THE EFFECT OF EXTRA-FRAMEWORK ALUMINUM IN DEALUMINATED ZSM-5 ZEOLITES ON THE TRANSFORMATION OF AROMATIC HYDROCARBONS

Article abstract of DOI:10.1135/cccc19950412

H-ZSM-5 zeolite was dealuminated by steaming under ''mild'' and ''severe'' conditions at 675 and 975 K for 71.5 and 0.5 h, respectively.It resulted to a comparable number of Al atoms released from the zeolite framework, although of their different nature.

Full text of DOI:10.1135/cccc19950412

4
12
Cejka et al.:
THE EFFECT OF EXTRA-FRAMEWORK ALUMINUM IN DEALUMINATED
ZSM-5 ZEOLITES ON THE TRANSFORMATION OF AROMATIC
HYDROCARBONS*
a,
a
a
b
Jiri CEJKA **, Alena VONDROVA , Blanka WICHTERLOVA , Hans G. JERSCHKEWITZ ,
c
d
Gunter LISCHKE and Ellen SCHREIER
a
J. Heyrovsky Institute of Physical Chemistry,
Academy of Sciences of the Czech Republic, 182 23 Prague 8, The Czech Republic
b
Center for Heterogeneous Catalysis,
Rudower Chaussee 5, D-12 484 Berlin, Germany
c
Technical University, D-15 752 Wildau, Berlin, Germany
Humboldt University, D-10 115 Berlin, Germany
d
Received October 20, 1994
Accepted December 5, 1994
H-ZSM-5 zeolite was dealuminated by steaming under “mild” and “severe” conditions at 675 and
9
75 K for 71.5 and 0.5 h, respectively. It resulted in a comparable number of Al atoms released from
the zeolite framework, although of their different nature. The “mild” zeolite steaming yielded Al
species easily removable from the zeolite by an acid leaching, in contrast to the zeolite steamed at
“
severe” conditions. The extra-framework Al species contributed to both a substantially higher para-
selectivity in toluene disproportionation and its alkylation with ethylene and to an increased rate of
side reactions leading to the zeolite deactivation by coke formation in toluene alkylation with
ethylene.
Dealumination of ZSM-5 zeolites by steaming is a way how to modify their activity
and to improve especially their selectivity with respect to particular hydrocarbon syn-
1
,2
thesis or its further transformation . The steaming process releasing evidently the
tetracoordinated framework aluminum into the extra-framework sites decreases the
number of bridging OH groups. It is evident that the degree of dealumination strongly
depends on the steaming conditions. With variation of temperature and time of steam-
ing, a broad range of Si/Al ratio is achieved. The structure of extra-framework Al
species strongly depends on the conditions of the steaming and a zeolite type. These Al
*
*
Presented at the XXVIth Czech and Slovak Symposium on Catalysis, Prague, November 7 – 8, 1994.
*The author to whom correspondence should be addressed.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Extra-Framework Aluminum
413
species, consisting of tetra-, penta- and octahedrally coordinated Al cations, range from
3
,4
a single charged Al complexes up to polymeric oxo-Al species .
It has been shown that simultaneous presence of bridging OH groups and Lewis sites
(
positively charged Al species) enhances zeolite oligomerization and cracking activ-
2
,3,5
ity . However, till now, the effect of extra-framework Al species formed by the
ZSM-5 zeolite steaming on its activity and para-selectivity in transformations of aro-
matics has not been investigated. It has been recently reported that the presence of
single metal cations (Al, Fe, Mn) located in the cationic sites slightly enhances zeolite
para-selectivity and, simultaneously, increases the rate of the zeolite deactivation by
6
coking . On the other hand, the presence of Fe cations attached to the “external” sur-
face of silylated ZSM-5 zeolite strongly increases the rate of subsequent isomerization
7
reactions, decreasing thus profoundly the resulting para-selectivity .
