Preparation and characterization of Zr-MCM-41 synthesized under microwave irradiation condition for acetylation of 1,2-dimethoxybenzene with acetic anhydride

Article abstract of DOI:10.1007/s11164-011-0353-4

The liquid phase of acetylation of 1,2-dimethoxybenzene with acetic anhydride has been investigated over a series of acid zirconium mesoporous materials (Zr-MCM-41) synthesized by a microwave irradiation method with different Si/Zr ratios (Si/Zr = 90, 60, 10) and characterized by several spectroscopic techniques such as: N2 physical adsorption, ICP, XRD, TEM, FT-IR, UV-vis and a temperature-programmed desorption (TPD) of pyridine. In fact, the catalyst Zr-MCM-41 (10) showed a greater performance in the acid-catalyzed acetylation of 1,2-dimethoxybenzene employing acetic anhydride as an acylating agent. Furthermore, the kinetics of the acetylation of 1,2-dimethoxybenzene over these catalysts have also been investigated. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0353-4

Res Chem Intermed (2012) 38:367–381
DOI 10.1007/s11164-011-0353-4
Preparation and characterization of Zr-MCM-41
synthesized under microwave irradiation condition
for acetylation of 1,2-dimethoxybenzene with acetic
anhydride
K. Bachari
M. Lamouchi
•
R. Chebout
•
R. M. Guerroudj
•
Received: 22 March 2011 / Accepted: 22 July 2011 / Published online: 14 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract The liquid phase of acetylation of 1,2-dimethoxybenzene with acetic
anhydride has been investigated over a series of acid zirconium mesoporous
materials (Zr-MCM-41) synthesized by a microwave irradiation method with dif-
ferent Si/Zr ratios (Si/Zr = 90, 60, 10) and characterized by several spectroscopic
techniques such as: N physical adsorption, ICP, XRD, TEM, FT-IR, UV–vis and a
2
temperature-programmed desorption (TPD) of pyridine. In fact, the catalyst
Zr-MCM-41 (10) showed a greater performance in the acid-catalyzed acetylation of
1
,2-dimethoxybenzene employing acetic anhydride as an acylating agent. Further-
more, the kinetics of the acetylation of 1,2-dimethoxybenzene over these catalysts
have also been investigated.
Keywords Zr-MCM-41 materials ꢀ 1,2-dimethoxybenzene ꢀ
Microwave irradiation ꢀ Acetylation ꢀ Acetic anhydride
Introduction
The microwave–hydrothermal synthesis of molecular sieves is a relatively new area
of research [1–7]. It offers many distinct advantages over conventional hydrothermal
synthesis. They include rapid heating to crystallization temperature due to volumetric
heating, resulting in homogeneous nucleation, fast supersaturation by the rapid
dissolution of precipitated gels and eventually a shorter crystallization time compared
K. Bachari (&) ꢀ R. Chebout ꢀ R. M. Guerroudj
Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques (C.R.AP.C),
Alger RP, BP 248, 16004 Alger, Algeria
e-mail: bachari2000@yahoo.fr
M. Lamouchi
Laboratoire de Catalyse et Environnement, EA 2598, MREI, Universit e´ du Littoral-Cote d’Opale,
145, Avenue Maurice Schumann, 59140 Dunkerque Cedex, France
123

