Vapor-phase alkylation of phenol with tert-butyl alcohol catalyzed by H3PO4/MCM-41

Article abstract of DOI:10.1016/S1872-2067(09)60085-1

The catalytic performance of Al-MCM-41 containing 5-35 wt H ₃PO₄ was studied for the vapor-phase alkylation of phenol with tert-butyl alcohol (TBA) from 383 to 493 K. 4-Tert-butyl phenol was produced as the main product with moderate selectivity. The product distribution depends on the reaction temperature, number of acid sites, and the Broensted to Lewis sites ratios. A lower molar ratio of reactants (TBA/phenol = 2) and a higher space velocity facilitated the production of 4-tert-butyl phenol. The influence of various parameters such as temperature, reactant feed molar ratio, feed rate, and time on stream were investigated for conversion yield and product selectivity.

Full text of DOI:10.1016/S1872-2067(09)60085-1

CHINESE JOURNAL OF CATALYSIS
Volume 31, Issue 7, 2010
Online English edition of the Chinese language journal
RESEARCH PAPER
Cite this article as: Chin J Catal, 2010, 31: 759–764.
Vapor-Phase Alkylation of Phenol with Tert-butyl Alcohol
Catalyzed by H3PO4/MCM-41
Mehran GHIACI*, Behzad AGHABARARI
Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran
Abstract: The catalytic performance of Al-MCM-41 containing 5–35 wt% H3PO4 was studied for the vapor-phase alkylation of phenol with
tert-butyl alcohol (TBA) from 383 to 493 K. 4-Tert-butyl phenol was produced as the main product with moderate selectivity. The product
distribution depends on the reaction temperature, number of acid sites, and the Brönsted to Lewis sites ratios. A lower molar ratio of reac-
tants (TBA/phenol = 2) and a higher space velocity facilitated the production of 4-tert-butyl phenol. The influence of various parameters
such as temperature, reactant feed molar ratio, feed rate, and time on stream were investigated for conversion yield and product selectivity.
Key words: alkylation; tert-butylation; phenol; tert-butyl alcohol; MCM-41
One of the most important alkyl phenols is 4-tert-butyl-
phenol, which has many applications [1–5]. The t-butylation of
phenol is a typical Friedel-Crafts alkylation and can be cata-
lyzed by acid sites. Numerous studies have been reported on
homogeneous and heterogeneous catalysis using several acid
catalysts such as Brönsted acids, Lewis acids, activated clay,
ionic liquids, and solid acid catalysts in supercritical carbon
dioxide [1–4,6,7]. However, homogeneous catalysts are envi-
ronmentally unfriendly, unselective, and require tedious
work-up for the separation of the catalyst. On the other hand,
acid catalysts supported on zeolites do not have any of these
problems and many have been used for the t-butylation of
phenol over zeolites such as Y [6,8] and mordenite [7,9] in the
gas or liquid phase.
produces carbon alkylated products [4,9].
In the last decade, the t-butylation of phenol over
H-AlMCM-41 [1], Fe-AlMCM-41 [4], sulfated AlMCM-41
with different SiO2/Al2O3 molar ratios [11], MeMCM-41 and
MeMCM-48 (Me = Ga, Fe, Al, or B) [12], Zn and Fe con-
taining AlMCM-41 [13] has been studied. In these studies, the
researchers have tried to find the best conditions for this reac-
tion to increase the selectivity for 4-TBP and to increase the
conversion of phenol [10,14].
In this work we designed a vapor-phase reaction using a
moderate solid acid catalyst for the synthesis of
tert-butylphenol because of the increasing demand for generic
procedures in vapor-phase chemistry and the broad range of
commercial applications of alkylated phenols. This was done
as part of our ongoing research concerned with the develop-
ment of an environmentally benign chemical process for fine
chemical synthesis. In this investigation H3PO4 was supported
on AlMCM-41 in different proportions. We studied the activity
of these catalysts toward the t-butylation of phenol in the vapor
phase and the results are discussed in detail. We demonstrate
that this new catalyst can act as a reusable heterogeneous
catalyst for the alkylation of phenol to the corresponding al-
kylated products.
