Controlled manipulation of selectivity between O- versus C-alkylation in methylation of phenol using ZrO2-WO3- SiO2 catalysts

Article abstract of DOI:10.1016/j.apcata.2018.05.026

Anisole and o-cresol are very important chemicals which could prepared from phenol using different catalytic routes favoring either O- or C-alkylation. In the methylation of phenol with methanol, the selectivity to either anisole or o-cresol could be manipulated using a suitable tailor-made catalyst having a proper balance of acidic and basic sites. In the current work, a series of compositions based on zirconium, tungsten and silicon oxides (xZrO₂-yWO₃-SiO₂) were prepared and employed as catalysts in fixed bed methylation of phenol. In the absence of silica as support, methylation of phenol over 20% WO₃-ZrO₂ and 40% WO₃-ZrO₂ favored predominantly C-alkylated product o-cresol. Among various combinations 10% ZrO₂-30% WO₃-SiO₂ gave the best results and selectivity to the desired product anisole. Effect of various parameters affecting selectivity to anisole or o-cresol were studied systematically. The mechanism of formation of different products with reference to catalyst structure and functionality was explored in detail. The process parameters could be optimized to get the desired product. Thus, the proper choice can be made to choose the selectivity of O-versus C-alkylation using a single catalyst. The time on stream study was carried out for 16 h to find that the catalyst was stable and could be regenerated.

Full text of DOI:10.1016/j.apcata.2018.05.026

Accepted Manuscript
Title: Controlled manipulation of selectivity between O-
versus C-alkylation in methylation of phenol using
ZrO -WO -SiO
2
3
2
Authors: Kalpesh H. Bhadra, Ganapati D. Yadav
PII:
S0926-860X(18)30254-0
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.05.026
APCATA 16675
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
3-3-2018
8-5-2018
23-5-2018
Please cite this article as: Bhadra KH, Yadav GD, Controlled manipulation of selectivity
between O- versus C-alkylation in methylation of phenol using ZrO -WO -SiO ,
2
3
2
Applied Catalysis A, General (2018), https://doi.org/10.1016/j.apcata.2018.05.026
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Revised Mns.: APCATA-D-18-00461
Sub: 8-May-2018
Controlled manipulation of selectivity between O-
versus C-alkylation in methylation of phenol using
ZrO -WO -SiO
2
3
2
Kalpesh H. Bhadra; Ganapati D. Yadav*
Department of Chemical Engineering
Institute of Chemical Technology
Nathalal Parekh Marg
Matunga
MUMBAI – 400 019
INDIA
*
Author to whom correspondence should be addressed
Email: gdyadav@yahoo.com, gd.yadav@ictmumbai.edu.in
Telefax: +91-22-3361-1001, Fax: +91-22-3361-1002/1020
1

Graphical Abstract
O
OH
OH
+
+
OH
H
3
50°C
0%ZrO - 30%WO - SiO
2
Anisole (major product)
+H HOH
1
2
3
OH
OH
400°C
Phenol Methanol
+
o-Cresol
2,6-Xylenol
(
major product)
Highlights




Synthesis of Zr, W and Si based mixed metal oxides for methylation of phenol.
10% ZrO
2
- 30% WO
3
-SiO
2
was the best for conversion of phenol and selectivity to anisole
catalyst was stable up to 8 h
10% ZrO
2
- 30% WO
3
-SiO
2
Single catalyst for O and C-alkylation with manipulation of process conditions.
Abstract
Anisole and o-cresol are very important chemicals which could prepared from phenol using
different catalytic routes favoring either O- or C-alkylation. In the methylation of phenol with
methanol, the selectivity to either anisole or o-cresol could be manipulated using a suitable tailor-
made catalyst having a proper balance of acidic and basic sites. In the current work, a series of
compositions based on zirconium, tungsten and silicon oxides (xZrO
and employed as catalysts in fixed bed methylation of phenol. In the absence of silica as support,
methylation of phenol over 20% WO -ZrO and 40% WO -ZrO favored predominantly C-
alkylated product o-cresol. Among various combinations 10% ZrO -30% WO
2
-yWO
3
-SiO
2
) were prepared
3
2
3
2
2
3
-SiO
2
gave the best
2

