the extent of shape selectiv-
ity. SEM observations revealed that the particular HZSM-5 sample
used in this study is composed of 1–2 ␮m aggregates of small pri-
mary crystallites that are in the range of 100–200 nm. Elemental
analysis confirmed the Si/Al ratio of 30, as reported by the manufac-
turer. The Brønsted acid density derived from IPA-TPD analysis was
0.523 mmol/g, in good agreement with the theoretical acid density
(0.538 mmol/g) that can be calculated from the Si/Al ratio of 30.
2.1. Catalyst preparation and characterization
NaZSM-5 (supplied by Süd-Chemie, Si/Al = 30) was subjected to
◦
three sequential exchanges with NH NO3 at 80 C. The resultant
NH ZSM-5 was transformed to H-form by calcination at 550 C for
4
4
◦
4
h.
The powder X-ray diffraction (XRD) pattern of the zeolite
was recorded on a Bruker D8 Discover diffractometer, equipped
with a Cu K␣ radiation source (ꢀ = 1.54056 Å). The morphology of
the zeolite was evaluated by high resolution scanning electronic
microscopy (SEM) using a Jeol JSM-880 system, equipped with
an X-ray elemental analyzer. The acid density was investigated
by conventional temperature programmed desorption of adsorbed
isopropylamine (IPA-TPD) as detailed elsewhere [36,37].
The coke deposited during reaction was characterized by tem-
perature programmed oxidation (TPO) of 30 mg samples of spent
catalyst, under a gas flow of 2% O /He (30 mL/min) [37]. The heating
3.2. Reaction pathway
3.2.1. Evolution of products with space time (W/F)
To elucidate reaction pathways, the evolution of products with
◦
space time was followed at 400 C. The variation of anisole (An)
conversion with W/F is shown in Fig. 1A. The major products
and minor product yields are plotted as a function of W/F in
Fig. 1B and C, respectively. Major products include methylanisole
isomers (MA), phenol (Ph), cresol isomers (Cr), and xylenol iso-
mers (Xol). Minor products include C1–9 aliphatic hydrocarbons
(mainly C1–5), aromatics (benzene, toluene, xylenes, trimethyl-
benzene), dimethylanisole isomers (with trace trimethylanisole
isomers), trimethylphenol isomers (included in xylenol due to their
small amounts and being not well separated), and heavy products
2
◦
ramp was 10 C/min. The signals of H O (m/z = 18), CO (m/z = 44),
2
2
and CO (m/z = 28) were continuously monitored by a mass spec-
trometer (MKS). Quantification was achieved by sending calibrated
CO2 and CO pulses (100 ␮L) into the detector by flowing He. Since
both CO and CO are formed during TPO, both contributions to total
carbon were considered in the analysis.
2
(
pentamethylbenzenes, naphthalene, methylated naphthalenes),
2
.2. Catalytic measurements
in good accordance with previous work on anisole conversion over
HZSM-5 [19,20]. Among the xylenol isomers, the 2,4-xylenol (2,4-
Xol) is the dominant isomer, accounting for ∼70% of the total
isomers; the other five isomers are distributed relatively evenly
in small amounts.
As shown in Fig. 1B, at low W/F, phenol and MA exhibit sim-
ilar yields, while other products remain in small quantities. At
higher W/F, phenol continues increasing, while methylanisole
passes through a maximum and then gradually drops. At the same
time, cresol starts small but its concentration picks up quickly and
becomes comparable to that of phenol at high W/F. The xylenol
The catalytic performance was evaluated using a quartz tube
reactor (0.25 in. o.d.) at atmospheric pressure. In each run (space
time of 0.5 h), the catalyst sample (60 mg, 40–60 mesh) was packed
in the reactor between two layers of quartz wool. The thermo-
couple was affixed to the outside wall of the reactor where the
catalyst was located. At the start of the experiment, the reactor
◦
temperature was increased at 10 C/min and held at the desired
value for 0.5 h in flowing He (20 mL/min) before reaction. When
the temperature stabilized, anisole (from Aldrich, 99.7%) was fed
by a syringe pump (kd scientific) at a liquid flow rate 0.12 mL/h and
vaporized before entering the reactor. All pipelines were heated
at 300 C to avoid condensation of either reactants or products.
The products were analyzed online in a gas chromatograph (GC
(2,4-Xol) starts with zero derivative and then increases, reaching a
plateau. As shown in Fig. 1C, the yields of minor products increase
slowly with W/F, starting with zero derivative. All of them appear
to be secondary and/or tertiary products.
◦
6
890, Agilent), equipped with a flame ionization detector (FID)
To determine which of the major products are primary, the
yields are plotted in Fig. 2 as a function of anisole conversion, by
either varying W/F (full symbols) or temperature (open symbols).