In this reaction, o-alkylation is kinetically favored because
of the –OH group on the phenol but this product is thermody-
namically unfavorable. On the other hand, ortho-tertiary butyl
phenol (2-TBP) readily isomerizes into the less hindered
p-isomer (4-TBP) [1,10]. Furthermore, in this reaction the
selectivity of the product depends on the nature of the acidic
sites present in the catalysts and the reaction temperature [7]. It
has been found that t-butyl phenyl ether (TBPE) is produced in
the presence of weak acidic catalysts as a major product. Under
strongly acidic conditions or high temperatures, the reaction
We have previously reported on the characterization of the
Received date: 26 January 2010.
*Corresponding author. Tel: +98-311-3913254; Fax: +98-311-3912350; E-mail: mghiaci@cc.iut.ac.ir (M. Ghiaci)
Foundation item: Research Council of Isfahan University of Technology.
Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(09)60085-1

Mehran GHIACI et al. / Chinese Journal of Catalysis, 2010, 31: 759–764
1
catalysts using 29Si, 27Al, 31P, 23Na, and H MAS NMR spec-
that the loading of H3PO4 onto Al-MCM-41 did not change the
regular arrangement of the uniform channels of the support
[17]. Interestingly, by loading approximately 30 wt% phos-
photungstic acid onto Al-MCM-41, the surface area of the
support decreases by less than 20% [10], but loading the same
support with approximately the same weight percent of H3PO4
results in the surface area decreasing to a larger extent. The
crystalline structure of the phosphotungstic acid (Keggin units)
and the amorphous structure of the phosphoric acid could
explain this difference. The 31P MAS NMR spectra of the 30
wt% H3PO4/Al-MCM-41(200) showed two peaks at δ = –10
and 0, which may correspond to the P atom in pyrophosphoric
acid or the terminal and monomeric PO43– groups, respectively.
From an analysis of the Si, Al, and P NMR signals, the forma-
tion of Si–O–Al–O–P bridges seems to be favored at the sur-
face of the Al-MCM-41 catalysts. The FT-IR spectra for the 30
wt% H3PO4/Al-MCM-41(70) containing adsorbed pyridine
shows the contribution of pyridine adducts from 1 650–1 450
cm–1. The formation of pyridinium ions, as shown by the ad-
sorption at 1 545 and 1 490 cm–1, is characteristic of Brönsted
acid sites and both Brönsted and Lewis acid sites, respectively.
A comparison of the spectra revealed that 30 wt% H3PO4/
Al-MCM-41(70) possesses a greater amount of acid sites than
the other catalysts. The concentration of Brönsted and Lewis
acid sites was calculated after evacuation at 473 K using the
extinction coefficient of the bands of Brönsted and Lewis acid
site adsorbed pyridine [18]. This revealed the presence of high
amounts of Brönsted and Lewis acid sites in 30 wt%
H3PO4/Al-MCM-41(70).
troscopy [15]. We have shown that H3PO4 loading on
AlMCM-41 induces a drastic decrease in the specific surface
area without any structural distortion of MCM-41. Therefore,
the main reason for choosing these catalysts for the t-butylation
reaction was that this reaction probably only occurs on the
external surface of the catalysts, and the acidity of the catalysts
as well as thermodynamic factors greatly affect the conversion
and selectivity.
1 Experimental
1.1 Preparation of the catalysts
The mesoporous molecular sieves Al-MCM-41 and
H3PO4/Al-MCM-41 were prepared in accordance with the
reported methods [16].
1.2 Catalytic activity
Catalytic tests were performed with the 5 wt% H3PO4/Al-
MCM-41(20) catalysts. The vapor-phase tests were carried out
using approximately 0.5 g of catalyst (20–40 mesh) in a feed
Pyrex reactor (i.d. 8 mm). The reactor was inserted into an
electric oven controlled by a J thermocouple and operated at
atmospheric pressure, and the temperature range was 383–493
K. The feed was tert-butyl alcohol (TBA):phenol (2:1 molar
ratio), which was introduced by an infusion pump (GENIE,
Kent Scientific Corporation) and nitrogen was used as the
carrier gas by regulation with a mass-feed meter. The catalyst
was previously activated in situ for 1 h at 673 K using a 25
ml/min N2 feed. In each experiment, the products were col-
lected after 8 h by condensing them in a trap containing salt-ice
and cooled to 258 K. Analyses were done off-line using a
Shimadzu gas chromatograph (model 14A) equipped with a
wide bone OV-17 (60 m) capillary column and an FID detector.