results and selectivity to the desired product anisole. Effect of various parameters affecting
selectivity to anisole or o-cresol were studied systematically. The mechanism of formation of
different products with reference to catalyst structure and functionality was explored in detail. The
process parameters could be optimized to get the desired product. Thus, the proper choice can be
made to choose the selectivity of O- versus C-alkylation using a single catalyst. The time on stream
study was carried out for 16 h to find that the catalyst was stable and can be regenerated.
Keywords Alkylation; Phenol; Methanol; Anisole; Mixed metal oxide; Tungsten-zirconia based
catalyst
1
. Introduction
Phenol and phenolic derivatives have numerous applications in industries particularly for alkylated
phenols. Methylation of phenol is extensively carried out to produce a variety of products such as
as anisole, methylanisole, cresols and xylenols because of their industrial and commercial
applications. In methylation of phenol, a number of O and C alkylated products are formed which
are valuable in different applications; for instance, o-cresol is used as an intermediate in the
synthesis of herbicides. 2,6 Xylenol is a monomer used for the synthesis of poly-(2,6-
dimethyl)phenylene oxide resin. Trimethyl phenol is used as a starting material for the synthesis
of vitamin E. Anisole and other methyl aryl ethers have applications as octane boosters for gasoline
[
1,2]. In conventional processes, alkylation of phenol to anisole is carried out using alkylating
agents such as methyl iodide and dimethyl sulfate [3,4]. However, these processes are not
environment friendly because alkali is used as a proton remover from phenol, the alkylating agents
are toxic and pollution is caused due to large quantities of dissolved solids in the aqueous effluent,
corrosion of equipment, etc. Extensive research has been carried out on alkylation of phenol using
different alcohols and ethers using solid acid catalysts. For instance, tert-butylation of phenol and
phenolic derivatives with tert-butanol and tert-butyl methyl ether [5,6], and isopropylation of
phenols with isopropyl alcohol to synthesize propofol (2,6-diisopropyl phenol) which is general
3

anesthetic [7,8]. Alkylation using alkene such as cyclohexene and isobutylene has also been
reported [9,10].
For methylation of substituted aromatics, methanol and dimethyl carbonate are used as alkylating
agents. Alkylation of phenol with dimethyl carbonate using solid base catalyst selectively produces
anisole but in batch process [11,12]. With methanol as an alkylating agent reactions are performed
in vapor phase either with solid acids or solid base catalysts. Numerous researches have been
reported on vapor phase methylation of phenol like alkali metal loaded silica [13], rare earth
promoted metal phosphates [14], sulfates supported on gamma-alumina [15] and alkali loaded
zeolites [16] which gave selective O-alkylated product whereas metal oxide catalysts such as MgO
[
17], Mg-Al-hydrotalcite [18], TiO
2
[19], silica-manganese mixed oxide [20], V
2
O
5
-ZrO [21], and
2
spinel catalysts like CoFe [22]
2
O
4
,
Cu [23], Mg/Fe/O [24] generated specifically C-
x
Mn3−x
O
4
alkylated products either o-cresol or 2,6-xylenol. Bronsted acid catalysts like Nafion-H resin, HY-
zeolite, H-beta zeolites and heteropoly acid (HPA) on silica generated mixtures of anisole, o-
cresol, xylenols and methyl anisole at lower temperatures [25-30]. In all of the above studies, it is
seen that these catalysts either give O- or C-alkylated product or mixture of both. But there is no
specific catalyst combination is available so far in literature as per our knowledge, which can
generate both kinds of products in selective amount by modifying process parameters.
The foregoing literature survey shows that numerous catalyst systems have been reported for
methylation of phenol. However, our objective was to design and develop a better catalyst which
could be used at milder conditions as well as to make predominantly either anisole or cresol
depending on the market and thus tuneability was very important. We initially targeted anisole as
the product of choice since the reported catalysts have shown very poor selectivity to anisole.
Anisole can be used to produce a number of products useful in fine chemical and pharmaceutical
industries. ZrO
2
-WO as a binary mixed oxide has been used for organic reactions. Vapor phase
3
methylation of phenol was planned in a fixed bed using a series of mixed oxides of Zr and W to
understand product profiles and to study selectivity between C-alkylation leading to o-cresol or
O-alkylation to anisole. Also there are no special reports are available on the use of ZrO
2
-WO -
3
SiO containing mixed metal oxide catalyst for vapor phase methylation reaction which is the
2
subject of current work. The manipulation of selectivity between anisole and 0-cresol using
different conditions in the same catalyst was also targeted.
4

Thus, we are reporting the synthesis of a series of xZrO
2
-yWO
3
-SiO catalysts and their application
2
in vapor phase methylation of phenol. The present study involves optimization of best catalyst
composition and process parameters such as WHSV (Weight hourly space velocity), mole ratio,
temperature and stability of best catalyst in terms of time of stream study (TOS).
2
. Experimental
2
.1 Materials
Phenol (LR grade), methanol (HPLC grade), zirconium oxychloride (LR grade), 25% ammonia
solution (all from S.D. fine chem., Mumbai, India), ammonium metatungstate (alfa aesar, Mumbai)
and aerosil pharma-300 (fumed silica, Evonik Inc., Mumbai) were procured.
2
.2 Catalyst synthesis and characterization
The mixed oxide catalysts of different compositions of ZrO
were prepared by impregnation technique. The total loadings of ZrO
0% w/w on silica. In each sample of catalyst, however the corresponding loadings of ZrO
WO varied from 0 to 40%.
2
and WO
3
supported on fumed silica
and WO were managed to
and
2
3
4
2
3
In typical synthesis of 10% ZrO
2
- 30% WO
3
- SiO , initially fumed silica (2.7 g) was dispersed in
2
methanol. Required amount of zirconium oxychloride (1.18 g) was added drop wise in aqueous
solution with continuous stirring. In the next stage, aqueous ammonia solution was added to the
above solution till the pH of solution reached to 9. Mixing of the solution was continued for 2 h.
Then water was evaporated with rotary evaporator and material was dried overnight at 110 °C.
Aqueous solution of ammonium metatungstate (1.43 g in 20 ml D/I water) was added drop wise
to the above dried material with continuous stirring for 3 h. Water was then evaporated and the
resulting dried material was calcined in air at 750 °C for 3 h. Similarly, other combinations of
variable percentages of ZrO
the described procedure as above and denoted as xZr-yW-SiO
percentage of corresponding metal oxide denoted with initials as Zr and W respectively.
2
and WO
3
were loaded on silica. The series of catalysts prepared by
2
where x and y represent the
5

