www.elsevier.com/locate/cattod

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(2008) 706–710

Catalysis Today 133–135

Basic catalyzed Knoevenagel condensation by FAU zeolites

exchanged with alkylammonium cations

Leandro Martins, Karla Moreira Vieira, Luiz Marcelo Rios, Dilson Cardoso *

Catalysis Laboratory, Chemical Engineering Department, Federal University of Sao Carlos, Rod. Washington Luıs, Km 235,

13565-905 Sao Carlos, SP, Brazil

Available online 4 March 2008

Abstract

This work concerns about ion exchange and basic properties evaluation of faujasite zeolites containing alkylammonium cations. The starting

materials were sodium FAU zeolites with two Si/Al ratios (1.4 and 2.5) and alkylammonium cations with different degrees of substitution of methyl

radical or alkyl chain length. Results showed that aqueous ion exchange equilibrium is readily achieved, even using (CH3)4N+ voluminous cation.

Due to the lack of space to diffuse inside of some cavities, none of the organic cations completely exchanged sodium cations in the zeolite, being the

exchange restricted to the large cavity. The maximum exchange degree diminishes with increase of the cation volume and aluminum content.

Micropores volume decrease linearly with increasing exchange degree and with size of the compensation cation. These organic cations containing

faujasites were evaluated as catalyst in the Knoevenagel condensation, leading to higher yields when Cs-FAU is used. This unique result opens

news perspectives for application of these highly basic and low-cost molecular sieves.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Ion exchange; Alkylammonium cations; Basic sites; Knoevenagel condensation

1. Introduction

The use of zeolites with basic properties in catalysis was

reported a long time ago [1,2]. More recently the interest in

these materials as catalysts is increasing considerably, due to

higher selectivity of base-catalyzed reactions [3,4], and in the

adsorption of weakly acid compounds [3]. Up to nowadays,

basic zeolites are obtained by substitution of proton, located in

ion exchange sites, by alkaline metals. Barthomeuf showed

that this ion exchange leads to acid–basic pairs, whose

character depends on the cation radius [5]. By comparing

measurements of acidity and basicity, Barthomeuf verified that

zeolites, e.g. faujasite (FAU), posses conjugated acid–basic

pairs. This means that as lower is compensating cation M+

acidity the higher is oxygen basicity. On its turn, cation M+

acidity diminishes when its volume is increased, being the

positive charge diluted in a higher volume (less electro-

negative). For example, for alkaline metals ion exchanged in

zeolites oxygen basicity increases in the following order

* Corresponding author. Fax: +55 16 3351 8266.

E-mail address: dilson@power.ufscar.br (D. Cardoso).

0920-5861/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.cattod.2007.12.043

Li < Na < K < Rb < Cs [6,7]. On the other hand, when

density of tetrahedral aluminum AlO4� is increased in the

zeolite framework (lower Si/Al ratio), besides formation of

more basic sites its strength is also increased.

Basic property of zeolites, as a consequence of ion exchange

with inorganic cations, has been studied in a number of papers

[1–7], but almost no attention was given to zeolites ion

exchanged with organic cations. With exception of two works

from the 1960’s [8,9], which studied uniquely ion exchange

isotherms of alkylammonium cations, no other paper described

the effect of organic cation in zeolite properties. Recently,

Kubota et al. [10] prepared Beta zeolite, which was used as

catalyst without going through calcination, i.e. with pores still

occluded with the structure directing template, the tetraethy-

lammonium cation. This as-synthesized zeolite showed to be an

excellent catalyst in the Knoevenagel condensation, which is

base-catalyzed [10]. This novel result indicates that zeolites

containing organic cations might have huge potential of

application, which has been reported only in a few papers [8,9]

and is not properly known yet.

Condensation reactions, which are catalyzed by various

basic solids, represent a class of reactions that are very

significant for the synthesis of organic compounds. Particularly,

L. Martins et al. / Catalysis Today 133–135 (2008) 706–710 707

Knoevenagel condensation is used in organic synthesis to

produce unsaturated compounds from molecules containing

carbonyl groups [11]. Since the reaction can be catalyzed by

solid bases possessing very different basic strength, it is used as

a probe reaction to compare the basic character of various

catalysts. Commercially, this reaction finds wide application in

the pharmaceutical industry, e.g. synthesis of antihypertensive

[12]. A long time ago, Corma et al. [13] performed

Knoevenagel condensation between benzaldehyde and ethyl

cyanoacetate and found that cesium exchanged faujasite

zeolites were more active than those containing only sodium.

