Study of the phenol methylation mechanism on zeolites HBEA, HZSM5 and HMCM22

Article abstract of DOI:10.1016/j.molcata.2010.05.014

The mechanism of gas-phase alkylation of phenol with methanol was studied on zeolites HBEA, HZSM5 and HMCM22. The nature, density and strength of the acid sites were determined by temperature-programmed desorption of NH₃ and FTIR spectroscopy of adsorbed pyridine. In all the cases, anisole, o-cresol and p-cresol were the primary products, but the initial product distribution greatly depended on the zeolite pore structure and surface acid properties. The complete reaction network was established by investigating the formation of secondary products (xylenols, methylanisoles, m-cresol) via the reactions of primary products with phenol and methanol. On HBEA, phenol was alkylated to primary and secondary products without significant diffusional restraints and the yield to dialkylated products increased when phenol conversion was increased. o-Cresol was consecutively alkylated to 2,4- and 2,6-xylenols (C-alkylation), and 2-methylanisole (O-alkylation). Anisole reacted with phenol to yield o- and p-cresol isomers, and with methanol to form methylanisoles. Selectivities to primary products on HZSM5 were similar to those determined on HBEA, but the formation of bulky intermediates leading to dialkylated compounds was hampered because of diffusional constraints. The alkylation of phenol with anisole to yield cresol isomers was completely suppressed on HZSM5. The particular pore structure of zeolite HMCM22 promoted the selective formation of p-cresol among the primary products of phenol methylation and drastically suppressed the consecutive reactions forming secondary products. In contrast, isomerization reactions between monoalkylated products were promoted on zeolite HMCM22 because of its high concentration of strong Br?nsted acid sites. The activity decay on stream was significant on the three zeolites, probably because of the formation of coke intermediates via both anisole dealkylation and methanol dehydration reactions.

Full text of DOI:10.1016/j.molcata.2010.05.014

Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Study of the phenol methylation mechanism on
zeolites HBEA, HZSM5 and HMCM22
∗
M.E. Sad, C.L. Padró, C.R. Apesteguía
Catalysis Science and Engineering Research Group (GICIC), Instituto de Investigaciones en Catálisis y Petroquímica-INCAPE-(UNL-CONICET),
Santiago del Estero 2654, (3000) Santa Fe, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 April 2010
Received in revised form 19 May 2010
Accepted 20 May 2010
Available online 27 May 2010
The mechanism of gas-phase alkylation of phenol with methanol was studied on zeolites HBEA, HZSM5
and HMCM22. The nature, density and strength of the acid sites were determined by temperature-
programmed desorption of NH3 and FTIR spectroscopy of adsorbed pyridine. In all the cases, anisole,
o-cresol and p-cresol were the primary products, but the initial product distribution greatly depended
on the zeolite pore structure and surface acid properties. The complete reaction network was estab-
lished by investigating the formation of secondary products (xylenols, methylanisoles, m-cresol) via the
reactions of primary products with phenol and methanol. On HBEA, phenol was alkylated to primary
and secondary products without significant diffusional restraints and the yield to dialkylated products
increased when phenol conversion was increased. o-Cresol was consecutively alkylated to 2,4- and 2,6-
xylenols (C-alkylation), and 2-methylanisole (O-alkylation). Anisole reacted with phenol to yield o- and
p-cresol isomers, and with methanol to form methylanisoles. Selectivities to primary products on HZSM5
were similar to those determined on HBEA, but the formation of bulky intermediates leading to dialky-
lated compounds was hampered because of diffusional constraints. The alkylation of phenol with anisole
to yield cresol isomers was completely suppressed on HZSM5. The particular pore structure of zeolite
HMCM22 promoted the selective formation of p-cresol among the primary products of phenol methy-
lation and drastically suppressed the consecutive reactions forming secondary products. In contrast,
isomerization reactions between monoalkylated products were promoted on zeolite HMCM22 because
of its high concentration of strong Brønsted acid sites. The activity decay on stream was significant on
the three zeolites, probably because of the formation of coke intermediates via both anisole dealkylation
and methanol dehydration reactions.