Powder X-ray diffraction pattern of all catalyst samples were recorded on X-ray diffractometer
Brucker) using Cu Kα using Cu Ka radiation (l = 1.540562). Infrared spectra were recorded on
(
Perkin-Elmer instrument and in each case the sample was referenced against a blank KBr pellet.
Surface area measurements and pore size distributions analysis were done by nitrogen adsorption
on Micromeritics ASAP 2020 instrument at an adsorption temperature 77 K, after pre treating the
sample under high vacuum at 250 ºC for 4 h. Scanning electron micrographs and elemental analysis
was done by FEI, QUNTA 200 instrument. The ammonia- TPD data were recorded for all catalyst
samples using AutoChem II 2920 TPD/TPR instrument (Micromeritics, USA) by using 10% NH
in He.
3
2
.3 Reactor setup and procedure
The vapor phase reaction of phenol with methanol was carried out in vapor phase reactor of the
dimensions ID 8 mm, OD 19 mm and length 400 mm (Chemito Technologies Pvt. Ltd., Nashik,
India). Reactor was provided with temperature sensor (T1) at the bottom to read the temperature
of reaction mixture during flow of reaction mass through the reactor. The reactor was equipped
with HPLC pump (1) for constant volume flow of reaction mixture, pre-heater supplied to just
before the reactor so that reaction mixture was vaporized before entering the reactor. MFC (mass
flow controller) for continuous and constant flow of nitrogen gas was connected to the output from
pre-heater (2) before entering the reactor. Nitrogen gas creates inert atmosphere also act as a carrier
gas. Input from MFC, pre-heater and HPLC pump were connected to the reactor. The reactor was
provided with furnace to heat it at higher temperature range up to 500 °C.Output from reactor was
connected to condenser (provided with chiller) to condense the vapors of reaction mass and
samples collected at time intervals from the condenser. Temperature of the entire system was
maintained by digital temperature controller, and pressure of nitrogen gas was regulated by a
regulator. The basic diagram of reactor set up is shown in Scheme 1.
3
3
3
. Results and discussion
.1 Catalyst characterization
.1.1 X-ray diffraction (XRD)
6

The XRD patterns of all catalysts are shown in Figure 1. XRD pattern of 40% ZrO
shows no sharp peaks of zirconia phases are observed. However, with introduction of WO
in next catalyst sample, 30% ZrO -10% WO -SiO , tetragonal phases of zirconia at 2θ values of
0° (111), 50° (220), and 60° (311) could be seen. Formation of tetragonal phases of ZrO , explains
the role of WO species in stabilizing the phases of zirconia [31-33]. However, WO peaks started
to be visible from 20% loading and above. Presence of peaks at 23-25° represent the monoclinic
phase of WO [34,35]. These intense peaks indicate the aggregation of WO species leading to
WO microcrystallites on the surface of ZrO -SiO . The average crystallite size of all catalyst
samples was calculated using Scherer equation. It was found that crystallite size increased with
increase in WO loading. The values of crystallite size are as follows. (a) 40% ZrO - SiO (N.D.)
b) 30% ZrO -10% WO - SiO (5.15 nm) (c) 20% ZrO - 20% WO -SiO (41.21 nm) (d) 10%
ZrO - 30% WO -SiO (47.24 nm) (e) 40% WO - SiO (32.31 nm). These structural changes are
thought to make the catalysts more effective in performance with increased tungstate species.
2
-Silica catalyst,
3
species
2
3
2
3
2
3
3
3
3
3
2
2
3
2
2
(
2
3
2
2
3
2
2
3
2
3
2
3
.1.2 FTIR spectroscopy
The FTIR results are shown in Fig.2a and 2b. No distinct peaks between Zr-O-Zr and
W-O-W vibrations were observed because of high silica content. Broad range peaks in the range
-1
of 3200-3600 cm represents the O-H stretching vibrations associated with silanol groups whereas
-1
the band near ~1630 cm are O-H bending vibrations due to the physically adsorbed water on
-1
silica surface. The band at 1100-1200 cm is due to siloxane bond (Si-O-Si). Apart from these
−
1
peaks, two peaks at 752 and 710 cm can be attributed to the W–O–W stretching vibration of
−
1
monoclinic WO
this band possibly is due to both crystalline and non-crystalline species. This observation is
supported by XRD pattern which showed the presence of monoclinic WO crystallites [32]. The
strong absorption peaks near 510 cm correspond to the Zr–O vibrational modes [39, 40].
3
(Fig. 2b) [36-38]. The weak band at 791 cm is assigned to W-O vibration and
3
-1
For further understanding, FTIR spectra of as such WO
Here few observations could be made. In case of ZrO , a sharp band near ~800 cm was observed
which became weak when observed in 40% ZrO -SiO . Similarly, a broad band was observed in
3
and ZrO
2
were compared (Figure 2b).
-1
2
2
2
-1
case of WO
3
in the range of 400-1000 cm . This broad band became mild and sharp in case of
7