Therefore, the aim of this study is to explore ion exchange of

Na-faujasite zeolites with alkylammonium cations and evaluate

catalytic activity of these materials in Knoevenagel condensa-

tion. We report significant improvement of the basic strength of

faujasite zeolites when it contains methylammonium cations,

compared with cesium cations, which is widely used to

generate zeolite basicity. This novel result opens new

perspectives for zeolite application in several fine chemistry

reactions and in separation processes in which the presence of

strong basic sites are desirable.

2. Experimental

2.1. Obtaining faujasites containing alkylammonium

cations

The starting materials were sodium faujasite zeolites (Na/Al

molar ratio 1) with two Si/Al molar ratio equal to 1.4 (Zeolite

X, manufactured by Merck) and 2.5 (Zeolite Y, manufactured

by Zeolyst). These sodium zeolites showed a micropore volume

of 0.25 mL/g.

To obtain the catalysts three consecutive ion exchanges were

done at 40 8C with 0.5 mol/L solution of the corresponding

organic cation. Then the sample was copiously washed with

deionized water and dried at 110 8C for 12 h. The chlorides of

cations used were (1) with different degrees of substitution,

(CH3)iNH4�i+ (0 � i � 4), and (2) linear monoalkyl-substi-

tuted, with different chain length, CyH2y+1NH3+ (1 < y � 4).

Ion exchange essays, to determine when ion exchange

equilibrium was achieved, were done in a 6 h period. Ion

exchange curve as a function of the time was determined by

withdrawing 10 mL samples from reaction medium under

vigorous stirring. After filtrating, the resulting sample was

washed successively times with deionized water.

The nomenclature used to identify the methylammonium

samples is Mei, where ‘‘i’’ index stands for the number of methyl

groups present in the methylammonium compound

(CH3)iNH4�i+. For example, Me0 stands for NH4

+ and Me3

stands for (CH3)3NH+. For the linear monoalkyl-substituted

cations, with different chain lengths, it was adopted the code Et1

and Pr1, which stand for ethyl and propyl groups, respectively.

2.2. Characterization

Ion exchange degree was calculated considering the Na/Al

molar ratio present in the zeolites. Na and Al present in the solid

were determined by chemical analysis in duplicate, by using

optical emission spectrometry through inductively coupled

plasma (ICP-OES).

N2 physisorption analysis was performed at N2 ebullition

temperature using a Quantachrome Co. (Nova-1200) equipment.

Before analysis, 50 mg of the sample were vacuum treated at

110 8C temperature for 2 h under nitrogen. This temperature was

chosen to prevent organic cations, present in the zeolite, to

decompose. Specific micropore volume was determined with t-

plot method [14]. In the case of Mei X zeolites, it was not possible

to determinate the micropore volume because these materials

were unstable during the drying step.

2.3. Knoevenagel condensation

Knoevenagel condensation is effected by treating a carbonyl

compound with an active methylene compound (Eq. 1) forming

unsaturated compounds of high molecular weight. Condensa-

tion reaction was accomplished in the liquid phase, without the

use of solvent, using benzaldehyde (compound (1), R C6H5) or

butyraldehyde (R C3H7) as carbonyl and ethyl cyanoacetate

(compound (2)) as methylene compound.

A typical reaction procedure is described as follows: to an

equimolar mixture (4.8 mmol) of benzaldehyde and ethyl

cyanoacetate was added 20 mg of the catalyst (2% in mass).

The reaction mixture was stirred for 3 h at 100 8C. Reagent and

product analysis, in the liquid samples, was performed with a

Varian chromatographic equipped with DB-1 capillary column

and flame ionization detector (FID). Other analysis conditions

were: injector temperature, 210 8C; column temperature,

140 8C for 12 min and 230 8C for 15 min; and He as carrier gas.

ð1Þ

R¼C3H7=DG298K¼ �44:6 KJ=mol

R¼C6H5=DG298K¼ �98:7 KJ=mol

3. Results and discussion

3.1. Ion exchange

Fig. 1 shows the degree of sodium cation ion exchange by

tetramethylammonium, Me4, as a function of time. From this

figure, it can be noticed that despite Me4 cation (Table 1) being

quite voluminous, the ion exchange kinetics reaches its

equilibrium quickly, i.e. in 50 min. Fig. 1 reports that, at

equilibrium, sodium ion exchange by Me4 cation reaches 22%.

This value is different from that depicted in Table 1 (i.e. 35%),

thus Fig. 1 shows only the first ion exchange procedure and

Fig. 1. Sodium ion exchange kinetics by tetramethylammonium in zeolite Yat a

concentration of 0.5 mol/L.