2.2 Catalytic studies
The vapor phase t-butylation of phenol with TBA was car-
ried out over Al-MCM-41 with SiO2/Al2O3 = 20, 70, and 150,
containing 5, 10, and 20 wt% H3PO4, respectively, from 383 to
493 K. The molar ratio chosen in these experiments for TBA to
phenol was 2:1. The major products were 4-tert-butyl phenol
(4-TBP), 2-tert-butyl phenol (2-TBP), and 2,4-di-tert-butyl
phenol along with less than 5% other products that were not
identified. We did not obtain any tert-butyl phenyl ether
(TBPE) even at high phenol conversion under the best reaction
conditions.
2 Results and discussion
2.1 Characterization
The physicochemical characterization of the catalysts was
similar to our earlier report [15]. However, other important
conclusions that we made from the characterization results are
outlined here. The XRD data indicated that the MCM-41
samples exhibit an ordered hexagonal structure characterized
by an intense reflection peak at a d spacing of 3.8 nm [16]. The
intensity of this low angle reflection peak did not change as the
H3PO4 loading increased. The specific surface area of the
samples calculated from the BET method ranges between
1 038 and 668 m2/g. Although the H3PO4 loading induces a
drastic decrease in the surface area, no structural distortions are
evident for Al-MCM-41. Consequently, it seems reasonable
The conversion of phenol over Al-MCM-41 with SiO2/
Al2O3 = 20, 70, and 150 was improved with an increase in
temperature. The maximum conversion was observed at 463 K
and when the temperature reached 493 K the conversion de-
creased by a few percent (Table 1).
We also observed that TBPE formed at 383 K but it was
completely absent at higher temperatures (small amounts of
ether formed at 413 K). In the early stages of the reaction, a
reversible O-alkylation occurs at a high rate because of the low
activation energy on almost all the acid sites (independent of
acid strength). The decrease in TBPE selectivity with increas-

Mehran GHIACI et al. / Chinese Journal of Catalysis, 2010, 31: 759–764
Table 1 Effect of temperature on the t-butylation of phenol over 5 wt% H3PO4/Al-MCM-41(20), 10 wt% H3PO4/Al-MCM-41(70), and 20 wt%
H3PO4/Al-MCM-41(150)
Product selectivity (%)
Catalyst
Temperature (K)
Phenol conversion (%)
TBPE
30.1
1.9
2-TBP
25.5
32.6
19.3
15.0
11.2
25.7
34.9
4-TBP
22.7
34.7
43.9
62.6
66.6
24.2
37.6
2,4-DTBP
21.2
Others
0.5
5 wt% H3PO4/Al-MCM-41(20)
383
413
443
463
493
383
413
35.7
40.2
44.9
47.7
36.8
47.5
49.4
30.1
0.7
—
35.8
1.0
—
19.1
3.3
—
17.9
4.3
10 wt% H3PO4/Al-MCM-41(70)
26.3
1.0
23.6
0.2
26.2
0.3
—
443
463
493
54.3
57.1
48.8
26.9
17.8
13.1
50.40
68.2
69.5
21.6
10.1
12.9
1.1
3.9
4.5
—
—
20 wt% H3PO4/Al-MCM-41(150)
383
413
33.6
37.4
29.5
3.3
—
23.8
30.7
22.2
36.2
24.2
29.2
0.3
0.6
443
463
493
48.6
54.1
41.9
31.6
15.6
18.6
39.4
61.0
56.8
27.5
21.6
20.2
1.5
1.8
4.4
—
—
Reaction conditions: TBA/phenol molar ratio = 2, WHSV = 1.41 h−1, time on stream = 2.5 h, catalyst = 0.5 g.
ing temperature may be due to rearrangement to a C-alkylated
product. It has been shown that such a rearrangement occurs on
heating or by contact with an acid catalyst [8,19].
