Modified HZSM-5 zeolites for the conversion of ethylene into
propylene and aromatics
Débora S. Fernandes*, Cláudia O. Veloso, Cristiane A. Henriques
Universidade do Estado do Rio de Janeiro, Instituto de Química, Programa de Pós-graduação em Engenharia Química, Rio de
Janeiro, 20550-900, Brazil.
*Corresponding author: [email protected]
Keywords: HZSM-5, ethylene conversion, acidity, metal inpregnation
1. Introduction
In order to reduce the amount of pollutants
associated with the production of petrochemicals,
ethanol has been extensively investigated as a raw
material for such products. Ethylene is the main
product of the conversion of ethanol in hydrocarbons
and can be considered a renewable product when
produced from ethanol dehydration. HZSM-5 zeolite
has shown a great activity to produce hydrocarbons
from ethanol and ethylene because of its properties
such as specific area, mesoporous volume and
acidity [1].
The reactions involved in ethylene transformation
over HZSM-5 catalyst are oligomerization,
cyclization, dehydrocyclization, cracking, and
hydrogen transfer. Variations in operating conditions
such as reactant partial pressure, reaction
temperature and contact time directly affect the
catalytic performance and consequently the obtained
products, especially propylene and aromatic [2].
To improve the catalytic performance some after
synthesis modifications can be made in zeolites such
as ion exchange and impregnation of metals. In this
work, a HZSM-5 zeolite with SiO2/Al2O3 molar ratio
equal to 30 was impregnated with Fe, La and W and
the influence of the addition of these metals in
ethylene conversion to propylene and aromatics in
predetermined conditions were verified.
2. Experimental Part
La-HZSM-5, Fe-HZSM-5 and W-HZSM-5 were
prepared by wet impregnation, using La(NO3)3.6H2O,
Fe(NO3)3·9H2O
and
(NH4)6H2W12O40.XH2O,
respectively. After impregnation, catalysts were
dried, and then calcined in an air flow at 500 C.
The effect of metal incorporation on zeolite acidity
was evaluated by temperature programmed
desorption of NH3. The samples were previously
submitted to an in situ treatment for the removal of
adsorbed impurities. Surface area and pore volume
of the samples were measured by N2 adsorption-
desorption (Micromeritics ASAP 2020) and the
crystallinity was determinate by X-ray diffractionXRD (Rigaku, Miniflex II, CuKα – 15 kV, 15 mA).
The catalytic tests were carried out in a fixed-bed
micro reactor at atmospheric pressure for 4.5 h using
HZSM-5 and impregnated HZSM-5. Ethylene was
fed with nitrogen into the reactor via a mass flow
meter (MKS-model 247 D). The products were
analyzed online by gas chromatography using a
Poraplot Q-HT column. The reaction conditions
were previously defined to promote high propylene
(pethylene = 0.12 atm; T = 500 °C; 0.15 h) and
aromatics (pethylene = 0.35 atm; T = 400 °C; 0.20
h) yields.
3. Results and discussion
The acid site density of the catalysts is shown in
Table 1. An increase of acid sits density can be
observed when W was added, while in presence of
Fe or La a reduction of these sites was noted. The
presence of La, W and Fe clearly increases the
amount of weak acid sites mainly due to the
decrease of intermediate acid sites, but for
FeHZSM-5 and LaHZSM-5 the amount of strong
acid sites also decreased.
Figure 1 shows XRD patterns of the samples. All
catalysts exhibited typical HZSM-5 zeolite structure,
suggesting that the impregnation of Fe, W and La
did not modify the framework structure of parent
HZSM-5 zeolite. Both specific surface area and pore
volume were not significantly altered by the
incorporation of metal species (Table 2).
Table 1. Acid properties
Catalyst
Acid site
density (mol
NH3 g-1)
Site strengh (%)
Weak
Intermediate
Strong
HZSM-5
1628
16
35
49
FeHZSM-5
1242
30
29
41
WHZSM-5
1736
33
19
48
LaHZSM-5
1049
35
23
42
Intensity (a.u.)
