The Role of External Acidity of Hierarchical ZSM-5 Zeolites in n-Heptane Catalytic Cracking

Article abstract of DOI:10.1002/cctc.201800086

The hierarchical ZSM-5 zeolite, which contains mesopores in addition to intrinsic micropores, is expected to enhance the bulky molecular diffusion and accessibility of acid sites. These sites, distributed in the internal surface (micropore) or external environment (including external surface and mesopore walls), play different roles in chemical reactions. To explore the effect of internal/external acidity on the reaction, in this study, a series of hierarchical ZSM-5 zeolites with similar pore structures but different external acid properties were synthesized. Tartaric acid was employed to poison the acidic sites in the external environment selectively and extract Al from the framework. The catalytic cracking of n-heptane (the main compound of naphtha) was chosen as a model reaction. The results show that elimination of external active sites tends to reduce the coke formation. The more external acid sites are selectively eliminated, the better catalytic performance can be achieved because of suppressing the bimolecular reactions of ethylene and propylene.

Full text of DOI:10.1002/cctc.201800086

Accepted Article
Title: The role of external acidity of hierarchical ZSM-5 zeolites in n-
heptane catalytic cracking
Authors: Xiaoxiao Zhang, Dang-guo Cheng, Fengqiu Chen, and Xiaoli
Zhan
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The role of external acidity of hierarchical ZSM-5 zeolites in n-
heptane catalytic cracking
Xiaoxiao Zhang, Dang-guo Cheng, * Fengqiu Chen and Xiaoli Zhan [a]
Abstract: The hierarchical ZSM-5 zeolite, which contains mesopore
in addition to the intrinsic micropore, is expected to enhance the
bulky molecular diffusion and accessibility of acid sites. These sites,
distributed in the internal surface (micropore) or external
environment (including external surface and mesopore wall), play
different roles in chemical reaction. To explore the effect of
internal/external acidity on the reaction, in this study, a series of
hierarchical ZSM-5 zeolites with similar pore structures but different
external acid properties were synthesized. Tartaric acid was
employed to poison the acidic sites in external environment
selectively and extract Al from the framework. The catalytic cracking
of n-heptane (the main compound of naphtha) was chosen as a
model reaction. The results show that elimination of external active
sites tends to reduce the coke formation. The more external acid
sites are selectively eliminated, the better catalytic performance can
be achieved because of suppressing the bimolecular reactions of
ethylene and propylene.
hierarchical zeolites are two main factors to determine the
catalytic performance during reactions. For hierarchical
zeolites, acid sites distributed in micropores or mesopores
may play different roles in the reaction. [26] Emdadi et al.
[27]
have applied hybrid lamellar bulk zeolite to catalytic
conversion of benzyl alcohol in mesitylene. They found that,
in the external environment (external surface acid sites and
acid sites in pore mouth region), the alkylation reaction rate
was monotonically decreased with increasing number of
poisoned external surface acid sites of zeolites, while the
etherification reaction rate was not influenced. Losch et al.
[28] found that external surface passivation of ZSM-5 zeolites
led to enhanced selectivity towards light olefins in MTO
[29, 30]
(methanol to olefins) reaction. Previous studies
have
also confirmed that less heavy coke might be formed on the
zeolite surface which was followed by a higher selectivity
[31]
towards light olefins in MTO reaction. Aramburo et al.
have gotten a conclusion that a lower number and strength
of acid sites located on the external surface of ZSM-5
zeolites resulted in a lower production of poly-aromatics
species and thus a positive effect on the stability of the
catalyst.
