Catechol O-methylation with dimethyl carbonate over different acid-base catalysts

Article abstract of DOI:10.1039/b605053k

A series of meso and microporous materials, previously described and characterised, were tested in the catechol O-alkylation process using methanol (MeOH) and dimethyl carbonate (DMC) as alkylating reagents. In this regard, interesting results in terms of catalytic activity and selectivity to the desired monomethylated product (guaiacol) compared to the dimethylated one (veratrole) were found for the majority of the catalysts. Moreover, DMC is a better methylating agent than methanol with respect to the conversion ratio of catechol and guaiacol. The presence of n-type nucleophilic centres (oxygen from the OH groups) together with π-type ones (aromatic ring) in catechol led only to O-alkylated (guaiacol and veratrole), whereas no C-alkylated products were found under the reaction conditions. AlPO₄ and, especially, AlPO ₄-Al₂O₃ systems showed the best performance in this alkylation process in comparison with silicoaluminophosphates (SAPO) and some acidic commercial zeolites (H-Y, H-β and H-ZSM-5). the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b605053k

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Catechol O-methylation with dimethyl carbonate over different acid–base
catalysts
Rafael Luque,w Juan Manuel Campelo,* Tomas David Conesa, Diego Luna,
Jose Maria Marinas and Antonio Angel Romero
Received (in Montpellier, France) 7th April 2006, Accepted 21st June 2006
First published as an Advance Article on the web 14th July 2006
DOI: 10.1039/b605053k
A series of meso and microporous materials, previously described and characterised, were tested
in the catechol O-alkylation process using methanol (MeOH) and dimethyl carbonate (DMC) as
alkylating reagents. In this regard, interesting results in terms of catalytic activity and selectivity
to the desired monomethylated product (guaiacol) compared to the dimethylated one (veratrole)
were found for the majority of the catalysts. Moreover, DMC is a better methylating agent than
methanol with respect to the conversion ratio of catechol and guaiacol. The presence of n-type
nucleophilic centres (oxygen from the OH groups) together with p-type ones (aromatic ring) in
catechol led only to O-alkylated (guaiacol and veratrole), whereas no C-alkylated products were
found under the reaction conditions. AlPO4 and, especially, AlPO4–Al2O3 systems showed the
best performance in this alkylation process in comparison with silicoaluminophosphates (SAPO)
and some acidic commercial zeolites (H-Y, H-b and H-ZSM-5).
Thus, solid materials are preferred, acid and/or basic cata-
Introduction
lysts being the best candidates for these reactions due to their
Alkylation reactions are among the most important reactions
easy separation and recycling after the reaction workup.
in organic chemistry.1–3 In this regard, while heterogeneously
Recently, an increasing interest in developing new synthetic
catalysed alkylations are widely employed in the production of
routes for high added value chemicals, minimising the waste
alkylbenzenes (ethylbenzene, toluene, xylenes, etc.), the use of
production as well as employing heterogeneous catalysis has
arisen.4 The methylation reaction of catechol, a typical acid-
stoichiometric reagents is still the common alkylation proce-
dure in the majority of the processes. The most common
catalyzed reaction, is of both industrial and academic rele-
methods use dimethyl sulfate or alkyl halides as alkylating
vance as well as an example of this type of process, using
agents in a reaction usually carried out in liquid phase. These
MeOH or DMC rather than an alkyl halide, over heteroge-
agents exhibit some problems associated with toxicity, corro-
neous catalysts. The distribution of products (O-/C-methy-
siveness, the need for stoichiometric amounts of base, salt
lation ratio) is largely dependent on reaction parameters such
waste and separation and recovery procedures. So, an impor-
as temperature, reactant concentration, acid–base properties
tant issue deals with the replacement of stoichiometric reac-
and the pore size of the catalyst used.
tions by catalytic ones, where the use of heterogeneous
The main products that can be obtained in the methylation
catalysis with vapour-phase reactants is strongly preferred.
process are guaiacol and veratrole, ring-methylated products
These important drawbacks can be overcome by replacing
(methylguaiacols and methylveratroles) as well as other pro-
stoichiometric reagents with non-toxic ones such as methanol
ducts (heavies), which come, mainly, from the condensation of
(MeOH) and/or dimethyl carbonate (DMC), even though
two or more catechol molecules (Scheme 1). Moreover,
these reagents are not as reactive as the previous ones, and
O-alkyl-catechols [guaiacol (gua) and veratrole (ver)] are im-
also replacing stoichiometric reactions by catalytic reactions.
