Basicity of Faujasites containing Alkyl-ammonium Linear Cations
Karina Arruda Almeida,a,b,* Dilson Cardosob
a
Federal University of Itajubá, Itajubá, 37500-00, Brazil
University of São Carlos, São Carlos, Zip code, Brazil
*Corresponding author: E-mail [email protected]
b Federal
Keywords: Faujasite, Knoevenagel, Alkyl-ammonium Cations, Basicity
1. Introduction
The basicity of zeolites is due to the presence of the
aluminate anion [AlO4]- in their frameworks. The
basic sites are associated with the oxygen atoms close
to the cations that compensate the negative charge [1].
The acid–base properties of zeolites can be classified
using the Lewis model, according to which the lower
the cation acidity, the higher is the anion basicity.
When the compensation cation is slightly
electronegative, the charge density of oxygen
increases enough to produce the basic properties. In
the case of the aluminosilicates, the higher the
number and concentration of Al atoms in the
structure, the greater is the number and the basic
strength of the oxygen atoms. Being no less
electronegative than the inorganic cation cesium, the
alternative found by Cardoso et al. [2] to enhance the
basicity of zeolite was ion exchange with organic
cations. Zeolites containing organic cations showed
basicity higher than that of zeolites containing cesium
cation.The objective of this paper was to evaluate the
basicity of zeolites X and Y exchanged with linear
alkyl-ammonium cations with different carbonic
chain lengths.
The sodium and aluminum concentrations in the
zeolites have been determined by inductively coupled
plasma atomic emission spectrometry (ICP-AES).
X-ray photoelectron spectroscopy (XPS) analyses
were performed using a VSW HA-100 spherical
analyzer, equipped with an AlK source (h=1486.6
eV), at a pressure less than 2.010-8mbar. The loading
effects have been adjusted by linear displacement of
the spectrum, so that the C1s signal had a binding
energy equal to 284.6 eV.
The Knoevenagel condensation (Scheme 1) has been
performed in 2-mL batch reactors. For these yield
tests, a reaction mixture containing equimolar
amounts (4.8 mmol) of butyraldehyde (1) and ethyl
cyanoacetate (2) have been prepared and diluted in
the same total volume of toluene as solvent. About
1mL of this mixture and 3 wt.% of catalyst was placed
in the vial and maintained under stirring at 50C for
1h. The reaction product has been analyzed using a
Varian gas chromatograph. For quantitative analysis
of the reaction components, linear fittings have been
built from the experimental data using the internal
standards method, with a confidence interval of 7%.
CH3CH2CH2
CN
C O +
H
2. Experimental Part
(1)
CN
CH3CH2CH2
+
C C
H2 C
COOEt
(2)
H
H2O
COOEt
(3)
Scheme 1: Knoevenagel condensation.
The catalysts were prepared using commercial
zeolites NaX (Merck) with Si/Al molar ratios equal to
1.4. The maximum degree of exchange was obtained
at 40ºC, using the procedure described previously [2].
Sodium ions present in zeolite were ionically
exchanged by methyl-, propyl-, butyl-, pentyl- and
hexylammonium, Table 1 shows the nomenclature
employed for the catalysts, according to the organic
cation and the zeolite used.
Table 1 – Nomenclature used for samples.
Cation
CH3NH3+
CH3-CH2NH3+
CH3-(CH2)2NH3+
CH3-(CH2)3NH3+
CH3-(CH2)4NH3+
CH3-(CH2)5NH3+
X zeolite
Me1X
Et1X
Pr1X
Bu1X
Pe1X
He1X
Y zeolite
Me1Y
Et1Y
Pr1Y
Bu1Y
Pe1Y
He1Y
Under the reaction conditions used, the Knoevenagel
condensation was always 100% for compound 3
(Scheme 1).
3. Results and discussion
Figure 1 shows the compound 3 (Scheme 1) yield in
the Knoevenagel condensation of the nbutyraldehyde with ethyl cyanoacetate, as a function
of the number of sites exchanged in the X and Y
zeolites. The comparison of figures reveals that, until
2.01019 sites exchanged, the zeolites X and Y
present the same yield for compound 3 (scheme 1),
about to 10%. From this value of exchanged sites, the
zeolite X is more active for cations 𝑀𝑒1+ , 𝐸𝑡1+ e 𝑃𝑟1+ .
From cation 𝐵𝑢1+, zeolite Y becomes more active up
to the exchanged sites equal to 4.01019. For
exchange values higher than 4.01019, zeolite X
becomes more active once again, due to a higher
number of exchanged sites, that is, a greater number
of active sites.
The maximum reached yield was 98% for catalyst
Et1X, Pr1X, Bu1X e Pe1X with exchanged number
sites of maximum values.