L. Martins et al. / Catalysis Today 133–135 (2008) 706–710708

sample Me4-Y was prepared in three consecutive ion

exchanges.

Tables 1 and 2 show that some cations should allow 100%

ion exchange, but none of the alkylammonium cations used was

able to exchange all sodium present in the zeolite. Similar

behavior, using ionic pairs Na–Rb and Na–Cs, was reported by

other researchers [15]. This limitation is attributed to two

stereo-spatial barriers that may take place simultaneously,

depending on cation volume. The first is due to the

inaccessibility of internal region of hexagonal prisms and

sodalite cavities to voluminous cations, thus restricting ion

exchange to the supercavity. The second barrier is related to the

lack of space between neighbor anions to voluminous cations

(i.e. Me3 and Me4) because they have kinetic diameter larger

than the distance between two negative neighboring charges,

induced by the incorporation of aluminum atoms in the zeolite

framework. Fig. 2 illustrates better this second barrier, then a

higher amount of framework aluminum, present in zeolite X,

makes difficult the ion exchange at neighboring aluminum

atoms.

From the data (1) volume occupied by the cation (VOC) and

(2) % of experimental exchange, available in Tables 1 and 2, the

theoretical micropore volume could be estimated. For example,

for the Me4-Y sample the calculated micropore volume was

Table 1

Estimated cation volume (VC), maximum ion exchange percent and micropore vol

Cation VC (A3)a VOCb (mL/g) %Expected exchangec %Experime

Na 3.6 . . . . . . 100

K 10.0 0.028 100 100

Cs 20.2 0.053 100 70

Me0 13.6 0.036 100 74

Me1 49 0.128 100 71

Me3 129 0.337 73.9 52

Me4 175 0.457 54.5 35

Et1 94 0.244 81.0 71

Pr1 115 0.299 76.7 65

a These data were obtained from reference [8].b Volume occupied by the cation considering 100% exchange.c Considering micropore volume of 0.250 mL/g of the zeolite.d % of expected exchange = (0.250/Voc) � 100.e Y = (AlO2)54.8(SiO2)137.2.

0.09 mL/g (0.25–0.457 � 0.35), however the measured value

was 0.14 mL/g (Table 1). This estimation was extended to the

others samples and irrespective to the cation exchanged,

calculated microporous volume is generally smaller than that

measured by N2 physisorption. This discrepancy could be

explained by an overestimation of the theoretical cation volume

(VC in Tables 1 and 2) exchanged in the zeolite porous.

Probably, the cations volume is reduced when they are located

in a constrained space.

Fig. 3 shows zeolite Mei-Y micropore volume measured by

N2 physisorption as a function of sodium exchange degree for

organic cations. As a result from voluminous organic cation

presence, for all Mei+ cations, i � 1, the micropores volume

decreases linearly and sharply with the exchange degree.

Similar behavior was also found by Romero et al. [16] in FAU

zeolites containing cesium. In Fig. 3 it can also be noticed that

for the same exchange degree, as expected, the sample

containing the more voluminous cation Me4 has the smallest

micropore volume. It can be further observed that the sample

containing Me3 cation (52% of ion exchange) was the one with

the smallest micropore volume. This low micropore volume

may be explained based on an already made observation:

considering that Me3 cation is less voluminous than Me4, for

rich-Al zeolites there is a possibility that two cations take close

aluminum sites, which occurs less frequently with Me4 cation.

As a result, the maximum exchange degree of Me3 cations is

higher (Table 1) and the available micropore volume is lower.

3.2. Knoevenagel condensation

Fig. 4 illustrates catalyst activity in Knoevenagel condensa-

tion, between benzaldehyde and ethyl cyanoacetate at 100 8C,

for faujasite zeolites containing maximum exchange degree of

methylammonium cations. During the reaction the presence of

a single product was always evidenced (compound (3), Eq. (1))

and the ratio between the consumed reactants was around unity

(stoichiometry 1:1 is maintained), indicating that subsequent

reactions which might occur did not take place at the reaction

conditions here used. This signifies that the selectivity is 100%

towards the product of the Knoevenagel reaction.

ume of Y zeolites

ntal exchanged Anhydrous unit cell formulae Micropore vol. (mL/g)

Na54.8(AlO2)54.8(SiO2)137.2 0.25

K54.8Y 0.24

Cs38.4Na16.4Y 0.20

(Me0)40.6Na14.2Y 0.21

(Me1)38.9Na15.9Y 0.19

(Me3)28.5Na26.3Y 0.11

(Me4)19.2Na35.6Y 0.14

(Et1)38.9Na15.9Y 0.10

(Pr1)35.6Na19.2Y 0.12

Table 2

Estimated cation volume (VC), maximum ion exchange percent and micropore volume of X zeolites