Introduction
In recent years, catalytic cracking of naphtha into light
alkenes has been attracted extensive attention due to its
advantages in products control and energy conservation.[1-6]
According to the reported catalysts candidate, ZSM-5,
containing abundant protonic sites, has been considered as
Despite of the acid property, the hierarchical pore
structure has also drawn extensive attentions in recent
[32]
years. Vu et al.
applied hierarchical ZSM-5 material to
the cracking of triglyceride-rich biomass and higher
one of the most promising ones for this process.[7,
8]
selectivity toward gasoline-range hydrocarbons was
[33]
However, the catalytic applications of ZSM-5 zeolites are
limited by mass transport in the microporous network when
the bulky molecules are involved in the reaction.[9-13] In this
regard, the introduction of mesopores into microporous
zeolite to obtain the hierarchical ones, possessing the
catalytic features of microporosity and the improved mass
transport consequence of mesoporosity,[14-20] would be an
optimal choice for its better performance.[21-25]
achieved. Xue et al.
found that hierarchical ZSM-5 was
proved to be more efficient in catalytic LDPE cracking due
to improved accessibility of large polymer molecules to the
active sites. Also, lots of efforts have been taken in our
previous studies [12, 34]. Different mesopore/micropore ratios
of hierarchical ZSM-5 zeolites with the acidity unchanged
have been prepared and the results showed that, in the
catalytic cracking reaction system, the higher mesoporous
Generally, the acidic property and the pore structure of
volume led to greater enhancement of selectivity of light
[12]
olefins.
A series of ZSM-5/ZRP-1 composite zeolites
have also exhibited higher selectivity of olefins in the n-
heptane catalytic cracking due to the presence of
mesopores.[34]
[a]
X. X. Zhang, Prof. D. G. Cheng, Prof. F. Q. Chen, Prof. X. L. Zhan
Zhejiang Provincial Key Laboratory of Advanced Chemical
Engineering Manufacture Technology,
However, the role of external environment acidity
(external surface acid sites and acid sites on mesopore
wall) of hierarchical ZSM-5 zeolites in n-heptane catalytic
cracking reaction remains unknown. Thus, qualitative as
well as quantitative characterizations of external acid sites
are essential in understanding the detailed reaction
mechanism and related selectivity features of the products
College of Chemical and Biological Engineering,
Zhejiang University, 38 Zheda Road, Hangzhou 310027, China
Tel: +86 571 87953382 Fax: +86 571 87951227
E-mail: dgcheng@zju.edu.cn (D.G. Cheng).
for
the
catalytic
cracking
reaction.
While
the
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characterization of the overall acidity of zeolites have been
widely reported, there are few reports that have discerned
the internal vs external acidity in terms of their type,
strength and amount of the hierarchical ZSM-5 zeolites. [35-
40]
phase. With decreasing of Si/Al ratio, the loss of crystallinity
is more obvious, indicating that more Al located on the
external surfaces or mesoporous walls are dealuminated,
but the crystal lattice is still well-maintained.
Herein, based on the adequate studies about the pore
Textural properties
[12, 34]
structures of hierarchical zeolites in our previous work,
the effect of external acidity of hierarchical zeolites on the
catalytic cracking reaction was thus explored in this work.
We prepared a series of hierarchical ZSM-5 zeolites with
similar pore structure but different external acid properties
by employing the organosilane APTES and adjusting Si/Al
ratios. And the dealumination process via tartaric acid
treatment has also been conducted to extract framework Al
on the external environment of zeolite. All physicochemical
properties of the prepared samples were also fully
characterized. The influence of external acidity in
hierarchical zeolites on the selectivity of light olefins and the
coke generation during the catalytic cracking of n-heptane
(the model compound of naphtha) was discussed in detail.
Table 1 summarizes the Si/Al ratios, the surface areas
and pore volumes of the samples, determined by ICP and
N2 adsorption-desorption test. For the parent hierarchical
ZSM-5 zeolites, the micropore volume and mesopore
volume are both not noticeably changed with Si/Al ratio
reduced. Comparing the textual properties of samples
before and after tartaric acid treatment, one can see that
the micropore volumes are almost similar and the
mesopore volume has a little increase. The former indicates
that the tartaric acid has no effect on the zeolitic framework.
And the latter can be ascribed to the creation of cavity and
external Si-OH groups as the result of the extraction of
[43]
external framework Al atom.