-
1
4
0% WO
3
-SiO
2
. The increase in band broadening near 800 cm is due to increasing content of
-WO -SiO combinations (Fig. 2a).
WO . This variation was observed in case of all ZrO
3
2
3
2
3
.1.3 BET Surface area and pore analysis
The textural properties of catalysts by nitrogen adsorption-desorption measurements are given in
2
3
Table 1. Fumed silica has a surface area of 306.1 m /g and pore volume of 0.774 cm /g and pore
diameter of 10.1 nm. Comparison of nitrogen adsorption-desorption isotherm of silica and various
catalysts are shown in Fig. (a-f) . The isotherms are of type IV with H1 hysteresis loop where
initially nitrogen is adsorbed on catalyst surface slowly at low relative pressure which indicates
the monolayer–multi layer adsorption on the surface. With increase in loading of zirconia and
tungstate, the corresponding surface area of SiO
decreased with marginal difference; however, important changes were found in pore sizes. Instead
of decreasing, on loading ZrO and WO on silica, pore size increased. It may be because initially
the surface and pores of silica are covered by zirconia and then loading of WO takes place on the
surface of ZrO -SiO which increases the pore size of the catalyst. High intensity peaks (23-25°)
2
was found to decrease. Pore volume was
2
3
3
2
2
in XRD (Fig. 1) represent crystalline phases of WO
catalyst.
3
supporting these changes on the surface of
3
.1.4 Acidity measurements
Acidity of catalyst samples was measured by temperature programmed desorption of ammonia
NH -TPD, Table 2). Two types of acidity were found in all catalyst samples. Weak acidity peaks
were observed in the range of 120-140 °C. The strong acidity peaks were observed around 670-
50 °C. These peaks indicate both type of acidic properties are present in catalysts. Increased WO
content increases the acidity. 40% WO -SiO showed highest acidity value of 0.687 mmol/g of
(
3
7
3
3
2
ammonia (combining weak and strong acidity values). Figure 4 shows the acidity pattern of
catalysts.
3
.1.5 SEM/EDXS analysis
8

SEM images of catalysts are shown in Figure 5. There are no marked differences in images of all
catalyst samples. Elemental composition of catalysts was confirmed by EDXS method. Typical
EDXS elemental composition for 10% ZrO
2
- 30% WO
3
-SiO
2
shown in Figure 6. Table 3
represents the compositions of xZrO -yWO -SiO
2
3
2
series catalysts. Since the silica used in the
catalyst preparation is fumed silica, irregular shape and sizes of particles are observed in the images
and the particle size also varied from 10 µm to 60 µm.
3
.2 Catalyst activity
3
.2.1 Material balance and product distribution
For material balance, reaction pathways shown in Scheme 2 were considered. But under all
experimental conditions used in this work, 2-methyl anisole (E) and 2, 4, 6-trimethyl phenol (F)
were not formed. Pathway 1 leads to anisole (B) which does not lead to 2-methyl anisole
subsequently. Pathway 2 goes to o-cresol (C) and subsequently 2,6- xylenols (D). Rearrangement
of anisole (B) to o-cresol (C) is one of the possible the reactions. It was observed that during the
course of reaction, selectivity of anisole decreased and that of o-cresol increased. It can be said
that the rearrangement reaction is one of most probable pathway for the formation of o-cresol apart
from direct C-alkylation of phenol. Since methanol was used in excess in all the sets of
experimental conditions, dimethyl ether was also formed due to dehydration of methanol,
formation of which is not shown in Scheme 2. Carbon balance was also done to find that formation
of products could be accounted for all organic compounds and ~10-12% carbon was deposited
after 6 h.
In order to understand the reaction mechanism and further improvement in the process to get
desired products like anisole and o-cresol following three sets of experiment were planned.
1
2
3
. Reaction of phenol and methanol
. Rearrangement of anisole to o-cresol to confirm rearrangement reaction
. Methylation of o-cresol to 2,6- xylenol. 3.2.2 Efficacy of catalysts
Independent experiments were also done to understand reaction mechanism and product profile
under identical conditions. As given before three sets of experiments were performed: (i) reaction
9

