A lewis acid site-activated reaction in zeolites: Thiophene acylation by butyryl chloride

Article abstract of DOI:10.1006/jcat.1998.2325

The acylation of thiophene by butyryl chloride has been studied in the liquid phase in the presence of 12 catalysts: HZSM-5, H-mordenite, and HY (ultrastable Y) with various framework Si/Al ratios, various numbers of Bronsted and Lewis sites, and various amounts of nonframework aluminum. The numbers of Bronsted and Lewis acid sites were obtained by FT-IR using chemisorbed ammonia, as described elsewhere. There is a correlation between the initial rates of reaction and the number of Lewis acid sites, whereas there is no correlation between the initial rates and the number of Bronsted sites. The correlation with the number of Lewis sites may be considered significant, since it expands on more than one order of magnitude. Poisoning of the catalyst probably results from the oligomerization on butyryl chloride. No carboxylic acid has been detected in the course of the reaction. No attempt has been made to determine the nature of the poisoning reaction.

Full text of DOI:10.1006/jcat.1998.2325

Journal of Catalysis 182, 257–263 (1999)
Article ID jcat.1998.2325, available online at http://www.idealibrary.com on
A Lewis Acid Site-Activated Reaction in Zeolites:
Thiophene Acylation by Butyryl Chloride
1
Yu. Isaev and J. J. Fripiat
Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin—Milwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201
Received September 16, 1998; revised October 15, 1998; accepted November 5, 1998
aluminum (NFAl) in HZSM-5 is low and the number of
The acylation of thiophene by butyryl chloride has been stud- Lewis sites is small (18), while with HY and H-mordenite
ied in the liquid phase in the presence of 12 catalysts: HZSM-5, (16–18) the content of NFAl and number of Lewis sites are
H-mordenite, and HY (ultrastable Y) with various framework Si/Al
ratios, various numbers of Brønsted and Lewis sites, and various
amounts of nonframework aluminum. The numbers of Brønsted
and Lewis acid sites were obtained by FT-IR using chemisorbed am-
monia, as described elsewhere. There is a correlation between the
possible cause explaining the very low activity of HZSM-5.
initial rates of reaction and the number of Lewis acid sites, whereas
there is no correlation between the initial rates and the number of
much larger.
At the time, restriction due to the molecular traffic in the
porous system in these zeolites, compared with that in the
Y or � molecular sieves, was invoked by Corma et al. as a
Since, as will be shown here, HZSM-5 becomes much more
active when steaming brings about NFAl in the porous
structure, while decreasing the number of Brønsted sites,
Brønsted sites. The correlation with the number of Lewis sites may
be considered significant, since it expands on more than one order
of magnitude. Poisoning of the catalyst probably results from the the role of Lewis acidity has to be reconsidered.
oligomerization on butyryl chloride. No carboxylic acid has been
Based on the study of acylation of benzofuran by acetic
detected in the course of the reaction. No attempt has been made to acid on Y-zeolites with framework (Si/Al) ratios between
determine the nature of the poisoning reaction.
�c 1999 Academic Press
4
.8 and 86, Richard, Carreyre, and P e´ rot (11–13) observed
a maximum activity for a ratio close to 15 and, simulta-
neously, and not surprisingly, a maximum in the coke for-
mation. They concluded that the maximum activity results
from a compromise between the number and the strength
of protonic acid sites.
INTRODUCTION
The acylation of aromatic molecules by acid chlorides
RCOCl is a classic example of an electrophilic substitu-
tion catalyzed by Lewis acids. A sustained attention to the
possibility of replacing the environmentally, unfriendly in-
organic chlorides (such as AlCl3, FeCl3 or SnCl4) by H- or
rare-earth metal-exchanged zeolites is evident in numerous
papers and reviews (1–5).
The main accent in those publications is on the yield in
acylated products and less on the nature of the acid sites
catalyzing the reaction. However, the influence of the
porosity and of the hydrophobicity on the acylation reac-
tion has also been studied (6–8), as has the influence of
the dealumination of the zeolite (8–14). Boreskova et al.
15) assumed that the reaction took place on Lewis sites,
while Corma et al. (8) in 1989 suggested the intervention
of protonic acidity on the basis of the activity of partially
sodium-exchanged HY zeolites. Interestingly, Corma et al.