Keywords:
Phenol methylation
Alkylation
Acid zeolites
Cresols
Anisole
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
anisole, cresols, xylenols and methylanisoles, at relatively low
temperatures (473–523 K) [6–12]. In general, catalysts containing
predominantly strong Brønsted acid sites favor the O-alkylation of
phenol relative to C-alkylation; for example, ultrastable HY zeo-
lite, Nafion-H resin, and HPA/SiO2 produce anisole/cresols ratios
between 2 and 9 [9,10,13]. On the contrary, zeolites contain-
ing similar amounts of Lewis and Brønsted acid sites (HBEA, HY)
and Lewis solid acids (SiO –Al O ) promote preferentially the C-
alkylation of phenol [6,12–14]. Regarding the para/ortho selectivity
for C-alkylated products, o-cresol is always formed preferentially in
comparison to p-cresol on amorphous acid catalysts or wide pore
zeolites such as HBEA, HY, and H-mordenites [8,10,13,15]. How-
ever, acid zeolites of narrow channels such as zeolite HMCM22
promote the selective formation of p-cresol [16,17]. All these
studies show that the reaction mechanism, and therefore the
product distribution, of phenol methylation greatly depend on
the nature and strength of surface acid sites and also on the
catalyst pore microstructure. Nevertheless, efforts devoted to elu-
cidate the exact requirements of density, nature and strength
of surface acid sites for selectively improving the reaction path-
The gas-phase methylation of phenol is an important indus-
trial reaction that forms valuable chemicals such as cresols,
anisole, and polyalkylated phenols. For example, cresol isomers
are widely used for the synthesis of pharmaceuticals, agrochem-
icals, herbicides and dyes, while 2,6-xylenol is the monomer
of several polymer and plastic applications [1–3]. The catalyst
selectivity to O-alkylated and C-alkylated products depends on
the active site nature (basic or acid sites), catalyst pore struc-
ture and phenol conversion level. Solid bases, in particular MgO,
form selectively ortho-C-alkylated products, namely o-cresol and
2
2
3
2
,6-xylenol, at temperatures between 673 and 773 K [4,5]. In con-
trast, solid acids such as silica-alumina, Nafion-H resin, HPA/SiO2,
and zeolites HBEA and HY, convert phenol to a mixture of
∗ Corresponding author.
E-mail address: capesteg@fiq.unl.edu.ar (C.R. Apesteguía).
URL: http://www.fiq.unl.edu.ar/gicic (C.R. Apesteguía).
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.05.014

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M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
ways leading from phenol to a desired alkylated product are still
needed.
and the NH3 concentration in the effluent was measured by mass
spectrometry in a Baltzers Omnistar unit.
Recently, we investigated the gas-phase methylation of phe-
nol on SiO –Al O and zeolites HBEA, HZSM5 and HMCM22 [17].
The nature of surface acid sites was determined by infrared
spectroscopy (IR) using pyridine as probe molecule and a Shi-
madzu FTIR-8101M spectrophotometer. The spectral resolution
2
2
3
Our goal was to explore the possibility of drastically improving
the reaction selectivity to obtain p-cresol. The development of a
novel process using solid catalysts is a highly desirable technologi-
cal target for the p-cresol synthesis because the current commercial
process involves the oxidation of toluene via sulfonation with sul-
furic acid and entails concerns related to corrosion and disposal
of spent base materials [1]. We observed that the para-selectivity
was dramatically increased by using zeolite HMCM22. In fact,
the p-cresol yield and the para-/ortho-cresol ratio on HMCM22
for 93% phenol conversion were about 58% and 3.4, respectively,
the highest values reported up to now for the p-cresol forma-
tion from methylation of phenol. In this paper, we have extended
these studies with the aim of ascertaining the complete reac-
tion network of phenol methylation on zeolites HBEA, HZSM5
and HMCM22. We first identified primary and secondary path-
ways involved in the reaction mechanism by modifying the contact
time. Then, we investigated the formation of secondary products
−
1
was 4 cm and 50 scans were added. Sample wafers were formed
2
by pressing 20–40 mg of the catalyst at 5 ton/cm and transferred
to a sample holder made of quartz. An inverted T-shaped Pyrex
cell containing the sample pellet was used. The two ends of the
short arm of the T were fitted with CaF2 windows. All the samples
were initially outgassed in vacuum at 723 K for 4 h and then a back-
ground spectrum was recorded after cooling the sample at room
temperature. Spectra were recorded at room temperature, after
admission of pyridine, adsorption at room temperature and evacu-
ation at 423 K. Difference spectra were obtained by subtracting the
background spectrum recorded previously.
2.3. Catalyst testing
The gas-phase alkylation of phenol (Merck, >99%) with
methanol (Merck, 99.8%) was carried out in a fixed bed tubu-
lar reactor at 473 K and 101.3 kPa. Samples were sieved to retain
particles with 0.35–0.42 mm diameter for catalytic measurements
and pretreated in air at 723 K for 2 h before reaction in order to
remove H O, hydrocarbons, and CO . A mixture of methanol (M)
(
xylenols, methylanisoles, m-cresol) via the reactions of primary
products with phenol or methanol. Specifically, we studied the
o-cresol methylation, the alkylation of phenol with anisole, the
anisole/methanol reactions and the pure anisole conversion reac-
tions. Results allowed us to establish and explain the differences in
the reaction mechanism of phenol methylation on zeolites HBEA,
HZSM5 and HMCM22. Reaction pathways are mainly influenced by
the zeolite pore microstructure but the Lewis/Brønsted acid sites
ratio and the strength of Brønsted sites also play a significant role.
2
2
and phenol (P) of M/P = 2:1 molar ratio was fed using a syringe
pump and vaporized into flowing N to give a N /(P + M) molar ratio
2
2
of 26.8. Standard catalytic tests were conducted at contact times
0
P
(
W/F ) of 56 or 112 g h/mol. Reaction products were analyzed by
on-line gas chromatography using an Agilent 6850 chromatograph
equipped with a flame ionization detector, temperature program-
mer and a 30 m Innowax column (inner diameter: 0.32 mm, film
thickness: 0.5 ␮m). Samples were collected every 20 min during 4 h.
