Acylation of Toluene with Acetic Anhydride over Beta Zeolites: Influence of Reaction Conditions and Physicochemical Properties of the Catalyst

Article abstract of DOI:10.1006/jcat.2000.2971

Acylation of toluene with acetic anhydride was carried out over Beta-type zeolites in a stainless steel autoclave. At low temperature, i.e., 150°C, and an arene/anhydride molar ratio 10-20, high yields of 4-methylacetophenone were obtained with selectivity close to 100%. The reaction is limited by the poisoning of the active sites by adsorption of the product and pore blockage due to coke -type products. Acid site accessibility is of paramount importance for this reaction, and the use of a nanocrystalline Beta zeolite as catalyst minimizes catalyst decay. This, combined with an optimum framework composition, allows the preparation of active and selective acylation catalysts based on Beta zeolite.

Full text of DOI:10.1006/jcat.2000.2971

Journal of Catalysis 195, 161–168 (2000)
doi:10.1006/jcat.2000.2971, available online at http://www.idealibrary.com on
Acylation of Toluene with Acetic Anhydride over Beta Zeolites: Influence
of Reaction Conditions and Physicochemical
Properties of the Catalyst
P. Botella, A. Corma, ,1 J. M. Lo´ pez-Nieto, S. Valencia, and R. Jacquot†
Instituto de Tecnolog´ıa Qu´ımica, UPV-CSIC, Universidad Polite´cnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain;
and †RHODIA, Centre de Recherches de Lyon, 85, Avenue des Fre`res Perret, B.P. 62, 69192 Saint-Fons Cedex, France
Received March 24, 2000; revised June 15, 2000; accepted June 15, 2000
aromatics. Materials such as pillared clays and rare-earth-
exchanged pillared clays (6), Keggin-type heteropoly-
acids (7), Nafion-H and Nafion/silica composites (8–10),
sulfated zirconia (6, 11), and graphite (12) have been proven
to catalyze this reaction. Acid molecular sieves have also
been widely used as acylation catalysts, and more specifi-
cally medium- and large-pore zeolites have been presented
as the ultimate catalysts for the future industrial production
of aromatic ketones (13). Basically, H-ZSM5 has been used
in acylation of less substituted aromatic molecules such
as benzene, phenol, and toluene (14–16), while zeolites
with larger pores such as H-Beta and HY have been used
extensively in acylation of arylethers such as anisole and
polysubstituted aromatics such as xylenes and mesitylenes
(13, 14, 17–21). A recent important development in this
field has been reported by RHODIA, which has established
the first industrial application of zeolites for the acylation
of anisole and veratrole (22, 23). Bulkier molecules such as
2-methoxynaphthalene have also been efficiently acylated
using large-pore zeolites as catalysts (24–26). However, it
must be pointed out that while the acylation of phenols and
aromatic ethers with alkyl/aryl chlorides is carried out with
very good yields and selectivities on zeolites, the results
obtained with low activated aromatic rings like toluene and
benzene are poorer, especially when less reactive acylating
agents, such as carboxylic acids and their anhydrides, are
used. On the other hand, the fast catalyst decay, produced
by “coke” deposition within the micropores (21, 27, 28),
strongly limits the use of these materials in commercial
processes. Nevertheless, we believe that it should be possi-
ble to improve the possibilities of using zeolites as acylation
catalysts through a better catalyst design. We have tried
in this work to optimize the physicochemical properties
of a Beta zeolite catalyst by optimizing parameters such
as framework Si/Al molar ratio, hydrophilic–hydrophobic
properties, and crystallite size. Moreover, the effects of
reaction conditions, i.e., temperature, catalyst concentra-
tion, and arene/anhydride molar ratio, on the catalytic
performance have also been studied.
Acylation of toluene with acetic anhydride was carried out over
Beta-type zeolites in a stainless steel autoclave. At low tempera-
ture, i.e., 150 C, and an arene/anhydride molar ratio 10–20, high
yields of 4-methylacetophenone were obtained with selectivity close
to 100%. The reaction is limited by the poisoning of the active sites
by adsorption of the product and pore blockage due to “coke”-type
products. Acid site accessibility is of paramount importance for this
reaction, and the use of a nanocrystalline Beta zeolite as catalyst
minimizes catalyst decay. This, combined with an optimum frame-
work composition, allows the preparation of active and selective
c
acylation catalysts based on Beta zeolite.