Cation VC (A3) VOC (mL/g) %Expected exchange %Experimental exchange Anhydrous unit cell formulaa Micropore vol. (mL/g)

Na 3.6 . . . . . . 100 Na80(AlO2)80(SiO2)112 0.25

Me0 13.6 0.050 100 71 (Me0)56.8Na23.2X nd

Me1 49 0.179 100 71 (Me1)56.8Na23.2X nd

Me3 129 0.471 53.1 31 (Me3)24.8Na55.2X nd

Me4 175 0.639 39.1 15 (Me4)12Na68X nd

a X = (AlO2)80(SiO2)112.

Fig. 2. Maximum exchange degree for different cation volume. (&) Zeolite Y

and (*) Zeolite X.

Fig. 3. Micropore volume for the zeolite Y containing different exchange

degree of alkylammonium cations.

L. Martins et al. / Catalysis Today 133–135 (2008) 706–710 709

Fig. 4a shows that yield to the product of the condensation

reaction (ethyl 2-cyano-3-phenylacrylate, compound (3),

R C6H5 in Equation (1)) is higher when Cs–Y zeolite is

used, instead of Na–Y or K–Y, as a consequence of its higher

Fig. 4. Product yield from Knoevenagel condensation using benzaldehyde (100 8C, 0

cations and (b) mono-substituted alkylammonium cations. The percentage value in

basicity [7]. When zeolites containing ammonium or methy-

lammonium cations are used, a further increase is seen in the

catalytic activity, despite the organic cations have a lower

micropore volume (Fig. 3). Among methylammonium cations,

Mel was the one that ensured highest condensation yield (85%),

possibly due to high exchange level [17,18].

An unexpected result was the higher activity for the sample

containing ammonium cation which gave a condensation yield

higher than that obtained for the cesium containing sample

(Fig. 4a). It was not found any report in the literature about the

use of zeolites containing ammonium cation as basic catalysis,

but as the volume of this cation is smaller than cesium, it was

expected a yield lower than obtained with cesium-FAU [13]. A

possible explanation could be the presence of covalent N–H

bonds in ammonium cation, which would guarantee lower

electronegativity to this cation and, consequently, higher

oxygen basicity. For alkylammonium cations, besides the

volume of the cation, the presence of electron donor methyl

groups [19] might contribute to the observed significant lower

electronegativity (Fig. 4).

Fig. 4b illustrates Knoevenagel yield by using mono-

substituted alkylammonium cations containing Zeolite Y as

catalyst. It can be noticed that for higher chain length lower is

catalyst activity. For the catalyst Et1(71%)-Y the lower yield, in

comparison to the catalyst Me1(71%) Y, is probably related to

the lower micropore volume available (Table 1) because both

samples have the same exchange degree. But for sample

Pr1(65%)-Y, besides lower micropore volume, higher sodium

content might have contributed to the lower activity.

Fig. 5 shows catalyst activity in Knoevenagel condensation,

between benzaldehyde and ethyl cyanoacetate at 100 8C, for

faujasite zeolites containing different exchange degree of

methylammonium cations. For the same degree of sodium

.02 g of catalyst and 3 h of reaction). Zeolite Y containing (a) methylammonium

brackets indicates the ion exchange degree.

Fig. 5. Product yield from Knoevenagel condensation using benzaldehyde as

carbonyl compound (100 8C, 0.02 g of catalyst and 3 h of reaction). Zeolite Y

with different exchange degree of methylammonium cations.

L. Martins et al. / Catalysis Today 133–135 (2008) 706–710710

exchange, for instance 35%, yield increases with cation volume

(Table 1), i.e. Me0 < Me1 < Me3 < Me4 [6]. It is noticed that,

differently of the micropore volume, which shows a linear

relationship with the exchange degree (Fig. 3), condensation

yield grows exponentially. Similar results were obtained by

Borgna et al. [20], that studying side chain alkylation of toluene

with methanol on Cs–Y zeolite did not observe linear

correlation between cesium exchange and catalytic activity.

For methylammonium cations it was expected that yield against

exchange degree curve presented a maximum, because despite

increase in the basic strength, micropore volume decreases with

exchange degree.

