Surface-Passivated Hierarchically Structured ZSM5 Zeolites: High-Performance Shape-Selective Catalysts for para-Xylene Production

Article abstract of DOI:10.1002/cctc.201800044

Hierarchically structured ZSM-5 zeolites (HSZ) were synthesized and used as high-performance catalysts for para-xylene (p-X) production by tuning their pore structures and external surface acidity, which was achieved by two independent passivation approaches: dealumination in oxalic acid solution and chemical liquid deposition of tetra-ethoxysilane (CLD of TEOS). The mesoporous structures of HSZ were well-preserved after dealumination or CLD of TEOS, and the external surfaces of HSZ were passivated significantly. Compared to dealumination, CLD is more efficient because not only the external surface could be passivated, but also the pore-openings of HSZ had been effectively narrowed, which both favored the enhancement of product p-X selectivity in ortho-xylene (o-X) isomerization. The constraint index (CI) of as-synthesized catalysts derived from the competitive reactions of ethylbenzene (EB) dealkylation and meta-xylene (m-X) isomerization and gravimetric adsorption measurements were introduced to investigate the extent of pore-narrowing by CLD of TEOS. Benefitting from the auxiliary mesopores and surface-passivation treatments, the optimized catalyst HSZ(Si0.5) showed remarkably enhanced catalytic activity over the microporous ZSM-5 counterpart in the model reaction of o-X isomerization.

Full text of DOI:10.1002/cctc.201800044

Accepted Article
Title: Surface--passivated hierarchically structured ZSM--5 zeolites:
highperformance shape-selective catalysts for para-xylene
production
Authors: Jian Lv, Zile Hua, Jian Zhou, Zhicheng Liu, Hangle Guo, and
Jianlin Shi
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To be cited as: ChemCatChem 10.1002/cctc.201800044
Link to VoR: http://dx.doi.org/10.1002/cctc.201800044
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10.1002/cctc.201800044
ChemCatChem
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Surface•passivated hierarchically structured ZSM•5 zeolites:
high•performance shape•selective catalysts for para•xylene
production
Jian Lv,[a,b] Zile Hua,*[a] Jian Zhou,[c] Zhicheng Liu[c], Hangle Guo[a,b] and Jianlin Shi*[a]
Abstract: Hierarchically structured ZSM-5 zeolites (HSZ) were
synthesized and used as high-performance catalysts for para-xylene
(p-X) production by tuning their pore structures and external surface
acidity, which was achieved by two independent passivation
approaches: dealumination in oxalic acid solution and chemical liquid
deposition of tetra-ethoxysilane (CLD of TEOS). Various materials
characterization techniques showed that the mesoporous structures
of HSZ were well preserved after dealumination or CLD of TEOS, and
more importantly, the external surfaces of HSZ were passivated
significantly. Compared to dealumination, CLD seems to be more
efficient because not only the external surface could be passivated,
but also the pore-openings of HSZ had been effectively narrowed,
which both favored the enhancement of product p-X selectivity in
ortho-xylene (o-X) isomerization. Constraint index (CI) of as-
synthesized catalysts derived from the competitive reactions of
ethylbenzene (EB) dealkylation and meta-xylene (m-X) isomerization
and gravimetric adsorption measurements were introduced to
investigate the extent of pore-narrowing by CLD of TEOS. Benefitting
from the auxiliary mesopores and surface-passivation treatments, the
optimized catalyst HSZ(Si0.5) showed remarkably enhanced catalytic
activity of ~42% and p-X selectivity of ~50%, simultaneously, in sharp
comparison to ~13% and ~45%, respectively, over the microporous
ZSM-5 counterpart in the model reaction of o-X isomerization.
over-production. Because these three isomers have similar
boiling points and molecular size, the product purification is rather
difficult and energy intensive [2]. Thus, selective production of p-X
through disproportionation, alkylation and isomerization are of
[1b, 2b, 3]
great significance and gaining more and more attentions
.