It is evident that phenol and MA are primary products initially
produced at comparable rates since the slopes at zero conver-
sion are finite and about the same for both. By contrast, cresol
and 2,4-xylenol appear as secondary products based on the zero
slope observed at anisole conversion approaching zero. The pri-
mary products methylanisole and phenol are expected to arise from
anisole disproportionation. As the anisole conversion increases, the
yields of cresol and 2,4-xylenol increase in a secondary step at the
expense of methylanisole.
and a 60 m Innowax capillary column. In parallel, the effluent was
trapped in methanol using an ice-water bath, and analyzed by
GC–MS (Shimadzu QP2010s) with the same Innowax column, using
reference standard compounds for identification. The space time
(
W/F), expressed in hours, is defined as the ratio between mass of
catalyst and the anisole mass flow rate. The conversion and yield
(
mol.%) were calculated based on the carbon balance.
To monitor the evolution of products as a function of space time
over a wide range, both catalyst amount (5–120 mg) and anisole
flow rate (0.12–0.36 mL/h, with a He/anisole molar ratio of 50 main-
tained) were varied. To test the effect of changing the carrier gas,
He was replaced by H2 in several runs, using a H /anisole molar
2
Based on these results and in agreement with previous stud-
ies [26,27,29], the following reactions are proposed (Scheme 1).
ratio of 50. All the gases used in this work were ultra high purity
grade, supplied by Airgas Inc. To test the effect of water addition to
the feed, deionized water was injected with another syringe pump
into the reactant line, keeping a water/anisole mass ratio of 1/4.
(1) Two anisole molecules disproportionate to phenol and methy-
lanisole; (2) subsequent reaction of methylanisole with phenol
yields two cresol molecules; (3) in turn, cresol and methylanisole
can form xylenol and phenol; and (4) phenol reacts with the anisole
feed, yielding cresol and another phenol molecule. In addition to
these major pathways, several minor pathways take place, includ-
ing the direct dealkylation of anisole and methylanisole [26,27,29].
The methyl groups eliminated appear as light gases, as observed.
Analysis of the evolution of the isomers of methylanisoles and
cresols provides further insight into the reaction pathways. Fig. 1D
3
. Results and discussions
3.1. Catalyst characterization
Phase purity of the zeolite was determined by powder X-ray
diffraction (XRD). The XRD pattern of the HZSM-5 sample con-

1
74
X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
Fig. 1. Effect of space time (W/F) on anisole conversion (A); major product yields (B); minor product yields (C); and methylanisole isomers and cresol isomers distributions
◦
(
D) over HZSM-5. Reaction conditions: T = 400 C, TOS = 0.5 h.
shows the evolution of the different isomers as a function of W/F,
while Table 1 summarizes the isomer distribution at very low W/F
(0.083 h), thus representing the distribution of primary products.
It is well known that while the meta- (m-) position is thermody-
namically most favored for substitution in phenol/anisole rings, the
ortho- (o-) position is kinetically favored since it is the most reac-
tive towards electrophilic substitutions [24,31,32]. Also, in the case
of zeolite-catalyzed reactions for which shape selectivity may play
a role, the para- (p-) position may be preferentially obtained. As
shown in Fig. 1D, it is clear that p-methylanisole is the most favor-
able primary product, but it is also consumed more effectively than
other isomers. The evolution of m-methylanisole with W/F sug-
gests that after the initial formation, some p-methylanisole may
isomerize (outside the zeolite channels) to the thermodynamically
preferred m-methylanisole. Very low yields of o-methylanisole are
obtained over the entire conversion range. In less confined envi-
ronments, e.g., amorphous SiO –Al O or rare earth exchanged
2
2
3
Y zeolites the yield of o-methylanisole by alkylation of anisole
with methanol is comparable or even higher than that of p-
methylanisole [23]. In this case, our results indicate that shape
selectivity is the predominant factor for methylanisole formation,
at least in the low conversion range.
Fig. 2. Effect of anisole conversion on major product yields over HZSM-5. The data
were derived from effect of W/F in Fig. 1B (closed symbol) and from effect of reaction
temperature in Fig. 5B (open symbol).

X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
175
Scheme 1. Proposed major reaction pathways of anisole conversion over HZSM-5 (see Table 2).