Main reaction products were cresols (o-, m- and p-cresols), anisole,
xylenols (dimethylphenols) and methylanisoles (MA); dimethyl
ether produced by methanol dehydration was also detected. Phe-
nol conversion (XP) was calculated as: XP = ꢂYi/(ꢂYi + YP), where
ꢂYi is the molar fraction of products formed from phenol, and YP
is the outlet molar fraction of phenol. The selectivity to product
i (Si, mol of product i/mol of phenol reacted) was determined as:
Si = [Yi/ꢂYi]. Yields (ꢃi, mol of product i/mol of phenol fed) were
calculated as ꢃi = SiXP. Additional catalytic tests were performed
using the same reactor unit to gain insight on the phenol alkylation
mechanism. Specifically, anisole (Merck, >99%) and o-cresol (Ane-
dra, >99%) were fed to the reactor together with phenol or methanol
at the same operating conditions detailed above for the alkylation
of phenol with methanol.
2
. Experimental
2.1. Catalyst preparation
Commercial zeolites HZSM5 (Zeocat Pentasil PZ-2/54) and
HBEA (Zeocat PB) were calcined in air at 723 K. Zeolite HMCM22
was synthesized according to Ref. [18], by using sodium alumi-
nate (Alfa Aesar, Technical Grade), silica (Aerosil Degussa 380),
sodium hydroxide (Merck, >99%), hexamethyleneimine (Aldrich,
9
9%) and deionized water as reagents. The molar composi-
tion of the synthesis gel was SiO /Al O = 30, OH/SiO = 0.18,
hexamethyleneimine/SiO = 0.35 and H O/SiO = 45. The gel was
2
2
3
2
2
2
2
transferred to a Teflon lined stainless steel autoclave, rotated at
0 rpm, and heated to 423 K in an oven for 7–10 days. After crystal-
5
lization, the solid was washed with deionized water, centrifugated,
dried at 373 K, and finally heated first in N2 and then in air at 773 K
for 15 h.
3. Results and discussion
2.2. Catalyst characterization
3.1. Catalysts characterization
The crystalline structure of the samples was determined by X-
◦
ray diffraction (XRD) in the range of 2ꢀ = 2–45 , using a Shimadzu
XD-D1 diffractometer and Ni-filtered Cu K␣ radiation (ꢁ = 1.540 Å).
BET surface areas (Sg) were measured by N2 physisorption at its
boiling point in a Micromeritics Accusorb 2100 E sorptometer.
Prior to N2 physisorption, samples were degassed for 3 h at 523 K.
Elemental compositions were measured using atomic absorption
spectroscopy.
Zeolites HBEA and HZSM5 were commercial samples while zeo-
lite HMCM22 was synthesized in our laboratory. The positions and
intensities of X-ray diffractograms (not shown here) correspond-
ing to both the HMCM22 precursor, i.e. the as-synthesized sample,
and calcined zeolite HMCM22 were in good agreement with those
published for the synthesis of HMCM22 zeolites [19]. The sample
physicochemical properties are shown in Table 1. The Si/Al molar
ratio varied between 12.5 (HBEA) and 20.0 (HZSM5). The BET sur-
Acid site densities were determined by temperature-
programmed desorption (TPD) of NH3 preadsorbed at 373 K.
2
face area of HBEA (560 m /g) was higher than those determined for
3
2
2
Samples (100 mg) were treated in He (60 cm /min) at 723 K for
HMCM22 (400 m /g) and HZSM5 (350 m /g).
2
h and then exposed to a 1% NH /He stream for 40 min at 373 K.
Sample acid properties were probed by TPD of NH preadsorbed
3
3
Weakly adsorbed NH3 was removed by flushing with He at 373 K
during 2 h. Temperature was then increased at a rate of 10 K/min
at 373 K and by IR spectra of adsorbed pyridine after admission at
298 K and evacuation at 423 K. Fig. 1 shows the NH3 TPD curves

M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
65
Table 1
Physicochemical and acid properties of zeolites HBEA, HZSM5 and HMCM22.