The results of N2
adsorption-desorption isotherms and pore size distributions
further confirm the characteristics of mesoporous structures
in all samples (Figure 2). It can be found that the hysteresis
loops appear on the isotherms due to the capillary
condensation in mesopores and the diameters of
mesopores for all samples are around 3.8 nm. [12] Also, from
the ICP results, when the hierarchical ZSM-5 zeolites were
selectively dealuminated from external environment with
tartaric acid, the change of the one with lower Si/Al ratio is
more obvious than the one with higher Si/Al ratio. This
difference is caused by the different Al amount located on
the external environment.
Results and Discussion
XRD patterns of the samples
Table 1 Textural property of the hierarchical ZSM-5 samples
Specific surface area
/(m2·g-1)
Pore volume
/(mL·g-1)
Sample
Si/Al
Total
External
Micropore
Mesopore
HZ-1
HZ-1-TA
HZ-2
48
60
69
77
88
365
390
370
398
354
411
101
129
106
119
93
0.136
0.135
0.136
0.144
0.135
0.150
0.235
0.272
0.223
0.316
0.243
0.284
Figure 1 XRD patterns of the samples
HZ-2-TA
HZ-3
Figure 1 exhibits XRD patterns of parent hierarchical
ZSM-5 zeolites with different Si/Al ratios and their
counterparts after selective dealumination. All the samples
are clearly crystalline and possess the classical MFI
features of ZSM-5 zeolite structure.[41] From the HZ-1 to HZ-
5, the diffraction peaks of samples become higher and
stronger as the Al content decrease gradually, which
suggests that the increasing Al content has a negative
effect on the crystallization rate.[42] Also, it is noted that the
dealumination step is accompanied by a loss in crystallinity
in each sample but not appearance of a new crystalline
HZ-3-TA
119
94
HZ-4
HZ-4-TA
HZ-5
100
104
111
113
378
391
387
389
97
116
95
0.146
0.141
0.150
0.141
0.228
0.234
0.220
0.241
HZ-5-TA
115
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tartaric acid. And the external acid amount of HZ-5 is reduced
from 0.017 mmol·g-1 to 0.015 mmol·g-1 for high Si/Al ratio. This
means that the lower Si/Al ratio is, the higher Al content the
external environment of zeolite can be eliminated, leading to the
more decrease of external acid amount. The low value of
external acid amount of HZ-n-TA samples also proves that the
tartaric acid was very effective for selective dealumination
process from the external surfaces and mesoporous walls.
Figure 4 shows the solid state 27Al MAS NMR spectra of the
parent hierarchical zeolites and the poisoned counterparts. The
peak at 55 ppm is assigned to the tetrahedrally coordinated
framework Al, while the peak around 0 ppm is due to an
[27]
octahedral coordination typical of extra-framework Al.
The
fraction of the extra-framework Al in theses zeolite samples
should be insignificant quantity since the peak around 0 ppm is
very small compared to that around 55 ppm in the spectra.
Observing from the peak intensity at 55 ppm of parent
hierarchical zeolites, there is no doubt that, with decreasing of
Si/Al ratio, more Al atoms are located in the framework. In
addition, the peak intensity at 55 ppm of the poisoned samples
become lower and weaker compared with that of parent ones
and the peaks at 0 ppm are not increased accordingly. This fact
clearly illustrates that selective dealumination from external
environment only gives rise to a reduction of framework Al
without generation of extra-framework Al. This also confirms that
acid sites presented on the external surfaces and mesoporous
walls are both important Brønsted acid, which are the main
active sites for reactions. Through the vertical comparison from
HZ-1-TA to HZ-5-TA, the fewer framework Al are dealuminated
from external environment and thus less reduction of Brønsted
acid amount as Si/Al ratio increases. This is accordance with
NH3-TPD results and the detailed integrated area of framework
Al in hierarchical zeolite is demonstrated in Table 3.
Figure 2 (a) N2 adsorption-desorption isotherms of all the samples (b)
Pore-size distributions of the prepared samples obtained by BJH model.
Acidity property
The acidic properties of the samples were investigated by
NH3-TPD. In Figure 3, two desorption peaks, at 160-180 °C and
340-380 °C, are observed for all samples, which are attributed to
desorption of NH3 adsorbed on the weak acid sites and strong
[12]
acid sites, respectively.
