Nafion/silica composite material reveals high catalytic potential in acylation reactions

Full text of DOI:10.1006/jcat.1999.2656

Journal of Catalysis 188, 230–232 (1999)
Article ID jcat.1999.2656, available online at http://www.idealibrary.com on
RESEARCH NOTE
Nafion/Silica Composite Material Reveals High Catalytic Potential
in Acylation Reactions
�
�
,1
Alfred Heidekum, Mark A. Harmer,† and Wolfgang F. Hoelderich
�
Department of Chemical Technology and Heterogeneous Catalysis, University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen,
Germany; and †E. I. DuPont de Nemours & Company, Wilmington, Delaware
Received May 26, 1999; revised August 2, 1999; accepted August 4, 1999
Friedel–Crafts acylations are very important reactions persed nanosized Nafion particles within a silica matrix,
for the synthesis of many organic substances of great in- have been reported to have a high catalytic potential in
terest in the field of fine chemistry (1). However, the tradi- several other reactions (13–17). These catalysts contain the
tional catalysts, such as AlCl3, BF3, and HF, lack in several advantages of both components, the acidic strength of the
respects. Their use in more than stoichiometric quantities Nafion resin and the simple accessibility of the high surface
combined with their toxicity and troublesome working-up area of silica (18). The combination results in high cata-
procedures cannot be called sustainable anymore (2, 3).
lytic activity at both moderate reaction temperature and
Consequently, several attempts have been made to re- low catalyst loadings, which often guarantees high selectiv-
place the traditional materials by heterogeneous catalysts ity to the product desired.
(
4–6). Although success has been noteworthy, in quite a
In the batch experiments 2.5 g of the carboxylic acid chlo-
number of cases the lower activity of heterogeneous cata- ride were added to a mixture of 0.5 g catalyst and 10 or 20 g
lysts has restricted applications. Zeolites are quite promis- aromatic compound, which was preheated in a two-necked
ing catalysts for the acylation of activated aromatic com- glass flask equipped with a reflux condenser and a magnetic
pounds, such as anisole (7–9). However, when using less stirrer. The reactions were carried out in a dry and inert ar-
activated aromatics at high temperatures, long reaction gon atmosphere. Both the carboxylic acid chlorides and the
times and large amounts of catalysts are needed to achieve aromatic compounds were purchased from Fluka and used
reasonable acylation. These reaction conditions often lead without further treatment. The aromatics were dried and
to a loss in selectivity. In addition, the use of highly reactive stored under argon before use. The Nafion resin (Nafion NR
carboxylic acid halogenide compounds as acylating agents, 50) was kindly provided by DuPont. The composite catalyst
as well as the anhydrous HCl, which is produced during (SAC 13) contained 13 wt% nanosized Nafion particles in
the reaction, may remove the aluminum species from the a porous silica matrix and was prepared as described previ-
zeolite framework (10).
ously (18). The average pore size diameter ranged between
Alternatively, acid-treated clays may be more active than 10 and 20 nm. The Nafion resin and the composite mate-
zeolitic structured materials though the problem of stability rial were dried at 413 K for 4 h under vacuum before use.
of the catalysts during the reaction still remains (11).
Products were analyzed by GLC and identified by authentic
However, just recently it was shown that even nonacidic samples synthesized according to the literature and by GC-
materials such as graphite exhibit encouraging activity in MS.
the catalysis of acylation reaction, when carboxylic acid
halogenides are used in combination with active aromatic and phenylpropionic acid chloride 2 is catalyzed by pure
compounds (12). Nafion resin NR 50and the Nafion/silica composite material
The acylation of anisole with phenylacetic acid chloride 1
In this paper we will show that Nafion/silica composite SAC 13 (see Eq. [1] in Scheme 1). Table 1 shows the results
catalysts serve as promising catalysts for the acylation of performed at different reaction temperatures and varying
even less active aromatic compounds. These very strong ratios of anisole 3 to the acid chlorides 1, 2. According to
solid acids, which can be made by entrapping highly dis- former results, it is to be expected that the composite mate-
rial is more active than the original acidic resin (14–17). By
1
using 1 in the presence of NR 50, the conversion increases
from 46 to 77% by increasing the temperature (runs 1–3,
To whom correspondence should be addressed. E-mail: hoelderich@
rwth-aachen.de.
230
0021-9517/99 $30.00
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NAFION/SILICA COMPOSITE MATERIAL
231
SCHEME 1
Table 1), whereas the composite material achieves 71 to steadily rises with increasing temperature and conversion.
1
5
00% conversion during the same rise in temperature (runs In the case of NR 50 the selectivity rises from 77% at 333 K
–7, Table 1), although it contains only 13% of the Nafion to 89% at 373 K. This behavior will be explained by a closer
resin. Surprisingly, the selectivity to the desired product 4 view of the side reactions.
TABLE 1
Acylation of Anisole by 1 and 2 Catalyzed by Original Nafion Resin NR 50
and Nafion/Silica Composite SAC 13
Selectivity to
Run
no.
Acid
cloride
Temperature
(K)
mass ratio
3/1, (2)
Conversion
(% )
Catalyst
4
5
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
333
353
373
373
333
353
373
373
333
353
373
373
333
353
373
373
4
4
4
8
4
4
4
8
4
4
4
8
4
4
4
8
NR 50
NR 50
NR 50
NR 50
SAC 13
SAC 13
SAC 13
SAC 13
NR 50
NR 50
NR 50
NR 50
SAC 13
SAC 13
SAC 13
SAC 13
46
68
77
60
71
77
87
89
97
81
86
85
97
89
100
100
43
62
69
54
61
85
99
72
77
81
94
78
86
85
98
1
1
1
1
1
1
1
100
Note. The anisole to acid chloride mass ratio is 4 and 8; the reaction time is 2 h.