WHZSM-5
FeHZSM-5
LaHZSM-5
HZSM-5
10
20
30
40
50
60
70
80
2  (degrees)
Figure 1. XRD patterns of HZSM-5, LaHZSM-5,
FeHZSM-5 and WHZSM-5.
but enhanced the formations of ethylene and C3+
olefins [3]. For LaHZSM-5, propylene yield
increased and BTX yield decreased, however a
significant increase in paraffins yield was also
observed. On the other hand, WHZSM-5 showed a
product distribution very similar to that of HZSM-5.
The behavior of La-HZSM-5 can be explained by
the lower density of acid sites that influences in the
reaction mechanism producing more propylene. The
same catalysts were tested using the reaction
conditions that enhance BTX yield, the results are
shown in Table 4.
Table 4. Product yield for ethylene conversion. BTX
conditions: p = 0.35 atm, T = 400 °C, = 0.20 h.
HZSM-5 FeHZSM-5
Table 2. Textural properties
Surface area
(m2 g-1)
Catalyst
Micropores
volume
(cm3 g-1)
Mesopores
volume
(cm3 g-1)
HZSM-5
344
0.154
0.072
FeHZSM-5
329
0.146
0.078
WHZSM-5
352
0.158
0.093
LaHZSM-5
343
0.150
0.089
HZSM-5 FeHZSM-5
WHZSM-5
LaHZSM-5
C1-C4
8.8
3.5
7.5
25.3
C3H6
44.7
35.0
44.4
48.5
=
28.6
11.2
24.3
29.6
C3H6
6.3
3.2
4.7
7.2
=
C4
10.9
5.2
8.6
12.0
C5+C5=
7.9
3.8
5.9
9.4
BTX
40.2
46.4
40.1
30.0
6.1
30.2
15.6
11.8
C6
Table 3. Product yield for ethylene conversion. Propylene
conditions: p = 0.12 atm, T = 500 °C, = 0.15 h.
LaHZSM-5
C1-C4
+
Tables 3 and 4 show product yield after 5 minutes of
reaction for propylene and aromatics experimental
conditions, respectively. The products formed were
propene (C3H6), butenes (C4=), a C5 (C5+C5=)
fraction, a fraction of compounds with more than six
carbon atoms (C6+), and a fraction of C1 to C4
paraffins (C1-C4) and aromatics (BTX) such as
benzene, toluene, xylenes. The production of heavier
hydrocarbons from ethylene occurs through
oligomerization steps whose intermediates are
carbocations which are converted through cracking,
isomerization, cyclization, dehydrocyclization and
hydrogen transfer reactions. In these reactions, the
experimental conditions and the competition
between acidity and shape selectivity of catalyst play
important roles.
WHZSM-5
Only FeHZSM-5 sample favored BTX production,
but a large amount of C6+ fraction was also observed.
In fact for all metal-impregnated zeolites, the
amount of C6+ fraction increased. When HZSM-5
was impregnated with W no change was noted on
BTX yield. On the other, LaHZSM-5 presented the
smallest BTX yield. The transformation of ethylene
occurs by a sequence of oligomerization and
cracking reactions. A catalyst with high acidity
converts hydrocarbons such as ethylene into higher
hydrocarbons and aromatics, for this reason
LaHZSM-5 presented a low BTX yield [1]. In this
condition, BTX yield obtained with HZSM-5 and
WHZSM-5 are similar.
4. Conclusions
The impregnation of Fe, W and La on ZSM-5 zeolite
modifies its acidity and consequently changes the
yield of products. The difference in acidity promotes
changes in the reaction route allowing different
yields. The other properties of the zeolites do not
undergo significant changes, which indicate that
acidity is the most important property.
C4
21.3
15.6
21.1
22.2
C5+C5=
10.2
8.7
9.6
12.8
Acknowledgments
BTX
10.0
21.3
12.7
6.1
The authors acknowledge the financial support from CAPES.
C6+
4.9
15.9
4.6
4.7
The presence of Fe resulted in a decrease of
propylene yield and a large increase of BTX yield.
This result is not in agreement with the conversion
of ethanol into hydrocarbons where Fe-loading
suppressed the formation of aromatic compounds
References
[1] Z. Song, A. Takahashi, M. Mimura,T. Fujitani, Catal. Lett.
2009, 131, 364.
[2] M. Inaba, K. Murata, M. Saito, I.Takahara, React. Kinet.
Catal. Lett. 2006, 88, 135
[3] M. Inaba, K. Murata, M. Saito, I.Takahara, Green Chem.
2007, 8, 638.