To increase selectivity towards para-isomer, catalysts with
excellent shape-selectivity are indispensable. In this respect,
ZSM-5 zeolites are well known and promising candidates
because of its 10-membered ring (MR) pore system with MFI
topography, the size of which is comparable to p-X molecules [4]
.
On the other hand, the sole presence of microporous structure in
purely ZSM-5 zeolites often leads to mass transfer limitation and
the internal acid sites of ZSM-5 cannot be utilized adequately. As
a result, its catalytic activity in bulky aromatic transformations is
far from being satisfactory. Besides, the micropores of ZSM-5 will
easily be blocked by coking, which could cause rapid catalyst
deactivation [5]. New catalysts with well-engineered pore structure
and dimension and excellent activity and high p-X selectivity are
highly desired.
It has been reported that introducing additional mesoporous
structures in microporous zeolites, which is known as
hierarchically structured zeolites (HSZ), will lead to much
facilitated mass transfer therein and enhanced anti-deactivation
ability [5, 6]. But in the meantime the newly introduced mesoporous
structure brings more external surface acid sites to the catalyst.
The external surface acid sites lack shape-selective control as
compared to acid sites located in the internal microporous
structure of ZSM-5 zeolites [7c], which is responsible for the
undesired side reactions and decrease the product p-X selectivity
Introduction
Para-Xylene (p-X) is a valuable aromatic compounds used
as the raw materials for the productions of terephthalate and
polyester [1]. In petrochemical industries, the aromatics, such as
xylene, are usually produced by catalytic reforming and naphtha
pyrolysis. In these process, thermodynamic equilibrium mixtures
of xylene isomers (o-/m-/p-) along with other aromatics are
produced, which means that the value-added target product p-X
will be obtained at the high cost of other less valuable isomers’
[3, 7]. So HSZ always show poorer para-selectivity as compared to
[8]
microporous zeolites in spite of their higher catalytic activity
.
The above characters make HSZ not suitable for functioning as
shape-selective catalysts. Thus, to enhance the para-selectivity
of HSZ while keeping its mesostructure and high catalytic activity,
one of the best way is to selectively passivate or de-activate its
external surface acid sites.
According to previous reports, the passivation of external
surface of ZSM-5 zeolites has been adopted by various post-
treatments, such as impregnating with B, Mg and P elements [2b,
9], coating of inert silicalite-1 layers [10a], chemical liquid deposition
[a]
Dr. J. Lv, Prof. Dr. Z. Hua, MS. H. Guo and Prof. Dr. J. Shi.
State Key Laboratory of High Performance Ceramics and Superfine
Microstructure
[2b,
3,
7a,
11]
(CLD) of tetra-ethoxysilane (TEOS)
and
mechanochemical approaches [7b]. However, in most cases, these
passivation treatments always lead to the decrease of zeolite
catalyst activity because of the pore blockage and/or loss of acid
sites. For example, Li et al. modified ZSM-5 zeolites with P
element and found that, in the reaction of toluene
disproportionation, the conversion of toluene decreased from
56.3% to 22.6% with the increase of P amount from 0.94 wt% to
4.86 wt%, while the product p-X selectivity increased from 23.4%
to 84.1% [9c]. By CLD of TEOS, the resultant ZSM-5 materials
Shanghai Institute of Ceramics, Chinese Academy of Sciences 1295
Dingxi Road, Shanghai 200050, PR China
E-mail: jlshi@mail.sic.ac.cn; huazl@mail,sic.ac.cn
Dr. J. Lv and MS. H. Guo
University of Chinese Academy of Sciences
19 Yuquan Road, Beijing 100049, P. R. China.
Dr. J. Zhou, Dr Z. Liu
Sinopec Shanghai Research Institute of Petrochemical Technology
1658 Pudong North Road, Shanghai 201208, P. R. China.
[b]
[c]
Supporting information for this article is given via a link at the end of
the document
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10.1002/cctc.201800044
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showed the decreased acid amount. Accordingly, in the followed
reaction of ethylbenzene (EB) disproportionation, the conversion
of EB decreased by ~10% [10b]. Therefore, it holds much promise
if we can combine the superior catalytic activity of HSZ and high
para-selectivity of surface-passivated zeolites in the reaction of o-
X isomerization.