portant as chemicals and chemical intermediates, in the fine
The development of cleaner processes is of special interest in
chemistry industry for the synthesis of flavouring agents,
fragrances, agricultural chemicals and pharmaceuticals,5
order to meet global competition and conform to environ-
mental specifications. The use of solid acids to replace waste
guaiacol being of special value.
generating soluble acids attained vast interest in recent years
Guaiacol and veratrole have been commonly synthesized
due to its non-corrosiveness, environmental benign nature,
from catechol and guaiacol methylation, respectively, either
reusability as well as minimization of undesired toxic wastes.
using corrosive and toxic reagents such as dimethyl sulfate and
NaOH as homogeneous catalyst5–7 or using an alkyl halide
Materials can also be tuned to give higher activity, selectivity
(methyl iodide) and NaOH in THF at room temperature.5–8
and longer catalyst life.
In homogeneous conditions, guaiacol has also been synthe-
sized using dimethyl carbonate and NaOH/NaI.8
Departamento de Quimica Organica, Facultad de Ciencias, Edificio
Marie Curie, Universidad de Cordoba, Cordoba, Spain. E-mail:
A wide range of materials have been investigated as cata-
qo1capej@uco.es; Fax: +34 957 212066; Tel: +34 957 218622
lysts for the process. These included pure and doped metal
w Current address: Clean Technology Centre, Department of Chem-
oxides, mixed metal oxides, sulfates, metal phosphates and
istry, The University of York, York, UK. Fax: +44 1904 432705;
Tel: +44 1904 434456; E-mail: rla3@york.ac.uk
zeolites.9–18 The activity–selectivity patterns as well as
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addition of an aqueous solution of either ammonia until
complete precipitation of the aluminium hydroxide. On the
other hand, a mixture of 238 g AlCl3 Á 6H2O, 70 mL H3PO4
(85 wt%) and 1000 mL distilled water were stirred and cooled
at 273 K, again adding an ammonia aqueous solution until the
aluminium phosphate, initially precipitated, was not able to
redissolve into the reaction medium (pH 4). Both solutions
were then mixed and more aqueous ammonia was added until
complete precipitation of the AlPO4–Al2O3 system. After 24 h,
the gel obtained was washed thoroughly with 2-propanol,
oven dried at 393 K for 24 h and finally calcined at 923 K
for 24 h.
These materials were designed as APAL-A-Y, where A was,
as for the AlPO4 materials, the precipitating H3PO4 solution
(aqueous ammonia) and Y is a number that refers to the
alumina wt% content in the materials (either 15 or 25).
Therefore, the APAL-A-15 material prepared was an AlPO4–
Al2O3 material (APAL) obtained after precipitation with
aqueous ammonia (-A) with a 15 wt% of alumina (-15).
Scheme 1 Catechol alkylation reaction scheme.
Likewise,
a fluorine modified AlPO4–Al2O3 material
optimum reaction temperature of the catalyst is primarily
governed by its acid–base properties and by the methylating
agent used, MeOH or DMC. Recently, some authors have
reported the vapour-phase alkylation of catechol with
DMC9–12 and MeOH,13–18 which leads, mainly, to the mono-
and di-methylated products. In this way, DMC was reported
to be a potential harmless and versatile methylating agent.
In this paper, we wish to join the latest contributions to this
field, presenting a greener approach to this process in which
heterogeneous catalysis and more environmentally friendly
reagents (DMC) have replaced the homogeneous conditions
as well as the stoichiometric reagents.
(APAL-A-15FM was synthesized by stirring the APAL-A-15
in an NH4F water–methanol solution for 20 min, containing
2.5 wt% of FÀ. Both materials (AlPO4 and AlPO4–Al2O3)
were calcined at 923 K for 3 h.19 Al2O3 and MgO were
obtained after thermal decomposition of the hydroxide pre-
cursors, generated from the nitrate aqueous solutions followed
by precipitation with ammonium hydroxide up to pH 7.
Microporous SAPO were obtained from fumed SiO2,
boehmite and H3PO4, in the presence of different amines such
as tri-n-propylamine (SAPO-5), di-n-propylamine (SAPO-11
and SAPO-31 and diethylamine (SAPO-41) by a hydrothermal
procedure at 473 K for 24 h followed by calcination in air at
873 K.20
Experimental
H-Y, H-ZSM-5, and H-b zeolites were commercially avail-
able from Zeolyst Inc. Textural properties of all catalysts are
summarised in Table 1.
