Ortho-alkylation of phenol with methanol using Pb-Cr promoted magnesium oxide catalysts

Article abstract of DOI:10.1002/jccs.200400206

In this study, Pb-Cr promoted magnesium oxide catalysts were used to catalyze the ortho-alkylation of phenol in the presence of excess methanol. The Cr/MgO catalyst exhibited a high conversion of phenol and a relatively high selectivity for the ortho-alkylation of phenol. The catalytic activity and the stability of Cr/MgO were improved by the addition of a fairly small amount of Pb. The Pb-Cr/MgO catalyst showed specificity for the ortho-alkylation of phenol, which was proved by a series of phenol derivative reactions with methanol.

Full text of DOI:10.1002/jccs.200400206

Journal of the Chinese Chemical Society, 2004, 51, 1407-1410
1407
Ortho-Alkylation of Phenol with Methanol Using Pb-Cr Promoted
Magnesium Oxide Catalysts
Jyh-Harng Kea (
), Jyh-Myng Zena* (
) and Fey-Long Wangb* (
)
aDepartment of Chemistry, National Chung Hsing University, Taichung, Taiwan, R.O.C.
bDepartment of Applied Chemistry, Providence University, Sha-Lu, Taichung, Taiwan, R.O.C.
In this study, Pb-Cr promoted magnesium oxide catalysts were used to catalyze the ortho-alkylation of
phenol in the presence of excess methanol. The Cr/MgO catalyst exhibited a high conversion of phenol and a
relatively high selectivity for the ortho-alkylation of phenol. The catalytic activity and the stability of
Cr/MgO were improved by the addition of a fairly small amount of Pb. The Pb-Cr/MgO catalyst showed spec-
ificity for the ortho-alkylation of phenol, which was proved by a series of phenol derivative reactions with
methanol.
Keywords: Pb-Cr promoted; Ortho-alkylation; Phenol.
INTRODUCTION
Alkylation of phenol with methanol results in a range of
were commercially available. Supported catalysts were pre-
pared by a simple impregnation method. A one-fold metal
promoted magnesium oxide (M1/MgO) catalyst was prepared
by impregnating MgO with a solution containing metal ni-
trate (M1(NO3)n). The metal content (wt%) was prepared
based on the concentration of the metal ions in the processing
solution. Impregnation took place in a water bath with stir-
ring at 100 °C to evaporate excess water, and the resulting
cake was dried at 110 °C in an oven overnight. It was finally
calcined by slow heating (10 °C/min) to 500 °C in air and
held for 6 h at the final temperature. A dual metal promoted
magnesium oxide (M2M1/MgO) catalyst was prepared by im-
pregnating M1/MgO with a solution containing M2 metal ions
in a similar fashion.
products, such as anisole, ortho-cresol, para-cresol, and 2,6-
dimethylphenol, which are extensively employed in industry
as intermediates for the synthesis of a variety of resins, herbi-
cides, insecticides, and other important chemicals.1,2 For ex-
ample, epoxy cresol novolac (ECN) and polyphenylene oxide
(PPO) are used in electronics and o-cresol & 2,6-dimethyl-
phenol derived industries, respectively.3,4 The catalysts em-
ployed for this reaction, including zeolites, g-alumina, sil-
ica-alumina, Nafion-H, phosphoric acid, ALPO, SAPO, metal
phosphates, acidic and basic oxides, result in a mixture of C-
and O-alkylated products.5-12
In previous work,13-15 we have found that 2,6-dimethyl-
phenol can be synthesized selectively from cyclohexanol
and/or cyclohexanone and methanol in one step using Cr/
MgO catalysts. In an extension of this study of reaction kinet-
ics, we have found that 2,6-dimethyl isomer is formed via a
phenol intermediate formed from cyclohexanol and/or cyclo-
hexanone. These results prompt us to study the ortho-alkyla-
tion of phenol using Cr/MgO with methanol as the methyl
group source. In this paper, we examine the ortho-alkylation
of phenol over Cr/MgO and the effect of the addition of Pb on
the activity of the catalyst.
The reaction of methanol and phenol was carried out in
a continuous down flow fixed-bed reactor. The reactor con-
sisted of a vertical quartz tube with an inside diameter of 1.8
cm. It was heated by an electric tubular furnace, and the tem-
perature was controlled with a PID temperature controller
with a sensor in the center of the catalyst bed. The catalyst (1
g) was packed in the reactor and pretreated to 400 °C in a
stream of nitrogen (30 mL/min). After pretreatment, the cata-
lytic reactions were carried out with a WHSV (g of feed/h g of
catalyst) of 0.4 at 300-460 °C and 1 atm. The stream of feed
was prepared as follows: a mixture of phenol and methanol
was fed by a syringe pump and vaporized in an evaporator.
The vapor was adjusted to a constant rate of 15 mL/min and
then diluted with nitrogen; the total flow rate was maintained
at 45 mL/min. Unless otherwise indicated, the molar ratio of
EXPERIMENTAL
All chemicals used were of reagent grade quality and
* Corresponding author. Fax: +886-4-22862547; e-mail: jmzen@dragon.nchu.edu.tw

