Zeolite as base catalyst: Nitroaldolic condensation

Article abstract of DOI:10.1006/jcat.1999.2759

The use of heterogeneous catalysts in producing petrochemicals and fine chemicals is one of the most successful lines in catalysis research. They may replace dangerous, corrosive, or toxic catalysts in some cases. The use of zeolites as main catalysts in petrochemical cracking reactions and in oil processing is a case of specific importance. The activity and selectivity of alkaline Y and X zeolites in promoting the nitroaldolic condensation of aromatic aldehydes with nitroalkanes affording nitroalkenes were studied. The reaction was promoted by the intrinsic basicity of the zeolites that increased with increases in both the aluminum content in the framework and the radius of the counter cation. The zeolite cages were microreactors, the walls of which were the basic sites whose number and strength could be controlled by choosing the chemical composition. The activity of substituted benzaldehyde revealed the influence of electronic effects. Steric factors played a critical role regarding the nitroalkanes; the combined effect of shape selectivity and basicity had to be considered in the heterogeneous nitroaldolic condensation.

Full text of DOI:10.1006/jcat.1999.2759

Journal of Catalysis 191, 348–353 (2000)
doi:10.1006/jcat.1999.2759, available online at http://www.idealibrary.com on
Zeolite as Base Catalyst: Nitroaldolic Condensation
�
,1
Roberto Ballini, Franca Bigi,† Elena Gogni,† Raimondo Maggi,† and Giovanni Sartori†
�
Dipartimento di Scienze Chimiche dell’Universit a` , Via S. Agostino 1, 62032 Camerino (MC), Italy; and †Dipartimento di Chimica
Organica e Industriale dell’Universit a` , Parco Area delle Scienze 17A, 43100 Parma, Italy
Received July 29, 1999; revised November 20, 1999; accepted November 20, 1999
basic properties can be obtained by alkali metals impreg-
In this work we have studied the activity and selectivity of al- nation (strongbasicsites) or byalkalimetalcation exchange
kaline Y and X zeolites to promote the nitroaldolic condensation (weak basic sites). The alkali metal cations compensate the
of aromatic aldehydes with nitroalkanes affording nitroalkenes.
The reaction is promoted by the intrinsic basicity of the zeolites,
which increases with an increase of both the aluminium content
in the framework and the radius of the counter cation. Indeed,
high yields and selectivities were obtained carrying out the reac-
faujasite has basic properties (3–5). Alkaline Y and X ze-
tions over CsNaX zeolite. The nature of the reactants, in particular,
nitroalkanes, affects the yield and shape selectivity effect was ob-
negative charge of the framework due to aluminium be-
ing tetrahedrally coordinated. The appearance of the pos-
itive and negative charge generates the acidobasic charac-
ter of the zeolites. Indeed, it has been shown that alkaline
olites have been used to catalyse Knoevenagel and aldol
condensations, alkylation reactions, dehydrocyclization of
served.
�c 2000 Academic Press
n-alkanes, and hydrogenation–dehydrogenation reactions
3).
In thiswork, we have studied the activityand selectivityof
Key Words: CsNaX zeolite; base catalyst; nitroaldolic condensa-
tion; nitroalkene.
(
alkaline Y and X zeolites, to promote the nitroaldolic con-
densation of aromatic aldehydes with nitroalkanes. The na-
ture of the sites involved in the reaction and shape-selective
effects have been studied.
Unsaturated nitrocompounds are important precursors
of a large variety of target molecules and possess biolog-
ical activity representative of insecticides (6), fungicides
1
. INTRODUCTION
The use of heterogeneous catalysts in the production of
petrochemicals as well as fine chemicals represents one of
the most successful lines of research in catalysis. In addition
to the known advantages they may replace dangerous, cor-
rosive, or toxic catalysts in some cases. It is possible to say
that they are potential environmentally friendly catalysts
(
7), and pharmacologically active substances (8). They are
powerful dienophiles in the Diels–Alder reaction and read-
ily undergo addition reactions with many different nucle-
ofiles. Due to the easy conversion of the nitro group into
a variety of diverse functionalities, nitroalkenes are quite
versatile compounds in synthetic organic chemistry (9). The
most versatile synthesis of nitroolefins involves the Henry
condensation reaction of a carbonyl compound with a ni-
troalkane, followed by dehydration of the �-nitroalcohol
formed. The Henry reaction is routinely accomplished un-
der mildly basic conditions (10) and several reagents have
been used for the dehydration step (11).
In view of the increased interest in the catalysis of organic
reactions by inorganic materials, different procedures to
obtain conjugated nitroalkenes catalysed by oxide, clay, or
mesoporous hybrid material have been reported, starting
from �-nitroalcohols (12) or directly from carbonyl com-
pounds (13).
(
1).
Solid acids are widely used to obtain selective processes
that minimise the waste production. Indeed, the use of ze-
olites as main catalysts in the petrochemical cracking reac-
tions and in oil processing, some of the largest processes
among the industrial chemical ones, represents a case of
particular importance (2, 3).
The studies of solid basic catalysts are not so extensive,
probably because most of the reactions of industrial interest
are catalysed by acids and in addition the generation of ba-
sic sites often requires high-temperature pretreatment and
catalyst deactivation may easily occur (3). Anionic clays
(
hydrotalcite-like), metal oxides, and cationic zeolites are
the most used solids with basic properties. Zeolites are
crystalline microporous metal oxides that could combine
the activity due to the basic sites and the geometrical con-
straints introduced by the molecular sieve. Zeolites with
Our interest in the use of heterogeneous catalysts for fine
chemicals preparation (14) has prompted us to explore the
potential of zeolites in the nitroaldolic condensation.
1
Fax: +39 0521905472. E-mail: bigi@unipr.it.
348
0021-9517/00 $35.00
Copyright �c 2000 by Academic Press
All rights of reproduction in any form reserved.