The situation is different for the cresol isomers. It is evident
that the production of o-cresol is dominant at low W/F, indicat-
ing that the relatively small size of cresol causes the electrophilic
substitution preference to prevail over shape selectivity. The yield
of p-cresol is higher than that of m-cresol at low W/F. At higher
W/F, the yield of m-cresol increases significantly via isomerization
to the thermodynamically most favored isomer [28–31].
tivity and eletrophilic substitution effects since 2,4-xylenol can be
formed from anisole reacting with p-cresol or o-cresol. Xylenols
might also be formed from disproportionation of two o-cresol
molecules; however, this contribution should be much lower than
that of the anisole–cresol reaction due to pore size limitation of
HZSM-5 since the formation of xylenol may be affected by both
transition state shape selectivity and product shape selectivity
[38,39].
The dominant isomer among the xylenols is 2,4-xylenol. This
preference could be the result of a combination of shape selec-
3.2.2. Kinetic model
A simple kinetic model was employed to quantify the con-
tribution of each of the proposed major reaction pathways and
Table 1
Product distributions (%) for anisole and phenol + anisole conversion over HZSM-
determine which paths are dominant. The reaction rate (r ) for each
i
◦
5
. Reaction conditions: W/F = 0.083 h, anisole or anisole + phenol (1:1), T = 400 C,
reaction i is assumed to follow the expression of r = k × C × CB,
i
i
A
TOS = 0.5 h. Kinetic model of 11-reaction model (a); bimolecular reaction model (b);
and unimolecular reaction model (c).
where k is the rate constant for reaction i, and C and CB are the
i
A
concentrations of the reactants A and B. A total 11 elementary reac-
tions are considered, as shown in Table 2, accompanied with the
Composition (%)
Feed
An
Model prediction of An + Ph
An + Ph
a
b
c
fitted k . The fitting of the kinetics data was done using the standard
i
non-linear least square (NLS) routine in Excel Solver.
Hydrocarbons
Aromatics
An
0.2
0.2
73
0
0.07
30
33
31
39
3
Table 2
MA
7.7
0.8
5
2.4
0.1
1.6
0.8
1.4
1.6
Proposed elementary reactions and fitted reaction rate constant ki over HZSM-5.
o-MA
p-MA
m-MA
Number
Reaction
Fitted ki
1.9
×10−5 L mol
−1 −1
−1
(h )
(
h
)
Dimethyl-An
Heavies
Ph
0.2
0
11
0
0
55
1
2
3
4
5
6
7
8
9
An + An → Ph + MA
Ph + MA → Cr + Cr
Cr + An → Ph + Xol
Ph + An → Cr + Ph
MA → Cr
0.032
0.25
0.20
0.16
53
11
53
13
54
4
Cr
5.7
3.5
1.4
0.8
9.5
4
3.2
2.4
0.093
0.38
0.37
0.093
0.35
0
o-Cr
p-Cr
m-Cr
Xol → Cr
Cr → Ph
An → Ph
Xol
1.9
1.6
0.26
2.1
2
0.2
1.3
1.3
0.4
An → Cr
2
,4-Xol
10
11
MA → Xol
Other Xol
Cr + Cr → Ph + Xol
0

1
76
X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
One of the questions that we had to address in developing the
kinetic model was whether the system could be described with uni-
molecular or bimolecular reactions, or both. For example, Jacobs et
al. [29] have suggested that the conversion of anisole on HZSM-5
can be described in terms of a rather simple combination of dis-
proportionation (i.e., 2An → Ph + MA) and unimolecular reactions,
i.e., isomerization (An → Cr, MA → Xol) and dealkylation (An → Ph,
MA → Cr). However, when we applied this model to the current
experimental data, the fit was poor (Fig. S1B). By contrast, when
bimolecular reactions were included the fitting improved dramat-
ically (Fig. S1A). Fig. S1A shows the goodness of the fit obtained
when bimolecular transalkylation steps were included. The mini-
mum number of steps needed to obtain a good fit was five, four of
them bimolecular transalkylations (Scheme 1) and one unimolec-
ular dealkylation (MA → Cr). It is clear that bimolecular reactions
are major contributors in the reaction scheme.
After a first approximation using 5 steps, the complete reaction
scheme was developed including 11 reaction steps (see Table 2).
They are: four bimolecular transalkylation steps (reactions (1)–(4)),
four unimolecular dealkylation steps (reactions (5)–(8)), and two
unimolecular isomerization steps (reactions (9) and (10)) and
cresol disproportionation (reaction (11)). As shown in Fig. 3, an
excellent fit of the experimental data is obtained with the pro-
posed set of reaction steps. The robustness of the fitting was tested
by varying the set of initial values for the adjustable parameters.
In each case, the minimum error was obtained with the same set
of final values. The physical implications of the parameters result-
ing from the fitting can be appreciated by plotting the variation
of the rates of the individual reactions as a function of W/F (Fig. 4).