Catalyst
Physicochemical properties
Surface acid properties
2
TPD of NH3 (␮mol/m2)
Si/Al
Surface area (m /g)
Pore size (Å)
IR of pyridine
L (area/g)a
B (area/g)a
B/L
HBEA
HZSM5
HMCM22
12.5
20.0
15.0
560
350
400
6.6 × 6.7; 5.6 × 5.6
5.1 × 5.5; 5.3 × 5.6
4.0 × 5.5; 4.1 × 5.1
0.90
2.20
1.18
272
341
176
282
337
560
1
1
3.2
a
By FTIR of pyridine adsorbed at 298 K and evacuated at 423 K (B: Brønsted sites; L: Lewis sites).
obtained for the zeolites used in this work. The TPD profile of
HZSM5consistedof apeakat about 531 K and abroad bandathigher
temperatures. Zeolite HMCM22 exhibited a qualitatively similar
TPD profile to that of HZSM5, showing two main NH3 desorption
bands centered at about 575 and 750 K, respectively. The TPD curve
corresponding to HBEA presented a single asymmetric broad band
with a maximum at around 610 K which is consistent with previous
results obtained on zeolites HBEA of similar Si/Al ratios [20]. The
2
total amount of desorbed NH (␮mol/m ) was measured by decon-
3
volution and integration of the TPD traces and it was taken as an
indication of the total acid site density. The results are reported
in Table 1. Zeolite HZSM5 exhibited the highest density of acid
2
2
sites (2.20 ␮mol/m ) in comparison to HMCM22 (1.18 ␮mol/m )
2
and HBEA (0.90 ␮mol/m ).
IR spectra obtained after admission of pyridine at 298 K and
evacuation at 423 K allowed us to establish the nature of sur-
face acid sites. Fig. 2 shows the IR spectra obtained on zeolites
HBEA, HZSM5 and HMCM22. The IR band assignment of the surface
species arising from the adsorption of pyridine on acid zeolites has
Fig. 2. FTIR spectra after pyridine adsorption at 298 K and evacuation at 423 K. Dot-
ted lines indicate the presence of Lewis (1450 cm ) and Brønsted (1540 cm ) acid
sites.
−
1
−1
−
1
been reported by several authors [21–24]. The band at 1540 cm
is characteristic of the formation of pyridinium ions on Brønsted
3
3
.2. Catalytic tests
−
1
acid sites while the band between 1440 and 1460 cm
arises
from coordinatively bonded pyridine on Lewis acid sites. The rela-
tive contributions of Lewis and Brønsted acid sites were obtained
by deconvolution and integration of the corresponding pyridine
absorption bands in Fig. 2. Results are given in Table 1. The amounts
of adsorbed pyridine on both Brønsted and Lewis sites of HBEA were
significantly lower than those adsorbed on HMCM22 and HZSM5,
thereby confirming that HBEA was the least acidic zeolite, as sug-
gested by the NH3 TPD curves in Fig. 1. The B/L ratios determined
on HBEA and HZSM5 evacuated at 423 K were close to one. Zeo-
lite HMCM22 contained the highest concentration of Brønsted acid
sites and also the highest B/L ratio (B/L = 3.2). This B/L ratio is con-
sistent with the results reported by other authors showing that
zeolites HMCM22 with Si/Al atomic ratios of about 15 contain B/L
ratios between 2 and 4.5 [25,26].
.2.1. Phenol methylation
Fig. 3 shows the evolution of phenol conversion and selectivi-
ties as a function of time obtained at 473 K on HZSM5 and typically
illustrates the time-on-stream behavior of the catalysts during the
reaction. Phenol conversion (XP) was initially 35% but continuously
diminished with the progress of the reaction reaching 8% after 3 h
on stream. Similar catalyst activity decay was observed on HBEA
and HMCM22 reflecting the in situ deactivation of the samples.
Fig. 3 shows that cresols and anisole were the main products on
HZSM5. The selectivity to cresols was about 50% and remained
approximately constant during the 4-h catalytic test. The selectivity
Fig. 3. Methylation of phenol. Phenol conversion (᭹) and selectivities as a function
of time on HZSM5. Selectivities to: cresols (ꢀ), anisole (ꢀ), xylenols (ꢁ), and MA (ꢁ)
Fig. 1. TPD profiles of NH3 on zeolites HBEA, HZSM5 and HMCM22. NH3 adsorption
at 373 K, heating rate: 10 K/min.
0
(473 K, 101.3 kPa, W/FP = 112 g h/mol, M/P = 2, N2/(P + M) = 26.8).

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M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
Fig. 4. Methylation of phenol. Yields as a function of contact time on HBEA, HZSM5 and HMCM22. Anisole (ꢀ), o-cresol (ꢂ), p-cresol (♦), m-cresol (ꢃ), xylenols (ꢁ), and MA
ꢁ). Operating conditions as in Fig. 3.
(
to anisole increased at the expense of xylenols and methylanisoles
tion depended on the type of zeolite used. In fact, Fig. 4 shows that
zeolite HMCM22 formed mainly p-cresol while zeolites HBEA and
HZSM5 produced mostly anisole and o-cresol (in similar yields).
(
MA), reaching 50% at the end of the run.