ZEOLITE AS BASE CATALYST
349
TABLE 1
Main Structural Characteristics and Chemical Composition of the Catalysts
(mmol/100 g of Hydrated Zeolite)
a
Surface area
Na
(mmol)
Li
(mmol)
K
Cs
(mmol)
2
� 1
Si/Al ratiob
Catalyst
(m g
)
(mmol)
HY (HSZ 330)
NaY (HSZ 320)
LiNaY
KNaY
RbNaY
CsNaY
NaX (13X)
CsNaX
460 � 10
570 � 10
n.d.c
n.d.
n.d.
3.0
2.8
2.8
2.8
2.8
2.8
1.23
1.23
—
290
173
73
69
98
—
—
128
—
—
—
—
—
—
—
—
210
—
—
—
—
—
—
—
—
—
147
—
134
n.d.
350 � 10
441
253
320 � 10
a
Measured by N2 physisorption following the BET procedure.
Data obtained from the furnishing.
Not determined.
b
c
2
. EXPERIMENTAL
Deactivation of the catalyst could occur, probably due to
the neutralisation of basic sites by carboxylic acid formed by
the oxidation of aldehyde, which occurs when the reaction
is carried out in the presence of air; hence, the reaction is
better under an inert atmosphere.
2
.1. Materials
Y-HSZ 320, 330, and 13X zeolites were purchased from
Tosoh Corp. and Fluka AG, Switzerland, respectively.
NaX and NaY zeolites were exchanged by lithium, potas-
sium, and cesium in a 1 M solution of the corresponding
The model reaction of 4-chlorobenzaldehyde and ni-
troethane was monitored between 1 and 23 h. Samples were
taken periodically, the catalyst was filtered and washed
with dichloromethane, and the course of the reaction was
followed by gas–liquid chromatography using 1,3,5-tri-tert-
butylbenzene asthe internalstandard added to the reaction.
�
metal chloride at 80 C for 60 min, using a liquid-to-solid
ratio of 10 (4). The samples were filtered, washed free of
chlorides, and dried at 773 K. The chemical composition
of the resulting materials showed that a partial substitu-
tion of the sodium atoms occurred, as expected for a single
cation-exchange treatment. The main characteristics of the
catalysts used are summarised in Table 1. Surface measure-
ments were obtained on a high-speed surface area analyzer
1
The H-NMR analyses of the products were carried out
with a Bruker AC300 or AMX400 spectrometer in CDCl3
with TMS as the internal standard. Chemical shifts are re-
ported in � (ppm) and are referenced to TMS. The chemical
structures of the products obtained in the model reaction
are presented in Scheme 1.
2
200 Micrometrics apparatus following the BET procedure
(
15).
2
.3. Spectroscopic-Spectrometry Data
of the Reaction Products
2
.2. Reaction Procedure
1
The reaction was carried out in a round-bottomed flask
(E)-1-(4-Chlorophenyl)-2-nitropropene (4ay). H-NMR:
fitted with a condenser; aldehyde (10 mmol), nitroalkane 7.95 (m, 1H, Ar–CH
=
=C), 7.36 (d, 2H, J = 8.6 Hz, ArH,
1
2
1
2
(
40 mmol), and zeolite (1 g) were heated at reflux (16) in
para system), 7.29 (d, 2H, J = 8.6 Hz, ArH,
para
�
an oil bath at 140 C under stirring for 24 h unless other-
wise stated. After cooling to room temperature, the cata-
lyst was filtered and washed with dichloromethane; the
filtrate was analysed by gas chromatography-mass spec-
trometry (GC-MS) (HP5890 Series 2 coupled to HP5971A
mass selective detector at 70 eV). The products were iso-
lated by silica gel chromatography with hexane/ethyl ac-
1
etate as an eluant and were characterised by H-NMR, GC-
MS, and m.p.
The yield and selectivity were determined on the basis
of the aldehyde fed by GC analysis on a capillary column
using the internal standard method and by silica gel chro-
matography.
SCHEME 1