of phenol and methanol, (ii) rearrangement of anisole to o-cresol to confirm rearrangement
reaction, and (iii) methylation of o-cresol to 2,6- xylenol. In particular, 20%WO -ZrO and 40%
WO -ZrO were prepared as per procedure mentioned in Section 2.2 without using fumed silica.
These two catalysts were also tested under the same reaction conditions. In case of 20%WO -ZrO
conversion of phenol after 6 h was 45% with anisole selectivity of 25% whereas o-cresol
selectivity was found to 53.7% (Figure (7a, 7b). Increasing amount of WO3, 40% WO -ZrO led
to 51.4 % conversion of phenol with selectivity of anisole and o-cresol found to be 38.4 % and
7.2%, respectively (Fig. 8a,8b). However, major drawback of these two catalysts was formation
of 2,6-xylenol, trimethyl phenol (TMP) and other alkylated derivatives of phenol. 2,6-Xylenol
formation was found 11.2% and 10.2% in 20% WO -ZrO and 40% WO -ZrO respectively.
Whereas other impurities and TMP together constituted 9.9% and 4.3% in the case of 20%WO
ZrO and 40% WO -ZrO2., respectively Then we thought of using SiO as a support and tested
in the reaction where anisole selectivity was improved as compared to the above catalysts. So, a
series of xZrO -yWO -SiO catalysts were prepared to tune the selectivity. Here the role of silica
3
2
3
2
3
2
,
3
2
4
3
2
3
2
3
-
2
3
.
2
2
3
2
in the ternary oxide system can be understood. Silica dilutes the activity of other two oxides and
limits the formation of impurities as well as increases the selectivity of anisole and o-cresol based
on variable compositions of other two oxides.
Initially a series of catalysts xZrO
2
- yWO
3
- SiO were tested to evaluate their activity and
2
selectivity in vapor phase methylation of phenol. For screening of catalysts, typical reaction
conditions were as follows. Phenol : methanol mole ratio-1:5, catalyst quantity - 2 g, temperature
-1
-
623 K, WHSV- 1 h and nitrogen gas flow- 30 ml/min. Reaction was continued for 6 h.
Conversion of phenol and anisole selectivity profiles are depicted in Figure 9(a) and (b). In results,
th
it was found that steady state condition was achieved only after 4 h in all catalyst combinations;
however, selectivity of anisole was found to decrease in all the combinations of catalysts except
2
0% ZrO
constant after the steady state (i.e. 3 h). In the case of 10% ZrO
condition was achieved after 4 h with 47 % conversion of phenol and 52 % anisole selectivity;
hence 10% ZrO - 30% WO - SiO was selected for further reaction parameters optimization. It
was observed that increasing amount of WO in catalyst increases the conversion of phenol as well
as improves the selectivity towards anisole but 40% WO -SiO gave lesser conversion because of
2
-20%WO
3
-SiO
2
, where conversion of phenol and selectivity of anisole remained
2
- 30% WO - SiO , steady state
3
2
2
3
2
3
3
2
1
0

presence of monoclinic WO
of decreasing activity [37]. The experimental results thus prove that only ZrO
supported on silica may not give good results but the combination of both the oxides can give
better conversion and selective formation of anisole. The tetragonal phase of ZrO is stabilized by
addition of WO (as revealed by XRD characterization) which further improved acidity of the
3
phase. Monoclinic WO
3
at 2θ=23- 25° is known as boundary point
2
or only WO
3
2
3
catalyst combinations (as found by ammonia TPD results). These two factors probably improved
activity of catalysts in terms of conversion of phenol and selectivity of anisole in a ternary mixture
of xZrO
2
-yWO
3
-SiO .
2
To understand the mechanism by which there is a decrease in anisole selectivity during the course
of reaction, two more sets of experiment were conducted.
Set-1 experiment included methylation of o-cresol at different temperatures with optimized
catalyst 10% ZrO
2
- 30% WO
3
- SiO
2
by maintaining o-cresol: methanol mole ratio as 1:5 and
gas flow maintained at 30 ml/min (Fig. 10a). The
-1
WHSV as 1 h , catalyst weight – 2 g and N
2
purpose of this set of experiment was to confirm the pathway of methylation of phenol. In the case
of o-cresol, o-methyl anisole (o-MAN) formed as highly selective product at 573K (72 %) and
6
23K (~64%) and its selectivity remained nearly the same throughout the course of reaction
(Figure 10a). However, rearrangement of o-methyl anisole (o-MAN) to 2,6-xylenol was observed
at 673 K with constant conversion of o-cresol (~34%) (Figure 10b). These results are in agreement
with the one obtained during methylation of phenol done by us.
Set-2 experiment included only anisole to be passed through the catalyst bed of 2 g 10% ZrO
2
-
3
0% WO - SiO at different temperatures, while other reaction parameters were the same as above.
3
2
The product profile in this reaction is somewhat complex to understand. At low temperature 573
K, anisole conversion found very low (~10% after 6 h of reaction), with higher selectivity for o-
th
th
methyl anisole (o-MAN, ~75% at 6 h) and phenol as another product (25% at 6 h) (Figure 11a
and 11b). The formation of o-MAN and phenol is possible through the first reaction given below
in Scheme 3. At 623 K, phenol formation is almost constant (~45%) whereas o-MAN selectivity
was found decreasing (from 50% to ~20% at the end of 6 h) with gain in selectivity of o-cresol
(
from ~7 % to 23 % at the end of 6 h) and 2,6-xylenol (from 0 % to ~12 % at the end of 6 h)
Figure 9b). Conversion of anisole was found to be ~57 % after 6 h. At 673 K, 94 % of anisole
(
1
1