To identify primary and secondary reaction products we inves-
tigated the effect of contact time on the product distribution. The
deactivation of all the samples, however, required that each data
point be obtained on a fresh catalyst and that initial yields be
obtained by extrapolating to initial time on stream. The yields at
t = 0 on zeolites HBEA, HZSM5 and HMCM22 are shown as a func-
tion of contact time in Fig. 4. The local slopes of the curves in Fig. 4
give the rate of formation of each product at a specific phenol con-
version and residence time. The nonzero initial slopes for anisole,
o-cresol and p-cresol show that they form directly from phenol and
are therefore primary products on the three zeolites. The direct
attack of methanol to phenol produces anisole by O-alkylation, and
o- and p-cresols by C-alkylation (Fig. 5). This later result shows
that methanol attacks the phenol ring by electrophilic substitu-
tion essentially in ortho and para positions, probably reflecting
the electron donor effect of the phenol OH group that increases
the electronic density of positions 2, 4, and 6 in the ring. On the
other hand, the zero initial slopes observed for m-cresol, xylenols
and methylanisoles in Fig. 4 for the three zeolites suggest that
these compounds are secondary products. Although the methy-
lation of phenol formed the same primary products on zeolites
HBEA, HZSM5 and HMCM22 (Fig. 5), the primary product distribu-
3.2.2. Catalytic tests using intermediate reaction products
In this work we particularly investigated the formation mech-
anism of secondary products for the phenol methylation reaction
with the aim of establishing the complete reaction pathway net-
work occurring on zeolites HBEA, HZSM5 and HMCM22. To achieve
this goal, we studied the conversion of primary to secondary prod-
ucts by feeding anisole or o-cresol to the reactor, with or without
methanol or phenol. Contact times of 27 g h/mol of reactant was
used on HBEA and of 112 g h/mol on HZSM5 and HMCM22. Simi-
lar initial phenol conversions (X P0 = 35%) are obtained using these
∼
contact times values for the phenol methylation reaction.
3.2.2.1. Reactants: o-cresol/methanol mixture. Fig. 6 shows the evo-
lution of conversion and selectivities during the alkylation of
o-cresol with methanol on zeolite HBEA. Main products were
xylenols (2,4- and 2,6-xylenol) and 2-methylanisole which are
formed by C- and O-alkylation of o-cresol, respectively (Reaction 1).
Minor amounts of p- and m-cresol formed from o-cresol isomeriza-
tion were also observed (Reaction 2). Qualitatively, similar results
were reported by Marczewski et al. [9] on ultrastable HY zeo-
Fig. 5. Methylation of phenol. Formation of primary products.

M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
67
Fig. 7. Reactant: anisole. Anisole conversion (᭹) and selectivities as a function of
time on HBEA. Selectivities to: phenol (ꢅ), o-cresol (ꢂ), p-cresol (♦), xylenols (ꢁ),
0
Fig. 6. Reactants: o-cresol/methanol mixture. o-Cresol conversion (᭹) and selec-
tivities on HBEA. Selectivities to: 2,4-xylenol (ꢄ), 2,6-xylenol (ꢁ), 2-MA (ꢁ),
and p-cresol (♦) (473 K, 101.3 kPa, Po-cresol = 1.013 kPa, methanol/o-cresol = 5,
and MA (ꢁ) (473 K, 101.3 kPa, Panisole = 1.013 kPa, W/F
= 27 g h/mol).
anisole
W/F0
= 27 g h/mol).
Besides, on HZSM5 the selectivity to 2,4-xylenol
o-cresol
(S2,4-xylenol = 80%) was clearly improved at the expense of 2,6-
xylenol. These differences in activity and selectivity observed
between HZSM5 and HBEA are explained on the basis of dif-
fusional (lower activity) and shape (lower selectivity to bulky
lites. The values of o-cresol conversion and selectivities obtained
by extrapolating at t = 0 the curves of Fig. 6 are given in Table 2.
Results obtained on zeolites HZSM5 and HMCM22 are also included
in Table 2. o-Cresol conversion was four times lower on HZSM5 in
comparison to HBEA, in spite that a higher contact time was used
for HZSM5 sample.
2
,6-xylenol) constraints existing in HZSM5 due to its narrower
pore structure (Table 1). In contrast with HBEA and HZSM5, zeolite
HMCM22 converted o-cresol mainly to p-cresol (Sp-cresol = 46%)
and also to m-cresol (Sp-cresol = 9%). This result shows that zeolite
HMCM22 is highly active for promoting isomerization reactions,
probably reflecting its high density of Brønsted acid sites (Table 1).
On the other hand, o-cresol conversion was three times higher
on HMCM22 than on HZSM5, in spite that more diffusional con-
straints are expected in HMCM22 for bulkier molecules because its
lower mean pore size. The superior activity obtained on HMCM22
reflects the particular ability of HMCM22 for converting o-cresol to
para and meta isomers. In fact, this isomerization reaction is less
Table 2
Catalytic results for o-cresol/methanol reactions.
Catalysts
Xo-cresol at t = 0 (%)
Selectivity at t = 0 (%)
p-Cresol
m-Cresol
2-MA
2,6-Xylenol
2,4-Xylenol
HBEA
HZSM5
HMCM22
20
5
15
2
1
46
2
1
9
18
9
5
33
9
10
45
80
30
T = 473 K, P = 101.3 kPa, Po-cresol = 1.013 kPa, and methanol/o-cresol = 5.
Table 3
Catalytic results for anisole conversion reactions.
