Organic Process Research & Development 2003, 7, 17−21
O-Methylation of Dihydroxybenzenes with Methanol in the Vapour Phase over
Alkali-Loaded SiO Catalysts: A Kinetic Analysis
2
Rajaram Bal, S. Mayadevi,* and S. Sivasanker
National Chemical Laboratory, Pune 411 008, India
Table 1. Composition and physicochemical properties of
alkali-loaded silica samples
Abstract:
The vapour-phase O-methylation of the dihydroxybenzenes
(DHBs; o-dihydroxybenzene (catechol), m-dihydroxybenzene
(resorcinol), and p-dihydroxybenzene (hydroquinone)) with
excess methanol has been investigated over alkali (Li, Na, K,
and Cs) oxide loaded-SiO2 as catalysts. The reaction takes place
in two consecutive steps, the formation of the monometh-
oxyphenol in the first step (rate constant, k1) and the dimeth-
oxybenzene in the second step (rate constant, k2). The two steps
have been assumed to follow a pseudo-first-order kinetics with
respect to the substrates (DHB in the first step and the
monomethoxy phenol in the second step) and the kinetic
parameters, k1, k2, tmax, and Rmax have been calculated. The
trends in the values of the above kinetic parameters have been
explained on the basis of the reactivity and electronic properties
of the three DHBs, the surface basicity of the catalysts, and
the mode of adsorption of the molecules on the support.
metal loading as oxide
(wt %)
S areab
(m2/g)
TPD of CO2
(mmol/g)c
catalysta
SiO2
-
166
104
99
91
70
-
Li(1.5)SiO2
Na(1.5)SiO2
K(1.5)SiO2
Cs(1.5)SiO2
2.25
4.65
7.05
21.1
0.062
0.071
0.078
0.079
a The number in parentheses denotes the mmol of alkali metal loaded per g
of SiO2. b Measured by N2 adsorption (BET method). c mmol of CO2 desorbed/g
of catalyst.
We have reported the selective vapour-phase O-methylation
of the dihydroxybenzenes, catechol, resorcinol, and hydro-
quinone with methanol over alkali (Li, Na, K, and Cs)-loaded
silica catalysts.13 We found that alkaline silica catalysts
produced O-alkylated products with high selectivity. We now
present a kinetic analysis of the O-methylation of dihydroxy-
benzenes over alkali metal oxides supported on SiO2 to
facilitate a better appreciation of the methylation reaction.
Methylation occurs in two steps, first the monomethoxy
product formation and then the dimethoxy product formation.
The two consecutive methylation reactions are analyzed by
assuming them to be two independent first-order reactions.
The kinetic analysis presented should assist in identifying
the optimal conditions for the production of the various
methylated compounds.
Introduction
O-Methylated dihydroxybenzenes are important synthetic
intermediates in the production of fine chemicals and
pharmaceuticals.1 They are conventionally synthesized by
methylation with dimethylsulphate2 or with methyl halide
in the presence of sodium hydride.3 These methylating agents
are corrosive and toxic. The greater desire for environmen-
tally safer processes have, in recent times, led to the
development of processes based on solid catalysts and
vapour-phase reactions in fixed-bed reactors. Vapour-phase
methylation of aromatic hydroxy compounds with methanol
over heterogeneous catalysts such as metal oxides, sulphates,
phosphates, and zeolites has been attempted.4-9 Fu et al.10-12
reported the vapour-phase O-methylation of catechol with
dichloromethane (DCM) over supported alumina catalysts.
Experimental Section
Materials and Methods. Fumed silica (Cab-osil, Fluka)
was used as the support for the alkali metal oxides. The alkali
metals were loaded (1.5 mmol/g) onto the support by an
impregnation procedure (incipient wetness method) using a
minimum amount of aqueous metal hydroxide/acetate (Li,
Na, K, and Cs).13 They were dried at 373 K for 6 h and
calcined at 773 K for 6 h in air. Granular catalysts (10-22
mesh) were prepared by pelletting the powder and crushing
to the desired size.
(1) Phenol derivatives. In Ulmann’s Encyclopedia of Industrial Chemistry;
Elvers, B., Hawkins, S., G., Eds.; VCH Verlagsgesellschaft: Weinheim,
1991; Vol. A19, p 354.
(2) Matsukuma, A.; Takgishi, I.; Yoshido, K. Jpn. Kokai Tokyo Tokkyo Koho
7357935, 1973.
(3) Tanabe, K.; Nishizaki, T. In Proceedings, 6th International Conference on
Catalysis; Bond, G. C., Wells, P. B., Tomkins, F. C., Eds.; The Chemical
Society: London, 1977; Vol. II, p 863.
(4) Santacesaria, E.; Grasso, D.; Gelosa, D.; Carra, S. Appl. Catal. 1990, 64,
83.
(5) Rao, V. V.; Chary, K. V. R.; Durgakumari, V.; Narayanan, S. Appl. Catal.
1990, 61, 89.
(6) Bezouhanova, C.; Al-Zihari, M. A. Appl. Catal. 1992, 83, 45.
(7) Marczewski, M.; Perot, G.; Guisnet, M. In Heterogeneous Catalysis and
Fine Chemicals; Guinset, M. et al., Eds.; Elsvier: Amsterdam, 1988; p 273.
(8) Yamanaka, T. Bull. Chem. Soc. Jpn. 1976, 49, 2669.
(9) Fiecher, E.; Olat, S.; Gesine, W. Wiss. Z. Uni. Rostock 1990, 39, 67.
(10) Fu, Y.; Baba, T.; Ono, Y. Appl. Catal., A 1998, 166, 419.
(11) Fu, Y.; Baba, T.; Ono, Y. Appl. Catal., A 1998, 166, 425.
(12) Fu, Y.; Baba, T.; Ono, Y. Appl. Catal., A 1999, 176, 201.
All the reactions were carried out in a vertical down-flow
glass reactor (15 mm i.d.) using 2 g of the catalyst. The zone
above the catalyst bed (∼15 cm long) was packed with
ceramic beads and served as a preheater. The reactor was
placed inside a temperature-controlled furnace (Geomeca-
nique, France). The reaction temperature was measured with
a thermocouple placed at the center of the catalyst bed. The
catalyst was activated in situ in flowing air (20 mL/min) at
773 K for 3 h and flushed with nitrogen, and the temperature
was adjusted to the desired reaction temperature (673 K).
(13) Bal, R.; Tope, B. B.; Sivasanker, S. J. Mol. Catal. A: Chem. 2002, 181,
161.
10.1021/op020054o CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/01/2002
Vol. 7, No. 1, 2003 / Organic Process Research & Development
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