Brönsted acid sites promotes side reactions of TBA such as
oligomerization and aromatization [1,20]. For this work, we
assumed that H3PO4 replaced most of the strong Brönsted acid
sites by moderate acid sites [15] and this decreases TBA oli-
gomerization [1,10].
The gradual increase in conversion of phenol from 383 to
463 K may be due to a predominant C-alkylation over
O-alkylation at higher temperatures and also the conversion of
O-alkylated to C-alkylated products. The major product was
found to be 4-TBP. A comparison of the product distribution at
383–463 K showed a low percentage of undesired products for
all the catalysts. This is important because the undesired
products may especially consume TBA without producing the
desired C-alkylated phenols, which leads to a low phenol con-
version. The decrease in conversion at higher temperature (493
K) may also be due to the partial blocking of pores and active
sites by coke or polybutene formation [10].
2.2.1 Effect of H3PO4
Different quantities of H3PO4 were loaded on Al-MCM-
41(70) using the incipient wetness technique. It has been found
that the selectivity for 2-TBP is highly dependent on the
amount of H3PO4 loaded on Al-MCM-41(70). With an increase
in H3PO4 loading from 5 to 30 wt%, the conversion of phenol
increased from 52% to 79%, respectively. The 35 wt%
H3PO4/Al-MCM-41(70) catalyst gave less conversion com-
pared with the 30 wt% H3PO4/Al-MCM-41(70) catalyst and
this is probably due to the masking of active sites. An optimum
selectivity for 4-TBP (~79%) at a conversion of about 79% was
obtained when 30 wt% H3PO4 was impregnated onto
Al-MCM-41(70) (Table 2).
The selectivity for 4-TBP was higher than for both 2-TBP
and 2,4-DTBP at temperatures higher than 383 K. The or-
tho/para ratio of the C-alkylated products reduces with an
increase in temperature. The increase in selectivity for 4-TBP is
proportional to the decrease in selectivity for 2-TBP and
2,4-DTBP, which is consistent with an increase in the rate of
dealkylation and it reaches an approximate equilibrium condi-
tion. Consequently, the rearrangement reaction during the
alkylation process will change the distribution of the alkyl
phenols. The continuous migration from o-alkylphenol to
p-alkylphenol is a thermodynamically driven process because
the latter is energetically more stable [19].
2.2.2 Effect of the SiO2 /Al2O3 ratio
To investigate the effect of Si/Al on the catalytic activity for
the alkylation of phenol with TBA in the vapor phase, we tested
different Al-MCM-41 (SiO2/Al2O3 ratio = 20–200) with 30
wt% H3PO4 at 463 K. As noted in the experimental section, the
catalysts were activated in situ for 1 h at 673 K under a 25
ml/min feed of N2 before the catalytic run. Therefore, no water
was present on the fresh catalyst surface. The experiments were
carried out at a molar ratio of TBA/phenol = 2.
A few reports [6,10] demonstrate that the observed increase
in para selectivity is accompanied by a decrease in phenol
conversion. In this respect, we think that the acidic strength of
the catalyst plays an important role. The presence of strong

Mehran GHIACI et al. / Chinese Journal of Catalysis, 2010, 31: 759–764
Table 2 Effect of H3PO4 weight percent on the t-butylation of phenol over Al-MCM-41(70)
Product selectivity (%)
Phenol conversion (%)
Catalyst
TBPE
—
2-TBP
27.2
20.6
17.8
14.6
12.2
8.1
4-TBP
61.1
67.8
68.2
70.0
71.2
75.2
79.1
72.2
2,4-DTBP
8.5
Others
3.2
Al-MCM-41(70)
45.3
52.2
57.1
65.5
62.5
70.7
78.6
75.4
5 wt% H3PO4/Al-MCM-41(70)
10 wt% H3PO4/Al-MCM-41(70)
15 wt% H3PO4/Al-MCM-41(70)
20 wt% H3PO4/Al-MCM-41(70)
25 wt% H3PO4/ Al-MCM-41(70)
30 wt% H3PO4/ Al-MCM-41(70)
35 wt% H3PO4/ Al-MCM-41(70)
—
8.7
2.9
—
10.1
10.3
11.2
3.9
—
5.1
—
5.4
—
12.3
12.5
15.8
4.4
—
5.7
2.7
—
6.1
5.9
Reaction conditions: catalyst = 0.5 g, temperature = 463 K, TBA/phenol molar ratio = 2, WHSV = 1.41 h−1, time on stream = 2.5 h.