3
50
BALLINI ET AL.
system), 2.35 (d, 3H, J = 1.1 Hz, CH3). MS: 199 (M + 2, 3),
+
1
97 (M , 8), 116 (50), 115 (100).
1
-(4-Chlorophenyl)-2-nitropropan-1-ol (3ay). 1H-NMR
(
400 MHz): 7.35 (m, 4H, ArH), 5.38 (d, 0.4 � 1H, J =
3
4
0
1
.6 Hz, CHOH), 5.02 (d, 0.6 � 1H, J = 8.8 Hz, CHOH),
.72 (qd, 0.6 � 1H, J = 8.8 and 6.8 Hz, CHCH3), 4.66 (qd,
.4 � 1H, J = 6.8 and 3.6 Hz, CHCH3), 2.8 (br s, 1H, OH),
.49 (d, 0.4 � 3H, J = 6.8 Hz, CH3), 1.32 (d, 0.6 � 3H,
SCHEME 2
+
J = 6.8 Hz, CH3). MS: 215 (M , 2), 170 (10), 168 (30), 143
(
23), 141 (79), 111 (26), 77 (100).
3
. RESULTS AND DISCUSSION
3
,5-Dimethyl-4-(4-chlorophenyl)-isoxazole (5ay). 1H-
1
NMR: 7.41 (d, 2H, J = 8.5 Hz, ArH, para system), 7.18 (d,
2
3.1. Catalyst Effect on Nitroaldolic Condensation
1
2
(
1
H, J = 8.5 Hz, ArH, para system), 2.39 (s, 3H, CH3), 2.25
2
+
s, 3H, CH3). MS: 209 (M + 2, 15), 207 (M , 47), 140 (26),
The nitroalkenes 4 are the products obtained by the con-
densation of benzaldehyde 1 and nitroalkane 2 (Scheme 2).
It is known that under basic catalysis the reaction starts
with the formation of the anion of nitroalkane followed by
its attack on the carbonyl group of the aldehyde.
38 (80), 103 (100).
2
.4. Scanning Electron Microscopy
Particles of zeolite CsNaX were arranged in spherical
When the model reaction between 4-chlorobenzalde-
clusters with an average diameter of about 2–3 �m. SEM hyde 1a and nitroethane 2y was carried out using the acid
investigations revealed the clusters had a fine structure zeolite HSZ 330, the nitroalkene 4ay was obtained in mod-
made of platelike single crystals with an average diame- erate yield, probably via carbonyl group activation (18)
ter of about 1 �m (see micrograph in Fig. 1). As confirmed (Table 2, entry 1). On the contrary, the employment of HSZ
by SEM images of NaX and CsNaX zeolites, the morphol- 320, a cationic NaY zeolite, gave the product 4ay in good
ogy and crystal size of the catalyst remained unchanged yield (62% ).
after cation exchange. NaY (HSZ 320) zeolite SEM im-
Aiming at promoting a base-catalysed process, al-
ages showed a uniform distribution of polyhedral crystals kali cation-exchanged Y and X zeolites were examined
of small size (� 0.5 �m) (17).
(Table 2).
FIG. 1. Scanning electron micrograph (SEM) of the CsNaX catalyst.