2000 Academic Press
Key Words: acylation; toluene; acetic anhydride; Beta zeolite;
methylacetophenone.
INTRODUCTION
Friedel–Crafts acylations are widely used in the manufac-
ture of aryl ketones, which are of interest in the synthesis of
a large number of fine chemicals such as drugs, fragrances,
dyes, and pesticides (1, 2). This process has been carried
out in industry by working under batch conditions using
acyl halides as acylating agents and homogeneous Lewis
acids such as anhydrous metal halides as catalysts (3). How-
ever, Lewis acids must be used in higher than stoichiomet-
ric amounts, and the catalyst must be destroyed at the end
of the reaction with a significant production of undesirable
wastes. Thus, it is highly desirable to develop new processes
that can reduce the environmental and economic problems
associated with the classical Friedel–Crafts acylation cata-
lysts (4, 5). These new processes will require replacement
of the homogeneous catalysts with friendly solid acids and
use of less hazardous acylating agents, i.e., carboxylic acids
and their anhydrides, instead of acyl halides.
Toward this purpose, different solid acids have been
tested as catalysts for the acylation of low-molecular-weight
1 To whom correspondence should be addressed. Fax: 34-6-3877809.
E-mail: acorma@itq.upv.es.
161
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

162
BOTELLA ET AL.
TABLE 1
Physicochemical Characteristics of H-Beta Zeolites
Acidity
(
mol py)c
Brønsted
Lewis
350 C
Area BET
Crystallinity
(% )
Crystal size
(
1
Sample
Si/Ala
(m2 g
)
m)b
250 C
350 C
400 C
250 C
400 C
CP811
A13
A16
A27
A38
B15
12.5
13
16
27
38
15
50
93
570
549
532
548
581
519
452
463
71
72
73
77
66
85
98
100
0.15
0.2
0.05
0.2
5
33
63
27
27
45
19
11
27
15
49
13
11
22
9
16
7
28
8
9
14
4
48
32
22
31
20
38
7
40
26
19
25
18
38
4
40
25
14
19
18
38
4
0.2
0.4–0.6
0.4–0.6
0.3–0.5
B50
B93
10
2
5
4
2
a As-made molar ratio.
b Average size determined from the SEM images.
c Determined from the infrared spectra of adsorbed pyridine after evacuation at 250, 350, and 400 C.
EXPERIMENTAL
Varian probe. 29Si spectra were recorded at 79.5 MHz using
a 55.4 pulse length of4.0 m and a recycle delayof20s, with
the sample spinning at 5 kHz. 27Al MAS NMR spectra were
Catalysts
recorded at 104.214 MHz with a 10 pulse length of 0.5
s
Several H-Beta zeolites have been tested as catalysts
for the acylation of toluene. A commercial Beta was pro-
vided by P.Q. Industries (sample CP811), and the rest of
the Beta samples were prepared in our laboratory. We
have named them according to the preparation procedure
(series A, B) and the as-made Si/Al framework molar ratio
(A13, A16, A27, . . .). Samples of series A were synthesized
hydrothermally at 140 C in basic medium in PTFE-lined
and a recycle delay of 0.5 s, at spinning rates of 7 kHz. The
27Alchemicalshiftsare reported relative to Al(H2O)36+. The
most relevant physicochemical properties of these zeolites
are summarized in Table 1.
Reaction Procedure
All experiments were carried out in a stainless steel
stainless steel autoclaves under rotation, using amorphous stirred 150-ml autoclave (Autoclave Engineerings) in the
silica (Aerosil 200, Degussa) as the silica source. Samples 140–250 C interval (typically, 150 C). A 1.00-g portion of
A13, A27, and A38 were prepared in the presence of alkali catalyst was activated in situ before the reaction by heat-
cations according to a reported procedure (29). Sample A16 ing for 2 h at 150 C in N2 flow (100 ml min 1). A mix-
isa nanocrystalline Beta zeolite synthesized in an alkali-free ture of 400 mmol of water-free toluene (Aldrich) and
basic medium (30). Samples B15, B50, and B93 were syn- 20 mmol of acetic anhydride (Riedel-de Ha¨en) was then
thesized in a similar way but using tetraethylorthosilicate added. The amounts of reactants were also varied when
(TEOS, Aldrich) as the silica source in a fluoride medium necessary to provide different catalyst concentrations and
at nearly neutral pH, according to a previously described arene/anhydride molar ratios. The time on stream (TOS)
method (31).