The aim of this investigation was to demonstrate the effect of the extra-framework
aluminum species on the acidic and catalytic properties of ZSM-5 zeolites in toluene
disproportionation and in its alkylation with ethylene. Moreover, the effect of the
presence of extra-framework species on the rate of the deactivation during catalytic run
is also discussed. The H-ZSM-5 zeolites dealuminated by steaming and those subjected
to acid leaching by nitric acid after dealumination are compared. The feasibility of the
extraction of extra-framework Al depends strongly on the temperature of the steaming
process. At lower temperatures, more extra-framework Al species are easily removed
1
from the zeolite, in contrast to the zeolites dealuminated at higher temperatures .
EXPERIMENTAL
The H-ZSM-5 zeolite dealumination by steaming and its subsequent extraction with an aqueous
1
HNO have been already described in detail elsewhere . The Na-ZSM-5 zeolite (original Si/Al 15.5)
3
was converted into its H-form by fourfold treatment with 0.2 M HNO in a batch procedure. The
3
H-ZSM-5 zeolite was heated in a stream of air and steam (water pressure of 90 kPa) at 675 K for
7
1.5 h (indicated as “mild” dealumination, D1) and at 975 K for 0.5 h (“severe” dealumination, D2)
to obtain zeolites of a comparable degree of dealumination. Each sample was subsequently treated
with 2 M HNO at 375 K resulting in a partial extraction of the extra-framework aluminum species
3
formed during the steaming process (D1ex, D2ex). The amount of the extracted aluminum was deter-
mined by chemical analysis in the acid solution. The amount of the remaining framework aluminum
was calculated from the ammonium ion-exchange capacity of dealuminated zeolites. The zeolites
mixed with a binder (35.0 wt.% of Aerosil-200 and 65 wt.% of the zeolite) were used for catalytic
testing.
X-Ray diffraction patterns were obtained with a HZG 4 diffractometer employing Ni-filtered
CuKα radiation. Zeolite characterization with respect to the bridging OH groups and the ammonia
adsorption by IR spectra (Specord M 85, Zeiss Jena) as well as temperature programmed desorption
8
,9
of ammonia (TPDA) are described in detail elsewhere
.
The zeolite void volume was estimated from the sorption capacity for argon measured in a static
apparatus at 13.30 kPa of argon at a temperature of 80 K on the samples pretreated at 720 K in
−4
vacuum of 10 kPa for 16 h.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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14
Cejka et al.:
The toluene reactions were performed in vapour phase, in a continuous glass down-flow micro-
reactor at atmospheric pressure (toluene concentration 18.5 vol.% in the feed, toluene to ethylene
molar ratio 3.8). Toluene disproportionation was carried out at a temperature of 770 K and WHSV
−1
−1
2
.7 h , and toluene alkylation with ethylene at 620 K and WHSV 10.0 h . The reaction products
were analyzed by means of an “on-line” high-resolution capillary gas chromatograph HP 5890 Series II
for details see refs1 ).
0,11
(
RESULTS AND DISCUSSION
Physico-Chemical Characterization
Although the dealuminated zeolites do not differ significantly with respect to their
framework silicon to aluminum (Si/Al ) molar ratio, and, therefore, to their number of
f
the extra-framework aluminum, the number of extractable extra-framework aluminum
atoms differ substantially (Table I). A long-time steaming at a lower temperature
clearly favours formation of the extra-framework aluminum species exhibiting higher
solubility in acidic solution. X-Ray diffraction patterns of all the investigated zeolites,
dealuminated and those after the subsequent extraction, show no significant changes
compared to original ZSM-5 (Fig. 1), indicating that no collapse of the zeolite structure
has proceeded either during dealumination procedures or during subsequent extractions.
A slight decrease in the unit cell parameters of dealuminated zeolites is in agreement
with the removal of aluminum from the framework and/or with an extraction of extra-
framework aluminum species from the channel system.