Here in this study, a strategy of passivating the external
surface of as-synthesized HSZ for enhanced activity and high
selectivity towards p-X production is reported. Firstly, HSZ was
synthesized by a mesoporogen-free synthetic route previously
reported by our group [12a]. Then the external surface acidity of
HSZ was fine-tuned by dealumination in oxalic acid solution or
CLD of TEOS. Compared to the typical micropore size (~0.55 nm)
of ZSM-5 zeolite, TEOS molecules (1.03nm) and trioxalato
aluminium complexes (0.64nm) [12b] are too large to freely diffuse
in the micropore systems and as a result, the external surface of
HSZ could be selectively passivated. The obtained samples were
characterized by TEM, SEM, N2 absorption/desorption, NH3-TPD
and catalytic cracking of 1,3,5-triisopropylbenzene (TIPB) to
illustrate the changes of its external acidity and pore structures
after passivation. In addition, constraint index (CI) about the
competitive reaction of EB dealkylation and meta-xylene (m-X)
isomerization was introduced to evaluate the extent of pore
narrowing caused by dealumination or CLD of TEOS. The impact
of surface passivation treatments on catalysts’ activity and para-
selectivity were investigated in the model reaction of ortho-xylene
(o-X) isomerization.
Fig.1 (a) XRD patterns and (b) N2 adsorption/desorption isotherms of all
as-synthesized materials.
m2g-1, 160 m2g-1 and 0.22 cm3g-1, respectively. In contrast, the
isotherms of HSZ series materials are the type-IV profile with a
hysteresis loop and an apparent uptake at elevated relative
pressures, indicating the presence of abundant mesoporous
structures. The most probable apertures of HSZ series are about
17 nm. Although the N2 sorption isotherms and pore size
distribution curves (Fig. S1) of surface-passivated HSZ materials
keep the original characteristics of parent HSZ, the lower SBET and
Sext values are obtained in comparison with those of parent HSZ.
In this regard, the decrease with SiO2-deposited HSZ is more
significant resulting from the partial blockage of micro-
/mesoporous channels, whereas the impact on sample HSZ(De-
Al) is insignificant. Specifically, the SBET, Sext and Vtotal values of
Results and Discussion
Fig. 1a is the XRD patterns of synthesized HSZ series
materials and microporous ZSM-5 zeolites in the 2 theta range of
5 ~ 50o. All of them demonstrate the typical and similar diffraction
patterns of MFI-type zeolite irrespective of post-synthesis
dealumination or CLD of TEOS treatment, indicating their high
[2a, 5,
crystallinity and high stability
12a]. Fig. 1b shows their N2
sorption isotherms and the textural properties are summarized in
Table 1. Among them, sample ZSM-5 exhibits the typical type-I
profile of microporous materials due to the absence of
mesoporosity. Its corresponding apparent surface areas (SBET),
external surface area (Sext) and total pore volume (Vtotal) are 370
Table 1. The physicochemical properties of as-synthesized materials.