First, it can be clearly seen that the reaction rates of the bimolecular
reactions (Fig. 4A) are much faster than those of the unimolecular
reactions (Fig. 4B). Among the bimolecular reactions, reaction (1)
dominates at the reactor inlet, as expected due to the high con-
centration of anisole. However, soon after, reaction (4) becomes
dominant. It is interesting to note that this reaction involves two
Fig. 3. 11-Reaction kinetic fittings of anisole conversion over HZSM-5. Symbols,
experimental data; lines, kinetic fittings. ꢁd is the total square deviations of fitted
data compared to experimental data.
2
reactants (anisole and phenol) which have significantly smaller
kinetic diameters than those of the other molecules, thus we can
expect a faster transport rate inside the zeolite channels. Among
the unimolecular reactions, the isomerization of An to Cr (reac-
tion (9) in Table 2) and the deakylation of An to Ph (reaction (8))
are more pronounced at low conversion; the deakylation of Xol to
Cr (reaction (5)) is noticeable at intermediate conversion and the
deakylation of Xol to Cr (reaction (6)) and Cr to Ph (reaction (7))
increase at high conversion.
It must be noted that the bimolecular reactions described in the
model do not necessary imply an elementary step. For example, the
step indicated as two An molecules yielding a Ph and a Cr molecules
Fig. 4. Comparison of predicted reaction rates of bimolecular (A) and unimolecular (B) reactions over HZSM-5 from the kinetic model and fitted parameters. See reaction
numbers in Table 2.

X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
177
Fig. 5. Effect of reaction temperature on anisole conversion (A); major product yields (B); minor product yields (C); and methylanisole isomers and cresol isomers distributions
D) over HZSM-5. Reaction conditions: W/F = 0.5 h, TOS = 0.5 h.
(
may in fact occur via an initial decomposition of anisole to Ph and
a surface methyl group [40], followed by reaction of this methyl
group with another anisole molecule, which yields Cr. However,
kinetically this path behaves as if it occurs as 2An to Ph + Cr.
It is well known that xylenes are readily isomerized over HZSM-
one would have expected that the addition of phenol would have
little effect on the formation of Cr. As shown in Table 1, the 11-
reaction model (a) and simplified bimolecular model (b) predict
a product distribution from the mixed feed that is closer to that
observed experimentally than that predicted by a unimolecular
reaction model (c).
5
. However, it is interesting to note that the isomerization of MA to
Xy (reaction (10)) does not seem to occur under the conditions of
this study.
3.3. Effect of varying operating conditions
3.2.3. Phenol co-feeding
3.3.1. Reaction temperature
As shown in Scheme 1, since phenol is both a primary product
The effects of varying reaction temperature were assessed using
an intermediate value of W/F in order to obtain a wide range of
conversions. Accordingly, the initial conversion as a function of
temperature is plotted in Fig. 5 at constant W/F = 0.5 h. It can be seen
of reaction (1) and a reactant for consecutive reactions (2) and (4),
co-feeding phenol with anisole can significantly affect the prod-
uct distribution. This experiment was tested at a low W/F (0.083 h)
to further confirm the proposed reaction pathways for HZSM-5.
The results are summarized in Table 1, accompanied by the kinetic
model prediction results. It is clear that the addition of phenol to
the feed reduces the formation of methylanisole since it competes
with the bimolecular transalkylation of anisole (reaction (1)), while
it enhances the formation of cresol via reactions (2) and (4), in
good agreement with the proposed major reaction pathways. If
the reaction followed an intramolecular rearrangement of An to Cr,
◦
that in the range 200–500 C the anisole conversion increases from
14% to 100%. In this range, the yield to methylanisole goes through
◦
a maximum at 325 C. By contrast, phenol, cresol and 2,4-xylenol
keep increasing with temperature and only start decreasing above
◦
550 C, at which temperature thermal cracking becomes significant
and causes coke deposition on the reactor walls and quartz wool,
in agreement with previous studies of anisole thermolysis in H2
flow [41]. Yields of dimethyanisoles and heavy compounds increase

1
78
X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
the reaction network involves a number of sequential reactions (as
those proposed in Scheme 1). If parallel (unimolecular) reactions
were more important, one could have expected that differences
in activation energies for each step would cause differences in the
product evolution as a function of conversion when the tempera-
ture is varied compared to the evolution realized by varying W/F at
constant temperature.
3.3.2. Time on stream
As shown in Fig. 6, the catalyst deactivates with time on stream
◦
at all temperatures. At temperatures below 450 C, the catalytic
activity drops quickly at the beginning of reaction, but then the
activity remains rather constant as a function of time. By contrast, at
◦
temperatures above 450 C, the rate of deactivation does not seem
to slow down, possibly related to the fast rate of coke formation.