8) observed that HZMS-5 calcined at 550 C has a very low
activity despite a large intrinsic Brønsted acidity. On cal-
cination at that temperature the content of nonframework
Gaare and Akporiaye (14), in the study of the Friedel–
Crafts acylation of anisole by acetyl chloride on dealumi-
nated Y-zeolite, observed an increase in the yield of the
p-substituted product as the Al content of the lattice de-
creased. The increase in activity was attributed to the in-
crease in hydrophobicity of the more siliceous lattice.
We must confess that we do not understand why hydro-
phobicity would represent an asset in reactions occurring
without production of water. In addition, none of the re-
ferred papers has given the number of Brønsted or Lewis
acid sites per gram of catalyst, which should be done as a
prerequisite to the discussion on their respective influences,
as should the heat of water adsorption be measured when
referring to hydrophobicity.
(
The goal of the present contribution is to study the na-
ture of the catalytic acid sites in the acylation of aromatics
�
(
(
thiophene and toluene) with acyl chloride (butyryl) in a so-
�
lution (chlorobenzene) at � 100 C on different zeolites. The
activities of steamed HZSM-5, H-mordenite, ultrastable Y,
and fluorinated and dealuminated USY with different Si/Al
ratiosand Brønsted and Lewissite contentswere compared.
1
To whom correspondence should be addressed.
257
0021-9517/99 $30.00
Copyright �c 1999 by Academic Press
All rights of reproduction in any form reserved.

2
58
ISAEV AND FRIPIAT
�
It will be shown that the initial rates correlate well with terval 115–350 C for 30 min. With knowledge of the ex-
+
the number of Lewis sites and not with the number of tinction coefficients of the NH4 and NH3 bending modes
Brønsted sites. The increased hydrophobicity which could and the integrated intensities, the absolute numbers of
be expected from the increase in the Si/Al framework ratio Brønsted or Lewis sites remaining at increasing temper-
�
may play a role, if no nonframework Al is formed.
atures were determined. At 115 C under vacuum, it is
The yield in acylated products is generally two orders of considered that all physically adsorbed species are re-
magnitude higher than the number of Lewis sites, show- moved. The number of Brønsted sites is obtained by fit-
+
ing that the catalytic reaction is not stoichiometric in the ting NH4 data with a Langmuir desorption isobar. Since
consumption of the sites.
the heat of chemisorption of ammonia on Lewis sites can
be described by a Freundlich isotherm, and consequently,
ln nads = a + b/T , the concept of site saturation is meaning-
EXPERIMENTAL
�
less. It has been suggested that at 115 C the correspond-
Materials
ing amount of chemisorbed NH3 represents the amount of
Lewis sites, because it is adsorbed with a differential heat
equal to or larger than 100 kJ/mol, up to � 200 kJ/mol (21).
The physical adsorption energy and the energy of most hy-
drogen bonds is much lower than 100 kJ/mol.
The initial mordenite and USY zeolites are commercial
PQ products with Si/Al ratios of 5.3 and 4.9, respectively.
ZSM-5isa commercialproduct ofAngarskneftorgsynthesis
Company. Toluene, thiophene, butyryl chloride, nitroben-
zene, and chlorobenzene of analytical purity were pur-
chased from VWR Scientific and used without further
purification.
Fluorinated USY zeolites were prepared according to
Panov et al. (19) in calcining USY impregnated with NH4F.
This treatment increases the framework Si/Al NMR ratio
and reduces the number of Brønsted and Lewis sites. With-
out fluorination an increase in the number of Lewis sites
results from an increase in that ratio. It was suggested that
the substitution of OH by F on the NFAl decreases the
probability of forming coordinately unsaturated Al (19).
The steaming of ZSM-5 was carried out in the quartz re-
actor. The sample was heated up to the temperatures shown
The accuracy or the number of Brønsted and Lewis sites
determined in that way and shown in Table 1 can be esti-
mated to be about 10% . The sum of these two populations
compared well with the ASTM test operated on samples
�
outgassed at 115 C (18). The characteristics of the Lewis
acidity will be commented on later.
In support of the accuracy of the FT-IR data, it is
worth mentioning the very good correlation found between
the absolute numbers of acetone and NH3 adsorbed on
Brønsted sites and the calculated number of protonic sites
(
22).