Acid sites (mmol/g)
Weak acid Strong acid
0.316 0.137
SBET
Sext
Vtotal
Vmeso
Vmicro
Entry
Samples
Si/Al
(m2g-1)
(m2g-1)
(cm3g-1)
(cm3g-1)
(cm3g-1)
1
2
HSZ
411
386
272
257
0.42
0.41
0.35
0.34
0.07
0.07
47
80
HSZ(De-Al)
0.183
0.114
3
4
5
6
HSZ(Si0.5)
HSZ(Si1.0)
HSZ(Si4.0)
ZSM-5
343
341
301
370
217
212
161
160
0.39
0.38
0.37
0.22
0.33
0.32
0.30
0.12
0.06
0.06
0.06
0.10
47
48
49
44
0.303
0.297
0.273
0.279
0.129
0.124
0.083
0.156
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HSZ(De-Al) and HSZ(Si0.5) are 386 and 343 m2g-1, 257 and 217
m2g-1, 0.41 and 0.39 cm3g-1, respectively. On the other hand, Fig.
2a-b and Fig. S3 are the SEM images of all materials. Apparently,
regardless of dealumination or CLD treatment, the materials
morphology remains unchanged. As shown in Fig. 2a-b and Fig.
S3, all materials consist of spherical particles of ~1 μm in diameter
in which abundant mesopores of 15-20 nm in size can be found
under their rough surfaces consistent with above N2 sorption
results. In TEM images of Fig. 2c &2d, the oriented lattice fringes
on the edge of particles and the discrete and bright SAED patterns
confirm the single-crystalline nature of synthesized HSZ series. In
contrast, microporous ZSM-5 shows typical coffin shape with
smooth surfaces (Fig. S3d).
species has been extracted (Table. 1), the decreases of high-
temperature band intensity and strong acid amount appear more
significant. Interestingly, because of the oxalic acid dealumination,
the broadened high-temperature desorption band of parent HSZ
become sharpened and the resultant profile of HSZ(De-Al) gets
similar to that of microporous ZSM-5, which confirms the
diminishing of medium strength acid sites and achievement of
selective dealumination on the HSZ external surfaces. In addition,
27Al MAS NMR analyses have also been conducted to investigate
the chemical states of Al species of synthesized HSZ series and
microporous ZSM-5. Similar to the typical 27Al MAS NMR profile
of microporous ZSM-5 (Fig. 3b), a major peak can be observed at
~53 ppm for HSZ series, assigned to the tetrahedrally coordinated
framework Al with strong Brønsted acidity [13], which means that
the chemical state of Al atoms is not obviously changed by the
surface-passivation treatments.
Fig 2. SEM images of (a) HSZ(Si0.5) and (b) HSZ(De-Al); High-resolution
TEM image and selected area electron diffraction pattern (inset) of (c)
HSZ(Si0.5) and (d) HSZ(De-Al).
Table 1 lists the Si/Al ratios of synthesized HSZ series and
microporous ZSM-5. Resulting from the successful acid
dealumination, sample HSZ(De-Al) possesses the highest Si/Al
ratio of 80, while the other materials keep the ratios between 47
and 49. Accordingly, Fig. 3a gives the NH3-TPD profiles of HSZ
series and microporous ZSM-5 and the calculated acid amounts
are listed in Table 1. All of HSZ series feature two desorption
bands similar to that of microporous ZSM-5, in which the high
temperature and low temperature band correspond to the strong
and the weak acid sites respectively [9b, 9c, 12a]. Because the strong
acidity of ZSM-5 zeolites originates from the isomorphous
substitution of framework Si by Al, the change of Al content and/or
Al coordination state would induce the variation of acid amount
and/or acid strength. For parent HSZ, the broadened high-
temperature bands reflect the presence of a large amount of
strong acid sites of relatively weaker acid strength resulting from
the disordered framework Al sites on the external surface of HSZ.
Previous reports also reported similar results, indicating that the
acid sites on external surface show different acidity from those
inside the microporous structures [18]. Accompanied by the
increase of SiO2-deposition amount, the gradual decrease of
high-temperature signal intensity and strong acid amount
confirms the successful passivation of external acid sites on HSZ.
On the other hand, for sample HSZ(De-Al), since about 40% of Al
Fig 3. (a) NH3-TPD profiles and (b) 27Al MAS NMR spectra of as-
synthesized materials.