For the parent hierarchical zeolites,
the intensity of desorption peaks at weak and strong acid sites
are decreased remarkably when the Si/Al ratios of zeolite
increase, which is due to less acid sites at lower Al
concentration. Moreover, each poisoned sample consequently
has lower acid amount, especially in weak acid sites and slightly
decreases in strong acid sites, comparing with the parent
zeolites. The difference between the poisoned and parent
sample results from the loss of acid sites caused by
dealumination process from external environment of the
samples. The lower Si/Al ratio, the more external acid sites are
accordingly dealuminated. The derived amount and strength of
strong and weak acid sites of the samples are listed in Table 2
and the external acid amount is also included. From the
determination by Coll (2, 4, 6-collidine), the external acid sites
of the parent hierarchical zeolites are gradually decreased as
Si/Al ratios increase. And the difference between the values of
external Al amount and total acid amount can be considered the
internal Al sites that are inaccessible to Coll. The external acid
sites of HZ-1 is decreased from 0.07 mmol·g-1 to a very low
value of 0.018 mmol·g-1, due to the dealumination process with
Figure 3 NH3-TPD curves of catalyst sample
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Table 2 Characteristics of acidity property in samples
Weak acid Strong acid
Total acid amount External Al
Samples
(mmol∙g-1)
(mmol·g-1)
Temperature (°C)
Amount (mmol·g-1)
Temperature (°C)
Amount (mmol·g-1)
HZ-1
HZ-1-TA
HZ-2
175
175
175
380
360
366
0.173
0.134
0.128
0.118
0.301
0.252
0.070
0.018
0.115
0.099
0.080
0.104
0.098
0.092
0.219
0.197
0.172
0.046
0.018
0.030
HZ-2-TA
HZ-3
168
167
164
363
337
348
HZ-3-TA
0.071
0.069
0.063
0.090
0.083
0.082
0.161
0.152
0.145
0.016
0.023
0.015
HZ-4
HZ-4-TA
HZ-5
161
161
160
338
347
349
0.056
0.055
0.081
0.080
0.137
0.135
0.017
0.015
HZ-5-TA
166
342
aggregated particles around 200 nm with rough surfaces for HZ-
5. To be specific, for the HZ-1 and HZ-2 with lower Si/Al ratio of
48 and 69, it is obvious that the aggregated particles are
consisted of nano units around 30 nm. For the HZ-3 and HZ-4
with medium Si/Al ratio of 88 and 100, the rough surfaces of
particles show that they are also composed of small nano units
around 30nm and these nano units connect with each other
more closely. And for the HZ-5, the highest Si/Al ratio of 111, it
also has rough surfaces but the size of the aggregates is around
200 nm. TEM micrograph of the parent hierarchical ZSM-5
zeolites further confirms the above results and it can be
concluded that the Al content greatly influences the morphology
[24]
of samples with the function of APTES
nano crystallites stack.
in term of the way of
Figure 4 Solid-state 27Al MAS NMR spectra of zeolite catalysts
Table 3 The integrated areas of framework Al at 55 ppm in solid-state
27Al MAS NMR spectra
Samples
HZ-1
Integrated area
99758
Δ Integrated area
40293
HZ-1-TA
HZ-2
59465
48713
6018
3193
2148
114
HZ-2-TA
HZ-3
42695
40915
HZ-3-TA
HZ-4
37722
36383
HZ-4-TA
34235
HZ-5
HZ-5-TA
28613
28499
Morphology features
SEM and TEM images of parent hierarchical ZSM-5 zeolites
and the corresponding selectively dealuminated ones are
represented in Figure 5. Comparing the parent hierarchical
zeolites with the dealuminated ones in SEM images, it can be
found that there is no change in morphology, which confirms that
the tartaric acid has no effect on the morphology features of
samples. For the parent hierarchical ZSM-5 zeolites, SEM
images show some differences in morphology, from big
aggregated ellipsoidal particles around 400 nm for HZ-1 to small
Figure 5 SEM and TEM images of hierarchical zeolite samples
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Catalytic performance
Coke formation
To further investigate the cause of improved selectivity of
ethylene and propylene, the coke analyses of tested HZ-1 and
HZ-1-TA at time on streams of 1.5 h and 9 h have been
conducted separately, seen in Figure 7. During the 1.5 h
reaction, there is only the heavy coke deposits located on
samples and the temperature when the carbonaceous residuals
are burnt off is around 550 °C. Moreover, the amount of coke
deposits on HZ-1-TA is negligible, comparing with that of HZ-1.