To quantify the amount of coke deposits after reaction, TPO was
carried out on spent HZSM-5 as a function of time on stream. Oxi-
◦
dation profiles obtained after several reaction periods at 400 C at
a W/F of 0.5 h are shown in Fig. S2. The shape of the TPO pro-
file does not change with time on stream, while the integrated
amounts of C formed after 0.5, 1.0 and 6.0 h on stream are 5.1,
Fig. 6. Anisole conversion as a function of time on stream over HZSM-5 at various
temperatures and constant W/F = 0.5 h.
5
.6, and 7.3 wt.%, respectively, indicating that most of the carbon
slowly with temperature and remain low over the entire range.
By contrast, formation of hydrocarbons and aromatics increases
rather quickly with temperature, due to the increase of secondary
reactions such as cracking with temperature.
It is interesting to compare the variation of product yield as a
function of anisole conversion brought about by either increasing
W/F or temperature. As shown in Fig. 2, the major product evolu-
tions fall on the same lines as a function of conversion whether this
is varied by increasing temperature (open symbols) or by increas-
is deposited at the beginning of the reaction. Elemental analysis
of spent catalyst in anisole conversion [18] showed that the coke
contains large amounts of oxygen, suggesting that the sources of
coke are probably oxygenates strongly adsorbed on the micropore
walls.
Fig. 7A and B shows the CO2 and H O evolution profiles during
2
TPO of the spent catalysts for different reaction temperatures. It
can be observed that when the reaction temperature is lower than
◦
450 C, the CO2 evolution profile is composed of two overlapping
◦
ing W/F (full symbols). This is valid up to temperatures close to
peaks, centered at approximately 545 and 637 C. With reaction
temperature further increased above 450 C, the two peaks merge
◦
◦
5
50 C, at which point thermal cracking becomes important.
Another similarity observed in the effect of varying conversion
into a larger one that shifts to higher oxidation temperatures, up to
◦
by either increasing temperature or W/F is observed in the distri-
bution of isomers of methylanisole and cresol. The trend shown
in Fig. 5D as a function of temperature is similar to that observed
in Fig. 1D as a function of W/F. This similarity is expected when
about 656 C. At the same time, the evolution profiles of water also
shift to higher temperatures, but the intensity does not increase
as rapidly as that of CO2 evolution, indicating that at higher reac-
tion temperatures the coke deposits are leaner in H. The results
◦
◦
◦
◦
◦
◦
Fig. 7. CO2 (A) and H2O (B) evolution profiles during TPO of spent HZSM-5. Reaction condition: (a) 200 C, (b) 350 C, (c) 400 C, (d) 450 C, (e) 500 C, (f) 550 C; W/F = 0.5 h;
TOS = 6 h except 550 C, TOS = 1 h for 550 C.
◦
◦

X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
179
Table 3
increased activity and stability [45,46]. More directly related to
our findings, Meusinger and Corma [46] found that increasing H2
pressure results in improved stability of HZSM-5 catalysts during
Amounts of carbon formed on the spent HZSM-5. Reaction conditions: W/F = 0.5 h,
TOS = 6 h.
◦
a
◦
Reaction temperature ( C)
Amount of carbon (wt.%)
ACO2 /AH2
O
n-heptane cracking at 270 C. They ascribed this phenomenon to a
200
350
400
450
500
550
3.4
6.0
7.3
9.6
12
1.5
2.1
2.3
2.6
3.6
3.4
reverse process of hydrocarbon activation over Brønsted acid sites,
that is, hydrogen reacts with adsorbed carbenium ions to form car-
bonium ions, which desorb as paraffins cleaning the surface. In such
a process, the Brønsted acid site is regained, leading to a higher sta-
b
bility than under N . Henriques et al. have also found that catalyst
11
2
stability is higher under H than under N during the conversion of
o-xylene at 350 C [47]. They found that H2 inhibits the formation
a
2
2
Integrated intensity ratio between CO2 signal and H2O signal of TPO of spent
◦
catalyst.
b
of refractory coke (insoluble in CH Cl ) by changing the balance
TOS = 1 h.
2
2
between hydrogenation and dehydrogenation of coke. These expla-
nations involve the activation of H2 on acid sites, which is much
less effective than on a typical metal catalyst and is more clearly
observed at high temperatures and pressures. The role of traces of
impurities either on the catalyst or from the gas phase should not
be discarded. If hydrogen reacts with traces of oxygenate impuri-
ties, it could generate traces of water, which, as shown below may
significantly enhance activity.
are quantified in Table 3. It is seen more clearly that the integrated
intensity of the CO /H O ratio increases with reaction temperature,
as the coke becomes more refractory.