Further, to demonstrate the variation of surface acidity of
HSZ materials before and after surface modification, a model
reaction of TIPB cracking has been carried out. The kinetic
diameter of TIPB molecule is about 0.95 nm, which is larger than
the micropore size of ZSM-5 zeolites of 0.55 nm. Therefore, the
TIPB cracking can only occur at the external surface of catalyst
[7b,
particles or at the micropore mouths
14]. As a result,
microporous ZSM-5 only offers the initial TIPB conversion of
~70%, as shown in Table 2. In contrast, parent HSZ exhibit a
higher initial conversion of 90% due to its abundant mesoporous
structures. Because of the selective dealumination of the external
surface area, the initial conversion with sample HSZ(De-Al)
decreases to 45% which is even much lower than that of
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microporous ZSM-5 counterparts. Comparatively, the initial
conversion of TIPB by SiO2-deposited HSZ gradually decreases
from 90% to 76%, 64% and 24% as the amount of deposited SiO2
is increased from 0.5 wt% to 1 wt% and 4 wt%, indicating that the
successful external surface passivation of HSZ by CLD of TEOS.
The above results demonstrate that all post-synthesis treatments
adopted in this research have achieved selective passivation on
the external surface of HSZ as expected.
HSZ(De-Al) is ~39%, which is three times that of microporous
ZSM-5, while both of them shows the approximately the same p-
X selectivity of ~45%. Considering the variation of materials
textural properties before and after dealumination treatment is
insignificant, as shown in Fig. 1a and Table 1, the enhanced para-
selectivity for HSZ(De-Al) could be mainly attributed to the
selective passivation of external surface active centers which
suppresses the possible non-selective isomer interconversion, in
consistence with the above NH3-TPD and TIPB cracking results
(Fig. 3a and Table 2). In comparison, the effect of SiO2 deposition
on HSZ on the o-X to p-X isomerization is more pronounced than
dealumination. As shown in Fig. 4b, the increase of SiO2-
deposition amount from 0.5 wt% to 4.0 wt% leads to the marked
increase of p-X selectivity from ~35% to ~62%, while the o-X
conversion decreases from ~56% to ~8%. Although the positive
effect of selective passivation on the external surface acid sites is
dominant as happened on sample HSZ(De-Al), the other critical
factor, i.e., pore-narrowing effect, should be considered in
enhancing the p-X selectivity over SiO2-passivated HSZ as
compared to that of microporous ZSM-5 because SiO2 deposition
could partially block the pore-openings and enlarge the diffusivity
difference of xylene isomers [15a-b]
.
In order to characterize the pore size variation and/or Al
distribution in zeolite catalysts, Constraint Index (CI) has been
designed basing on the competitive reactions of two reactants
with differentiated molecular size [15]. Among them, Bhat et al had
ever studied the pore size reduction of zeolites caused by silica
deposition through the competitive reaction between EB
dealkylation and m-X isomerization and the CI was defined as
Log(1-XEB)/Log(1-Xm-X) in which XEB and Xm represent the
-X
conversions of EB and m-X, respectively [15a]. Higher CI means
more significant pore-narrowing. Herein, the competitive reaction
between EB dealkylation and m-X isomerization was adopted to
clarify the pore-narrowing effect and the subtle variation of pore-
openings of surface-passivated HSZ. As shown in Table 2, the
conversions of EB and m-X over microporous ZSM-5 are 25% and
16%, respectively, and the calculated CI is 1.65, which is close to
previous results reported by Bhat et al [15a]. For parent HSZ, due
to the introduction of additional mesoporous structures and
consequently the decreased diffusion limitation of the relative
bulky m-X molecules, higher conversion of m-X of 30% has been
obtained as expected, while the conversion of EB decreased to
16% resulting from the competitive adsorption of m-X molecules
and the much higher activation energy of EB dealkylation (~186
KJ/mol) as compared to that of m-X isomerization (47~54 KJ/mol)
[16]. Consequently, the corresponding CI of HSZ decreases to 0.49.