ZEOLITE AS BASE CATALYST
351
TABLE 2
Indeed, high yield (80% ) and selectivity (91% ) were ob-
served carrying out the reaction over CsNaX zeolite as the
catalyst (Table 2, entry 8). These results are in agreement
with the observation that the intrinsic basicity of zeolites
with the same crystalline structure, in our case a faujasite-
type structure, depends on the chemical composition, i.e.,
on the Si/Al ratio and type of counter cation (5). As seen
from SEM images the NaX and CsNaX catalyst have sim-
ilar crystal dimensions. This enables us to correlate the ac-
tivity with the basicity of the zeolite catalyst, excluding a
reaction control by the internal diffusion. These data are in
accordance with the statement that in an exchanged CsNaX
catalyst most of the basic sites are able to abstract protons
with pKa around 10 (4).
Reaction of Benzaldehydes with Nitroethane in the Presence
of Different Catalysts
Entry Aldehyde
X
Catalyst Product Yield (% ) Select. (% )
1
2
3
4
5
6
7
8
9
0
1
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
HY
NaY
4ay
4ay
4ay
4ay
4ay
4ay
4ay
4ay
4by
4cy
4dy
36
62
50
57
63
65
74
80
68
65
86
63
79
67
74
78
81
92
91
90
90
93
The model reaction between 1a and 2y in the presence
of the CsNaX zeolite was monitored between 1 and 23 h
LiNaY
KNaY
RbNaY
CsNaY
NaX
(
Fig. 2). It is interesting to observe that the diattack by-
product 5ay is formed at the beginning of the reaction and
the amount did not increase in the time.
CsNaX
CsNaX
3
.2. Mechanism
1
1
OCH3 CsNaX
NO2 CsNaX
The reaction mechanisms occurring on the surfaces are
suggested to be essentially the same as that in homoge-
neous basic conditions (3). Indeed, the intermediacy of
2
-nitroalkanol, which suffers loss of water to give the corre-
In good accordance with the results reported for Kno-
sponding nitroalkene, was evidenced using half the amount
of the CsNaX catalyst. In that case 1-(4-chlorophenyl)-2-
nitropropanol (3ay, Scheme 1) could be isolated as the main
product.
evenagel condensation by Corma et al. (4), the reactivity
increases from lithium to cesium and from Y to X zeolites.
The basic strength of alkali ion-exchanged zeolites follows
the order Li < Na < K < Cs because the average oxygen
charge increases with an increasing ionic radius of the ex-
changed cation (5). Further, increasing the aluminium con-
tent of the zeolite strengthens the basic character and this
accounts for the higher activity of X than Y zeolites.
3
.3. Influence of the Nature of the Reactant
The condensation was carried out using different aro-
matic aldehydes and the results from Table 2 show that the
FIG. 2. Condensation between 1a and 2y on the CsNaX catalyst.

3
52
BALLINI ET AL.
TABLE 3
CONCLUSIONS
Effect of Nitroalkane Chain Length
Alkali metal cation-exchanged X and Y zeolites, in par-
ticular CsNaX, are active and selective catalysts for the con-
densation of benzaldehydes and nitroalkanes. The activity
of the zeolites increases with an increase of the aluminium
content in the framework and increase in the radius of the
counter cation. Indeed, the zeolite cages are microreactors,
the walls of which are the basic sites whose strength and
number can be controlled by selecting the chemical com-
position.
The activityofsubstituted benzaldehydesshowsthe influ-
ence of electronic effects. Regarding the nitroalkanes, steric
factors play an important role, showing that the combined
effect of basicity and shape selectivity have to be considered
in this heterogeneous nitroaldolic condensation.
Yield (% )
Yield (% )
By-product
Entry
Nitroalkane
n
Product
4
1
2
3
4
5
6
2x
2y
2z
2u
2v
2w
0
1
2
3
5
8
4ax
4ay
4az
4au
4av
4aw
30
56
81
92
23
15
54a
b
8
b
3
—
—
—
a
Michael addition product.
Isoxazole derivative.
b
ACKNOWLEDGMENTS
The authors are grateful to Dr. Giancarlo Salviati (Istituto MASPEC,
presence of an electron acceptor group on the aromatic CNR, Parma) who performed the SEM investigation. Thanks are due
ring of benzaldehyde increases the yield; meanwhile, the to the Ministero dell’Universit a` e della Ricerca Scientifica e Tecnologica
(
MURST), Italy, the Consiglio Nazionale delle Ricerche (CNR), Italy,
presence of an electron donor group decreases the yield
Table 2, entries 8–11). In all the cases the isomer E was
obtained and isolated (19).
and the University of Parma (National Project “Stereoselezione in Sintesi
Organica. Metodologie ed Applicazioni”) for financial support and to Mr.
Pier Antonio Bonaldi (Lillo) for technical collaboration.
(
The effect of the nitroalkane chain length was investi-
gated (Table 3). It should be noted that nitromethane has a
particular reactivity, as already observed by other authors
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