was 4 h, and samples were taken at different TOSs and an-
The Al content in the samples was determined by atomic alyzed. By recycling the catalyst, it was observed that some
absorption spectrophotometry (Varian spectrAA-10 Plus). deactivation occurs during the reaction. Thus, in all exper-
Crystallinity was measured by powder X-ray diffraction, us- iments, a larger number of samples were taken during the
ing a Phillips PW1710 diffractometer with CuK radiation, first hour of reaction. Products were analyzed by GC in a
and compared with a highly crystalline standard sample. Fisons 8000 Series instrument equipped with a 25-m capil-
The acidity of the samples was established by the stan- lary cross-linked 5% phenylmethylsilicone column and an
dard pyridine adsorption–desorption method (32). The FID detector, using nitrobenzene as an internal standard.
surface area of the catalysts was calculated by the BET/BJH The products were also identified by mass spectrometry in
method with N2 adsorption/desorption performed at 77 K a Varian Saturn II GC–MS model working with a Varian
in a Micromeritics ASAP 2000 instrument. The crystal size Star 3400 gas chromatograph.
was determined from the SEM images obtained in a JEOL
6300 scanning electron microscope.
When the mass balances were performed at shorter times
on stream, an excess of acetic acid was found. This was
Solid-state MAS NMR spectra were acquired in a Varian formed by hydrolysis of the anhydride, due to the small
VXR-400S WB spectrometer, using a 7-mm RT CP/MAS amounts of water retained in the catalysts even after the

H-BETA ZEOLITE FOR TOLUENE–ACETIC ANHYDRIDE ACYLATION
163
activation step (19). Such hydrolysis was taken into ac-
count in calculating the conversion of acetic anhydride,
the yield of methylacetophenone, and the mass balance
of anhydride, according to the equations proposed by Ro-
han et al. (27). Furthermore, “coke”-like products were ad-
sorbed on the catalyst during the reaction, and the catalyst
became dark brown. Catalysts were extracted in a micro-
Soxhlet with CHCl3 over 12 h. The solvent was evapo-
rated and the remaining organic material was weighted,
diluted with toluene, and analyzed by GC–MS. Among the
adsorbed products we found acetic acid, methylacetophe-
none, and some minor compounds, basically diacyltoluene,
coming from the consecutive acylation of monoacylated
products. Although their percentages were usually very
small, they were considered for the calculation of the total
mass balance. Carbon determination in the used catalyst
allowed us to quantify the nonextractable organic deposit
by thermogravimetric and C-elemental analysis performed,
respectively, in a NEST instrument using calcined Kaolin
as reference material and in a Fisons EA 1108 CHSN-O ap-
paratus. Ultraviolet–visible spectroscopy (Varian Cary 5G
UV–vis-NIR spectrophotometer) was used to study the na-
ture of the functional groups present in the coke. Taking
into account all these fractions it was possible to obtain
mass balances 95% .
FIG. 1. Variation of yield (ᮀ) and selectivity (᭹) to MAP with re-
action temperature obtained during acylation of toluene with acetic an-
hydride (AA) over Beta zeolite CP811. Experimental conditions: cata-
lyst/AA weight ratio = 1; toluene/AA molar ratio = 20; TOS = 240 min.
selectivity to MAP with the reaction temperature at a time
on stream (TOS) of 240 min. It can be seen that the yield of
MAP shows a maximum at 150 C and decreases when the
reaction temperature is further increased. We have found
100% selectivity to MAP when the reaction is carried out
at low temperature, i.e., 150 C, but it decreases up to 80%
at 250 C.
The influence of TOS on the MAP yield is shown in
Fig. 2. It can be seen that the yield of MAP first increases
with increasing TOS but remains practically constant af-
ter 120 min. In addition, an increase in the yield of MAP
is also observed when the catalyst/AA weight ratio is in-
creased from 0.5 to 1. It is then apparent that an important
deactivation of the catalyst occurs during the reaction.