1408 J. Chin. Chem. Soc., Vol. 51, No. 6, 2004
Ke et al.
Table 1. Conversion and selectivity data for phenol reaction with
WHSV = 0.4 h-1 and methanol:phenol = 10:1 at 1 atm
and 400 °C catalyzed by M-Cr/MgO catalysts
phenol to methanol in the feed was 1:10. Products were iden-
tified by gas chromatography-mass spectrometry (GC-MS)
and were analyzed quantitatively with a GC attached to the
reaction system through a gas-sampling valve. The columns
used in the GC were a SP2100 column (3 m) and a Chrosieve
II column (H2, CO and CO2 analysis).
Promoter
Metal
Selectivity (%)
Conversion
(%)
o-cresol 2,6-DMP 2,4,6-TMP Others
Pb
Na
Bi
Mo
Al
In
Tl
Pd
Pt
98.3
97.6
95.5
92.6
88.2
85.6
85.1
81.3
80.8
63.7
56.7
69.0
13.6
8.3
9.5
70.8
63.7
64.8
67.8
67.1
47.2
56.7
58.8
51.1
54.1
62.3
28.4
15.2
26.9
23.9
18.9
17.9
29.0
23.8
11.3
24.2
12.4
10.5
3.0
0.5
1.2
2.7
0.4
2.7
1.4
1.8
0.7
1.6
0.9
1.8
3.4
13.2
12.3
21.7
17.5
29.2
23.2
32.6
25.4
65.3
RESULTS AND DISCUSSION
Activities of magnesium supported catalysts
The results of the alkylation of phenol with methanol
over various promoted magnesium oxide catalysts after reac-
tion for 2 h are given in Table 1. The reaction of phenol and
methanol yielded 2-methylphenol (o-cresol), 2,6-dimethyl-
phenol (2,6-DMP) and 2,4,6-trimethylphenol (2,4,6-TMP).
The activity of Cr/MgO is improved by the addition of a
second metal on its surface. As can be seen in Table 1, Pb-
Cr/MgO exhibited the most promising characteristics in the
catalytic synthesis of 2,6-DMP. The catalyst properties can
be enhanced only by the addition of a small amount (0.05
wt%) of Pb, and excess addition results in a decrease in activ-
ity and selectivity. At higher Pb loading, the Pb-Cr/MgO cat-
alyst produces various xylenol isomers, shows lower selec-
tivity for the formation of 2,6-DMP, and is similar in catalytic
activity to Pb/MgO.
Eu
Ag
-
Fig. 2.
A similar phenomenon can be found in the reaction by
using Cr/MgO as a catalyst. The results suggest that the car-
bon deposition is the main factor that decreases the catalyst’s
activity.
Fig. 1 shows the phenol conversion and selectivity to
alkylated phenols as a function of time in the reaction of phe-
nol and methanol using Cr/MgO and Pb (0.05 wt%)-Cr/MgO,
respectively. As can be seen, Pb (0.05 wt%)-Cr/MgO shows a
higher activity than Cr/MgO at the same reaction time. Both
catalysts show a deactivation during the reaction, but the ac-
tivity of Cr/MgO decays faster than that of Pb (0.05 wt%)-
Cr/MgO; the selectivity to 2,6-DMP decreases, while the se-
lectivity of o-cresol increases. Fresh Cr/MgO shows an activ-
ity similar to Pb (0.05 wt%)-Cr/MgO that has been in perfor-
mance for 5 h, indicating that the product distribution is a
function of conversion. The results suggest that the addition
of Pb to Cr/MgO increases the activity of alkylation.
In order to clarify the factor that caused the decay of the
catalyst, we examined the activity variation of the catalyst by
re-oxidation. When the reaction proceeded for a few hours,
the feed stream was closed and a stream of air was introduced
to the reaction system to oxidize the catalyst for 1 h. The cata-
lytic reaction was then restarted and the activity of Pb-Cr/
MgO was almost fully recovered. This cycle was repeated at
least six times, with the same trend observed. The variation of
phenol conversion and selectivity to products are shown in
Fig. 1. Variation of phenol conversion and selectivi-
ties of products with time on stream in the reac-
tion of phenol and methanol (1:10) over Cr/
MgO and Pb-Cr/MgO at 440 °C; (Ž) phenol
conversion, (d) ortho-cresol, and («) 2,6-DMP;
open symbols for Cr/MgO and Ž filled symbols
for Pb-Cr/MgO.