This difference indicates that carbonaceous species are
To illustrate the effect of external acidity of hierarchical ZSM-5
zeolites on catalytic performance by adjusting Si/Al ratios, the
prepared samples and external-poisoned counterparts were
tested with catalytic cracking of n-heptane reaction as a model,
as shown in Figure 6. For the parent hierarchical ZSM-5
zeolites, the catalytic activity is enhanced with increasing acid
amount of zeolites caused by the increasing Al content. Hence,
the conversion of n-heptane is increased as the order of HZ-
5<HZ-4<HZ-3<HZ-2<HZ-1. On the contrary, for the selectivity of
ethylene and propylene, it is decreased as the order of HZ-
5>HZ-4>HZ-3>HZ-2>HZ-1. One reason is that the increase in
framework Si/Al ratio results in a decrease in the hydrogen
preferentially built up in the external environment of the
[40]
hierarchical zeolite
and with the absence of external acid
sites for HZ-1-TA, little coke deposits are formed. When the
reaction was extended to 9 h, the coke amount of HZ-1-TA was
also significant less than that of HZ-1. In addition, despite the
almost complete removal of external acid sites, the coke species
(including light coke and heavy coke species) is very similar
between the HZ-1 and HZ-1-TA with catalytic steam cracking
reaction proceeding. This behaviour confirms that external acid
sites give rise to coke formation and partially determine the rate
of coke generation. [40]
transfer/cracking rate ratio, which is favourable for the
[26, 44]
improvement of selectivity of ethylene plus propylene
.
When the hierarchical ZSM-5 zeolites were selectively
dealuminated from external environment, the catalytic
performance is therefore changed. Firstly, the n-heptane
conversions on all samples are decreased after tartaric acid
treatment and the parent zeolite with lower Si/Al ratio selectively
dealuminated leads to more obvious decrease of conversion.
This is due to the fact, for the parent zeolite with lower Si/Al
ratio, the more Al atoms located on the external environment
can be eliminated (Table 2), leading to the greater decrease of
acid amount for the poisoned counterparts. However, the Al
content and thus the acid amount is still as the order of HZ-1-
TA>HZ-2-TA>HZ-3-TA>HZ-4-TA>HZ-5-TA (as seen in Table 2
and Figure 3). This leads to the conversions of all the poisoned
samples are also increased as the order of HZ-5-TA<HZ-4-
TA<HZ-3-TA<HZ-2-TA<HZ-1-TA. Besides the change of n-
=
=
heptane conversion, the selectivity of light olefin (C2 +C3 ) is
also significantly improved after tartaric acid treatment as
presented. Comparing the HZ-1 with HZ-1-TA, the selectivity of
ethylene and propylene is increased by around 8% with absence
of external acid sites.
Figure 7 Thermogravimetric analysis curves of tested HZ-1 and HZ-1-TA
Mechanism analysis
The reason for less coke deposits on HZ-1-TA is because that
the more acid sites on external environment are poisoned,
leading to more inhibition of undesirable secondary reactions.