Synthesis of materials
AlPO4 catalysts (designed as AP-X) were obtained by pre-
cipitation, from aluminium chloride and H3PO4 aqueous
solutions, with aqueous ammonia (X = A), ethylene oxide
(X = E) or propylene oxide (X = P).19 A mixture of 79.2 g
AlCl3 Á 6H2O, 23.2 mL H3PO4 (85 wt%) and 334 mL distilled
water were gently stirred and cooled at 273 K, followed by the
dropwise addition of an aqueous solution of either ammonia
(AP-A), propylene oxide (AP-P) or ethylene oxide (AP-E),
respectively. The addition was continued until complete pre-
cipitation of the AlPO4 (pH B 7). After 24 h, the material was
filtered, washed thoroughly with 2-propanol, oven dried at 393
K for 24 h and subsequently calcined at 923 K for 3 h.
Therefore, the AP-A material obtained was an AlPO4 material
obtained after precipitation with aqueous ammonia (-A).
Thus, the use of ethylene (-E) and propylene (-P) oxides
instead of aqueous ammonia as precipitating reagents led to
materials named AP-E and AP-P, respectively.
Characterisation
Materials have been previously synthesized and characterised
as reported in the literature.19,20
Surface acid–basic measurements. The surface acidity was
measured in a dynamic mode by means of the gas-phase
adsorption of pyridine (Py) at 473 and 573 K, respectively,
using a pulse chromatographic technique.21 The surface basi-
city was measured by means of the liquid-phase (298 K)
adsorption of benzoic acid (BA) and phenol (PH) using a
spectrophotometric method.22 Data is presented in Tables 2
and 3, respectively.
Catalytic testing
The vapour-phase alkylation of catechol with DMC was
carried out in a vertical down-flow reactor (5 mm, i.d.) at
atmospheric pressure and at temperatures in the range
573–623 K. Temperature was measured with an iron–constan-
tan thermocouple placed in the middle of the catalyst bed. The
catalyst samples (ca. B30 mg) were pretreated at 573 K for 1 h
in an N2 flow (50 mL minÀ1). The catechol–DMC mixture
(2 M, 40 mL minÀ1) was fed from the top through a syringe
AlPO4–Al2O3 catalysts (15 and 25 wt% Al2O3, respectively)
were obtained by adding Al(OH)3 to a reaction medium where
the precipitation of the AlPO4 (from AlCl3 Á 6H2O and H3PO4)
was initiated by the addition of aqueous ammonia. A mixture
of 156 g Al(NO3)3 Á 9H2O and 498 mL distilled water were
gently stirred and cooled at 273 K, followed by the dropwise
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Table 1 Textural properties [surface area (SBET), pore volume (Vp)
and pore diameter (Dp) of different meso- and microporous materials
Table 3 Surface base properties of the AlPO4 and AlPO4–Al2O3
systems (15–25 Al2O3 wt%), and the metal oxides (MgO and Al2O3)
for the benzoic acid (BA) and phenol (PH) titration in liquid phase
at 473 K
a
a
Vmp /
SBET
/
Vp/
mL gÀ1
Smp
/
Catalysts
m2 gÀ1
Dp/A
m2 gÀ1
mL gÀ1
Basicity/mmol gÀ1
AP-A
AP-P
AP-E
APAL-A-25
APAL-A-15
APAL-A-15FM
MgO
109
228
242
244
267
165
23
151
183
110
115
91
17.6
13.6
8.6
6.2
11.8
17.0
—
8.2
—
—
—
0.48
0.75
0.52
0.37
0.77
0.70
0.09
0.31
0.08
0.07
0.08
0.07
0.06
—
—
—
—
—
—
—
—
—
128
65
63
117
83
—
—
—
—
—
—
—
—
—
—
—
0.12
0.06
0.03
0.04
0.06
—
Catalyst
BA
PH
AP-A
AP-P
AP-E
APAL-A-25
APAL-A-15
APAL-A-15FM
MgO
200
166
266
535
402
395
81
56
120
166
161
148
Al2O3
SAPO-5
SAPO-11
SAPO-31
SAPO-41
SAPO-34
H-Y-5.2
H-b-75
a
a
—
556
—
194
Al2O3
a
—
—
—
—
Not measured.
119
660
650
400
—
—
—
—
known compounds in calibration mixtures of specified
compositions.