was converted and hence more side products were observed like other isomers of xylenol, 2,4,6-
trimethyl phenol and duranol (Figure 11c). The increased formation of o-cresol is a result of
reactions 2 and 4 given in Scheme 3. Phenol is the product in most of the steps (1, 2 and 3) whereas
o-MAN is formed initially and then involved in formation of o-cresol. These observations prove
that there is a rearrangement of aryl ethers to respective phenolic compound and rest of the
products formed are because of following set of reactions given in Scheme 3. The results are
comparable to the one obtained by Prasomsri et al [38].
From the above sets of experiments, it can be concluded that O-methylation of phenolic molecules
is a first step of reaction and then there is a rearrangement of anisole to o-cresol (in case of
methylation of phenol) and o-MAN to 2,6- xylenol (in case of methylation of o-cresol).
3
.3 Optimization of process parameters
3
.3.1 Effect of WHSV
-1
Effects of WHSV were studied in the range from 1 to 3 h , maintaining mol ratio, phenol :
methanol (1:5), catalyst weight (10% ZrO - 30% WO - SiO ) – 2g and temperature maintained
2
3
2
-1
at 623 K in presence of nitrogen flow rate- 30 ml/min. With increase in WHSV from 1 to 3 h ,
conversion of phenol decreased (Figure 12). The probable reason for decreasing conversion is the
lesser contact time. However, the selectivity of anisole increased with increased WHSV. Because
of lesser contact time, the rearrangement of anisole to o-cresol is inhibited. In comparison, WHSV
-1
th
of 1 h gave higher conversion of phenol (47%) as well as good anisole selectivity (52%) at 6 h;
-1
hence further reactions were carried out with WHSV of 1 h .
.3.2 Effect of mole ratio
Mole ratio studies were carried out in range from 1:3 to 1:9 (phenol: methanol) over optimized
3
-1
catalyst, 10% ZrO
2
- 30% WO
3
-SiO (2g) and optimized WHSV of 1 h . The temperature of
2
reaction was set to 623 K. The results are shown in Figure 13 (a) and (b). With increase in molar
ratio from 1:5 to 1:9, the conversion of phenol was found to decrease because lower molar
concentration of phenol (hence lower partial pressure of phenol) passed through the catalyst bed
during the reaction and hence conversion was found to decrease [39]. However, anisole selectivity
1
2

was lowered in case of increasing mole ratio from 1:5 to 1:7 and 1:9. This inversion in selectivity
of anisole may be due to increased concentration of methanol which provides more methyl groups
to attack on reactive ortho positions of phenol ring resulting in formation of o-cresol and 2,6-
xylenol. With this conclusion, in case of 1:3 mol ratio, the conversion was expected to increase
but it did not show any such increment (Figure 13a). The probable reason for lower conversion
(5.2%) may be due to excess molar concentration of phenol in reaction mixture lead to more coke
formation and hence conversion was consistently decreased due to blockage of the active sites.
Hence, mole ratio 1:5 was optimized for further study of temperature effects.
3
.3.3 Effect of temperature
Temperature plays a crucial role in deciding conversion and selectivity of products in either liquid
phase or vapor phase reaction. In temperature studies, reactions were carried out at temperature
between 573- 673 K, maintaining the optimized reaction conditions of mole ratio of 1:5 and WHSV
-1
of 1 h in each study. It was observed from the results (Figure 14a) that increased temperature
673 K) leads to lower conversion of phenol. The experiments were repeated and it was found that
(
there was reduction in porosity due to carbon deposition on the catalyst surface. Because formation
of o-cresol and xylenols followed by carbonization the selectivity to anisole decreased at 673 K.
So the regenerated catalyst was found to give the same conversion and selectivity for 1 h and
thereafter the same trend was observed as regards more byproducts and carbon formation.
At lower temperature of 573 K, conversion was less. Conversion and selectivity profile at various
temperatures are shown in Fig 14 (a) and (b).
3
.3.4 Time on stream (TOS) study
To understand the activity and stability of catalyst, time on stream (TOS) studies were conducted
at optimized conditions of mole ratio of 1:5 (phenol: methanol), catalyst weight (2 g), WHSV (1
-1
h ) and temperature (623 K) and nitrogen gas flow 30 ml/min. The reaction was continued for 16
h to check the stability of catalyst. Conversion of phenol was marginally decreased with time;
th
however, from 10 h onwards conversion started to decrease and reached to the final conversion
of 38 % (Figure 15a); similar trend was observed in selectivity of anisole (Figure 15b). It was
decreased linearly up to first 5 h and then remains stable up to 8 h. It means catalyst has stability
1
3