In comparison, because of the reduction of active acid site
amounts by the dealumination treatment, the m-X and EB
conversions over sample HSZ(De-Al) consistently decrease to
25% and 13%, respectively. But interestingly, the calculated CI is
0.48, almost the same as that of parent HSZ, which reflects the
maintenance of pore structures of HSZ through the post-synthesis
dealumination treatment. In addition, although the conversion of
m-X over SiO2-deposited HSZ series decrease gradually from
30% of parent HSZ to 7% with the increase of SiO2-deposition
amount up to 4.0 wt%, the variation of EB conversion is non-
unidirectional, which first decreases to 13%, then increases to
Fig 4. (a) Conversion of o-X (b) Selectivity of p-X over as-synthesized
materials in the model reaction of o-X isomerization (HSZ ■, HSZ(De-Al)
□ , HSZ(Si0.5) ▲, HSZ(Si1.0) △, HSZ(Si4.0) ●, ZSM-5 ○).
Fig. 4 shows the catalytic performance of HSZ series and
microporous ZSM-5 in the isomerization of o-X to p-X. In general,
besides the xylene isomers (o-/m-/p-X), there is no other
hydrocarbons detected in the effluent. And during the testing
period of 24 h, the deactivation of all catalysts is insignificant. As
previously reported [8a], benefitting from the auxiliary mesoporous
structures, parent HSZ exhibits an initial conversion of o-X as high
as 56%, which is four times that of microporous ZSM-5
counterparts (~13%). However, suffering from its larger external
surface area and consequently more external acid centers, the
product selectivity of p-X with parent HSZ is as low as ~35%,
which is significantly lower than 46% over ZSM-5. This means that
the undesired external surface acid sites of HSZ which lacks
shape-selective control promote side reactions, leading to the
lowered product p-X selectivity. Fortunately, when HSZ(De-Al) or
SiO2-passivated HSZ materials (except sample HSZ(Si4.0)) were
used, superior catalytic performances, not only the high activity
but also the enhanced or comparable p-X selectivity, were
achieved. Specifically, the conversion of o-X over sample
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25% and finally decreases again to 23%, demonstrating the
complicated effects of SiO2-passivation on the materials acidity
and textural properties. Specifically, for the relative bulky m-X
molecules, both the decrease of surface acid site amounts and
the pore narrowing effect by SiO2 deposition contribute to the
decrease of m-X conversion. However, in the EB conversion,
although the decrease of external active acid site amounts in
SiO2-passivated HSZ would result in lowered catalyst activity, its
small molecule size allows EB molecules to diffuse much more
freely in the micropore channels than bulky m-X molecules, and
consequently resulting in its higher concentration in micropore
channels in comparison with m-X molecules, especially in the
pore-narrowed HSZ series. In addition, the amorphous deposited
SiO2 coating on zeolite surfaces might enhance the adsorption
possibility of organic reagents from gas phase to the catalyst
surface and then into the micropores, which may also favor the
diffusion of EB molecules within the micropore channels due to its
relative small size [17a, b]. As a result, when HSZ(Si0.5) was used,
the EB conversion decreased slightly from 16% to 13% (by ~18%),
whereas the conversion of m-X decreased much more
significantly from 30% to 18% (by ~40%). Moreover, although the
m-X conversion over HSZ(Si1.0) catalyst with higher amount of
SiO2 passivation continues to decrease to 14%, the EB
conversion in contrast increases to 25% over the same catalyst.
Correspondingly, the calculated CI are 0.70, 1.90 and 3.60 (Fig.
S4), respectively, for sample HSZ(Si0.5), HSZ(Si1.0) and
HSZ(Si4.0), which confirms the continuous pore-narrowing effect
by SiO2-deposition in HSZ.