As shown in Fig. 1, phenol conversion over 30 wt%
H3PO4/Al-MCM-41(70) was higher than that for the other
catalysts. The catalysts with a high hydrophilic character (low
Si/Al framework ratio) are the most active catalysts because of
their improved adsorption properties for polar molecules [7].
Therefore, we chose Al-MCM-41(70) as the best catalyst for
further studies.
2,4-DTBP. From Fig. 2, the selectivity for 4-TBP decreases
with an increase in the TBA content of the feed. The extent of
decrease in 4-TBP selectivity was drastic when the molar ratio
was changed from 2:1 to 3:1. At higher TBA to phenol molar
ratios the selectivity for 4-TBP decreased and the undesired
product selectivity increased. From the results of phenol con-
version and 4-TBP selectivity (desired product), a molar ratio
of 2:1 was chosen as the optimum for further studies.
2.2.3 Effect of molar ratio
2.2.4 Effect of WHSV
Figure 2 demonstrates the effect of TBA to phenol molar
ratio on the t-butylation reaction of phenol over the 30 wt%
H3PO4/Al-MCM-41(70) catalyst. It can be seen from the figure
that phenol conversion increases with an increase in the TBA to
phenol molar ratio. This data is in agreement with the literature
[10] because the increase in conversion might be due to in-
creased chemisorption of the TBA over the catalyst surface and
the greater availability of t-butyl cations near the chemisorbed
phenol. However, the increase in TBA content leads to a de-
crease in monoalkylated product selectivity because of a pos-
sible secondary alkylation reaction resulting in the formation of
The conversion of phenol increased with an increase in
WHSV from 0.564 to 1.41 h−1 and beyond this (1.692 h−1), a
decrease was observed (Fig. 3). This increase in phenol con-
version suggests the clustering of phenol at a lower WHSV,
which suppresses alkylation whereas at higher WHSV the
dispersal of phenol facilitates high conversion. At higher than
1.41 h−1, the conversion decreases because of the rapid diffu-
sion of phenol from the surface of the catalyst, which may be
responsible for the reduced reaction. Therefore, a negative
WHSV effect was clearly observed at higher than 1.41 h−1. The
selectivity for 4-TBP and 2-TBP remains nearly the same with
80
70
80
70
X(phenol)
S(2-TBP)
60
60
X(phenol)
S(4-TBP)
S(2,4-TBP)
S(2-TBP)
S(4-TBP)
50
50
S(2,4-TBP)
40
15
10
5
30
20
10
0
0
20 40 60 80 100 120 140 160 180 200
SiO2/Al2O3 molar ratio
1
2
3
4
TBA/phenol molar ratio
Fig. 1. Effect of SiO2/Al2O3 molar ratio on the t-butylation of phenol
over 30 wt% H3PO4/Al-MCM-41(70). Reaction conditions: catalyst = 0.5
g, temperature = 463 K, TBA/phenol molar ratio = 2, WHSV = 1.41 h−1,
time on stream = 2.5 h.
Fig. 2. Effect of TBA/phenol molar ratio on the t-butylation of phenol
over 30 wt% H3PO4/Al-MCM-41(70). Reaction conditions: catalyst = 0.5
g, temperature = 463 K, WHSV = 1.41 h−1, time on stream = 2.5 h.