This is confirmed by in situ DRIFTS in our previous study [43] that
the increased mesopore volume of ZSM-5/mordenite zeolites
caused by hydrothermal treatment led to the fast-bimolecular
reaction. To further investigate the role of external acid in
catalytic cracking reaction, two standards are employed to
distinguish the monomolecular cracking mechanism and
bimolecular reaction mechanism on different acid sites. The first
one is olefin to paraffin ratio (O/P) in the C2-C6 product. [26] The
Figure 6 Conversion of n-heptane and selectivity of ethylene and propylene
over parent samples (the solid star) and selectively dealuminated counterparts
(the hollow star)
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Table 4 Product selectivity in the n-heptane catalytic cracking over samples
Selectivity (%)
Sample
O/P
CMR
=
=
=
=
=
C1
C2
C2
C3
C3
iC4
3.4
2.8
3.2
2.9
3.2
3.0
2.8
2.6
2.5
2.3
nC4
4.8
3.4
4.0
3.0
4.9
2.6
2.7
3.2
2.5
2.7
C4
C5
C5
C6
C6
HZ-1
HZ-1-TA
HZ-2
5.5
3.8
5.2
4.1
4.8
4.2
4.5
3.9
4.0
3.5
4.5
5.0
4.6
5.2
4.9
5.3
5.0
5.2
5.2
5.3
19.8
24.4
23.8
26.3
24.5
25.5
25.0
26.2
25.8
26.2
6.2
6.5
6.2
6.9
6.0
6.9
6.3
6.8
6.2
6.3
34.2
37.3
36.5
38.2
37.3
39.7
38.4
39.8
38.8
40.3
11.4
13.7
8.8
0.8
0.9
0.9
0.6
0.6
0.8
0.7
0.6
0.5
0.5
0.5
0.9
0.3
0.7
0.3
0.8
0.4
0.8
0.5
0.6
0.6
0.8
0.6
0.2
0.8
0.3
0.7
0.5
0.2
0.4
0.4
0.6
0.4
0.4
0.5
0.5
0.4
0.4
0.2
0.2
3.3
4.0
3.6
4.1
3.5
4.1
4.0
4.1
4.5
4.6
8.7
11.9
10.5
12.3
10.7
11.7
12.3
13.6
14.0
15.2
HZ-2-TA
HZ-3
10.8
9.7
HZ-3-TA
HZ-4
11.2
8.7
HZ-4-TA
HZ-5
10.6
11.6
12.5
HZ-5-TA
second one is cracking mechanism ratio (CMR) proposed by
Wielers et al., [45] which is defined as follows:
fast bimolecular reactions in the external environment builds the
obstacle to arrive the higher selectivity of ethylene and
propylene. This is hence responsible for the diminution of the
selectivity of target product. Accordingly, to take benefits from
improved mass transfer in hierarchical zeolite, selective
deactivation of the external acid sites is required.

n(C1) 
n(C2)
CMR

(1)
n(i  butane)
Where n (C1) is the mole number of methane, ∑n (C2) total
mole number of ethane and ethylene and n (i-butane) the mole
number of isobutane.
The results of the product selectivity for all samples are
demonstrated in Table 4. Taking HZ-1 and HZ-1-TA as
examples, of particular importance here is the selectivity of light
olefins (including all the olefins in the C2-C6 product) of HZ-1-TA
which is about 10% higher than that of HZ-1. As the
consequence, a significant change in the olefin to paraffin ratio
(O/P) in the C2-C6 product is found [26]. The O/P ratio of HZ-1-
TA (4.0) is higher than that of HZ-1 (3.3), which indicates a
decrease in the hydrogen transfer/cracking rate ratio with
elimination of external acid sites. On the other hand, the
implication of the CMR is to denote the ratio of monomolecular
cracking mechanism and bimolecular reaction mechanism. The
results show that CMR of HZ-1-TA (11.9) is also higher than that
of HZ-1 (8.7), indicating that monomolecular cracking
mechanism is more predominantly than bimolecular mechanism
in the n-heptane steaming cracking over HZ-1-TA. The same
principle also applies to other samples in Table 4. All above
analyses strongly confirm that the internal acid sites are main
active sites for the monomolecular cracking reaction, while the
external acid sites hold the dominate position for the undesirable
Scheme 1 The proposed mechanism of n-heptane catalytic cracking over
different active sites of hierarchical zeolites
Conclusions
A series of hierarchical zeolites with similar pore structures
but different external acid properties have been
successfully synthesized. The influence of external acid
sites on the catalytic cracking of n-heptane has also been
fully elucidated based on reprocessing of external acid sites
in the absence and presence of tartaric acid treatment on
hierarchical ZSM-5 zeolites. The acid treatment caused
selective removal of the framework Al on external
environment, giving selective deactivation of external acid
sites. A combination of hierarchical structure and tartaric
acid treatment can provide a more efficient hierarchical
ZSM-5 catalyst, exhibiting higher selectivity of light olefins
as well as high resistance to coke formation during n-
heptane catalytic cracking. Our results also suggest that the
lower Si/Al ratio of hierarchical ZSM-5 zeolites is, the more
external acid sites can be dealuminated through tartaric
acid treatment, with which better performance and thus
bimolecular reactions such as alkylation, hydrogen transfer and
[1, 46]
isomerization because of less space constrain.