H-ZSM-5-30
—
a
Microporous surface area and volume obtained from the Harkins
and Jura method.
Results and discussion
pump (Hardward 4400-001) at a space velocity (W/F) of 220 s
and was vaporized prior to passing it through the catalyst bed
in the presence of a flow of nitrogen carrier gas (50 mL minÀ1).
In order to prevent any condensation of reactant and pro-
ducts, all connections were heated at 548 K. Blank runs at 723
K showed that under the experimental conditions used in this
work, thermal effects could be ignored.
Previous results of experiments performed using a pulse
chromatographic method showed that DMC was an efficient
methylating agent compared to MeOH, since the alkylation
reaction can be carried out at relatively lower temperatures
with improved selectivity to the desired O-alkylated products
(Table 4).11,12,23 Also, the molar ratio catechol : DMC is an
important issue to consider, increasing markedly the selectivity
to guaiacol at lower molar ratios (1 : 2 vs. 1 : 4).12,23
The products were sampled on-line and analysed in a
FISONS GC 8000 series by using a column (2m  3mm) of
10% OV-1 on Chromosorb WS DMCS 80/100. The alkylated
catechols were also characterized by GC-MS, finding the
guaiacol and veratrole as main products. Under the reaction
conditions, no C-alkylated products (3 and 4-methylcatechols)
were found.
Similarly, DMC has been reported as a non toxic and
environmentally benign chemical,24 as the lowest photochemi-
cal ozone creator potential (POCP) among common volatile
organic compounds (VOCs) (2.5, ethylene = 100),24,25 stres-
sing the greenness of our protocol.
Finally, response factors of the reaction products were
determined with respect to catechol from GC analysis using
Surface acid–basic properties
Starting with the acid properties, AlPO4–Al2O3 systems ex-
hibited improved surface acid properties compared to AlPO4
materials, increasing at the same time, in the systems, when
decreasing their alumina content (from 25 to 15 wt%). As
expected, when increasing the temperature from 473 to 573 K,
the materials’ surface acidity, as measured by pyridine titra-
tion, decreased as a consequence of the activation of the
strongest acid sites that are the only ones retaining the
adsorbed base (Table 2). In addition, these materials also
exhibited surface basic sites (Table 3), thus showing their
bifunctional character.19 In terms of basicity, the Al2O3
Table 2 Surface acidity properties [pyridine (Py) and 2,6(DMPy)
titration at different temperatures] of different meso- and microporous
materials
Py at 473 K/ Py at 573 K/ DMPy at 573 K/
mmol gÀ1
Catalyst
mmol gÀ1
mmol gÀ1
AP-A
AP-P
AP-E
APAL-A-25
APAL-A-15
81
166
240
118
156
15
23
33
41
61
99
4
16
29
17
50
30
APAL-A-15FM 201
—
18
a
a
a
Table 4 DMC vs. MeOH performance [total conversion (XT) and
selectivity to guaiacol (Sgua)] in the catechol O-alkylation reaction over
different AlPO4 materialsa
MgO
Al2O3
—
—
10
145
95
65
10
—
—
—
—
—
—
—
—
—
SAPO-5
SAPO-11
SAPO-31
SAPO-41
SAPO-34
H-Y-5.2
H-b-75
H-ZSM-5
a
—
—
—
—
—
—
—
—
MeOH 2 M
Catalyst Temp./K XT Sgua
DMC 2 M
Temp./K XT Sgua
17
326
278
373
AP-A
AP-P
AP-E
a
573
11.7
7.3
5.5
87.2
80.6
93.3
523
24.9
32.5
27.9
87.8
93.1
86.0
No base adsorption.
Pulse chromatographic method (amount = 2 mL).