Catalyst
Xanisole at t = 0 (%)
Selectivity at t = 0 (%)
Phenol
o-Cresol
17
10
8
m-Cresol
p-Cresol
Xylenols
MA
HBEA
HZSM5
HMCM22
80
31
100
55
59
60
0
1
1
10
10
30
11
0
0
7
20
1
T = 473 K, P = 101.3 kPa, and Panisole = 1.013 kPa.

6
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M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
affected by diffusional restrictions because it involves the forma-
tion of smaller intermediates in comparison to alkylation reactions
forming xylenols or methylanisoles. In summary, our results show
that the activity and selectivity of o-cresol methylation on acid zeo-
lites greatly depend on both the zeolite structure and the density
and nature of surface acid sites.
suggests that cresols are in part formed from MA dealkylation
(Reaction 5) as it has been proposed by Jacobs et al. [27].
3.2.2.2. Reactant: anisole. Fig. 7 shows the catalytic results
obtained when pure anisole was fed to the reactor on zeolite HBEA.
Anisole formed mainly phenol and also methylanisoles, o-cresol, p-
cresol and minor amounts of m-cresol and xylenols. The selectivity
to phenol remained approximately constant while the selectivity to
methylanisoles increased at the expense of xylenols and cresol iso-
mers. The initial anisole conversion (80%) decreased to 25% at the
end of the 4-h run. The selectivities determined by extrapolating
at t = 0 the curves of Fig. 7 are shown in Table 3. The catalytic val-
ues obtained on HZSM5 and HMCM22 are also included in Table 3.
Initial anisole conversions on HBEA and HMCM22 were signifi-
cantly higher than on HZSM5. The three zeolites formed initially
similar amounts of phenol, between 55 and 60%. Phenol may be
obtained from anisole either by disproportionation (self-alkylation
of anisole) giving also methylanisoles (Reaction 3) or by anisole
dealkylation forming phenol and olefins (Reaction 4) [27].
In contrast, Reaction 5 does not take place on HMCM22 because
this zeolite did not form MA from anisole (Table 3). Direct anisole
isomerization (Reaction 6) formed o- and p-cresol on the three zeo-
lites, but the selectivity to p-cresol was particularly significant on
HMCM22 (Sp-cresol = 30%).
3.2.2.3. Reactants: anisole/phenol mixture. The evolution of anisole
conversion and selectivities on HBEA when an anisole/phenol mix-
ture was introduced to the reactor is presented in Fig. 8. In this case,
phenol is a reactant but also a product of the reaction, so that the
selectivity to phenol (SP, mol of P produced/mol of anisole reacted)
ꢀ
out
in
out
in
in
was determined as (YP − Y )/[ Yi + (YP − Y )], where YP
P
P
out
and YP are the molar fractions of phenol in the feed and in the
products, respectively, and
ꢀ
Yi is the molar fraction of products
formed from anisole, excepting phenol. Cresols may be formed
now via two reaction pathways: (i) intramolecular rearrange-
ment of anisole (Reaction 6) and (ii) alkylation of phenol with
anisole (Reaction 7). Regarding this later reaction, previous studies
[
9,15] have specifically reported that anisole may produce cresols
on solid acids by alkylating phenol, in particular by considering
that anisole is a more effective alkylating agent than methanol
[
28].
Results in Table 3 show that in all the cases the selectivity to
phenol was clearly higher than those to MA thereby indicating that
phenol is mainly formed from anisole dealkylation. The presence of
heavy olefins among the products was always detected which can
explain the significant activity decay observed on stream on all the
samples; olefins are well known intermediates for the formation of
coke.
Fig. 7 shows that HBEA produced initially about 10% of xylenols,
probably by methylanisole rearrangement. In contrast, zeolite
HZSM5 did not form xylenols, thereby suggesting that its narrow
channels hampered the formation of bulky intermediates involved
in xylenol formation. Fig. 7 also shows that on HBEA the selectivity
to MA increased at the expense of cresol isomers as anisole con-
version decreased with the progress of the reaction. This result
Catalytic results obtained at t = 0 on HBEA, HZSM5 and HMCM22
are presented in Table 4. It is observed that on HBEA the
selectivity to cresol isomers (Scresols = 72%) increased in compar-
ison with the values obtained by feeding pure anisole (Table 3,
Scresols = 27%), thereby showing that phenol is effectively alky-
lated by anisole. Besides, formation of xylenols and methylanisoles
was practically suppressed on HBEA, probably because the
presence of phenol among the reactants hinders the anisole
disproportionation reaction (Reaction 3). In contrast, the selec-
tivities to cresol isomers on HZSM5 and HMCM22 (Table 4)
were slightly lower than those obtained using pure anisole as
reactant (Table 3). This result strongly suggests that cresols
are essentially formed from direct anisole isomerization (Reac-
tion 6) and not via phenol alkylation (Reaction 7) when an

M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
69
Table 4
Catalytic results for anisole/phenol reactions.