As illustrated in Fig. 4, ion exchange by cations of lower

electronegativity than sodium, i.e. bulky cations, leads to higher

yield in Knoevenagel reaction. This is probably related to a

higher framework oxygen basicity as stated by others authors

[3]. Another method for increasing zeolite basicity is by adding

framework aluminum, because this way negative framework

charge density is increased. Fig. 6 shows how varies yield in the

condensation between ethyl cyanoacetate and butyraldehyde

over zeolites Y and X, with Si/Al ratio of 1.4 and 2.5,

respectively. The use of butyraldehyde as carbonyl compound

is interesting because it allows the reaction to be processed at

lower temperature (60 8C), due to its higher reactivity. As can

be seen in Fig. 6, for all cations, zeolite X is more active than

Fig. 6. Product yield from Knoevenagel condensation using butyraldehyde as

carbonyl compound (60 8C, 0.03 g of catalyst; Zeolite X: 1 h of reaction and

Zeolite Y: 3 h of reaction). The percentage value in brackets indicates the ion

exchange degree.

zeolite Y, as a consequence of its higher basic strength. Samples

containing cations Cs and Me1 had the same ion exchange

degree, differently from samples exchanged with Me3 and

Me4. Despite the lower exchange degree of these cations in

zeolite X (i.e. 31 and 15%, for cations Me3 and Me4,

respectively) yield in the condensation reaction was approxi-

mately three times superior when zeolite X is used.

4. Conclusions

During sodium ion exchange for alkylammonium cations,

which have larger volume, chemical equilibrium is quickly

reached. For steric reasons, none of the alkylammonium cations

replaced all Na cations present in the faujasite zeolite, and the

ion exchange being restricted to the large cavity.

Using faujasite containing ammonium cations, with lower

acidity than cesium, the catalyst activity increased in

Knoevenagel condensation as a result of its higher basicity.

When Y and X faujasite zeolites, ion exchanged with

methylammonium cation Me1, it was attained the highest

activity indicating that might exist an optimal relationship

between cation acidity and the volume it occupies.

Acknowledgements

Authors wish to thank Fapesp and CNPq for awarding

doctorate and master scholarship to L. Martins and K.M. Vieira,

respectively.

References

[1] P.B. Venuto, P.S. Landis, Adv. Catal. 18 (1968) 331.

[2] Y. Ono, Stud. Surf. Sci. Catal. 5 (1980) 19.

[3] D. Barthomeuf, Catal. Rev. 38 (1996) 521.

[4] H. Hattori, Chem. Rev. 95 (1995) 537.

[5] D. Barthomeuf, J. Phys. Chem. 88 (1984) 42.

[6] U.D. Joshi, P.N. Joshi, S.S. Tamhankar, V.V. Joshi, C.V. Rode, V.P.

Shiralkar, Appl. Catal. A: Gen. 239 (2003) 209.

[7] L. Martins, D. Cardoso, Quim. Nova 29 (2006) 358.

[8] B.K.G. Theng, E. Vansant, J.B. Uytterhoeven, Trans. Faraday Soc. 64

(1968) 3370.

[9] R.M. Barrer, R. Papadopoulos, L.V.C. Rees, Inorg. Nucl. Chem. 29 (1967)

2047.

[10] Y. Kubota, Y. Nishizaki, H. Ikeya, M. Saeki, T. Hida, S. Kawazu, M.

Yoshida, H. Fujii, Y. Sugi, Micropor. Mesopor. Mater. 70 (2004) 135.

[11] J. Weitkamp, M. Hunger, U. Rymsa, Micropor. Mesopor. Mater. 48 (2001)

255.

[12] G. Marciniak, A. Delgado, G. Leclerc, J. Velly, N. Decker, J. Schwartz, J.

Med. Chem. 32 (1989) 1402.

[13] A. Corma, V. Fornes, R.M. Martın-Aranda, H. Garcıa, J. Primo, Appl.

Catal. 59 (1990) 237.

[14] B.C. Lippens, J.H. de Boer, J. Catal. 4 (1965) 319.

[15] D.H. Olson, Zeolites 15 (1995) 439.

[16] M.D. Romero, G. Ovejero, A. Rodrıguez, J.M. Gomez, Micropor. Meso-

por. Mater. 81 (2005) 313.

[17] L. Martins, D. Cardoso, BR Pat. PI 0505706, 2005.

[18] L. Martins, R.T. Boldo, D. Cardoso, Micropor. Mesopor. Mater. 98 (2007)

166.

[19] F.A. Carey, R.J. Sundberg, Advanced Organic Chemistry, Plenum, New

York and London, 1990.

[20] A. Borgna, S. Magni, J. Sepulveda, C.L. Padro, C.R. Apesteguia, Catal.

Lett. 102 (2005) 15.