2
2
3.3.3. Carrier gas
As shown in Fig. 8, while the initial anisole conversion using
H2 or He as a carrier gas is the same, the deactivation under H2
seems to be only slightly slower than under He. TPO of spent cat-
alysts (Fig. S3) showed that the carbon deposition is slightly lower
under H2 (6.65 wt.%) carrier gas than under He (7.26 wt.%). This
result indicates that H2 may play a role in keeping the catalyst sur-
face somewhat cleaner and having a moderate effect in preventing
deactivation.
When the catalyst contains a metal function in addition to the
acid function, it is well known and easy to understand that the
presence of dihydrogen in the gas phase reduces the rate of carbon
accumulation [42,43]. However, the effect of hydrogen gas is not
as obvious for the case of metal-free zeolites. Nevertheless, it has
been reported in several studies that when an inert carrier gas such
as He or N2 is replaced by hydrogen the catalyst stability improves
On the other hand, as shown in Fig. S4, the presence of H2 does
not seem to affect the distribution of products. Both, under He and
H , the initial major products are phenol, cresol and 2,4-xylenol but
2
their concentration drops with time on stream while methylanisole
initially increases and then drops, following the same trend as that
seen when varying conversion by changing either temperature or
W/F. When plotting the yield of major products as a function of
anisole conversion (see Fig. 9) one can see that the yield of all major
products fall on the same lines for both carrier gases, suggesting
that hydrogen has no effect on the reaction pathway.
3.3.4. Water addition
Since large amounts of water are present in bio-oil [2], the
effect of adding water to the anisole feed was investigated. For
a standard run with a W/F of 0.5 h, at which the conversion in
the absence of water was about 80%, the addition of controlled
amounts of water resulted in an increase in activity that led to 100%
anisole conversion. Only after for 4 h on stream were slightly lower
conversions observed (see Fig. S5). Interestingly, the TPO analy-
sis (Fig. S6) of the spent samples after 6 h on stream showed that
the addition of water did not result in less coke, but rather some-
what higher carbon content (8.2 wt.%) than that obtained under
[
44–47]. While the experimental fact has been observed clearly, the
explanation of this effect has been less unanimous. For example,
H2 activation has been attributed to the presence of metal impu-
rities in the zeolite. Kanai et al. [44] studied metal-free zeolites
(
Y, Mordenite, ZSM-5) and found them active for ethylene hydro-
◦
genation at rather low temperatures (350 C) and pressures, which
they explained in terms of the presence of Fe ions. By contrast,
at high temperatures and pressures, conversion of Na ions into
Brønsted acid sites has been proposed to occur, which could explain
Fig. 9. Effectofcarrier gason major product yieldsas a function ofanisole conversion
◦
Fig. 8. Effect of carrier gas on anisole conversion over HZSM-5. Reaction conditions:
W/F = 0.5 h, T = 400 C.
over HZSM-5. Reaction conditions: W/F = 0.5 h, T = 400 C. Closed symbol, He carrier;
open symbol, H2 carrier.
◦

1
80
X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
Fig. 10. Effect water addition on anisole conversion over HZSM-5. Reaction con-
ditions: W/F = 0.083 h, 400 C. Full symbols: no water; open symbols: continuous
water addition; half-full symbol: water addition on–off–on.
Fig. 11. Effect of water addition on major product yields as a function of anisole
conversion over HZSM-5. Reaction conditions: W/F = 0.083 h with water added and
W/F = 0.5 h without water condition. The variable conversion data was obtained at
varying time on stream. Full symbols: no water; open symbols: with water addition.
◦
similar conditions in the absence of water (7.2 wt.%). As shown in
Fig. 10, in order to quantify the effect of water on activity more
precisely, additional runs were conducted at lower initial conver-
sions by reducing the W/F to 0.083 h. It can be seen that the initial
anisole conversion in the presence of water is ∼2.5 times higher
than without water. Interestingly, the deactivation rate does not
seem to vary with and without water. Therefore, the effect of water
cannot be ascribed to a cleaning of coke precursors and a conse-
quent increase in the number of active sites exposed at a given
time. Another important result is also included in Fig. 10. It can be
seen that when the addition of water was suddenly stopped during
the time on stream, the anisole conversion dropped to exactly the
same level as that obtained without the addition of water, keeping
the same behavior during the 2-h period during which water was
not injected. However, the anisole conversion recovered quickly
once the water addition was resumed. Moreover, the points lie
on the same line as that for which water was kept in the feed
for the entire reaction time. These results imply that the positive
effect of water is reversible and not associated with the catalyst
deactivation. Water is apparently an active species in the reaction
process.