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K. Bachari et al.
to conventional autoclave heating [1–7]. It is also energy efficient and economical [1–
]. On the other hand, Friedel–Crafts aceylation of aromatic compounds and aromatic
7
heterocyclic compounds is an excellent example of electrophilic substitution
catalyzed by acidic or basic catalysts, which has been widely used in the industry
for the production of various organic value-added intermediates. Preparation of
aromatic ketones has received much attention in recent years because of their
commercial importance, being used as intermediates in the synthesis of pharmaceu-
ticals (naproxen, dextromethorphan, ibuprofen), dyes, fragrances, and agrochemicals
[
catalysts such as AlCl , and BF , strong mineral acids like H SO , HF, or supported
8]. Conventionally, these reactions are carried out in the presence of homogeneous
3
3
2
4
Lewis acid catalysts, using acid chloride or anhydride as an acylating agent [9].
Regrettably, the use of the above catalysts in the industry causes a lot of
environmental-related problems as they are highly corrosive in nature and cannot
be regenerated for further use. Moreover, the separation of the homogeneous catalysts
from the product mixture is difficult and often a stoichiometric quantity excess of the
catalysts is required as the Lewis acid catalysts form a complex with the product
ketones, which is then destroyed in the hydrolysis step required for product isolation.
To avoid the above environmental-associated problems, it is desirable to develop a
catalyst process which is stable, environmentally friendly, and recyclable. A class of
interesting catalytic systems which has been widely used these days and can eliminate
the corrosion and environmental problems is the heterogeneous solid Br o¨ nsted acid
zeolites and zeotypes molecular sieves, which are highly efficient, sustainable,
recyclable, and ecofriendly. Indeed, the acetylation of anisole and 1,2-dimethoxy
benzene leads to the synthesis of p-acetylanisole and 4-acetylveratrole, which are
commercially important products and are being used as the precursors of a sun
protector and of a component in an insecticide formulation [10], respectively.
Different catalysts, including zeolites [11–14], heteropoly acids [15, 16], sulfated
metal oxides [17] and AlSBA-15 [18], have been investigated for the Friedel–Crafts
alkylation and acetylation reactions. Indeed, in the present study, we have studied the
liquid phase of acetylation of 1,2-dimethoxybenzene with acetic anhydride over
Zr-MCM-41 catalysts synthesized by a microwave irradiation method.
Experimental
Synthesis of the catalysts
Microwave–hydrothermal (M–H) synthesis of Zr-MCM-41 mesoporous molecular
sieves with different Si/Zr ratios (Si/Zr = 90, 60, 10) was performed using a
MARS5 (CEM, Matthews, NC, USA) microwave digestion system. This system
operates at a maximum power of 1,200 W. The power can be varied from 0 to
1
00%, and is controlled by both pressure and temperature to a maximum of 350 psi
and 513 K, respectively. A 2.45-GHz microwave frequency was used which is the
same as that in domestic microwave ovens. The syntheses were carried out in
double-walled digestion vessels which have an inner liner and cover made up of
Teflon PFA and an outer strength vessel shell of Ultem polyetherimide. In a typical
1
23

Preparation and characterization of Zr-MCM-41
369
synthesis, 21.32 g of sodium metasilicate (Na SiO ꢀ9H O, CDH) was dissolved in
2
3
2
60 g of water. The reaction mixture was stirred for 2 h. Meanwhile, cetyltrimeth-
ylammonium bromide (5.47 g, CTABr, OTTO Chemie) and zirconium sulfate
(
Zr(SO ) ꢀ4H O, Merck) were dissolved in 20 g of distilled water. Then, the
4
2
2
resultant solution was added dropwise to the sodium metasilicate solution. The final
mixture was stirred for 1 h. The pH of the gel was adjusted by using 2 M sulfuric
acid (H SO ; 98% Merck) and it was stirred for another 3 h. The gel thus obtained
2
4
was allowed to crystallize under microwave–hydrothermal conditions at 373 K for
h. The crystallized product was filtered off, washed with warm distilled water,
dried at 383 K and finally calcined at 813 K in air for 6 h.
2
Characterization techniques
The X-ray diffraction (XRD) patterns of samples were recorded with a powder XRD
instrument (Rigaku D/max 2500PC) with Cu Ka radiation (k = 0.154 18 nm). It was
operated at 40 kV and 50 mA. The experimental conditions correspond to a step
width of 0.02° and a scan speed of 1°/min. The diffraction patterns were recorded in
the 2h range of 1–10°. Fourier transform infrared spectra of samples were recorded
on a Nexus FT-IR 470 spectrometer made by Nicolet (USA) with the KBr pellet
-
1
technique. The effective range was from 400 to 4,000 cm . Specific surface area
and pore size were measured by using a NOVA2000e analytical system made by
Quantachrome (USA). The specific surface area was calculated by the Brunauer–
Emmett–Teller (BET) method. Pore size distribution and pore volume were
calculated by the Barrett–Joyner–Halenda (BJH) method. Transmission electron
microscopy (TEM) morphologies of samples were observed on a Philips TEMCNAI-
12 with an acceleration voltage of 100–120 kV. Diffuse reflectance UV–Vis spectra
were recorded on a UV-3100 spectrophotometer made by Shimadzu (Japan) with
spherical diffuse reflectance accessory, using BaSO as a reference. The zirconium
4
content in the samples was determined by inductively coupled plasma (ICP)
technique (Vista-MAX, Varian). The density and strength of the acid sites of the
different Zr-MCM-41 samples were determined by the temperature-programmed
desorption (TPD) of pyridine. About 100 mg of the materials were evacuated for 3 h
-
5
at 523 K under vacuum (P \ 10 kPa). Thereafter, the samples were cooled to
room temperature under dry nitrogen followed by exposure to a stream of pyridine in
nitrogen for 30 min. Subsequently, the physisorbed pyridine was removed by heating
the sample to 393 K for 2 h in a nitrogen flow. The temperature-programmed
desorption (TPD) of pyridine was performed by heating the sample in a nitrogen flow
(
50 mL/min) from 393 to 873 K with a rate of 10 K/min using a high-resolution
thermogravimetric analyzer coupled with a mass spectrometer (SETARAM setsys
6MS). The observed weight loss was used to quantify the number of acid sites
1
assuming that each mole of pyridine corresponds to one mole of protons.
Catalytic testing
Acetylation of 1,2-dimethoxybenzene with acetyl chloride was carried out under
liquid phase conditions. The liquid phase reaction set up consists of two-necked
123