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Table 5 Total conversion (XT, mol%), product selectivities (Si,
mol%) and reaction rate (k 10À4, sÀ1) in the catechol O-alkylation
reaction with DMC over different catalysts (I)a
4 h on stream
12 h on stream
Catalyst
AP-A
Temp./K XT Sgua Sver k 10À4 XT Sgua Sver k 10À4
573
2.9 81.9 18.1 1.4
5.3 90.8 9.2 2.5
3.4 93.7 6.3 1.6
5.9 80.1 19.9 2.9
11.3 80.0 20.0 5.6
7.2 85.6 14.4 3.5
8.1 90.0 10.0 4.0
9.7 89.3 10.7 4.8
1.8 82.1 17.9 0.9
3.4 91.1 8.9 1.6
2.9 95.3 4.7 1.4
4.0 79.8 20.2 2.0
7.2 80.5 19.5 3.5
6.9 88.7 11.3 3.3
4.0 89.2 10.8 1.9
5.0 88.4 11.6 2.4
623
573Pyb
AP-P
AP-E
573
623
573Pyb
573
623
573Pyb
8.3 94.1 5.9 4.1
7.2 95.0 5.0 3.5
c
c
MgO
573
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
c
c
Al2O3
573
APAl-A-25
573
5.9 98.9 1.1 2.9
3.9 98.0 2.0 1.8
623
573Pyb
5.7 98.4 1.6 2.6
d
3.4 96.7 3.3 1.6
d
—
—
—
—
—
—
—
—
APAl-A-15
573
10.9 98.3 1.7 5.3
8.2 96.1 3.9 4.0
8.0 97.7 2.3 3.8
5.1 93.6 6.4 2.4
9.0 98.0 2.0 4.2
6.9 97.4 2.6 3.3
3.2 82.0 18.0 1.5
7.9 98.7 1.3 4.0
623
573Pyb
11.2 100.0
—
5.3
APAl-A-15FM 573
11.1 97.6 2.4 5.4
7.5 92.6 7.4 3.6
623
573Pyb
10.6 100.0
—
5.4
a
b
F = 1.33 Â 10À6 mol sÀ1; W/F = 220 s; 2 M catechol in DMC. Deactivated
with pyridine. No reaction under the stated conditions. Not measured.
c
d
incorporation to the AlPO4 provided systems AlPO4–Al2O3
with a higher number of basic centres in comparison with the
AlPO4.
On the other hand, the acidity measurements carried out for
the microporous materials (SAPO and zeolites) proved that
these materials display a greater number of strong acid
sites when compared to AlPO4 and AlPO4–Al2O3 catalysts
(Table 3).
Fig. 1 Deactivation curves with time on stream; conversion (XT,
mol%) and guaiacol selectivity (Sgua, mol%) of AlPO4 materials in the
catechol O-methylation reaction at 573 K (J = XT, & = Sgua) and
623 K (K = XT, ’ = Sgua), respectively.
the deactivation curves of the different catalysts as a function
of the reaction time. Under the reaction conditions (including
catechol conversion o20%), as we have pointed out before,
the main products obtained were the O-alkylated ones, that is,
guaiacol and veratrole for AlPO4, AlPO4–Al2O3, SAPO-5 and
commercial zeolites.
Catechol O-methylation
Catechol conversions, reaction rate constants and product
selectivities of the alkylation of catechol with DMC are
collected in Tables 5 and 6. Moreover, Fig. 1 and 2 showed
Table 6 Total conversion (XT, mol%), selectivity to guaiacol (Sgua
,
mol%) and reaction rate (k 10À4, sÀ1) in the catechol O-alkylation
AlPO4 and AlPO4–Al2O3 systems. Data summarised in
Table 5 manifest that the reaction rate constants follows the
order AP-A o AP-P o AP-E on AlPO4 catalysts, as the
surface acidity increases in this order, without a noticeable
change in terms of selectivity to guaiacol. The incorporation of
Al2O3 to AlPO4 developed AlPO4–Al2O3 systems that exhib-
ited increased activity as compared to AlPO4 catalysts (Table
5). In a similar way as for the AlPO4 materials, the catalytic
activity increases when increasing the surface acidity/basicity
as well as with the temperature, following the order APAL-
A-25 o APAL-A-15 BAPAL-A-15FM. In terms of selectiv-
ity, these materials showed an improvement in the guaiacol
selectivity, obtaining selectivities close to 100% to the mono-
methylated product. In spite of the better performance of these
AlPO4–Al2O3 systems compared to the AlPO4 ones, a tem-
perature increase not only did not improve the catalytic
reaction with DMC over different catalysts (II)a
4 h on stream
12 h on stream
Catalyst
SAPO-5
Temp./K XT Sgua k 10À4 XT Sgua
k 10À4
c
c
573
623
573
573
573
573
573
573
—
1.9 96.9 0.9
2.0 96.4 0.9
—
—
—
—
—
0.8
0.8
—
—
—
—
0.3
0.4
b
1.7 96.5
1.7 96.0
SAPO-11
SAPO-31
SAPO-41
SAPO-34
H-Y-5.2
H-b-25
b
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
b
b
0.6 99.0 0.4
1.0 84.3 0.5
1.5 95.0 0.7
0.7 82.9
0.9 94.3
H-ZSM-5-30 573
F = 1.33 Â 10À6 mol sÀ1; W/F = 220 s; 2 M catechol in DMC. No
a
c
reaction under the stated conditions. The difference to 100 corre-
sponds to the veratrole selectivity (Sver).