up to 8 h and hence it has to be regenerated to get consistent selectivity of anisole. From 8 h
onwards, o-cresol increases until 16 h and remain constant. The decrease in conversion may
probably due to the formation of coke on the surface of catalyst. The presence of carbon was
confirmed and quantified from EDXS analysis. The total carbon content was found to be ~12%.
Fig. 16 depicts the comparison of SEM image of fresh and spent catalyst after continuing reaction
for 16 h. As per SEM images, the smooth surfaces of fresh catalyst are observed as rough in case
of spent catalyst. The change in surface morphology is because of coke formation on the surface
of catalyst. The elemental composition of fresh and spent catalyst showed differences as shown in
Table 4. The carbon content in the spent catalyst was found up to ~12%. This generation of carbon
on the surface of catalyst reduces the activity of catalyst after 16 h. Spent catalyst was also
characterized by XRD and ammonia-TPD techniques to compare with the fresh catalyst as shown
in Fig. 17 and 18, respectively. XRD comparison showed less intensity peaks in spent catalyst
which supports the formation of coke on the surface of catalyst. However, there were no phase
changes when compared with the fresh catalyst (Table 5). Ammonia –TPD thermograms of fresh
and spent catalyst are given Fig. 18. The decrease in acidity of catalyst is due to coke formation
on the surface of catalyst.
4
. Conclusion
Methylation of phenol to anisole or o-cresol is industrially important. Vapor phase methylation
was carried out successfully with a series of novel mixed metal oxide catalysts xZrO -yWO -SiO
Amongst the series of catalysts, 10% ZrO -30% WO -SiO was proved to be the best catalyst in
2
3
2
.
2
3
2
terms of conversion and selectivity towards anisole. At 623K, anisole was found as a major product
but on increasing temperature up to 673K, o-cresol selectivity increased. Increasing mole ratio
leads to increased cresol selectivity. So, the reaction parameters were optimized by carrying out
detailed studies. For anisole formation, optimized conditions are (1:5 Phenol: methanol mol ratio),
-1
WHSV- 1 h and 623 K for 8 h. The great advantage of the catalyst is the switching of selectivity
for desired product by manipulating reaction parameters. The catalyst 10% ZrO
2
- 30% WO
3
- SiO
2
was also found stable up to 8 h. Thus, methylation of phenol using 10% ZrO - 30% WO
2
3
- SiO
2
is
environmentally viable and benign process and it opens up the freedom of manipulating the
1
4

selectivity to either anisole or o-cresol using a single catalyst through choice of proper reaction
parameters.
Conflict of Interest Statement
The authors declare no conflict of interest.
Acknowledgements
K.H.B. thanks UGC for providing JRF under its BSR programme in Green Technology.
G.D.Y. received support from R.T. Mody Distinguished Professor Endowment and J.C. Bose
National Fellow of Department of Science and Technology, Government of India.
1
5

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1
8

(
e)
(
d)
(
c)
(
b)
(
a)
1
0
20
30
40
50
60
70
80
Angle (2 theta)
Figure1. XRD of xZrO
WO - SiO , (c) 20% ZrO
SiO
2
- yWO
3
-SiO
2
series catalysts (a) 40% ZrO
2
- SiO
2
, (b) 30% ZrO -10%
2
3
2
2
-20% WO
3
- SiO2, (d) 10% ZrO -30% WO
2
3
- SiO
2
and (e) 40% WO -
3
2
1
9

4
0% WO -SiO
3 2
1
0% ZrO -30% WO -SiO
2 3 2
2
0% ZrO -20% WO -SiO
2
3
2
3
0% ZrO -10% WO -SiO
2
2
3
4
0% ZrO - SiO
2 2
4000 3500 3000 2500 2000 1500 1000
500
-
1
Wave number (cm )
Figure 2(a). FTIR comparison of xZrO
2
-yWO
3
-SiO
2
series catalysts
4
0% WO - SiO2
3
WO3
4
0% ZrO - SiO2
2
ZrO2
4000 3600 3200 2800 2400 2000 1600 1200
800
400
Wave number (cm-1)
Figure 2(b). FTIR comparison of ZrO
2
and WO
3
with 40% ZrO
2
-SiO
2
and 40% WO
3
- SiO
2
2
0

(
b) 40% ZrO - SiO
(
a) Aerosil -300 (SiO2)
2 2
750
600
450
300
150
0
500
400
300
200
100
0
Qty. Adsorbed (cm³/g STP)
Qty. Adsorbed (cm³/g)
0
0.2
0.4
0.6
0.8
1
1.2
0
0.2
0.4
0.6
0.8
1
1.2
Relative Pressure (P/P0)
Relative Pressure (P/Po)
(
c) 30% ZrO - 10% WO - SiO
2 3 2
(d) 20% ZrO - 20% WO -SiO
2 3 2
6
4
3
1
00
6
00
50
Quantity Adsorbed (cm³/g)
Qty. Adsorbed (cm³/g)
50
00
50
0
4
300
150
0
0
0.2
0.4
0.6
0.8
1
1.2
0
0.2
0.4
0.6
0.8
1
1.2
Relative Pressure (P/Po)
Relative Pressure (P/Po)
(
e)10% ZrO - 30% WO - SiO
2
3
2
6
00
(f) 40%WO - SiO
3
2
4
00
00
00
00
0
Qty. Adsorbed (cm³/g)
Qty. Adsorbed (cm³/g)
450
300
150
0
3
2
1
0
0.2
0.4
0.6
0.8
1
1.2
Relative Pressure (P/Po)
0
0.2
0.4
0.6
0.8
1
1.2
Relative Pressure (P/Po)
Figure 3. N
2
adsorption-desorption isotherms of xZrO
2
-yWO
3
-SiO series catalysts at 77 K
2
2
1