370
K. Bachari et al.
5
0-mLround bottom flask duly fitted with a condenser in one end for cooling while
the other vent is closed with a Teflon septum for collecting samples by glass syringe
at regular intervals. The whole system was kept in a thermostated oil bath with a
magnetic stirrer coupled to a heating plate. In a typical reaction, the catalyst was
added to a solution of 2.6 g 1,2-dimethoxybenzene (18.9 mmol), 0.4 g acetyl
chloride (3.8 mmol) and 50 mL chlorobenzene together with 1 g nitrobenzene as
internal standard. Then, the reaction mixture was heated to the required temperature
and the samples were collected at regular time intervals. The collected samples were
analyzed periodically by a gas chromatograph (HP-6890) equipped with a FID
detector using a DB-5 capillary column. The products were confirmed with GC–MS
(
HP-5973) analysis.
Results
Characterization of the samples
The small angle X-ray diffraction patterns of Zr-MCM-41 (Si/Zr = 90, 60, 10)
synthesized by the microwave irradiation method are shown in Fig. 1. The
Zr-MCM-41 (90) sample gives a very strong (100) peak followed by (110) and
(
200) lower intensity peaks. All three distinct Bragg reflections at low angles were
Fig. 1 XRD patterns of Zr-MCM-41 materials in the domain of 1–10° (2h) prepared by microwave–
hydrothermal (M–H) process: a Zr-MCM-41 (90), b Zr-MCM-41(60), c Zr-MCM-41(10)
1
23

Preparation and characterization of Zr-MCM-41
371
indexed on the hexagonal lattice. However, when the zirconium content ratio
increased from 60 to 10, the intensities of the long-range ordered peaks were
gradually reduced. However, compared with the pattern of the typical MCM-41
mesoporous molecular sieve [19], it can be noted that the d100 spacing in Zr-MCM-
4
1 samples is significantly larger than MCM-41 (for both as-synthesized and
calcined samples) and shows the incorporation of Zr in the MCM-41 structure. The
zirconium content in Zr-MCM-41 is increased with Zr content in the synthesis gel
(
Table 1). The porosity of the samples was evaluated by N adsorption isotherms.
2
Figure 2 shows the N2 adsorption–desorption isotherms of the three calcined
samples of Zr-MCM-41. The specific surface areas and pore size distributions and
pore volumes calculated by the BET and BJH methods are summarized in Table 1.
According to Fig. 2, the N adsorption–desorption isotherms of the two samples Zr-
2
MCM-41(90) and Zr-MCM-41(60) exhibit typical type IV isotherms with hysteresis
loop caused by capillary condensation in mesopores, manifesting that the two
samples possess the obvious mesoporous structure [19]. There are three well-
defined stages in the isotherms of the two samples that may be identified. The
isotherms show a step at the relative pressure (p/p ) of ca. 0.3–0.4, characteristic of
0
capillary condensation of uniform mesoporous materials, showing that the two
samples have uniform pore size distribution and larger pore volume. The isotherms
corresponding to p/p \ 0.3 are due to a monolayer adsorption of nitrogen on the
0
Table 1 Physicochemical properties of different Zr-MCM-41 (M–H) samples
Sample
Chemical analysis
Si/Zr (gel)
S
BET
,
-1
m g
Pore volume,
Pore diameter,
nm
2
3
cm g
-1
Si/Zr
Zr-MCM-41
Zr-MCM-41
Zr-MCM-41
10
60
90
10.7
59.5
92.4
686
918
0.66
0.88
0.98
2.9
2.7
2.6
1150
2
Fig. 2 N physical adsorption–
desorption isotherms of
Zr-MCM-41 materials prepared
by microwave–hydrothermal
(
M–H) process: a Zr-MCM-41
90), b Zr-MCM-41(60),
(
c Zr-MCM-41(10)
123