With regard to the toluene/anhydride molar ratio
(T/AA), Fig. 3 shows that the yield of MAP increases when
the T/AA ratio is increased to 20, and then remains practi-
callyunchanged. Thisisprobablydue to the inhibitingeffect
ofMAP, which can be stronglyadsorbed on the catalyst (27).
RESULTS AND DISCUSSION
Friedel–Crafts acylation of toluene with acetic anhydride
(AA) is a very selective process for the production of
4-methylacetophenone 1 (see Scheme 1). The reaction in-
volves the production of one molecule of acetic acid (HAc)
for every molecule of anhydride consumed. Small amounts
of 2- and 3-methylacetophenone (2 and 3, respectively)
were also found in the product mixture, especially at high
conversions, due to the acylation of the less favored posi-
tions in the aromatic ring. Since the amount of these com-
pounds was never above 2% of the total yield in acylated
products, we have considered in those cases the total yield
of methylacetophenones (MAP) calculated as the sum of
yields of the three isomers.
As the first step we studied the influence of the reaction
conditions on the yield of MAP, using a commercial Beta ze-
olite (CP811). Figure 1 shows the variation of the yield and
FIG. 2. Variation of yield of MAP with TOS obtained during acyla-
tion of toluene with acetic anhydride (AA) over Beta zeolite CP811 at
a catalyst/AA weight ratio of 1 (᭹) or 0.5 (ᮀ). Experimental conditions:
temperature = 150 C; toluene/AA molar ratio = 20.
SCHEME 1

164
BOTELLA ET AL.
FIG. 3. Variation of yield of MAP with toluene/AA molar ra-
tio obtained during acylation of toluene with acetic anhydride (AA)
over Beta zeolite CP811. Experimental conditions: temperature = 150 C;
catalyst/AA weight ratio = 0.5; TOS = 240 min.
This inhibiting effect would be less significant for mixtures
richer in toluene, because the large excess of toluene helps
to desorb the ketone formed. T/AA ratios larger than 20
do not improve significantly the yield in MAP and conse-
quently we have used throughout this work a T/AA ratio
of 20.
The acylated product is adsorbed on the acid sites due
to its higher polarity and, consequently, the adsorption of
the toluene on the surface and in the pores of the catalyst
is restricted and the reaction rate decreases. Moreover, the
adsorbed MAP can further react, giving larger by-products,
SCHEME 2
the Beta zeolite catalyst needs to be improved not only in
activity but also in catalyst decay.
Influence of Zeolite Properties
From what we have observed up to now, we can conclude
which remain adsorbed and further increase the diffusion that both the total number of acid sites and the adsorption
problems of reactants within the channels of the catalyst, properties of the zeolite should have direct effects on the
producing a decrease of the catalytic activity. The forma- activity and decay of the catalyst. If this is so, one zeolite
tion of those bulkier products increases when the reaction variable that influences both the total number of Brønsted
temperature is raised from 150 to 250 C, since higher re- acid sites and the adsorption properties of the catalyst is
action temperature favors consecutive acylation, dispro- the framework Si/Al ratio. Indeed, increasing the frame-
portionation, alkylation, and decarboxylation. Some of the work Si/Al ratio decreases the total number of Brønsted
by-products formed during acylation of toluene with Beta
CP811 are indicated in Scheme 2. Those were barely seen
in the liquid phase, but they were present in the zeolite
pores and were analyzed and identified by GC–MS when
the zeolite was subjected to Soxhlet extraction using chloro-
form. The diffuse reflectance spectrum of the catalyst used
showed the typical band of acetophenones and polyaromat-
ics, which were absent in the fresh sample (Fig. 4). Thermo-
gravimetric and elemental analyses revealed that the prod-
ucts retained in the zeolite pores accounted for 5–8 wt% .