Sample
Xanisole at t = 0 (%)
Selectivity at t = 0 (%)
Phenol
o-Cresol
45
6
5
m-Cresol
p-Cresol
Xylenols
MA
HBEA
HZSM5
HMCM22
61
31
88
24
85
69
3
1
3
24
6
23
0
0
0
4
2
0
T = 473 K, P = 101.3 kPa, Panisole = 1.013 kPa, and phenol/anisole = 3.
Brønsted acid sites [30]. Catalytic results obtained at t = 0 on HBEA,
HZSM5 and HMCM22 are presented in Table 5. Zeolites HBEA and
HZSM5 yielded mainly phenol that would be essentially formed
by anisole dealkylation (Reaction 4). Nevertheless, formation of
phenol via Reaction 9 cannot be excluded because of the high
methanol/anisole molar ratio in the feed (methanol/anisole = 5).
In contrast with zeolites HBEA and HZSM5, HMCM22 formed
significant amounts of p-cresol (Table 5). Fig. 9 shows that the selec-
tivity to phenol increased with time on stream at the expense of
p-cresol, thereby suggesting that p-cresol is formed from the con-
secutive methylation of phenol.
Fig. 8. Reactants: anisole/phenol mixture. Anisole conversion (᭹) and selectivities
on HBEA. Selectivities to: phenol (ꢅ), o-cresol (ꢂ), p-cresol (♦), MA (ꢁ), and m-cresol
(
ꢃ) (473 K, 101.3 kPa, phenol/anisole = 5, W/F0
= 27 g h/mol).
anisole
anisole/phenol mixture is contacted with zeolites HZSM5 and
HMCM22.
3
.2.2.4. Reactants: anisole/methanol mixture. Finally, we fed to the
As already observed in Fig. 4, methylation of phenol on HMCM22
produces mainly p-cresol because its narrow pore structure ham-
pers the formation of bulkier o-cresol isomer.
reactor an anisole/methanol mixture and determined the zeolites
activity and selectivity. Fig. 9 shows the results obtained on zeolite
HMCM22. The activity decay on stream was significant on HMCM22
and the anisole conversion decreased from about 100% at t = 0 to
4
4% at the end of the run. Dymethylether (DME) and hydrocarbons
3
.3. Reaction networks for phenol methylation
were detected among the reaction products which suggests that
the formation of coke via methanol dehydration and consecutive
DME conversion to hydrocarbons (Reaction 8) may be responsible
for the catalyst deactivation observed in Fig. 9. Reaction 8 has been
widely investigated on solid acids [29] and it has been reported
that the formation of hydrocarbons increases with the strength of
Based on our catalytic results detailed above and previous
works [9,10] we postulated in Figs. 10–12 the reaction mecha-
nism pathways of phenol methylation on zeolites HBEA, HZSM5
and HMCM22, respectively. Let us first analyze the phenol methy-
lation mechanism on HBEA (Fig. 10). Phenol is alkylated to primary
and secondary products without significant diffusional restraints
on HBEA and the reaction proceeds yielding increased amounts
of dialkylated products (xylenol isomers) as phenol conversion
increases (Fig. 4). Phenol initially reacts with methanol via two par-
allel alkylation reactions: by O-alkylation phenol is transformed
to anisole and by C-alkylation yields o- and p-cresols. m-Cresol
isomer is not formed directly from phenol. Initially, HBEA forms
similar amounts of anisole and o-cresol, and the o-cresol/p-cresol
ratio is close to 2, corresponding to the statistic attack of the
aromatic ring by methanol in ortho and para positions. o-Cresol
is consecutively alkylated mainly to 2,4- and 2,6-xylenols (C-
alkylation), but also forms minor amounts of 2-MA (O-alkylation)
(Table 2). o-Cresol isomerization to m-cresol is not significant.
In the reaction network of Fig. 10 we assumed that p-cresol is
alkylated to the corresponding dialkyl isomers (2,4-xylenol and
4
-MA) by similar reaction pathways than o-cresol (we did not
perform catalytic runs using p-cresol as a reactant). Anisole may
undergo several consecutive reactions, but under the operating
conditions of phenol methylation (simultaneous presence of phe-
nol and methanol) our results show that the most important anisole
Fig. 9. Reactants: anisole/methanol mixture. Anisole conversion (᭹) and selectivi-
ties on HMCM22. Selectivities to: phenol (ꢅ), o-cresol (ꢂ), p-cresol (♦), m-cresol (ꢃ)
(
473 K, 101.3 kPa, methanol/anisole = 5, W/F0
= 112 g h/mol).
anisole

7
0
M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
Table 5
Catalytic results for anisole/methanol reactions.