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Fig. 2 Deactivation curves with time on stream; conversion (XT,
mol%) and guaiacol selectivity (Sgua, mol%) of AlPO4–Al2O3 systems
in the catechol O-methylation reaction at 573 K (J = XT, & = Sgua
and 623 K (K = XT, ’ = Sgua), respectively.
)
activity in the materials but also led to a decrease in the total
conversion and the guaiacol selectivity. This decrease was even
more perceptible in the more acidic APAL-A-15FM material
due to an increase in the coke formation throughout the
reaction course. In general, these materials deactivate with
time on stream, losing almost 50% of the catalytic activity
after 12 h of reaction, particularly the AlPO4 materials (Fig.
1), whereas the AlPO4–Al2O3 systems were slightly less de-
activated (around 35–40%) after 12 h (Fig. 2).
Fig. 3 Deactivation curves with time on stream; conversion (J =
XT) and guaiacol selectivity (& = Sgua) of fresh AlPO4 materials
versus (K = XT) and (’ = Sgua) of AlPO4 pyridine treated (20 mL
minÀ1, 15 min) in the catechol O-methylation reaction.
of the DMC (once activated), the residual methyl hydrogen
carbonate decomposes to form CO2, which is probably ad-
sorbed in the catalysts basics sites, causing the MgO and Al2O3
quick deactivation.
In any case, the deactivation rate is a function of the
reaction temperature as well as of the surface acid/basic
properties of the catalysts. In this sense, the higher the reaction
temperature and/or the materials surface acidity, the higher
the deactivation rate. Nevertheless, in most cases, the product
distribution does not change noticeably with time on stream,
guaiacol being the main reaction product.
So, medium to strong acidic sites are possibly required for
the O-methylation of catechol with DMC. However, from this
evidence, basic sites are also required in addition to acidic sites
in order to attain catalysts with high activity.
Therefore, in acid/basic materials such as the AlPO4 and
AlPO4–Al2O3 systems, the O-alkylation takes place in acid/
basic pairs as has been proposed before,12,18 whereas the
C-alkylation takes place, generally, in strong basic sites.
Microporous SAPO and zeolitic materials. On the other
hand, microporous materials such as silicoaluminophosphates
(SAPO) and commercial zeolites exhibited a poor performance
in the catechol alkylation reaction (Table 6). SAPO materials
showed very low conversion values, increasing the catalytic
activity, in general, when the surface acidity increases, whilst a
temperature increase has no significant effect for these systems.
High selectivities were achieved for the monomethylated pro-
duct. It is noticeable that zeolitic materials proved themselves
as very poor catalysts for the catechol alkylation reaction. The
extremely low catalytic activity of zeolites is attributed to the
rapid deactivation of the catalysts, basically due to their high
surface acidity and lower pore diameter in comparison with
the mesoporous ones, so that, the pores of zeolites were
probably blocked by the oligomeric products.
MgO and Al2O3 oxides. Moreover, MgO, a basic metal
oxide, as well as Al2O3, an acid/basic metal oxide, exhibited a
poor performance in the O-methylating process (less than 3%
conversion), being completely deactivated after 3 h of reaction,
indicating that the basic or acid properties of the catalysts by
themselves (if not strong enough) did not play an important
role in the reaction. This may be caused by the strong
adsorption of CO2 at MgO and Al2O3 basic sites and due to
the low surface area and acidity values for the MgO and
Al2O3, respectively. Indeed, Jyothi et al.12 have reported that,
after the alkylation process takes place via nucleophilic attack
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Table 7 Total conversion (XT, mol%), selectivities to guaiacol
(Sgua, mol%) and reaction rate constants (k 10À4, sÀ1) of the AlPO4
reused in the catechol O-methylation processa
2 h on stream
6 h on stream
Catalyst Temp./K Carrier XT Sgua k 10À4 XT Sgua k 10À4
AP-A
AP-P
AP-E
573
573
573
623
Air
O2
Air
4.1 80.5 1.9
5.2 77.9 2.5
7.4 88.9 3.7
9.4 88.8 4.7
2.4 79.4 1.1
3.2 79.5 1.5
4.2 88.3 2.0
5.5 88.1 2.7
a
F = 1.33 Â 10À6 mol sÀ1; W/F = 220 s; 2 M catechol in DMC.