In the
presence of external acid sites, the primary products such as
ethylene and propylene can undergo further reactions that lead
to formation of the coke precursor (benzene, toluene and
xylenes) [46, 47] and the coke deposits, as proposed in Scheme 1.
This suggests that, even though mesoporous structure is of
great importance to decrease the diffusion resistance to obtain
more ethylene and propylene compared with the solely
micropore zeolites [12], the external acid sites impede further
generating even more light olefins. The negative contribution of
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10.1002/cctc.201800086
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higher the selectivity of ethylene and propylene can be
eventually achieved. The present study is exemplified that
the external acidity in meso-/microporous ZSM-5 zeolites is
vital in impacting the selectivity of ethylene and propylene
during the n-heptane catalytic cracking reaction.
θ angle values in the range of 3-50 ° and recorded with a
step size and a counting time of 0.02 ° and 0.3 s,
respectively. The Si and Al contents of the samples were
determined by PerkinElmer ICP-OES 8000. N2 adsorption-
desorption test was carried out on a Micromeritics ASAP
2020 analyzer at 77 K. The specific surface area of the
sample was calculated using the Brunauer, Emmett and
Teller (BET) method. The micropore volume was obtained
by t-plot method. The micropore and mesopore size
distributions were determined via Horvath-Kawazoe (HK)
and Barrett-Joyner-Halenda (BJH) method, respectively.
Transmission electron microscopy (TEM) images were
obtained with JEM-2100 electron microscope operating at
200 kV and scanning electron micrographs (SEM) were
carried out on SU-8010 to investigate the morphology of
samples. The acidity of the samples was determined by
ammonia temperature programmed desorption (TPD) in a
Experimental Section
Chemicals
The chemicals used were as follows: tetraethoxysilane
(TEOS, 98 %), aluminum nitrate nonahydrate (Al
(NO3)3·9H2O, AR), tetrapropyl-ammonium hydroxide
(TPAOH, 25 wt% aqueous solution), (3-aminopropyl)
triethoxysilane (APTES, 98 %) and tartaric acid (C4H6O6, 99
%). Tartaric acid was purchased from Aldrich and the other
chemicals were purchased from Sinopharm Chemical
Reagent Co., Ltd and used as received.
Micromeritics AutoChem
Ⅱ 2920 system using He as
carrier gas. Previous to the test, the samples were
outgassed under a helium flow (30 mL·min-1) with a heating
rate of 20 °C·min-1 up to 500 °C and kept at this
temperature for 60 min. After cooling to 100 °C, an
ammonia flow of 20 ml·min-1 was passed through the
sample for 60 min. Once the physisorbed ammonia was
removed by flowing helium at 100 °C for 60 min, the
chemisorbed ammonia was determined by increasing the
temperature with a heating rate of 10 °C·min-1 up to 600 °C.
The ammonia concentration in the effluent helium stream
was measured with a thermal conductivity detector (TCD).
Thermogravimetric analysis was tested by using TA-Q500
TGA system. A coke sample around 15 mg was first
dehydrated at 100 °C and then heated from 100 °C to 800
°C under an oxidative atmosphere (N2 +O2) at a rate of 10
°C·min-1. Solid-state 27Al MAS NMR spectra were recorded
under magic angle spinning (MAS) at ambient temperature
on an Infinity plus 300WB spectrometer equipped with a 4
mm MAS probe.
