Evidence for weak base site participation in the vapour phase
methylation of catechol over solid base catalysts
ab
b
b
b
Venkataraman Vishwanathan,* Steven Ndou, Lucky Sikhwivhilu, Neville Plint, K. Vijaya
Raghavan and Neil J. Coville*
a
b
a
Indian Institute of Chemical Technology, Hyderabad 500-007, India
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg,
Wits 2050, South Africa. E-mail: ncoville@aurum.chem.wits.ac.za
b
Received (in Cambridge, UK) 15th February 2001, Accepted 2nd April 2001
First published as an Advance Article on the web 24th April 2001
The vapour phase alkylation of catechol over supported
caesium catalysts gives good selectivity to guaiacol forma-
tion, and TPD studies indicate that this result can be
correlated with the presence of weak basic sites on the
catalyst.
The vapour phase alkylation of catechol with methanol was
carried out in a vertical flow-type reactor at 623 K at
atmospheric pressure (ca. 650 mm Hg). Before the start of the
experiment, the catalyst (ca. 2 g) was activated in the reactor at
673 K for 1 h in nitrogen and then the solid was cooled to the
reaction temperature. A pre-mixed catechol–methanol (1+3
w/w ratio) mixture was fed from the top of the reactor at a fixed
Vapour phase alkylation of catechol is commercially important
for the production of oxy-alkylated products, namely guaiacol
and veratrole. These are synthetic intermediates used in the
production of flavourings, fragrances and pharmaceuticals.1
Studies on phenol alkylation have shown that product selectiv-
ity depends on the acid–base property of the catalyst surface.2
The addition of metal ions to alumina is known to generate new
2
1
rate of 5.1 ml min (methanol-free catechol flow rate 0.95 mol
2
1
h
) by means of a Sage syringe pump. After 1 h the liquid
products were analysed by GC (FID) using a DB-1 capillary
column. The reaction products were further confirmed by GC–
13
MS (VG-11-250 data system) and C NMR (Brucker AC-400)
spectroscopy.
3
active sites which are basic in nature. Available literature on
The reaction data for the vapour phase alkylation of catechol
over unsupported and supported catalysts are shown in Table 1.
the vapour phase alkylation of catechol over solid base
catalysts, though limited, suggests that the basic sites are
primarily responsible for the formation of C-/O-alkylated
2 3 2 2
The acidic oxides (Al O , SiO and TiO ) do not show much
guaiacol formation. However, the basic oxide MgO shows a
selectivity of 65% for guaiacol. This suggests that the basic sites
on the catalyst surface are involved in the formation of guaiacol.
It is interesting to note that caesium oxide alone shows a low
activity and selectivity towards guaiacol formation. However,
4
,5
products. This is further supported by the report that CO
2
adsorption over the basic sites leads to catalyst deactivation.6,7
However, there is no evidence to suggest that the weak sites
over the base catalysts are primarily important for the formation
of mono oxy-methylated products, in particular guaiacol. Here,
for the first time, such a correlation is proposed, i.e. that a
correlation exists between the weak base sites on a catalyst
surface and selectivity to guaiacol.
impregnation of caesium on to the acidic oxides (Al
and TiO ), increased the selectivity for guaiacol significantly.
2 3 2
O , SiO
2
Both alumina and caesium-modified alumina show the highest
reaction activity as compared to the other unsupported and
supported oxides. This suggests that the stronger and larger
number of acidic sites on the catalyst surface promotes more
ring alkylation than side (O-) alkylation. In the case of both
alumina and caesium-modified alumina a significant amount of
polyalkylated products is also formed. The formation of phenol,
and to a lesser extent diphenyl ether, indicates that both
dehydration and ring alkylation are the two competitive
The supported caesium catalysts were prepared by impreg-
nating Al O , SiO and TiO , all of commercial origin, with an
2 3 2 2
appropriate amount of an aqueous solution of caesium hydrox-
ide (10% Cs by mass). The resulting solids were dried and
calcined in air at 673 K for 4 h. Temperature programmed
2
desorption (TPD) was performed using CO gas at a heating rate
of 10 K min2 in the temperature range 373–973 K.
1
Table 1 Reaction data on unsupported and supported cesium catalysts
Selectivity (%)
Guaiacol
Veratrole
Activity
C-alkylated
products
mol h g21
2
1
a
Othersb
Catalyst
TiO
Al
SiO
MgO
Cs
2
0.718
2.341
0.133
0.106
0.053
0.426
1.409
0.505
10
18
20
65
16
37
58
75
2
3
36
73
33
15
6
10
28
4
42
6
4
2
O
2
3
43
15
17
12
1
5
2
O
57
41
13
5
10 wt% Cs
10 wt% Cs
10 wt% Cs
2
2
2
O/TiO
O/Al
O/SiO
2
2
O
2
3
16
a
b
DOI: 10.1039/b101497h
Chem. Commun., 2001, 893–894
This journal is © The Royal Society of Chemistry 2001
893