2
32
HEIDEKUM, HARMER, AND HOELDERICH
TABLE 2
time of two hours as well as the reaction temperature may
be increased to ensure a higher conversion at a selectivity
which is still on a high level. On the other hand, it is obvi-
ous that in contrast Nafion NR 50 shows a poorer catalytic
performance.
Acylation of Different Aromatic Compounds
with 1, Selectivity >97%
In conclusion, the Nafion/silica composite material rep-
resents a promising tool for acid-catalyzed reactions.
Entrapping nanosized Nafion particles in a silica-matrix ef-
fectively enhances the accessibility of the acid sites in com-
parison to the original material, Nafion NR 50. The acyla-
tion of anisole with carboxylic acid chlorides is performed
very satisfactorily with short reaction times and very high
selectivity. Even less active aromatic compounds such as m-
and o-xylene are successfully acylated. It may be expected
that because of their further optimization of the reaction
conditions, Nafion/silica composite materials constitute
competitive alternatives to traditional homogeneous cata-
lysts.
Note. Reaction conditions: 2.5 g 1, 20 g aromatic compound, 0.5 g cata-
lyst, T = 373 K; the reaction time is 2 h.
According to Eq. [2] in Scheme 1, the acylation prod-
ucts 4, 5 can be transformed to the chlorinated derivatives
and 7, respectively. This equilibrium reaction is shifted
to the right-hand side by the irreversible hydrolysis of
the chlorides by the produced water, according to Eq. [3]
6
ACKNOWLEDGMENTS
(
Scheme 1). Both the chlorinated derivatives 6, 7 and the
free carboxylic acids 8, 9 constitute the main byproducts
during the acylation reaction.
The authors are grateful to DuPont de Nemours & Company for finan-
cial support and for the permission to publish this work.
By enhancing the reaction temperature, the solubility of
the hydrogen chloride in the reaction solution is distinctly
reduced; consequently the selectivity to the desired prod-
ucts 4, 5 is enhanced. By increasing the amount of the aro-
matic component, the selectivity is raised even further, to
values of 94 to 98% , according to runs 4, 8, 12, and 16 in
Table 1. When using phenyl propionic acid chloride 5 as acy-
lating agent, a ring closure reaction to form the 4-methoxy
indanon 10 can take place according to Eq. [4] (Scheme 1).
In the stoichiometric AlCl3 catalyzed reaction this self-
acylation makes up for the main side reaction, whereas in
the heterogeneously catalyzed reaction it is detected only
in traces, less than 1% selectivity.
The acylation of anisole over heterogeneous catalysts is
often investigated due to the activated character of the aro-
matic compound refering to electrophilic substitution reac-
tions. However, as a competitive alternative to highly ac-
tive homogeneouscatalysts, heterogeneouscatalystsshould
have the potential to catalyze acylation reactions with lesser
activated aromatic compounds. Therefore, we studied alter-
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