372
K. Bachari et al.
walls of the mesopore. The near-horizontal section beyond p/p [ 0.4 represents the
0
multilayer adsorption on the outer surface of the particles. In addition, from Fig. 2,
we can observe that the isotherms of the sample Zr-MCM-41(90) have a sharper
capillary condensation step at the same p/p , illustrating that it possesses more
0
uniform pore size distribution as compared with the isotherms of the other sample.
Moreover, the isotherms of the sample Zr-MCM-41(10) did not fit into typical type
IV isotherms; this is attributed to the partial damage of the mesoporous structure
caused by the overfull zirconium ions incorporated into the MCM-41 mesoporous
2
molecular sieve. However, combined with the surface area data (686 m /g) listed in
Table 1 and the XRD data (Fig. 1), the sample Zr-MCM-41(10) still has a partial
mesoporous structure, but the ordering is poor. Figure 3 presents the pore size
distribution curves of the three calcined samples Zr-MCM-41 synthesized by the
microwave irradiation method. As shown in Fig. 3, narrow and sharp peaks can be
observed in a pore size range of ca. 2–3 nm for the samples Zr-MCM-41(90) and
Zr-MCM-41(60), and the intensity of the peak is also strong as compared with that
of the sample Zr-MCM-41(10), indicating that the two samples also have uniform
pore size distribution. Additionally, as the zirconium content in the sample
increased, the intensity of the pore size distribution peak became weak, which
reflects that the increase of zirconium ions incorporated into the silica framework of
MCM-41 mesoporous molecular sieve caused the partial distortion of the
mesoporous framework, resulting in the irregular pore size distribution and the
poor mesoporous ordering. From Table 1, we can conclude that the specific surface
area and pore volume of the resulting samples gradually decreased as the zirconium
content increased, and that the pore size is in the range of 2.5–2.9 nm. Combined
with the results of XRD and N adsorption isotherms, it is reasonable to conclude
2
that the mesoporous ordering of the sample Zr-MCM-41 synthesized by the
microwave irradiation method gradually decreased with the increase of the
zirconium content incorporated into the mesoporous framework. Figure 4 presents
the FT-IR spectra of the synthesized Zr-MCM-41(60) sample before and after
Fig. 3 Pore size distribution
curves of the of Zr-MCM-41
materials prepared by the
microwave–hydrothermal
(
(
M–H) process: a Zr-MCM-41
90), b Zr-MCM-41(60),
c Zr-MCM-41(10)
1
23

Preparation and characterization of Zr-MCM-41
373
Fig. 4 FT-IR spectra of the Zr-MCM-41(60) material prepared by the microwave–hydrothermal (M–H)
process: a before calcination and b after calcinations
-
1
calcination at 813 K in air for 6 h. As shown in Fig. 4, the band at 3,500 cm is the
characteristic band of the water adsorbed on the sample surface. The bands at
-
,620–1,640 cm are aroused by the flexion vibration of the OH bond [20]. The
1
1
band at 1,080 cm is from the asymmetric extension vibration of Si–O–Si. The
-
1
-
band about 810 cm is due to the corresponding symmetric vibration of the Si–O–
1
-
Si bond, while the band at 460 cm is assigned to rocking vibration of the Si–O–Si
1
-
1
bond. The bands at 2,921, 2,850 and 1,480 cm are the characteristic bands of the
surfactant alkyl chains. After the sample Zr-MCM-41(60) was calcined at 813 K in
-
1
air for 6 h, the bands at 2,921, ,2850 and 1,480 cm disappeared, certifying that the
template had been effectively removed. The diffuse reflectance UV–vis spectra of
the three samples Zr-MCM-41 are presented in Fig. 5. As shown in Fig. 5, it can be
observed that the samples Zr-MCM-41 all show one single peak at ca. 205–220 nm
and the intensity also increases with the increase in the Zr content of the samples.
Generally, the position of the adsorption peak associated with ligand to metal charge
2
-
4?
transfer (O ? Zr ) spectra depends on the ligand field symmetry surrounding
the zirconium center. And the electronic transitions from oxygen to tetra-
4
?
coordinated Zr
require a higher energy as compared with hexa-coordinated
one. Therefore, it may be suggested that the adsorption peak at ca. 205–220 nm for
the samples Zr-MCM-41 is probably attributed to the presence of one type of
4
?
zirconium species-tetra-coordinated Zr . It is probably that the Zr (IV) species has
been incorporated into the framework of the sample MCM-41 [21]. The TEM
images of the samples Zr-MCM-41(90), Zr-MCM-41(60) and Zr-MCM-41(10) are
shown in Fig. 6. It can be observed that two samples exhibited the hexagonal arrays
structure, indicating that the three samples synthesized under microwave irradiation
condition possess the mesoporous framework. However, the mesoporous ordering
123