−1
The IR band at 3 740 cm reflects terminal SiOH groups of the Aerosil binder and
the H-ZSM-5 zeolite (Fig. 2). As expected, the dominant band at 3 610 cm−1 (bridging
I
1
2
3
4
5
4.0
10.8
17.6
24.4
31.2
2ϑ , °
FIG. 1
X-Ray diffraction pattern of parent and modified zeolites: 1 original H-ZSM-5, 2 D2, 3 D2ex, 4 D1,
D1ex
5
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Extra-Framework Aluminum
415
SiOHAl groups) of the parent H-ZSM-5 is substantially decreased by dealumination, in
accord with the release of Al atoms from the framework. On the other hand, after
−1
dealumination, both zeolites exhibit a band at 3 660 cm (Fig. 2), characteristic for the
OH groups bonded to the extra-framework aluminum cations. Because of its relatively
TABLE I
Characteristics of zeolites
Degree of
extraction
%
a
b
c
Sorption
capacity
−1,d
−1,d
Catalyst
Si/Alf
Si/Ale
TPDA
e
1 616 cm
1 460 cm
D1
108.2
108.2
85.9
18.1
23.6
18.9
19.9
–
23.2
–
0.23
0.25
0.23
0.20
3.98
4.04
4.05
3.95
0.05
0.00
0.06
0.03
0.28
0.31
0.32
0.22
D1ex
D2
D2ex
118.7
5.2
a
b
c
Si/Al framework molar ratio; Si/Al extra-framework molar ratio; mmol OH/g, corresponding
f
e
d
−1
to the high temperature peak of TPDA; normalized intensity of the IR bands: 1 616 cm – NH3
coordinated to Lewis sites, 1 460 cm – NH ; sorption capacity for argon (mmol/g).
−1
+ e
4
3
610
A
0.2
3
740
3
660
1
2
3
FIG. 2
4
5
IR spectra (OH vibration region) for the parent,
dealuminated and acid extracted zeolites: 1 original
H-ZSM-5, 2 D1, 3 D1ex, 4 D2, 5 D2ex
3
800
−1 3 400
ν, cm
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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16
Cejka et al.:
low intensity, the aluminum species generated by the dealumination process are sup-
posed to be rather highly aggregated, non-hydroxylated species. Moreover, with the
H-ZSM-5 zeolite dealuminated under “mild” conditions, its acidic extraction causes
removal of a substantial amount of the extra-framework aluminum species, while the
degree of the extra-framework aluminum species removal from the zeolite steamed at
higher temperatures is much lower (Table I). It should be stressed that no visible col-
lapse of the structure after dealumination and extraction was observed, as indicated by
X-ray diffraction.
IR spectra of adsorbed ammonia on zeolites (after ammonia adsorption the zeolites
were subjected to evacuation at room temperature for 18 h to remove weakly held
ammonia) confirm the above results. A comparison of the normalized absorption in-
tensity at the band maximum (Table I) shows that the acidic extraction of dealuminated
zeolites substantially decreases the amount of adsorbed ammonia on the Lewis sites
−1
(
1 616 cm ), in accordance with the amount of Al extracted into the acidic solution.
The degree of extraction of extra-framework Al species is more pronounced with the
+
−1
“
mildly” dealuminated zeolite. A slight increase in the NH band (1 460 cm ) of the
4
D1 zeolite after acidic extraction indicates that cationic extra-framework aluminum
+
species are replaced by H Brønsted acid sites. On the other hand, for the “severely”
dealuminated zeolites, the number of Brønsted sites is lower after the acid extraction of
the zeolite. It again indicates that different extra-framework aluminum species are
formed during the dealumination via “mild” and “severe” conditions. In the case of
“
mild” dealumination, extra-framework species are charge balanced cations in the
ZSM-5 structure, and via acid treatment a part of them is substituted by protons. As for
severely” dealuminated zeolite, the number of Si−OH−Al groups is lower after extrac-
“
tion in acidic media (as demonstrated by both IR of OH groups and TPDA), which can
be explained only by a leaching of framework Al atoms from the D2 dealuminated
zeolite. However, it should be emphasized that both X-ray diffraction data as well as
sorption capacities for argon do not indicate some collapse of the structure during both
dealumination and extraction processes (Fig. 1, Table I).