To further evaluate the effect of water addition, we plotted
in Fig. 11 the yield of major products for the run with water
addition as a function of anisole conversion and compared these
data with those obtained without the addition of water. In con-
trast with all the previous comparison of product distribution as a
function of conversion, which was the same under different con-
ditions, a clear difference is observed here. While the trends for
cresol and 2,4-xylenol yields do not change with water addition,
a significant increase in phenol yield at the expense of methy-
lanisole was observed at all conversion levels when water was
added. One could ascribe this enhancement to the hydrolysis of the
methoxy group in anisole and methylanisole. Since at the entrance
of the reactor the anisole concentration is highest, the hydroly-
sis of anisole not only produces more phenol than without water,
but also reduces the contribution of the bimolecular reaction (1),
thus decreasing the yield of methylanisole. While the conversion of
methylanisole may be increased by hydrolysis, the reduced concen-
tration of methylanisole makes the increase in cresol concentration
to be lower. Therefore, the overall effect of water is an increase in
anisole conversion and phenol yield with decreased methylanisole
yield.
4
. Conclusions
Anisole conversion over HZSM-5 has been studied as a model
compound in the evaluation of catalysts for bio-oil refining. Anisole
contains a methoxy group that provides interesting chemistry. The
kinetic analysis indicates that both bimolecular and unimolecular
reactions are important in the reaction pathway. At low contact
times and higher feed concentrations the reaction is dominated by
bimolecular steps involving transalkylation of anisole. Secondary
bimolecular reactions involving cresol, phenol and methylanisole
become important at higher space time. Several parallel unimolec-
ular reactions also take place. Shape selectivity is evident in the
distribution of methylanisole isomers, which are relatively large.
By contrast, the distribution of cresol isomers is dominated by
electrophilic substitution at low conversions and then by thermo-
dynamic equilibrium.
The same product distribution is obtained with varying conver-
sion by reaction parameters such as reaction temperatures, space
time, presence of hydrogen carrier, or catalyst deactivation. By con-
trast, a significant change in product distribution was observed
when water was added to the feed. In addition, the presence of
water in the feed was found to improve the activity of the zeolite
without altering the stability, probably due to the participation of
methoxy group hydrolysis.
Acknowledgements
Financial support from the National Science Foundation EPSCOR
(0814361), the Department of Energy (DE-FG36GO88064), and the
Oklahoma Bioenergy Center is greatly appreciated.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.03.018.
References
[
[
1] D. Mohan, C.U.P. Pittman Jr., P.H. Steele, Energy Fuels 20 (2006) 848–889, and
references therein.
2] G.W. Huber, S. Iborra, A. Corma, Chem. Rev. 106 (2006) 4044–4098, and refer-
ences therein.

X. Zhu et al. / Applied Catalysis A: General 379 (2010) 172–181
181
[
[
[
[
3] D.E. Resasco, S. Crossley, AIChE J. 55 (2009) 1082–1089.
4] D.C. Elliott, Energy Fuels 21 (2007) 1792–1815.
5] E. Furimsky, Appl. Catal. A 199 (2000) 147–190.
[26] P. Beltrame, P.L. Beltrame, P. Carniti, A. Castelli, L. Forni, Appl. Catal. 29 (1987)
327–334.
[27] M. Marczewski, J.-P. Bodibo, G. Perot, M. Guisnet, J. Mol. Catal. 50 (1989)
211–218.
6] C. Zhao, Y. Kou, A.A. Lemonidou, X.B. Li, J.A. Lercher, Angew. Chem. Int. Ed. 48
(
2009) 3987–3990.
7] A. Pinheiro, D. Hudebine, N. Dupassieux, C. Geantet, Energy Fuels 23 (2009)
007–1014.
[28] M. Marczewski, G. Perot, M. Guisnet, Stud. Surf. Sci. Catal. 41 (1988) 273–282.
[29] J.M. Jacobs, R.F. Parton, A.M. Boden, P.A. Jacobs, Stud. Surf. Sci. Catal. 41 (1988)
221–230.
[
1
[
[
8] T.R. Viljava, E.R.M. Saari, A.O.I. Krause, Appl. Catal. A 209 (2001) 33–43.
9] C.A. Fisk, T. Morgan, Y.Y. Ji, M. Crocker, C. Crofcheck, S.A. Lewis, Appl. Catal. A
[30] R.F. Parton, J.M. Jacobs, H. Van Ooteghem, P.A. Jacobs, Stud. Surf. Sci. Catal. 46
(1989) 211–221.
3
58 (2009) 150–156.
10] T.R. Carlson, G.A. Tompsett, W.C. Conner, G.W. Huber, Top. Catal. 52 (2009)
41–252.