Hierarchical zeolite preparation
Details of the preparation have been described in our
[12]
previous work.
The typical experimental procedure is as
follows: hierarchical ZSM-5 precursor solutions were
prepared according to the following molar composition: 60
SiO2: x Al2O3: 16.8 TPAOH: 1500 H2O (x = 0.75, 0.5, 0.375,
0.3, and 0.25). The obtained solutions were pre-crystallized
under reflux and stirring at 90 °C for 12 h and then aged at
ambient temperature for 12 h. APTES was added then with
a fixed proportion (n (APTES): n (SiO2) = 0.05) and the
silanized solution was kept stirring at 90 °C for another 8 h.
The resulting solution was transferred into stainless steel
autoclaves for hydrothermal process at 170 °C for 120 h.
The products were washed with deionized water for several
times and obtained by centrifugation, dried overnight at 80
°C and calcined at 600 °C for 5h. The samples synthesized
with the Si/Al ratio of 40, 60, 80, 100 and 120 were named
as HZ-n (n=1, 2, 3, 4 and 5).
The number of external acid sites in hierarchical zeolites
was determined by the chemisorption of 2, 4, 6-collidine
(Coll) using a high-accuracy intelligent gravimetric analyser
(Hidden IGA, sensitivity = 0.001 mg). Basic organic amines
can be adsorbed at the acidic Al sites in zeolite.
Considering the molecular size, Coll is clearly too bulky
(0.74 nm diameter) to enter the microporous structure of
ZSM-5 zeolite. Therefore, it is reasonable to assume that no
more than one Coll molecule can be adsorbed per Al site at
the external surface and the mesopore walls. Based on the
assumption, the adsorption of Coll over hierarchical ZSM-5
zeolites can be ascribed to the acid site density at the
external environment. The experimental procedure is briefly
described here. Prior to the adsorption of Coll, the zeolite
sample was degassed overnight under a vacuum at 350 °C.
An excess amount of Coll was injected and vaporized in the
chamber at 200 °C. After being cooling to the room
temperature, the sample was exposed to the Coll until
saturation and was subsequently evacuated at 200 °C for 6
h to remove any weakly adsorbed Coll molecules. The
weight changes before and after adsorption were recorded
Selective dealumination process
The hierarchical zeolites with different Si/Al ratios were
dealuminated by the procedure using tartaric acid. Tartaric
acid has a kinetic molecular diameter of 0.68 nm, which
causes
a diffusion limitation for dealumination inside
micropores (the average pore size is 0.54 nm). For the
tartaric acid treatment, 1 g of sample was added to 20 mL
of 1 M of tartaric acid solution. The slurry was stirred at 60
°C for 4 h. After dealumination, the zeolites were filtered,
washed with deionized water, and dried at 100 °C for
overnight. The samples were denoted as HZ-n-TA (n=1, 2,
3, 4 and 5). Here, TA refers to “tartaric acid”.
Characterization of the samples
The powder X-ray diffraction (XRD) patterns were
obtained by a Shimadzu XRD-6000 diffractometer (Cu Kα
radiation, λ = 0.154 nm). Measurement was achieved for 2
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10.1002/cctc.201800086
ChemCatChem
FULL PAPER
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bed reactor (9 mm i.d.) over 100 mg catalysts at
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Keywords
:
catalytic cracking
•
external acid site
zeolites
•
hierarchical pores
•
heterogeneous catalysis
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
The hierarchical ZSM-5 zeolite without
external acid sites can restrain the
bimolecular reaction of ethylene and
propylene and thus lead to the
Xiaoxiao Zhang, Dang-guo Cheng,
Fengqiu Chen and Xiaoli Zhan [a]
*
Page No1. – Page No.9
enhancement
propylene selectivity.
of
ethylene
and
The role of external acidity of
hierarchical ZSM-5 zeolites in n-
heptane catalytic cracking
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