Catalyst
Xanisole at t = 0 (%)
Selectivity at t = 0 (%)
Phenol
o-Cresol
m-Cresol
p-Cresol
Xylenols
MA
HBEA
HZSM5
HMCM22
60
60
98
72
80
39
6
9
14
0
0
5
6
7
42
0
0
0
16
4
0
T = 473 K, P = 101.3 kPa, Panisole = 1.013 kPa, and methanol/anisole = 5.
conversion pathways are the phenol/anisole reaction to yield o-
and p-cresol isomers (Reaction 7, Table 4), anisole methylation to
methylanisoles (Table 5) and anisole dealkylation to phenol and
olefins (Reaction 4, Tables 3 and 4). Thus, anisole reaches a max-
imum concentration with increasing residence time (Fig. 4); for
phenol conversion values higher than 80% anisole is detected only
in trace amounts. Anisole disproportionation is not an important
reaction and produces only minor amounts of MA (Reaction 3,
Table 4). In Fig. 10 we also include the methanol dehydration path-
way to DME that is consecutively converted to hydrocarbons.
The reaction network for methylation of phenol on HZSM5 is
shown in Fig. 11. The density of surface acid sites on HZSM5 is
higher than on HBEA, but the distribution of Lewis and Brønsted
sites is similar on both zeolites (L/B ratio about 1). The observed
differences in product distribution between HBEA and HZSM5 are
essentially caused by diffusional constraints operating in HZSM5
that impede the formation of bulky intermediates leading to dialky-
lated compounds. Selectivities to primary products (anisole, o- and
p-cresol) on HZSM5 are similar to those determined on HBEA.
But the methylation rate of o-cresol to xylenols, in particular 2,6-
xylenol, severely decreases on HZSM5 (Table 2). Regarding the
anisole conversion reactions, the alkylation of phenol with anisole
to yield cresol isomers is suppressed (Table 4) whereas the for-
mation of MA from anisole methylation drastically diminishes
(Table 5).
The product distribution of phenol methylation on HMCM22 is
significantly different than those observed on HBEA and HZSM5
(Fig. 4). The particular pore structure of zeolite HMCM22 promotes
the selective formation of p-cresol among the primary products
and drastically suppresses the consecutive reactions forming sec-
ondary products. The reaction pathways for phenol methylation
on HMCM22 are presented in Fig. 12. Zeolite HMCM22 has unique
pore architecture with two independent pore systems. One pore
system is tridimensional and composed of 12-member ring (MR)
supercages (18.2 Å × 7.12 Å × 7.1 Å) connected by 10-MR windows
(4.0 Å × 5.5 Å). The second, bidimensional, is composed of inter-
connected sinusoidal 10-MR channels (4.0 Å × 5.0 Å) and does not
contain any cages [19,31]. The small pores of zeolite HMCM22
can change the product distribution of phenol methylation reac-
tion according to the ease of diffusivity of product molecules. In
fact, only molecules that can diffuse through the 10-MR pores of
HMCM22 may reach the internal active sites of this zeolite because
the 12-MR supercages are connected to the exterior through a
10-MR window. Difference of the transport rates of p- and o-
Fig. 10. Reaction kinetic network for phenol methylation on HBEA. Dotted lines indicate minor relevance of the reaction pathway.

M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
71
Fig. 11. Reaction kinetic network for phenol methylation on HZSM5. Dotted lines indicate minor relevance of the reaction pathway.
cresol isomers in the 10-MR pores of HMCM22 may explain the
drastic para-selectivity improvement observed in this work for
the gas-phase methylation of phenol on HMCM22 as compared
to HZSM5 or HBEA. Our results on HMCM22 reveal that phe-
nol can go inside the sinusoidal 10-MR channels and form cresol
isomers but o-cresol would diffuse at much smaller rates than p-
cresol. In a previous work [17], we calculated the initial formation
HBEA and HZSM5, but reached values of 4.1 and 7.7 on HMCM22.
Thus, zeolite HMCM22 suppresses the formation of anisole and
also significantly decreases the o-cresol formation rate, but yields
p-cresol at high rates. Formation of secondary dialkylated com-
pounds such as MA and xylenols were completely suppressed
on HMCM22 (Fig. 4 and Tables 3 and 5). In contrast, isomeriza-
tion reactions between monoalkylated products were promoted
on HMCM22 (Tables 2 and 5) because of its high concentration of
strong Brønsted acid sites. Thus, formation of m-cresol from con-
0
0
0
rates of primary products (r ) and observed that r
/r
i
p-cresol o-cresol
and r0
0
/r
ratios were about 0.7 and 0.6, respectively, on
p-cresol anisole
Fig. 12. Reaction kinetic network for phenol methylation on HMCM22. Dotted lines indicate minor relevance of the reaction pathway. The broad line indicates major relevance
of the reaction pathway.

7
2
M.E. Sad et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 63–72
secutive isomerization of o-cresol reached about 12% at X0 = 95%
∼
Acknowledgements
P
(
Fig. 4). Finally, it must be noted that the activity decay on HMCM22
during the methylation of phenol was similar to that observed on
HBEA, thereby suggesting that the reactions forming coke inter-
mediates such as methanol dehydration are not suppressed to any
significant extent on HMCM22.
We thank the Universidad Nacional del Litoral (UNL), the
Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT)
and the Consejo Nacional de Investigaciones Científicas y Téc-
nicas (CONICET), Argentina, for the financial support of this
work.
4
. Conclusions
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[
1