374
K. Bachari et al.
Fig. 5 Diffuse reflectance UV–vis spectra of the samples Zr-MCM-41 prepared by the microwave–
hydrothermal (M–H) process: a Zr-MCM-41 (90), b Zr-MCM-41(60), c Zr-MCM-41(10)
of the sample Zr-MCM-41(10) is poor as compared to that of the samples Zr-MCM-
4
1 (60) and Zr-MCM-41 (90). Additionally, it can also be observed that there are no
particles and/or clusters containing zirconium species on the surface of the three
samples in Fig. 6. Furthermore, the acid site distribution and acid amounts of Zr-
MCM-41 synthesized under the microwave irradiation condition were determined
using temperature-programmed desorption (TPD) of pyridine; the data are presented
in Table 2. Weak (423 and 633 K), moderate (633–743 K) and strong ([743 K)
acid sites are found in all samples. The weak acid sites are attributed to surface
hydroxyl groups and the medium and the strong acid sites probably originate from
the incorporation of zirconium atoms into the MCM-41 walls. It is interesting to
note that the number of weak acids sites decreases with decreasing Si/Zr ratio.
However, the amount of medium and strong acid sites decreases with increasing Si/
Zr ratio. It should be noted that the total number of acid sites (medium and strong
acid sites) of Zr-MCM-41 (10) prepared by the microwave–hydrothermal (M–H)
process is higher than that of Zr-MCM-41 (60) and Zr-MCM-41 (90).
Catalytic activity
Catalytic performances of Zr-MCM-41(M–H) materials with different Si/Zr ratio
in the acetylation of 1,2-dimethoxybenzene with acetic anhydride
The acetylation of 1,2-dimethoxybenzene has been carried out over Zr-MCM-41
(
M–H) materials with different Zr contents (Si/Zr = 90, 60, and 10) using acetic
anhydride as an acylating agent. The acetylation of 1,2-dimethoxybenzene with
acetic anhydride generates acetic acid in the products, which resulted from
the acetic anhydride utilization. Only one product is obtained, which is
3
,4-dimethoxyacetophenone, due to the fact that both the ortho positions of
1
23

Preparation and characterization of Zr-MCM-41
375
Fig. 6 TEM images of the Zr-
MCM-41(90), Zr-MCM-41(60)
and Zr-MCM-41(10) materials
prepared by the microwave–
hydrothermal (M–H) process
123