We then confirmed that methylacetophenone is strongly
adsorbed in the zeolite pores and can undergo a number of
consecutive reactions that produce bulkier molecules which
cannot diffuse out of the zeolite. Those products not only
block the acid sites but also produce pore blocking with the
corresponding loss of catalytic activity. Soxhlet extraction
FIG. 4. UV–vis spectra of CP811, A13, and A16 samples before (B)
with chloroform cannot extract all the by-products within
and after (A) their use in the acylation of toluene with acetic anhy-
the pores, and calcination in the presence of air becomes
necessary to remove the organic residues. It is apparent that
dride. Experimental conditions: temperature = 150 C; catalyst/AA weight
ratio = 0.5; toluene/AA molar ratio = 20; TOS = 240 min.

H-BETA ZEOLITE FOR TOLUENE–ACETIC ANHYDRIDE ACYLATION
165
FIG. 6. 27Al MAS NMR spectra of calcined samples A13, A27, and
A38. The peak at 0 ppm is associated with octahedral nonframework Al
present in the sample.
The higher the framework Al content of the as-synthesized
zeolite sample, the higher the dealumination. Therefore, the
differences in the final number of hydroxyl bridging groups,
and consequently in the Brønsted acidity of the three sam-
ples studied, are smaller than expected taking into account
the original framework Si/Al ratio (see pyridine results in
Table 1). In any case, the acidity results are in good agree-
ment with the catalytic activity of the three Beta sam-
ples, which, even though close, follows the order A13 >
A27 A38 (see Table 2).
At this point, we prepared a more hydrophobic Beta zeo-
lite by carrying out the synthesis in fluoride media (sample
B15) (31). 29Si MAS NMR results (Fig. 5) show that the
number of defects is indeed lower in sample B15 than in
the corresponding A13 sample. Moreover, both samples
FIG. 5. 29Si MAS NMR spectra of calcined samples A13, A27, A38,
and B15. The band at 101 ppm is related to silanol groups present on
zeolite surface.
acid sites and increases the hydrophobicity of the mate-
rial. To explore the influence of the Si/Al ratio, three Beta
samples with Si/Al ratios of 13 (A13), 27 (A27), and 38
(A38) were synthesized by a conventional method (29)
that produces samples with a large number of defects. These
are detected by 29Si MAS NMR (Fig. 5) from the band at
101 ppm, which corresponds to Q3Si (3Si, 1OH) groups.
The presence of those groups strongly decreases the po-
tential hydrophobicity of the zeolite and, consequently, we
should not expect large variations in this property from
sample A13 to sample A38. With respect to changes in
acidity with the framework Si/Al ratio, we can see that the
three zeolite samples suffer dealumination during the calci-
nation process necessary to remove the structure-directing
agent. This dealumination increases the framework Si/Al
ratio while producing a certain amount of extraframework
Al shown at 0 ppm in the 27Al MAS NMR spectra (Fig. 6).
The dealumination is also shown by the 1450 cm 1 IR band
corresponding to pyridine coordinated to Lewis acid sites
(Table 1), which corresponds to extraframework Al (33).
TABLE 2
Friedel–Crafts Acylation of Toluene with Acetic Anhydride
on H-Beta Zeolitesa
AA conversion
(wt% )b
MAP yield
(wt% )
MAP selectivity
(wt% )
Catalyst
CP811
A13
A16
A27
A38
B15
58
58
83
56
55
56
38
28
46
54
80
49
50
54
35
28
100
100
100
100
100
100
92
B50
B93
89
a Experimental conditions: T = 150 C; catalyst/AA weigth ratio = 0.5,
1 : 2; toluene/AA molar ratio = 20; TOS = 240 min.
b The difference between the conversion of acetic anhydride (AA)
and the yield of MAP indicates the extension of the hydrolysis of the
acetic anhydride.

166
BOTELLA ET AL.