Thus, the results shown in Table 1 provide a safe basis
for assigning the catalytic activity to either one of the pos-
sible acid sites. In Fig. 2 the errors corresponding to the
determination of the Lewis sites are indicated for the three
top values. For the others these errors correspond approx-
imately to the thickness of the symbols.
�
in Table 2 at the rate of 2 C/min in oxygen and exposed to
a flow of oxygen coming through water kept at 80 C. At
various lengths of time the steam was exchanged with the
flow of dry oxygen and the sample was cooled down.
�
Characterization of Materials
Catalytic Activity Measurements
All catalysts were characterized by X-ray diffraction.
XRD spectra were obtained by using a Scintag X-ray diff-
ractometer (CuK� radiation) and a scan rate of 5 /min. The
Catalyst activation. The zeolite was heated in the flow
�
of dry oxygen up to 150 C for 2 h and kept for 1 h at this
�
�
temperature. The temperature was then increased to 250 C
full sorption, N2 adsorption, and desorption measurements
were performed on an automated physisorption instrument
over 2 h and kept at this temperature for 1 h. Finally, the
catalyst was heated to 450 C over 4 h and kept at this tem-
perature for 2 h.
�
(
Omnisorp 100, Coulter Corp.). The Si/Al ratio in the modi-
29
fied zeolites was obtained from Si NMR data, as described
previously (16–18).
Acylation process. The reaction was carried out under
All the physical characterizations (XRD and N2 porosity the atmosphere of dry argon in a glass flask with a con-
�
2
� 2
measurements) confirm that the modifications brought denser. The mixture of 10 mol thiophene and of 10 mol
about by the dealumination treatments were those ex- butyryl chloride in 50 ml chlorobenzene was heated to
�
� 3
pected (19, 20) and that they did not deeply alter the lattice 100 C. Nitrobenzene (8.8 � 10 mol) was used as the
structure.
internal standard to follow the changes in the concentra-
The measurements of the acidity of these zeolites were tions. The pretreated zeolite (0.15 g for the study of thio-
carried out as described previously (18), with FT-IR spec- phene) was added to the stirred solution, carefully prevent-
troscopy using NH3 as a probe molecule. The FT-IR spec- ing its contact with atmospheric moisture. Samples were
tra were recorded on zeolites having chemisorbed NH3 analyzed periodically by GC (Perkin-Elmer 8500,. TC-
1
and outgassed at five different temperatures in the in- detector) using a 6 � 8 -in. packed column (10% OV-101 on

LEWIS ACID SITE-ACTIVATED REACTION IN ZEOLITES
259
Chromosorb W-HP, 80/100). It should be emphasized that
TABLE 1
no carboxylic acid has been detected with GC/MS. Blank
tests, without catalyst, at the same temperature have shown
no reaction in the acylation of either toluene or thiophene.
Numbers of Brønsted and Lewis Sites in the Catalysts Used
in This Work (mmol/g)
Catalyst
Brønsted sites
Lewis sites
NMR, Si/Al
Analytical Treatment of the Experimental Data
1
. HZSM-5 (initial)
0.81
0.81
0
0
16
� 16
Let Y be the yield (% ) in acylated product. If CA and C0
(t)
A
2. HZSM-5 steamed
�
are the concentrations in thiophene (Th) at time t and time
t = 0, respectively, we define the initial rate as
for 60 min at 550 C
3
. HZSM-5 steamed
0.71
0.28
0.07
0.2
0
17.4
36.4
40.5
34.7
n/a
�
for 15 min at 600 C
�
!,
�
�
(
A
t)
4. HZSM-5 steamed
0.11
0.13
0.12
0.16
0.17
dY
dt
C
�
=
100 d 1 �
dt.
for 30 min at 600 C
0
A
C
t→0
5. HZSM-5 steamed
�
for 45 min at 600 C
It is observed that the consumption of thiophene from the
liquid phase matches the formation of 2-butyryl thiophene
6. HZSM-5 steamed
�
for 75 min at 600 C
7
8
. HZSM-5 steamed
0.1
(
2-BTh) very well .
�
for 120 min at 600 C
. HZSM-5 steamed
On the opposite, the consumption of butyryl chloride is
0.06
n/a
always higher than the yield in 2-BTh. Oligomerization of
butyryl chloride probably occurs and this contributes to the
poisoning of the catalyst.