Mehran GHIACI et al. / Chinese Journal of Catalysis, 2010, 31: 759–764
80
75
80
70
60
50
40
30
20
10
0
70
65
X(phenol)
S(2-TBP)
S(4-TBP)
S(2,4-TBP)
X(phenol)
S(2-TBP)
S(4-TBP)
S(2,4-TBP)
20
15
10
5
0
0.5
1.0
WHSV (h−1)
1.5
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Time on stream (h)
Fig. 3. Effect of WHSV on the t-butylation of phenol over 30 wt%
H3PO4/Al-MCM-41(70). Reaction conditions: catalyst = 0.5 g, tempera-
ture = 463 K, TBA/phenol molar ratio = 2, time on stream = 2.5 h.
Fig. 4. Effect of time on stream on the t-butylation of phenol over 30
wt% H3PO4/Al-MCM-41(70). Reaction conditions: catalyst = 0.5 g,
temperature = 463 K, TBA/phenol molar ratio = 2.
an increase in WHSV. This observation indicates that the
catalyst may be useful in maintaining the selectivity of the
products over a WHSV range.
The data in Table 3 shows a comparison of the results ob-
tained over 30 wt% H3PO4/Al-MCM-41(70) and the other
catalysts used in this reaction. In fact, the catalyst used in this
study might be one of the best catalysts with regards to phenol
conversion and selectivity for 4-TBP (yield of 4-TBP) and with
regards to the simplicity of the synthetic method.
2.2.5 Effect of time on stream
Catalyst deactivation by coke deposition or by the formation
of polybutenes was not of consequence in this study. Therefore,
the selectivity for 4-TBP was almost constant (Fig. 4) and the
slight decrease in conversion or selectivity did not appear to be
important. The selectivity for 2,4-DTBP showed a slight in-
crease with an increase in time on stream as a result of the
slight decrease in selectivity for 4-TBP. 2-TBP may not be a
significant contributing factor to the selectivity for 2,4-DTBP
as it did not show a decrease in selectivity with time on stream.
From these observations, we conclude that the adsorption of
4-TBP or 2-TBP is largely prevented compared to phenol and
hence both the 2-TBA and 4-TBP that remain in the vapor
phase react with the tert-butyl cation [21].
3 Conclusions
In this work, we demonstrate the applicability of a new
catalyst containing moderate and weak Brönsted acid sites for
the alkylation of phenol. A series of H3PO4 loaded Al-MCM-41
catalysts were synthesized and characterized by various NMR
methods. The vapor phase alkylation of phenol over these
catalysts showed that the catalyst containing 30 wt% H3PO4
supported on Al-MCM-41 had the highest activity. The ab-
sence of multialkylation and the catalyst’s high activity over 15
h on stream as well as negligible catalyst deactivation are
important characteristics of the catalyst.
Table 3 Comparison between 30 wt% H3PO4/Al-MCM-41(70) and the other catalysts for phenol conversion and 4-TBP selectivity
Product selectivity (%)
Catalyst
Phenol conversion (%)
Yield of 4-TBP
Ref.
TBPE
—
2-TBP
6.0
4-TBP
83.7
79.1
79.2
75.2
80.9
86.5
50.9
76.5
68.8
76.0
83.4
25.7
2,4-DTBP
10.3
12.5
14.2
14.4
11.3
6.7
20 wt% PW/Al-MCM-41
30 wt% H3PO4/Al-MCM-41(70)
30%PW/SBA-15
Fe,Al-MCM-41
88.7
78.6
70.1
70.1
62.9
57.8
97.1
63.6
69.4
57.0
35.9
80.6
74.24
62.17
55.52
52.71
50.89
50.00
49.42
48.65
47.74
43.32
29.94
20.71
[10]
this work
[21]
[4]
—
5.7
6.6
10.4
7.7
AlSBA-15 (45) (Ex)
Sulfated zirconia
Zeolite, HY
[22]
[20]
[8]
—
—
6.8
2.0
47
H-GaMCM-48
10.1
6.4
10.5
9.8
[12]
[6]
HY
0.5
16.0
—
20 wt% PW/AlPO
Al-MCM-41(56)
15% MPA/ZrO2
7.9
—
[18]
[1]
8.1
3.9
11.5
55.2
[19]

Mehran GHIACI et al. / Chinese Journal of Catalysis, 2010, 31: 759–764
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