0.022
0.020
0.018
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
(
(
(
(d)
(e)
a)
b)
c)
-0.002
0
100 200 300 400 500 600 700 800 900
Temperature (deg.C)
Figure 4. Ammonia-TPD thermograms of xZrO
2
-yWO
3
-SiO
2
series of catalysts. (a) 40% ZrO
2
-
-
SiO
SiO
2
2
, (b) 30% ZrO
2
-10% WO
3
- SiO
2
, (c) 20% ZrO - 20% WO
2
3
- SiO2, (d) 10% ZrO - 30% WO
2
3
and (e) 40% WO
3
- SiO
2
2
2

Aerosil-300
40% ZrO
2
- SiO
2
3
0% ZrO
2
-10% WO
3
-SiO
2
20% ZrO
2
-20% WO
-SiO
3 2
1
0% ZrO
2
-30% WO
3
-SiO
2
3 2
40% WO -SiO
Figure 5. SEM images of fumed silica and xZrO
000x
2
- yWO
3
- SiO series catalysts at magnification
2
2
2
3

Figure 6. EDXS analysis of xZrO
2
- yWO
3
- SiO series catalysts
2
2
4

7
6
5
4
3
2
1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0
.0
0
1
2
3
4
5
6
7
Time (h)
Figure 7(a). Phenol conversion profile with 20% WO
3
-ZrO
2
catalyst. Reaction conditions -
-1
Phenol: methanol (1:5) mole, catalyst – 2 g, WHSV – 1 h , temperature – 623 K, N flow – 30
2
mL/min
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
1
2
3
4
5
6
7
Time (h)
Anisole
o-Cresol
Tmp
2
,6-Xylenol
Other
Figure 7(b). Selectivity profile with 20% WO
3
-ZrO
2
catalyst. Reaction conditions - Phenol:
-
1
methanol (1:5) mole, catalyst – 2 g, WHSV – 1 h , temperature – 623 K, N flow – 30 mL/min
2
2
5

60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
1
2
3
4
5
6
7
Time (h)
Figure 8(a). Phenol conversion profile with 40%WO
3
-ZrO
2
catalyst. Reaction conditions - Phenol:
flow – 30 mL/min
-
1
methanol (1:5) mole, catalyst – 2 g, WHSV – 1 h , temperature – 623 K, N
2
50.0
40.0
30.0
20.0
10.0
0.0
0
1
2
3
4
5
6
7
Time (h)
Anisole
Other
o-Cresol
Tmp
2,6-Xylenol
Figure 8(b). Selectivity profile with 40%WO
3
-ZrO
2
catalyst. Reaction conditions - Phenol:
flow – 30 mL/min
-
1
methanol (1:5) mole, catalyst – 2 g, WHSV – 1 h , temperature – 623 K, N
2
2
6

60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
1
2
3
4
5
6
7
Time (h)
4
2
4
0%ZrO2-SiO2
30%ZrO2-10%WO3-SiO2
10%ZrO2-30%WO3-SiO2
0%ZrO2-20%WO3-SiO2
0% WO3-SiO2
Figure 9(a). Phenol conversion profile with different catalysts. Reaction conditions - Phenol: methanol
-1
(
1:5) mole, catalyst – 2 g, WHSV – 1 h , temperature – 623 K, N flow – 30 mL/min, time – 6 h
2
2
7

100.0
80.0
60.0
40.0
20.0
0
1
2
3
4
5
6
7
Time (h)
4
2
4
0%ZrO2-SiO2
30%ZrO2-10%WO3-SiO2
10%ZrO2-30%WO3-SiO2
0%ZrO2-20%WO3-SiO2
0% WO3-SiO2
Figure 9(b). Anisole selectivity profile with different catalysts. Reaction conditions- Phenol:
-1
methanol (1:5) mole, catalyst – 2 g, WHSV – 1 h , temperature – 623 K, N
2
flow – 30 mL/min,
time – 6 h
2
8

80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
Time (h)
Conversion o-Cresol, 573 K
Conversion o-Cresol, 673 K
selectivity of o-MAN, 623 K
Conversion o-Cresol, 623 K
Selectivity of o-MAN, 573 K
selectivity of o-MAN, 673 K
Figure 10(a). o-Cresol conversion and o-methyl anisole selectivity during methylation of o-cresol
with 10%ZrO -30%WO -SiO at different temperatures. Reaction conditions- o-Cresol: methanol
1:5) moles, catalyst – 2 g, WHSV – 1 h-1, N flow – 30 mL/min
2
3
2
(
2
2
9