�
at 700 C
9. H-mordenite
10. USY initial
0.82
1.42
0.83
0.42
0.62
0.21
1.43
1.0
0.88
0.78
n/a
4.9
6.6
12.1
14
1
1
1
1. USY1.5F
2. USY3F
3. USY5F
For practical purposes, the initial rate of acylation is ob-
tained from the linear regression of Y versus time for a short
2
time. The standard deviation (R ) is usually better than 0.9
for at least five experimental points, and the error on the
initial rate is that on the slope of the regression.
Since this contribution deals solely with the initial rates
and the nature of the acid sites catalyzing the acylation reac-
tion, we did not try to analyze the catalyst as to the nature of
the coke formed during the reaction. Also, we did not try to
measure the adsorption of reagents or reaction products by
the zeolites. Such studies have been performed by Richard
et al. (12, 13).
experimental conditions. Although butyryl chloride is an
active acylation reagent (7), the yield of the reaction never
exceeded � 30% on USY, USY1.5F, and USY5F catalysts,
which, as shown later, are the most active catalysts. In these
catalysts, the numbers of Brønsted and Lewis sites are for-
tuitously close to one another and, thus, at this preliminary
stage of the study the activity cannot be assigned to either
kind of site (Table 1). The initial USY with the larger num-
ber of Brønsted and/or Lewis sites is the most active. As
shown in Fig. 1, the reaction slows down earlier on USY
then it does for USY5F which contains the lowest number
of Brønsted and/or Lewis sites.
RESULTS
Acylation of Toluene
Preliminary experiments were performed in the acyla-
The interpretation of these observations is simple, if we
tion of toluene with butyryl chloride (BC) to define the admit that a poisoning reaction occurs more rapidly on the
�
FIG. 1. Acylation of toluene at 100 C versus reaction time, catalyzed by USY, USY1.5F, and USY5F (see Table 1).

2
60
ISAEV AND FRIPIAT
most active catalyst. As confirmed by GC/MS the prod-
It is not a simple matter to evaluate the extent of the sec-
uct of the reaction is p-butyryltoluene; no other product ondary reaction. BC can be adsorbed by the zeolite and we
is detectable in the liquid phase. However, deactivation of do not know the amount of 2-BTh remaining in the catalyst.
Y-zeolites has been reported for similar reactions (8, 11–13) A very rough estimate of the extent of the polymerization
and oligomerization of acetylchloride (25) has been sug- reaction is between 5 and 10% of the initial BC content for
gested.
the majority of the reactions listed in Table 2. The initial
Additionally, we have found that when USY is used as rates of formation of 2-BTh are given in Table 2.
�
catalyst at 110 C and for � 5 h the yields of p-butyryltoluene
The initial ZSM-5, which does not contain Lewis sites in
increase quasi-linearly with the weight of catalyst. The yield measurable amounts, has very low activity in the acylation
was 21% with 1 g of catalyst, 9% with 0.5 g of catalyst, and of thiophene. Mordenite is much more active. The highest
�
4% with 0.25 g catalyst. The reaction of thiophene being activity was observed in the presence of USY with the max-
less easily poisoned, the complete set of 12 catalysts shown imum Lewis site content. Those results indicate an increase
in Table 1 was studied under the experimental conditions in the acylation rate with the number of Lewis sites, irre-
detailed earlier and in the presence of (0.15 g) catalyst.
spective of the pore size. Differences in the structure of the
catalyst might additionally affect the catalytic activity of the
zeolite. It has been shown (20) that steaming of ZSM-5 for
Acylation of Thiophene
�
different times at increasing temperatures up to 800 C and
There are several examples of the use of zeolites as cata- fluorination of USY do not appreciably modify the pore
lysts for the acylation of aromatic heterocycles (11–13, 24). structure.