Catalytic Tests
The toluene conversions and product compositions observed in toluene alkylation with
ethylene and toluene disproportionation for the dealuminated zeolites are given in
Tables II and III. The overall selectivity to ethyltoluenes in toluene alkylation with
ethylene exceeds 96 wt.%, while toluene disproportionation to benzene and xylenes is
accompanied by a significant dealkylation leading to formation of about 20% of C – C
1
4
aliphatics. A substantially higher para-selectivity in both reactions has been found for
both xylenes and ethyltoluenes, compared to thermodynamic equilibrium concentration
1
2
(
39.7 – 43.8 vs 24.3 at 770 K for p-xylene and 48.5 – 82.7 vs 33.4 at 620 K for
1
3
p-ethyltoluene ). The toluene conversion in individual reactions has been practically of
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Extra-Framework Aluminum
TABLE II
Toluene alkylation with ethylene (WHSV 10.0 h , temperature 620 K, toluene to ethylene molar
ratio 3.8, time-on-stream 15 min)
417
−1
Catalyst
Parameter
D1
D1ex
19.9
D2
D2ex
17.7
Toluene conversion, c, %
Selectivity, vol.% to
p-ethyltoluene
m-ethyltoluene
o-ethyltoluene
benzene
16.3
16.8
81.7
13.9
–
47.0
46.9
0.5
1.0
1.1
0.4
0.1
1.6
1.4
1.6
79.8
15.5
–
69.8
25.3
0.1
0.6
1.3
0.2
–
0.5
1.4
0.2
–
0.6
p-xylene
0.1
m-xylene
0.2
o-xylene
<0.1
1.2
ethylbenzene
higher aromatics
1.1
1.2
3.7
1.2
1.4
6.5
1.7
a
Coking , % = (C − C135)/C15
2.5
15
a
Coking expresses a decrease in toluene conversion from time-on-stream of 15 min to 135 min
relative to toluene conversion at 15 min of time-on-stream.
TABLE III
−1
Toluene disproportionation (WHSV 2.7 h , temperature 770 K, time-on-stream 15 min)
Catalyst
Parameter
D1
5.4
D1ex
9.2
D2
5.4
D2ex
6.4
Toluene conversion, c, %
Selectivity, vol.% to
benzene
40.4
18.0
14.0
4.1
–
44.5
11.4
19.5
8.1
40.6
16.9
13.6
4.8
–
43.8
13.6
15.6
6.3
p-xylene
m-xylene
o-xylene
ethyltoluenes
higher aromatics
C1 – C4 aliphatics
0.6
0.7
–
0.3
–
–
23.5
7.6
15.6
12.2
24.1
4.6
20.0
5.5
a
Coking , % = (C − C135)/C15
15
a
See footnote in Table II.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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18
Cejka et al.:
the same value for both dealuminated zeolites D1 and D2 and no substantial differences
in the zeolite para-selectivity have been observed. This corresponds also to the same
number of framework Al atoms and extra-framework Al species for these zeolites. As
expected, the lower para-selectivity in xylenes compared to ethyltoluenes has been
found for toluene disproportionation which proceeds at a higher temperature. More-
over, smaller differences in transport rates among smaller xylene isomers can be ex-
pected in contrast to more bulky ethyltoluenes.
It is evident that the presence of the extra-framework aluminum species formed by
the steaming of the zeolite affects substantially its catalytic behaviour. After the extrac-
tion of a part of the extra-framework species, particularly that exhibiting a positive
charge, the toluene conversions in both reactions are higher compared to those of the
steamed zeolites (D1ex and D2ex vs D1 and D2, respectively). It means that the simul-
taneous presence of Brønsted and Lewis sites has no positive effect on the rate of
transformations of aromatic hydrocarbons, as the Dex zeolites clearly contained the
lower amount of extra-framework Al species. Surprisingly, for both Dex zeolites, the
higher toluene conversion (in both reactions) was observed, even though for D2ex com-
pared to D2, the lower number of bridging OH groups was found in TPDA and IR.
Therefore, for both dealuminated zeolites treated in acid media, the increase in the
conversion can be explained only by an increase in the diffusion rate of reactants and
products resulting from the removal of some extra-framework aluminum species from
the zeolite channels, and for “mildly” dealuminated zeolite also by a partial replacing
of the extra-framework aluminum charged species by the bridging OH groups (cf.