FIG. 7. Thermogravimetric (A) and thermodifferential (B) patterns of Beta zeolites obtained on samples used in catalytic tests: (a) CP811,
(b) A13, and (c) A16. Conversion of AA and the yield of MAP obtained with these samples are comparatively shown in Table 3.
have practically the same Si/Al ratio as the as-made zeo- that the activity of sample A16 is much higher than that of
lite, but B15 should be more hydrophobic. In principle, a any of the other samples studied. Moreover, from what has
more hydrophobic zeolite should allow toluene to diffuse been discussed before, it can be expected that the crystal
and adsorb better, thereby helping in the desorption of ace- size of the catalyst could also have an impact on the catalyst
tophenone. It is remarkable that, although the total number decay. In this way, the catalyst deactivation has been stud-
of Brønsted sites is higher in B15 than in A13 owing to the ied by comparing the amount of residues left in samples
higher stability upon calcination of the former, the catalytic CP811, A13, and A16 after reaction.
activity of B15 is similar to that of A13. “Defect-free” Beta
The amount of coke deposited on the catalyst has been
samples with Si/Al ratios of 50 and 93 were also synthesized determined by thermogravimetric (TG) and differential
(Table 1), and these should have stability similar to that thermal analyses (DTA), and the results are shown in
of B15 but should be more hydrophobic. We can see now Figs. 7a and 7b, respectively. From a comparative purpose,
(Table 2) that the catalytic activity and selectivity in the Table 3 shows the conversion of AA and the yield of
B series decrease when the framework Si/Al ratio is in- MAP obtained on the studied zeolites. Similar coke de-
creased, regardless of the increase in hydrophobicity when posits (7 wt% ) were obtained by TG with CP811 and A13
the Si/Al ratio is increased. Therefore, we must conclude samples, while the amount of coke in A16 is lower (5 wt% )
that neither the higher hydrophobicity nor the higher acid- at similar toluene conversions. On the other hand, when
ity of sample B15 produces an important increase in the the DTA diagrams obtained in the presence of air are com-
catalytic activity for the acylation of toluene with acetic an- pared (Fig. 7b), both CP811 and A13 zeolites present two
hydride. This unexpected result can be explained by taking
into account that even though sample B15 has a higher total
number of acid sites than A13, it also has a larger crystal size
(Table 1) and this may have a negative influence on the dif-
fusion of reactants and products within the catalyst pores.
Thus, since diffusion is an important issue during acyla-
tion reactions, at least in the liquid phase, we could expect
(25) the zeolite crystallite size to have an important impact
on the conversion and catalyst decay during acylation of
toluene with Beta zeolites.
If we take this into account, the logical next step was
to synthesize a Beta zeolite with smaller crystallites and
high stability upon calcination. This was achieved through
a special synthesis procedure (30) that produced one sam-
TABLE 3
Coke Formation in H-Beta Zeolites during the Friedel–Crafts
Acylation of Toluene with Acetic Anhydride at Different Levels of
Conversiona
AA conversion
(wt% )b
MAP yield
(wt% )
Coke deposit
(wt% )c
Catalyst
CP811
58
38
58
83
48
46
31
54
80
42
7.0d
6.6
A13
A16
7.0d
6.9
4.9d
a Experimental conditions: T = 150 C; catalyst/AA weigth ratio = 0.5,
ple (A16) with very small crystallites and a high density of 1 : 2; toluene/AA molar ratio = 20.
b The difference between the conversion of acetic anhydride (AA) and
acid sites (Table 1). The small crystallites should allow faster
diffusion of the products out of the catalyst and this, com-
bined with the high acidity, should provide an important
increase in the activity and a decrease in the rate of catalyst
deactivation. The results presented in Table 2 clearly show
the yield of MAP indicates the extension of the hydrolysis of the acetic
anhydride.
c The coke formed during the reaction has been determined from the
TG experiments.
d Data corresponding to samples of Fig. 7.

H-BETA ZEOLITE FOR TOLUENE–ACETIC ANHYDRIDE ACYLATION
167
formed and the production of coke deposits limits the ex-
tent of the reaction.
Catalytic activity is strongly influenced by the Brønsted
acidity of the zeolite. In this way, low framework Si/Al ratios
lead to better yields in MAP.
Considering the intrapore diffusion problems caused by
the adsorption of different products, better results could
be obtained with nanocrystalline stable Beta zeolite with
lower nominal framework Si/Al ratios. This type of catalyst
minimizes catalyst decay by coke deposition, with the corre-
sponding improvements in activity and time of catalyst use.
ACKNOWLEDGMENTS
The authors thank the Comisio´n Interministerial de Ciencia y Tec-
nolog´ıa (CICYT) in Spain (Projects MAT 97-0561 and MAT 97-1016-
C02-01) for financial support.
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