Ortho-Alkylation of Phenol
J. Chin. Chem. Soc., Vol. 51, No. 6, 2004 1409
Table 2. Conversion and selectivity data for the reaction of
various substrates with substrates; methanol = 1:10 at 1
atm and 440 °C catalyzed by Pb(0.05 wt%)-(Cr(2.5
wt%)/MgO
Reaction of phenol derivatives and methanol over
Pb-Cr/MgO
The results of the alkylation of phenol and various
mono and dimethylphenols with methanol over Pb (0.05
wt%)-Cr/MgO after 2 h are summarized in Table 2. The reac-
tion of phenol and methanol yields o-cresol, 2,6-DMP and
2,4,6-TMP (run 1). According to the stoichiometry of the for-
mation of o-cresol and 2,6-DMP from phenol and methanol,
o-cresol is a primary product that is formed by the addition of
a methyl group to phenol. Therefore, 2,6-DMP is a secondary
product formed by the further reaction of o-cresol with meth-
anol. For simplification, 2,4,6-TMP is classified as a second-
ary product. Therefore, the ratio of the secondary product to
the primary product is about 1:1.
Selectivity (%)a
runs substrate Conv. (%)
DMP
TMP tetraMP others
1
2
phenol
o-cresol
82.0
98.8
93.8
71.5
92.9
98.6
87.6
94.4
95.6
37.8
38.8
(2,6)
81.29
(2,6)
54.12
5.7
(2,4,6)
17.6
(2,4,6)
46.7
-
49.3b
1.2
5.4
4.0
4.7
6.0
1.6
9.5
-
3
m-cresol
p-cresol
2.79
(2,5+2,3) (2,3,6) (2,3,4,6)
4
52.04
(2,4)
-
43.9
(2,4,6)
75.8
-
5
2,3-DMP
2,4-DMP
2,5-DMP
3,4-DMP
3,5-DMP
2,6-DMP
19.5
(2,3,6) (2,3,4,6)
When monomethylphenols (o-, m- and p-cresol), in-
stead of phenol, are used in the reaction, DMP and TMP be-
come the primary and secondary products, respectively. In
the reaction of o-cresol and methanol, the reaction yields
2,6-DMP as the main product with a small amount of 2,4,6-
TMP produced (run 2). Thus, the ratio of the secondary prod-
uct to the primary product is about 1:4, indicating that the
secondary product is more difficult to be formed. In contrast,
when o-cresol, m-cresol or p-cresol is used instead of phenol,
the ratio of the secondary product to the primary product in-
6
-
-
-
-
-
93.9
(2,4,6)
84.6
(2,3,6) (2,3,4,6)
28.7 31.8
(2,3,4) (2,3,4,6)
9.2 90.8
(2,3,5) (2,3,5,6)
-
7
13.7
8
9
10
100
-
(2,4,6)
a The numbers in the parentheses indicate the locations of methyl
groups.
b o-cresol.
100
creases. In the reaction of m-cresol and methanol, the ratio of
the secondary product (2,3,6-TMP) to the primary product
(2,5-DMP + 2,3-DMP) is 1 (run 3), and in the reaction of p-
cresol and methanol, a similar value in the ratio of the second-
ary product (2,4,6-TMP) to the primary product (2,4-DMP)
(run 4) is found. The results suggest that the alkylation of
phenol derivatives with methanol over Pb-Cr/MgO is limited
to the o-positions of the phenol skeleton.
Air
Air
Air
80
60
40
20
0
To verify the postulation that the alkylation of phenol
derivatives over Pb-Cr/MgO occurs at o-positions, we further
examined the reactions using various DMP isomers including
2,3-, 2,4-, 2,5-, 3,4-, 3,5-, and 2,6-DMP. Results are also
given in Table 2 (runs 4-10). According to the positions of
methyl groups on the benzene ring of phenol, these DMP iso-
mers may be categorized into three groups. The first group,
2,3-, 2,4-, and 2,5-DMP have an o-position vacancy on the
benzene ring. The second group, 3,4- and 3,5-DMP, pos-
sesses double vacancy o-positions. Furthermore, 2,6-DMP is
a reactant without an o-position vacancy. The alkylation of
2,3-, 2,4-, and 2,5-DMP with methanol, selectively yielded
corresponding o-methylated products (2,3,6-, 2,4,6-, and
0
4
8
12
16
20
Time on stream (h)
Fig. 2. Effect of air treatment on phenol conversion
and selectivities of products in the reaction of
phenol and methanol (1:10) over Pb-Cr/MgO at
400 °C; (Ž) phenol conversion, (t) ortho-
cresol, and (s) 2,6-DMP.

1410 J. Chin. Chem. Soc., Vol. 51, No. 6, 2004
Ke et al.
2,3,6-TMP, respectively) (runs 5-7). The reaction of 3,4-
DMP and methanol yields 2,3,4-TMP and 2,3,4,6-TetraMP
(run 8), and the reaction of 3,5-DMP and methanol yields
2,3,5-TMP and 2,3,5,6-TetraMP (run 9). As postulated,
2,6-DMP shows very poor reactivity (run 10), owing to the
fact that methyl groups already occupy the o-positions of
phenol. These results show the ability of Pb-Cr/MgO to cata-
lyze the o-alkylation of phenol with methanol.
Received May 12, 2004.
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Financial support by the National Science Council of
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