80
80
60
Me1X
Me1Y
40
Yield (%)
100
Yield (%)
100
60
20
20
0
0
0
1
2
3
4
5
6
7
8
Bu1X
Bu1Y
40
9
0
19
Number of sites exchanged x 10
(a)
100
3
4
5
6
7
8
9
19
(d)
80
60
Et1X
Et1Y
40
Yield (%)
Yield (%)
2
100
80
20
60
Pe1X
Pe1Y
40
20
0
0
0
1
2
3
4
5
6
7
8
9
0
19
Number of sites exchanged x 10
1
2
3
4
5
6
7
8
9
19
Number of sites exchanged x 10
(b)
(e)
100
80
80
60
Pr1X
Pr1Y
40
20
Yield (%)
100
Yield (%)
1
Number of sites exchanged x 10
60
He1X
He1Y
40
20
0
shaped curves. Initially, zeolite X is more active
because it has more sites. However, from the cation
buthyl, steric effects become more intense on zeolite
X, so the zeolite Y becomes more active.
Table 2 – Crystallographic sites distribution by unit cell
on zeolites NaX and NaY [3].
Sites
I
II
III
Total
NaX (Si/Al = 1,3)
16
32
38
86
NaY (Si/Al = 2,5)
16
32
8
56
The binding energy of the electrons with oxygen
nucleus, Eb[O1s], is an indicative of its basic strength.
The lower Eb[O1s] value, the greater is the
availability of electrons, and hence the greater the
Lewis basicity of oxygen [2]. Table 2 shows the
Eb[O1s] energy values for attributed to oxygen of the
aluminate anions compensated by alkyl-ammonium
cations (MAlO). These values were determined from
the deconvolutions of XPS spectra for zeolites
containing different cations. It is noted that the values
of Eb[O1s] decrease for methyl- for ethyl- cation on the
two zeolites, however, they keep constant, showing that
the basicity does not increase significantly with the
carbonic chain increase. The observed conversions
variation is due to the exchanged sites numbers (Table 3).
0
0
1
2
3
4
5
6
7
8
9
19
0
1
2
3
4
5
6
7
8
9
19
Number of sites exchanged x 10
Number of sites exchanged x 10
(c)
(f)
Figure 1. Yield of compound 3 (Scheme 1) as a function
of the exchanged sites numbers.
It has been noted that for zeolite X the yield increases
suddenly at level from 3.0 up to 4.01019 exchanged
sites. These results may be explained by the
distribution of crystallographic sites of these zeolites
(Table 2). Sites type I are located at the hexagonal
prisms and are unreachable to the alkyl-ammonium
cations, thus they are not basic sites. Sites type II are
close to the ring plans of six members that form the
sodalite cavity. Sites type III are located in the great
cavity near the four member rings that form the
supercavity. Sodium cations belonging to sites type II
and III may be exchanged by alkyl-ammonium
cations. The zeolite X unit cell has 30% more sites of
type III than zeolite Y (Table 2).
Due to the hydrophilicity of the −𝑁𝐻3+ group, the
ionic exchange must occur firstly on the internal
sites II and III of the supercavities. Due to the alkylammonium cations are voluminous they difficult the
reagents access to catalytic sites. After changing all
cations belonging to the supercavities, the ionic
exchange starts to occur at the zeolites external
surfaces. As the external sites are more accessible to
the reagents, the activity increases significantly at
values of number of sites exchanged near to 3  1019.
Therefore, high slope regions are observed in the S-
Table 3 – Eb[O1s] zeolites values from XPS spectra.
Sample
Number of sites/g
(10+19)
Eb[O1s] (eV)b
MAlOa
Conversion
(%)
4.73
530.20
Me1X
4.80
530.13
Et1X
4.58
530.00
Pr1X
4.35
530.03
Bu1X
4.47
529.99
Pe1X
3.97
530.07
He1X
4.77
530.60
Me1Y
5.01
530.60
Et1Y
4.61
530.52
Pr1Y
3.42
530.52
Bu1Y
4.37
530.52
Pe1Y
3.59
530.48
He1Y
aM = Me +, Et +, Pr +, Bu +, Pe + e He +. bError = ± 0,1 eV.
1
1
1
1
1
1
80
86
91
88
80
31
12
16
15
51
80
79
4. Conclusions
The results indicate that from methyl- cation the carbonic
chain increase does not enhance the sites basic strength.
The catalytic site accessibility determines the zeolite
catalytic activity containing the linear alkyl-ammonium
cations.
Acknowledgments
The authors acknowledge the financial support from Capes.
References
[1] D. Barthomeuf, Micropor. Mesopor. Mater. 66 (2003) 1.
[2] K. A. Almeida, R. Landers, D. Cardoso. J. Catal., v. 294, p.
151-160, 2012.
[3] D.W. Breck, J. Chem. Education, v.41, p. 678-689, 1964.