The reaction has an almost quantitative yield and high se-
As shown in Fig. 3, the initial rates of acylation increase
lectivity. As said before, the equality between the amount with increasing number of Lewis sites. Even if one con-
of thiophene consumed and the amount of 2-BTh as well siders that the 12 catalysts distribute themselves into five
as a larger than expected consumption of BC are the two groups (no Lewis site, HZSM-5 with slightly increasing con-
general characteristics of the reaction. In Fig. 2 are repre- tent in Lewis sites, H-mordenite, fluorinated USY, and ini-
sented a few examples of the evolution of the yield Y with tial USY), the trend is clear. Starting ZSM-5 and ZSM-5
time.
steamed for 15 min have the lowest rates of acylation. In
FIG. 2. Yield (% ) of the acylation of thiophene versus reaction time.

LEWIS ACID SITE-ACTIVATED REACTION IN ZEOLITES
261
TABLE 2
for the HZSM-5 samples, while it increases with that num-
ber for the USY, USY3F, and USY5F catalysts in which the
numbers of Lewis and Brønsted sites vary in the same way.
H-mordenite and the initial HZSM-5 have the same num-
ber of Brønsted sites, but the initial rate of the reaction is
Parameters of the Acylation Reaction of Thiophene with Butyryl
Chloride at 110 C Listed by Order of Increasing Content of Lewis
�
Acid Sites (see Table 1)
Lewis acid sites
(mmol/g)
dY(% )/dt, about 25 times larger on H-mordenite than in HZSM-5, in
Catalyst
t → 0
good correlation with the difference between their content
of Lewis sites. As far as the activity of HZSM-5 is con-
cerned, the explanation presented by Corma et al. (8) deals
with the hydrophobicity of the lattice. Still, the initial rate
of reaction is multiplied by a factor of 10 after calcination
. Initial HZSM-5, st.a
. HZSM-5 steamed
for 60 min at 550 C, st.
. HZSM-5 steamed
for 15 min at 600 C, st.
. HZSM-5 steamed
for 30 min at 600 C, st.
. HZSM-5 steamed
for 75 min at 600 C, st.
. HZSM-5 steamed
for 45 min at 600 C, st.
. HZSM-5 steamed
for 120 min at 600 C, st.
. HZSM-5 steamed
1
2
0
0
0.9 � 0.5
0.5 � 0.1
�
3
4
6
5
7
8
9
0
0.6 � 0.1
8.4 � 1.1
7.6 � 0.5
10.1 � 0.9
14.6 � 1.3
16.3 � 0.9
�
�
at 700 C, as compared with that of HZSM-5, calcined at
0.11
0.12
0.13
0.16
0.17
�
�
600 C for 15 min.
In conclusion, assigning the activity of acid zeolites to
Lewis activity fits the experimental observation satisfacto-
rily. Still, among the remaining questions is the role of ex-
ternal or internal surfaces: Does reaction take place in the
whole volume of the catalyst or only on the external sur-
face? Two USY catalysts with different particle sizes were
compared. One is composed of granules between 28 and
�
�
�
�
at 700 C, st.
. H-mordenite, st.
0.21
0.78
0.88
1.43
28.3 � 1.8
76.4 � 18.4
63.5 � 4.4
91.0 � 13.2
3
5 mesh and the other of granules larger than 60 mesh.
1
1
1
3. USY5F, n.st.
2. USY3F, n.st.
0. USY, n.st.
Both those catalysts exhibited the same activity, suggesting
that the process does not take place only on the external
surface. However, the conclusion would be valid only if we
had compared identical catalysts with different crystal sizes.
We did not have such materials and we doubt that they can
be prepared.
a
st., stoichiometric mixture; n.st., mixture of 2 butyryl chloride for
1
thiophene.
samples steamed for 30–75 min the rate is 3–10 times higher.
ZSM-5 steamed at 700 C has maximal activity. The change
�
DISCUSSION
in activity in the set of fluorinated zeolites shows the same
tendency. The most active catalyst in this row is the initial
USY.
Recently (25), the state of the acylating reagent acetyl
chloride was studied by 13C NMR on protonic and metal
The dependence with respect to the number of Lewis oxide-exchanged Y and ZSM-5. Besidessignalsattributable
sites is clear for the 12 catalysts that were studied, while the to the adsorbed reagent, which are close in position to those
variation with respect to the number of Brønsted sites is observed in liquid acetyl chloride, a strong signal is ob-
incoherent. The same initial experimental rates are plotted served near 183 ppm which could be assigned to an acetate-
versus the Brønsted acid site content in Fig. 4. The activity like structure resulting from the reaction of an acylium
+
decreases with an increase in the number of Brønsted sites R–C� O ion with a lattice oxygen. This signal is observed
�
FIG. 3. Initial rates of thiophene acylation at 110 C versus the content of Lewis sites. The error bars are estimated as explained in the text.