Table I), as well. Although a part of extra-framework aluminum species has been
removed from the zeolites via treatment in acid media, no substantial differences has
been found in the sorption capacities of these two samples, compared to dealuminated
ones. The expected changes in the transport rate of reactants and products are reflected
2
0
4
c, %
1
8
3
2
1
FIG. 3
Time-on-stream (t) dependence of toluene
conversion c (%), in toluene alkylation with
1
6
−1
ethylene (WHSV 10.0 h , temperature 620 K,
toluene to ethylene molar ratio 3.8), for: 1 D1,
0
40
80
120
t, min
2
D1ex, 3 D2, 4 D2ex
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Extra-Framework Aluminum
419
also in the higher para-selectivity in xylenes and ethyltoluenes for the steamed zeolites
which contain in their channels the higher amount of the extra-framework Al species
compared to the acid extracted dealuminated zeolites.
The catalytic effect of the extra-framework aluminum species (Lewis sites) is clearly
demonstrated for toluene disproportionation in Table III. The enhancement of the rate
of toluene dealkylation leading to C – C aliphatics for dealuminated zeolites is evident
1
4
compared to the extracted zeolites.
Deactivation
The presence of the extra-framework Al species, especially those exhibiting electron-
acceptor properties, affects also the rate of zeolite deactivation. Even though no signi-
ficant effect of simultaneous presence of Brønsted and Lewis sites has been observed
during the transformation of aromatics, the catalytic effect of the extra-framework Al is
clearly demonstrated in the enhancement of dealkylation reaction of toluene during its
disproportionation. The conversion of toluene to C – C products is substantially de-
1
4
creased, especially when comparing D1 and D1ex (from ca 24 to ca 16%, Table III)
after leaching particularly of charged extra-framework Al species. For both couples of
dealuminated and extracted zeolites (D1–D1ex and D2–D2ex) a decrease in conversion
is lower (for Dex compared to D), although the level of conversion is higher. Two
effects likely contribute to this finding: ethylene oligomerization, leading expectably to
coke formation, is supported by the simultaneous presence of Brønsted and Lewis sites.
Moreover, the presence of the extra-framework species contributes to the retainment of
the formed products in the zeolite channels, increasing their contact time and, thus,
enhancing their further transformation leading to high molecular weight products. This
finding is in agreement with the higher para-selectivity and faster deactivation found
for H-ZSM-5 zeolites, where a part of protons was substituted for Fe, Al and Mn ca-
6
tions . On the other hand, with toluene disproportionation over steamed zeolites (Table III)
the deactivation is lower due to the increase in the rate of dealkylation. The smaller
molecules can then easily diffuse out of the pores of these zeolites. This effect probably
plays a major role in comparison with the lowering of transport hindrances after partial
aluminum extraction.
CONCLUSIONS
It has been found that the presence of the extra-framework aluminum species in the
zeolite channels formed via zeolite dealumination by steaming plays an important role
in the catalytic transformation of aromatics. All the extra-framework Al species cause
a decrease in the zeolite inner volume and, thus, contribute to the increased para-selec-
tivity. This is accompanied also by the retainment of the reaction products inside the
zeolite channels and, therefore, in a longer reaction contact time, causing further trans-
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

4
20
Cejka et al.:
formation of reaction products That leads to the formation of high-molecular weight
products – coke, and zeolite deactivation. Besides the effect of the extra-framework Al
on the geometry of the zeolite inner volume, the extra-framework Al species, bearing a
positive charge, Lewis sites, contribute to the catalytic activity of the zeolite. They
enhance ethylene oligomerization in toluene alkylation with ethylene, leading to high-
molecular weight products and to zeolite deactivation by coking. Therefore, the posi-
tive effect of the presence of the extra-framework Al species on the enhancement of
para-selectivity is, on the other side, compensated by the faster zeolite coking and by
tendency to dealkylation. Some contribution of Lewis sites to the transformation rate of
aromatics was not evidenced.
The authors greatly appreciate the running of XRD patterns by U. Steinike in the X-ray Analytical
Laboratory. The work of J. C. and B. W. was supported by the Grant Agency of the Academy of Sciences
of the Czech Republic (Grant No. 440408).
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