Guaiacol selectivities were also affected, increasing, in gen-
eral, for the poisoned materials in comparison with the fresh
ones. This increase in the guaiacol selectivity was more
noteworthy for AlPO4 materials (Fig. 3). Results pointed out
that the unavailability of the strong acidic centres over the
catalysts surface after the pyridine deactivation, led to an
improvement of the total conversion and guaiacol selectivity
with time on stream thus supporting the fact that the strong
acid/basic centres were basically involved in deactivation
processes (coke formation) and di-methylation (veratrole
Fig. 4 Deactivation curves with time on stream; conversion (J =
XT) and guaiacol selectivity (& = Sgua) of fresh AlPO4–Al2O3 systems
versus (K = XT) and (’ = Sgua) of AlPO4–Al2O3 pyridine treated
(20 mL minÀ1, 15 min) in the catechol O-methylation reaction.
The high acid Brønsted sites strength in these materials led
to a preferential parallel catechol adsorption over the catalysts
surface as shown by Tanabe et al.,26 possibly causing a
disrupted interaction between the p-electrons within the aro-
matic ring and the Brønsted acid sites, therefore leading to the
molecule destruction and coke formation. This coke formed
deposits over the catalyst surface, thus blocking the active acid
centres, which caused the quick zeolites deactivation in the
early reaction stages.
In summary, AlPO4 and AlPO4–Al2O3 systems showed far
higher catalytic activity and guaiacol selectivity than micro-
porous SAPO, and commercial zeolite materials, in spite of
their higher acid properties.
Pyridine poisoning experiments. Poisoning experiments were
carried out, prior to the alkylation process, by deactivating the
materials with pyridine. Pure pyridine (20 mL minÀ1; 15 min)
was passed through the catalyst bed with an N2 flow of 50 mL
minÀ1 at 573 K. The catalyst was later stabilised for 30 min
under these conditions, before the O-methylation process took
place. Data are summarised in Table 5 and Fig. 3 and 4.
In general, we can assert that the pyridine poisoning treat-
ment definitely has an effect over the materials catalytic
activity. In terms of activity, in most cases, initial total
conversion for the materials remained almost unchanged or
slightly diminishes, which makes sense if one considers that
pyridine is adsorbed on the acid centres over the catalyst
surface, therefore deactivating these centres. However, cata-
lysts treated with pyridine experienced a lower deactivation
with time on stream compared to the fresh materials (Fig. 3
and 4). Such a different behaviour became more noticeable
when increasing the reaction time (Table 5).
Fig. 5 Reactivation experiments; Conversion (K = XT) and guaiacol
selectivity (’ = Sgua) of regenerated AlPO4 versus (J = XT) and
(& = Sgua) fresh AlPO4 materials.
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formation), whereas weak to medium strength acid /basic
centres were responsible for the mono-methylation process.
References
1 J. Cejka and B. Wichterlova, Catal. Rev. Sci. Eng., 2002, 44, 375.
2 K. Sreekumar and S. Sugunan, Appl. Catal., A, 2002, 230, 245.
3 C. Perego and P. Ingallina, Green Chem., 2004, 6, 274.
4 J. H. Clark and D. J. Macquarrie, in Handbook of Green Chemical
Technology, Blackwell Science, Oxford, 2002.
5 G. Dorothea, in Phenol Derivatives, Ullmann’s Encyclopedia of
Industrial Chemistry, ed. E. Barbara, H. Stephen and S. Gail,
VCH, Verlagsgesellschaft, Weinheim, 1991, p. 354.
6 A. Matsukuma, I. Takagishi and K. Yoshido, Jpn. Pat. Appl.,
7357935.
Reusability experiments. Reusability experiments were car-
ried out after completion of the O-methylation reaction. In
this sense, after 20 h on stream, catalysts were regenerated in a
carrier (air or oxygen, 50 mL minÀ1) oxidant flow, increasing
the temperature from 573 to 873 K (10 K minÀ1). After
stabilizing the materials at 873 K for 1 h, the methylating
process was performed again. Results are shown in Table 7
and Fig. 5, respectively.
7 A. Barbara, N. Stoochnoff and L. Benoiton, Tetrahedron Lett.,
1973, 1, 21.