376
K. Bachari et al.
Table 2 Density and strength of acid sites of Zr-MCM-41 (M–H) catalysts with different Si/Zr ratios (Si/
Zr = 90, 60, 10)
Sample
Acid sites (mmol/g)
Weak (423–633 K) Medium (633–743) Strong ([743) Total (medium and
strong acid sites)
Zr-MCM-41 (10) 0.539
Zr-MCM-41 (60) 0.601
Zr-MCM-41 (90) 0.748
0.191
0.123
0.107
0.224
0.179
0.145
0.415
0.302
0.252
1
,2-dimethoxybenzene are sterically crowded for electrophilic reaction; acetylation
preferentially occurs at the fourth position which is the most favored and para to one
of the methoxy substituents. No products other than 3,4-dimethoxyacetophenone
were found in the product mixture. The conversion and the selectivity in the
acetylation of 1,2-dimethoxybenzene over Zr-MCM-41 (M–H) catalysts with
different Si/Zr ratio (Si/Zr = 90, 60, and 10) at a reaction temperature of 333 K and
under the standard reaction conditions (stoichiometric ratio 1,2DMB/AC = 5,
catalyst weight = 0.1 g and a reaction time of 2 h) are given in Table 3. The
activity of the catalysts is in the following order: Zr-MCM-41 (M–H) (10) [ Zr-
MCM-41 (M–H) (60) [ Zr-MCM-41 (90). In fact, it is found that Zr-MCM-41(10)
is the mainly efficient catalyst, show much higher conversion of acetic anhydride as
compared with that of other catalysts studied. The Zr-MCM-41(10) catalyst
registers the complete acetic anhydride conversion and 100% selectivity to 3,4-
dimethoxyacetophenone after 95 min of reaction. The very higher activity of
Zr-MCM-41(10) than other Zr-MCM-41 (M–H) catalysts may be mostly attributed
by the fact that the catalyst exhibits higher acidity than other Zr-MCM-41 (M–H)
catalysts (Table 2). On the other hand, the lower activity of Zr-MCM-41 (M–H)
(
90) which has the highest surface area and larger pore volume is mainly due the
presence of lower number of active sites on the porous surface. Elsewhere, it was
interesting to compare the solids with zeolites catalysts investigated earlier under
similar conditions. The activity of the catalysts is in the following order: Zr-MCM-
41(M–H) (10) [ BEA [ MFI. Indeed, it must be noted that, although the acidity of
the zeolites catalysts is much higher than that of the Zr-MCM-41 (M–H) (10), the
catalytic activity of the later is superior to that of zeolites catalysts. Thus, it has to be
Table 3 The conversion and the selectivity of 1,2-dimethoxybenzene over Zr-MCM-41 (M–H) catalysts
with different Si/Zr ratio (Si/Zr = 90, 60, and 10) at a reaction temperature of 333 K, stoichiometric ratio
1,2DMB/AC = 5, catalyst weight = 0.1 g and a reaction time of 2 h
Sample
Acetic anhydride
conversion (%)
Selectivity to 3,
4-dimethoxyacetophenone (%)
Zr-MCM-41 (M–H) (10)
Zr-MCM-41 (M–H) (60)
Zr-MCM-41 (M–H) (90)
100.0
88.3
66.5
100.0
100.0
100.0
1
23

Preparation and characterization of Zr-MCM-41
377
concluded that the accessibility of the active sites by the reactant molecules is
higher in Zr-MCM-41 (M–H) (10) than that of the zeolite catalysts, which is
supported by their enormous specific surface area and the specific pore volume, and
large pore diameter with well-ordered pore structure. These exceptional textural
characteristics of the Zr-MCM-41 (M–H) (10) are accountable for its higher
catalytic activity and the results further confirm that the catalysts with high surface
area, and large pore diameter is highly important for acetylation of substituted
aromatic compounds. As the Zr-MCM-41 (M–H) (10) catalyst was found to be
highly active, we opted to choose the catalyst for investigating the effect of other
reaction parameters such as the reaction temperature, the stoichiometric ratio
1
,2DMB/AC, weight of the catalysts and the recycling of the catalysts on the
conversion of acetic anhydride and the selectivity in the acetylation of 1,2-
dimethoxybenzene.
The effect of reaction temperature in the acetylation of 1,2-dimethoxybenzene
with acetic anhydride
The effect of reaction temperature on the conversion of acetic anhydride in the
acetylation of 1,2-dimethoxybenzene over Zr-MCM-41 (M–H) (10) at different
reaction times is shown in Table 4. As expected, an increase in the reaction
temperature had a favorable effect on the conversion of the acetic anhydride. The
conversion of acetic anhydride increases with increasing the reaction temperature.
However, the selectivity to 3,4-dimethoxyacetophenone was not changed with
increasing the reaction temperature. The selectivity of the product was almost 100%
for all the reaction temperature studied. It must be noted that the rate of the reaction
at lower temperature is rather slow. On the other hand, at the reaction temperature
more than 333 K, the rate of the reaction is very fast. The apparent rate constant of
the reaction at different reaction temperature was calculated using the pseudo-first
order rate law:
Log ½1=1 ꢁ xꢂ ¼ ðka= 2:303Þ ðt ꢁ t0Þ
where k is the apparent first-order rate constant, x the fractional conversion of
a
acetic anhydride, t the reaction time and t the induction period corresponding to the
0
time required for reaching equilibrium temperature. A plot of log [1/1 - x] as a
function of the time gives a linear plot over a large range of acetic anhydride
conversions. As expected, the apparent rate constant for the acetylation reaction was
Table 4 Catalytic activities of Zr-MCM-41 (M–H) (10) at different temperatures: 313, 333 and 353 K,
stoichiometric ratio 1,2DMB/AC = 5, catalyst weight = 0.1 g
a
Time (min)
Temperature (K)
Selectivity to
,4-dimethoxyacetophenone (%)
Apparent rate constant
3
-1
a
k (910 min )
3
3
3
3
a
13
33
53
210.2
95.0
40.1
100.0
100.0
100.0
13.2
40.3
94.4
Time required for complete conversion of acetic anhydride
123