[31] N.S. Chang, C.C. Chen, S.J. Chu, P.Y. Chen, T.K. Chuang, Stud. Surf. Sci. Catal. 46
(1989) 223–230.
[
2
[32] G. Moon, W. Böhringer, C.T. O’Connor, Catal. Today 97 (2004) 291–295.
[33] M. Bregolato, V. Bolis, C. Busco, P. Ugliengo, S. Bordiga, F. Cavani, N. Ballarini, L.
Maselli, S. Passeri, I. Rossetti, L. Forni, J. Catal. 245 (2007) 285–300.
[34] W. Wang, P.L. De Cola, R. Glaeser, I.I. Ivanova, J. Weitkamp, M. Hunger, Catal.
Lett. 94 (2004) 119–123.
[
[
[
[
[
11] T.R. Carlson, T.P. Vispute, G.W. Huber, ChemSusChem 1 (2008) 397–400.
12] M.C. Samolada, A. Papafotica, I.A. Vasalos, Energy Fuels 14 (2000) 1161–1167.
13] J.D. Adjaye, N.N. Bakhshi, Fuel Process. Technol. 45 (1995) 185–202.
14] P.T. Williams, P.A. Horne, J. Anal. Appl. Pyrol. 31 (1995) 39–61.
15] P. Chantal, S. Kaliaguine, J.L. Grandmaison, A. Mahay, Appl. Catal. 10 (1984)
[35] N. Ballarini, F. Cavani, L. Maselli, A. Montaletti, S. Passeri, D. Scagliarini, C. Flego,
C. Perego, J. Catal. 251 (2007) 423–436.
3
17–332.
[
[
16] A.G. Gayubo, A.T. Aguayo, A. Atutxa, R. Aguado, M. Olazar, J. Bilbao, Ind. Eng.
Chem. Res. 43 (2004) 2619–2626.
17] A.G. Gayubo, A.T. Aguayo, A. Atutxa, R. Aguado, J. Bilbao, Ind. Eng. Chem. Res.
[36] W.E. Fameth, R.J. Gorte, Chem. Rev. 95 (1995) 615–635.
[37] X.L. Zhu, L.L. Lobban, R.G. Mallinson, D.E. Resasco, J. Catal. 271 (2010) 88–98.
[38] F.E. Imbert, S. Gnep, M. Guisnet, J. Catal. 172 (1997) 307–313.
[39] F.E. Imbert, M. Guisnet, S. Gnep, J. Catal. 195 (2000) 279–286.
[40] P. Cheung, A. Bhan, G.J. Sunley, E. Iglesia, Angew. Chem. Int. Ed. 45 (2006) 1617.
[41] I.W.C.E. Arends, R. Louw, P. Mulder, J. Phys. Chem. 97 (1993) 7914–7925.
[42] M. Santikunaporn, J.E. Herrera, S. Jongpatiwut, D.E. Resasco, W.E. Alvarez, E.L.
Sughrue, J. Catal. 228 (2004) 100–113.
4
3 (2004) 2610–2618.
[
[
[
[
18] J.D. Adjaye, N.N. Bakhshi, Biomass Bioenergy 8 (1995) 131–149.
19] P.D. Chantal, S. Kaliaguine, J.L. Grandmaison, Appl. Catal. 18 (1985) 133–145.
20] P.A. Horne, P.T. Williams, Renew. Energy 7 (1996) 131–144.
21] M.A. Peralta, T. Sooknoi, T. Danuthai, D.E. Resasco, J. Mol. Catal. A 312 (2009)
7
8–86.
22] S. Namba, T. Yashima, Y. Itaba, N. Hara, Stud. Surf. Sci. Catal. 5 (1980) 105–
11.
[43] M.A. Coelho, W.E. Alvarez, E.C. Sikabwe, R.L. White, D.E. Resasco, Catal. Today
28 (1996) 415–429.
[
1
[44] J. Kanai, J.A. Martens, P.A. Jacobs, J. Catal. 133 (1992) 527–543.
[45] S. Senger, L. Radom, J. Am. Chem. Soc. 122 (2000) 2613–2620.
[46] J. Meusinger, A. Corma, J. Catal. 152 (1995) 189–197.
[47] C.A. Henriques, A.M. Bentes Jr., P. Magnoux, M. Guisnet, J.L.F. Monteiro, Appl.
Catal. A 166 (1998) 301–309.
[
[
23] L.B. Young, US Patent 4,371,714, (1983).
24] S. Balsama, P. Beltrame, P.L. Beltrame, P. Carniti, L. Forni, G. Zuretti, Appl. Catal.
1
3 (1984) 161–170.
[
25] R. Renaud, P.D. Chantal, S. Kaliaguine, Can. J. Chem. Eng. 64 (1986) 787–791.