᭜
, HZSM-5; ᭿, mordenite; ᭹, USY.

2
62
ISAEV AND FRIPIAT
�
FIG. 4. Initial rates of thiophene acylation at 110 C versus the content of Brønsted sites. The lines are shown to outline the incoherent behavior.
The error bars on the rates would be the same as in Fig. 3. The error bars on the content of Brønsted sites (not shown) should be smaller by a factor of
at least 2 than those on the Lewis sites.
for alkali and alkali-earth exchanged Y-zeolite as well as for ter calcination with the extent of Na–H exchange. As early
HZSM-5. Both forms play a role in the acylation of toluene as 1982, it was shown that Na–NH4Y exchanged at 70%
�
and according to the authors of this study, Bos a´ cˇ ek et al. and calcined at 400 C underwent about 50% dealumina-
(
25), these observations are “in agreement with the role of tion (27).
donor–acceptor complexes between acylium ion precursors
and Lewis acids.”
The number of Lewis sites is an intricate function of the
NFAl content, the activation temperature, the length of
Thus, a direct correlation between the number of Lewis treatment, the dryness of the medium in which activation
acid sites and the initial rate of reaction of BC with thio- is performed, etc. It has also been shown (18) (i) that the
phene is not unexpected. Indeed, such a correlation derived degree of dispersion of the Lewis sites, defined as the ratio
from the results in Table 2 and shown in Fig. 3 strongly sug- of the number of Lewis sites to the number of NFAl, fluc-
gests that the difference in reactivity of the different zeo- tuates considerably according to the nature of the zeolites,
lites is due principally to their Lewis acid site contents, and and (ii) that the strength of the Lewis sites, estimated from
much less to differences between their lattices. In addition, the vibration frequency of the NH3, �2 symmetrical bending
there is an apparent lack of correlation with the density in mode, is larger in dealuminated HZSM-5 and H-mordenite
Brønsted sites (Fig. 4).
than in USY.
This does not mean that the Brønsted sites have no role.
Thus, it is not surprising that the linearity of the regres-
The acidic bridging OH could well act as adsorption sites as sion shown in Fig. 3 is far from perfect. On the opposite,
shown in the condensation of acetone (26). Also, the nature the spread of the strength of the Brønsted sites is smaller as
of the catalytic sites for BC oligomerization reactions is estimated from the spread of the heat of NH3 chemisorp-
open to speculation, since no information pertaining to this tion (28). Therefore, the dispersion of the results shown in
area is available so far. It cannot be excluded that Brønsted Fig. 4 is not likely to be explained by this spread. As for the
sites are involved in the oligomerization reaction.
hydrophobicity of materials, recent results by Wakabayashi
Our conclusion on the overwhelming role of Lewis acid- et al. (29) show that water adsorbs preferentially on Lewis
ity in acylation contradicts the opinion of Corma et al. (8), sites in HZSM-5. Therefore, a dealuminated HZSM-5
who by a series of experiments with Y and HZSM-5 con- would be more hydrophilic than the starting material.
cluded that the active sites are the Brønsted sites. They, in-
deed, observed an excellent correlation between the initial
ACKNOWLEDGMENTS
rate of formation of 1-(4-methoxy phenyl)-2 phenylethane
+
+
and the degree of Na , NH4 exchange on three Y-zeolites,
namely, exchanged at 100, 50, and 21% . It was assumed that
The financial support of DOE Grant DE-FG02-90 ER1430 is gratefully
acknowledged. We also want to thank Dr. A. Sklyarov, Director of the Ad-
the number of protonic sites is a linear function of this ex- vanced Analytical Facilities of the University of Wisconsin—Milwaukee,
for stimulating discussion.
change. However, this assumption would be correct only if
no NFAl and silicon were formed during the calcination at
�
5
50 C for 3 h performed after drying the exchanged zeolite.
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temperatures as low as 400 C, and it may be suggested that
the amount of NFAl and number of Lewis sites increases af-
�
1
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LEWIS ACID SITE-ACTIVATED REACTION IN ZEOLITES
263
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