378
K. Bachari et al.
-
3
-3
to 94.4.10 min
-1
increased from 13.2.10
temperature from 313 to 363 K. The activation energy for the Zr-MCM-41(10)
with increasing the reaction
-
1
calculated from an Arrhenius plot, was found to be 28.2 kJ mol . From these
results, it is found that the reaction temperature of 333 K is the best temperature for
Zr-MCM-41(10) and is preferred for the subsequently catalytic studies.
The effect of the stoichiometric ratio 1,2DMB/AC on the acetylation of 1,2-
dimethoxybenzene with acetic anhydride
The effect of the stoichiometric ratio 1,2DMB/AC on the conversion of acetic
anhydride was studied over Zr-MCM-41 (M–H) (10) at a reaction temperature of
3
1
33 K as a function of reaction time and the results are shown in Table 5. When the
,2DMB/AC ratio was varied, the conversion of acetic anhydride was significantly
affected. The conversion of acetic anhydride is increased by increasing the
,2DMB/AC from 1 to 10 at a reaction temperature of 333 K, while the selectivity
1
to 3,4-dimethoxyacetophenone is almost constant in all the cases. It was found that
the increase of the 1,2DMB/AC ratio may support the adsorption of a greater
amount of 1,2-dimethoxybenzene on the catalysts surface compared to acetic
anhydride molecules.
The effect of the weight of the catalysts in the acetylation of 1,2-dimethoxybenzene
with acetic anhydride
The weight of the catalysts was varied to study its influence on the conversion of
acetic anhydride in the acetylation of 1,2-dimethoxybenzene over Zr-MCM-41 (M–
H) (10). The weight of the catalysts in the reaction mixture was changed from 0.02
to 0.2 g. Table 6 shows the influence of the weight of the Zr-MCM-41 (M–H) (10)
catalyst on the conversion of the acetic anhydride in the acetylation of 1,2-
dimethoxybenzene. The concentration of the catalyst in the reaction mixture has a
huge impact on the conversion of acetic anhydride. The conversion of the acetic
anhydride increases when increasing the weight of the catalyst from 0.02 to 0.2 g at
the reaction temperature of 333 K and the stoichiometric ratio 1,2DMB/AC = 5,
whereas maintaining 100% selectivity to 3,4-dimethoxyacetophenone. This could
be mostly due to the fact that the accessibility of the surface acidic sites is larger at
Table 5 The conversion and the selectivity in the acetylation of 1,2-dimethoxybenzene over Zr-MCM-
4
3
1 (M–H) (10) catalyst with different stoichiometric ratio 1,2DMB/AC at a reaction temperature of
33 K, catalyst weight = 0.1 g
a
Time (min)
Stoichiometric ratio
1,2DMB/AC
Selectivity to
3,4-dimethoxyacetophenone (%)
Apparent rate constant
3
-1
a
k (910 min )
1
5
1
a
255.3
95.0
64.3
100.0
100.0
100.0
10.1
40.3
70.2
0
Time required for complete conversion of acetic anhydride
1
23