As shown in Fig. 5, all AlPO4 materials are regenerated to a
variable extent. Thus, the degree of reactivation is higher for
the less acidic AlPO4 catalyst (AP-A). On the other hand,
selectivities remained almost unchanged with time on stream
for both reactivated and fresh materials. Finally, we also
found that the catalyst regeneration was independent of the
carrier employed in the reactivation procedure.
8 M. M. Marcello, R. Ugo and R. Franco, Ing. Chim. Ital., 1985,
21, 6.
9 Y. Fu, T. Baba and Y. Ono, Appl. Catal., A, 1999, 176, 201.
10 Y. Fu, T. Baba and Y. Ono, Appl. Catal., A, 1999, 178, 219.
11 M. B. Talawar, T. M. Jyothi, T. Raja, B. S. Rao and P. D. Sawant,
Green Chem., 2000, 2, 266.
12 T. M. Jyothi, T. Raja, M. B. Talawar and B. S. Rao, Appl. Catal.,
A, 2001, 211, 41.
13 L. Kiwi-Minsker, G. Jenzer, L. Pliasova and A. Renken, Stud.
Surf. Sci. Catal., 1999, 121, 159.
14 F. Cavani and T. Monti, Catalysis of Organic Reactions, Marcel
Dekker, New York, 2000, p. 123.
Conclusions
15 F. Cavani, T. Monti and D. Paoli, Stud. Surf. Sci. Catal., 2000,
130, 2633.
16 R. Bal, B. B. Tope and S. Sivasanker, J. Mol. Catal. A: Chem.,
2002, 181, 161.
17 X. M. Li, W. Zhang, G. Liu, L. Jiang, X. Zhu, C. Pan, D. Jiang
and A. Tang, React. Kinet. Catal. Lett., 2003, 79, 365.
18 X. Zhu, X. Li, M. Jia, G. Liu, W. Zhang and D. Jiang, Appl.
Catal., A, 2005, 282, 155.
19 A. Blanco, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas and
A. A. Romero, J. Catal., 1992, 137, 51.
Amorphous AlPO4–Al2O3 catalysts exhibited higher catalytic
activity and guaiacol selectivity than amorphous AlPO4, micro-
porous SAPO-5 and zeolitic materials. Moreover, AlPO4 and
AlPO4–Al2O3 catalysts exhibited greater activity and selectivity
when strong acid sites were deactivated with pyridine. Due to
the presence of strong acid sites, H-ZSM-5, H-Mordenite,
H-b and H-Y zeolites rapidly deactivate (due to coke forma-
tion) in comparison with AlPO4 and AlPO4–Al2O3. Conse-
quently, our results indicate that strong acid sites are not
required for the O-methylation of catechol and that weak to
medium strength pairs of acid/basic sites are responsible for
the mono-methylation process.
20 J. M. Campelo, F. Lafont, J. M. Marinas and M. Ojeda, Appl.
Catal., A, 2000, 194, 85.
21 J. M. Campelo, D. Luna, R. Luque, J. M. Marinas, A. A. Romero,
J. J. Calvino and M. P. Rodrıguez-Luque, J. Catal., 2005, 230, 327.
22 F. M. Bautista, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas
and A. A. Romero, React. Kinet. Catal. Lett., 1998, 63, 261.
23 (a) J. M. Campelo, D. Luna, R. Luque, J. M. Marinas and A. A.
Romero, Proceedings of the 4th Green Chemistry Conference,
Generalitat de Catalunya, Barcelona, Spain, 2002; (b) R. Luque,
Tesis de Licenciatura, Cordoba, 2003.
Acknowledgements
This research was subsidized by grants from the Direccion
General de Investigacion (Projects CTQ2004-21662-E and
CTQ2005/04080/BQU), Ministerio de Ciencia y Tecnologıa,
FEDER funds, and from the Consejerıa de Educacion Ciencia
(Junta de Andalucıa).
24 F. Rivetti, Green Chemistry: Challenging Perspectives, ed. P. T.
Anastas and P. Tundo, Oxford University Press, 2000, p. 201.
25 M. E. Jenkin and G. D. Hayman, Atmos. Environ., 1999, 33, 1275.
26 K. Tanabe, in Catalysis by acid and bases, ed. B. Imelik, C.
Naccache, G. Coudurier, Y. Ben Taarit and J. C. Vedrine, Elsevier,
Amsterdam, 1985, p. 1.
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1234 | New J. Chem., 2006, 30, 1228–1234