Fig 5. Normalized amount of adsorption profiles of (a) m-X and (b) EB
over HSZ and passivated samples under 0.5 mbar at 25 ℃ in a short time
domain.
Table. 2 Results of TIPB cracking and competitive reactions between EB
and m-X over as-synthesized materials and ZSM-5.
EB and m-X competition reaction
with above N2 sorption and CI results. However, for SiO2-
TIPB
deposited
HSZ(Si0.5) and HSZ(Si4.0), the diffusion time
Samples
Conversion
(%)
EB
Conversion
(%)
m-X
Conversion
(%)
constants decreased from 7.04×10-5 s-1 to 5.24×10-5 s-1 and 1.66
×10-5 s-1, respectively, which strongly suggest that the diffusion of
m-X in modified HSZ is significantly retarded resulting from the
CLD-of-TEOS-induced pore-narrowing. On the other hand, when
the EB molecules with smaller kinetic diameter as compared to
m-X were adopted as sorbates, the diffusion time constants of EB
in HSZ was 1.54×10-4 s-1, significantly higher than the diffusion
time constants of m-X. After CLD of TEOS, the diffusion time
constants of EB in HSZ(0.5) (1.56×10-4 s-1) and HSZ(4.0) (1.70×
10-4 s-1) were nearly unchanged as compared to HSZ (1.54×10-4
s-1), which means although the pore-openings of HSZ were
narrowed by CLD, EB molecule still can diffuse into the
micropores easily due to its small molecule size. Accordingly, the
conversion of EB in the above competitive reactions would
decrease at a slower rate or even was promoted after CLD of
TEOS because the diffusion of m-X was retarded by pore-
narrowing while the diffusion of EB molecules were nearly
unaffected. Indeed, we can observe that the EB conversion
decreased slightly from 16% of HSZ to 13% of HSZ(Si0.5), and
then increased to 25% of HSZ(1.0). Similarly, in o-X isomerization,
the pore-narrowing of HSZ enlarges the diffusivity differences
among xylene isomers, which selectively retains bulkier o-X and
m-X molecules in the micropores, facilitates their further
CI
HSZ
90
45
16
13
30
25
0.49
0.48
HSZ(De-Al)
HSZ(Si0.5)
HSZ(Si1.0)
76
64
13
25
18
14
0.70
1.90
HSZ(Si4.0)
ZSM-5
24
70
23
25
7
3.60
1.65
16
To further verify the pore-narrowing effect caused by post-
synthesis dealumination and CLD of TEOS treatment, gravimetric
adsorption measurements was conducted using m-X and EB as
probe molecules. The normalized amount of m-X and EB uptake
was shown in Fig. 5. It can be observed that there was a good
linear relationship between the normalized uptake amount (Mt/M
∞) and t1/2, which is consistent with previous reports [17a, c]. The
diffusion time constants (D/r2) can be derived from slopes (6·π-
1/2·(D/r2)1/2) of fitting lines, as shown in Fig. 5 and Table. 3. For
bulky m-X molecules, parent HSZ and sample De-Al got the
approximate diffusion time constants of 7.04×10-5 s-1 and 7.24×
10-5 s-1, indicating that dealumination does not lead to the
significant change on HSZ pore structures, which is consistent
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10.1002/cctc.201800044
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crushed and transferred into three crucibles. We put each crucible
into a Teflon-lined stainless autoclave. Subsequently, 0.7 g
deionized water was added into the bottom of Teflon liner, but
outside the crucible container. The autoclave was heated in an
oven at 150 ℃ for 10 hours. The final product was washed by
deionized water for 3 times and dried in an oven at 80 ℃ overnight.
Then, the organic structure-directing-agents was removed by
calcination at 550 ℃ for 6 h.
transformation to p-X and results in the enhanced para-selectivity
in the products.
Table. 3 Diffusion time constants of m-X and EB in parent and surface
passivated HSZ.