Preparation and characterization of Zr-MCM-41
379
Table 6 The conversion and the selectivity in the acetylation of 1,2-dimethoxybenzene over Zr-MCM-
1 (M–H) (10) catalyst with different weight of the catalyst at a reaction temperature of 333 K, stoi-
chiometric ratio 1,2DMB/AC = 5
4
a
Time (min)
Weight of the
catalyst (g)
Selectivity to
3,4-dimethoxyacetophenone (%)
Apparent rate constant
)
3
(9 10 min
-1
k
a
0
0
0
a
.02
.1
289.5
95.0
58.9
100.0
100.0
100.0
8.9
40.3
79.8
.2
Time required for complete conversion of acetic anhydride
higher catalyst concentrations, which supports the increase in the conversion of
acetic anhydride.
Recycling of the catalysts
The recyclability has been done over Zr-MCM-41 (M–H) (10) in the acetylation of
1
,2-dimethoxybenzene at a reaction temperature of 333 K, a reaction time of 2 h,
and a 1,2DMB/AC ratio of 5. After the reaction, the catalyst was filtered, washed
several times with acetone and dried in an oven at 393 K. Then, the catalyst was
activated at 823 K for 6 h under oxygen atmosphere. The recyclability experiments
were carried out twice and the procedure for the activation was repeated every time
after the reaction. The results on the recyclability are given in Table 7. The catalyst
shows approximately similar conversion after two cycles, without any transforma-
tion in the selectivity of the products. This shows that the catalyst is very stable
under the specified reaction conditions, and able to be recycled.
Applications to other aromatic compounds
The acetylation of other substrates with acetic anhydride was accomplished using
Zr-MCM-41(10) under the optimized reaction conditions. The information
concerning the reaction conditions and the results are presented in Table 8. The
catalyst shows amazing performances in the acetylation of aromatics used in the
reaction. In the case of acetylation of m e´ thoxybenz e` ne (anisole), it was observed
that the time required for complete conversion of acetic anhydride over Zr-MCM-41
Table 7 Effect of recycling the catalyst in the acetylation of 1,2-dimethoxybenzene over Zr-MCM-41
(M–H) (10) at a reaction temperature of 333 K, stoichiometric ratio 1,2DMB/AC = 5, catalyst
weight = 0.1 g
a
Time (min)
Catalyst
Selectivity to
,4-dimethoxyacetophenone (%)
Apparent rate constant
3
-1
a
k (9 10 min )
3
Fresh
95.0
99.2
100.0
100.0
100.0
40.3
37.6
33.9
First reuse
Second reuse
a
102.5
Time required for complete conversion of acetic anhydride
123

380
K. Bachari et al.
Table 8 Acetylation of different aromatic substrates over Zr-MCM-41 (M–H) (10) catalyst at a reaction
temperature of 333 K
a
Time (min)
Substituent
Reaction products (Selectivity (%))
1
,2-dimethoxybenzene
95
27
3,4-dimethoxyacetophenone (100%)
p-methoxyacetophenone (98.5%)
o-methoxyacetophenone (1.5%)
Methoxybenzene (anisole)
2-Methoxy naphthalene
37
1-acetyl-2-methoxynaphthalene (97.4%)
6
-acetyl-2-methoxynaphthalene (2.6%)
a
Time required for complete conversion of acetic anhydride
(
M–H) (10) catalyst is 27 min with 98.5% selectivity to (p) 4-methoxyacetophenone,
while the rest is (o) 2-methoxyacetophenone. The data on the acetylation of
-methoxynaphthalene with acetic anhydride under nitrobenzene as solvent are also
presented in Table 8. In fact, the selectivity to 1-acetyl-2-methoxynaphthalene and
-acetyl-2-methoxynaphthalene of 97.4 and 2.6%, respectively, was observed. The
2
6
formation of the sterically hindered, but kinetically favored, product, 1-acetyl-2-
methoxynaphthalene, has also been observed for the zeolite catalysts with greater
external surface areas and the proton exchanged MCM-41 catalyst at low reaction
temperature because the position 1 is considered to be the most activated one [22,
2
3]. Therefore, we consider the preferential formation of 1-acetyl-2-methox-
ynaphthalene in the Zr-MCM-41 (M–H) catalysts system could be mostly due the
fact that the materials possess large surface area, pore volume and pore diameter
which support the formation of the kinetically favored 1-acetyl-2-methoxynaphtha-
lene rather than the thermodynamically favored 6-acetyl-2-methoxynaphthalene.
Conclusion
Ordered hexagonal Zr-MCM-41 mesoporous molecular sieves with high specific
surface area were successfully synthesized via the microwave irradiation method.
After the calcination, the template was effectively removed. The Si/Zr molar ratio is
a key factor influencing the textural properties and structural regularity of Zr-MCM-
4
1 mesoporous molecular sieves. High zirconium content is unfavorable to the
formation of the Zr-MCM-41 with a highly ordered mesoporous structure. These
samples have many medium acid sites. The study of the liquid phase of acetylation
of 1,2-dimethoxybenzene and other two aromatic compounds with acetic anhydride
using Zr-MCM-41 solids shows that these catalysts have remarkable activities and
can also be reused in this reactions for several times.
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