Diffusion
time
Samples
constant
HSZ
De-Al
HSZ(Si0.5)
HSZ(Si4.0)
(s-1)
m-X
7.04×10-5
1.54×10-4
7.24×10-5
1.60×10-4
5.24×10-5
1.56×10-4
1.66×10-5
1.70×10-4
Ion exchange of HSZ: 1.0 g synthesized HSZ was added into
100 ml NH4Cl solution (1 M). and stirred at 80 ℃ for 4h. This
procedure was repeated 3 times. Then, ion-exchanged samples
were dried and calcined at 550 ℃ for 4 h to make the catalysts to
be H-type.
EB
Conclusions
Dealumination of HSZ: 1.0 g HSZ (H-type) and 20 ml oxalic
acid solution (2M) were add into three-necked flask. It was stirred
at 500 rpm under reflux for 10 h. Then the sample was filtered and
washed by deionized water. The as-collected powder was dried
at 120 ℃ for 6 h. The above procedure was repeated for 3 times.
At last, the sample was calcined at 550 ℃ for 4 h. The as-
synthesized catalyst was denoted as HSZ(De-Al).
In summary, by tailoring the external surface acidity and pore
structures of HSZ via dealumination in oxalic acid solution or CLD
of TEOS, we synthesized surface-passivated HSZ which
performs well in the reaction of o-X isomerization. As compared
to dealumination, CLD approach seems to be more efficient in
enhancing the shape-selectivity of HSZ because not only the
external surface of HSZ has been passivated, but also its pore-
openings have been effectively narrowed as demonstrated by EB
and m-X competitive reactions and gravimetric adsorption
measurements, both of which are beneficial to enhancing the p-X
selectivity in the model reaction of o-X to p-X isomerization. Over
the optimally tailored HSZ sample, HSZ(Si0.5), both the relatively
high catalytic activity (~42% conversion) and enhanced shape-
selectivity (~50% p-X selectivity) have been simultaneously
achieved, which are remarkably higher than the conversion
(~13%) and p-X selectivity (~45%) of microporous ZSM-5
counterparts.
Chemical liquid deposition of tetra-ethoxysilane (CLD of
TEOS): 1.0 g HSZ (H-type) and 25 ml n-hexane were added into
a three-necked flask. It was stirred at 500 rpm under reflux. Then
specific amount of TEOS (equal to 0.5 wt%, 1 wt% and 4 wt%
SiO2) was added. The reaction was kept for 1 h. Subsequently the
n-hexane was removed by evaporation and the resultant
materials was calcined at 550 ℃ for 4 h. The as-synthesized
catalysts were denoted as HSZ(Si0.5), HSZ(Si1.0) and
HSZ(Si4.0), respectively.
Characterization
The X-ray diffraction patterns (XRD) of synthesized materials
were collected on a Rigaku D / Max 2200PC diffractometer
equipped with Cu Kα radiation source (40 kV and 40 mA). The
scanning rate and the angular step size were 5° min-1 and 0.02°
respectively. The TEM images of synthesized materials were
measured on a JEOL-2010F microscope at 200kV. SEM images
Experimental Section
Chemicals
were conducted on a FEI-Magellan 400L. Si/Al ratios of
synthesized materials were determined by the Agilent
Technologies 725 inductively coupled plasma optical emission
Tetra-ethoxysilane (TEOS, AR grade) was purchased from
Shanghai Lingfeng Chemical Co. Aluminum isopropoxide
(Al(iPrO)3, AR grade) was purchased from Adamas reagent Co.
Ltd. Tetrapropylammonium hydroxide (TPAOH, 25 wt% in water)
was obtained from Yixing Dahua Chemical Co. Ammonia chloride
(NH4Cl, AR grade), n-hexane (AR grade) and oxalic acid (AR
grade) were purchased from Sinopharm Chemical Reagent Co.
O-xylene, p-xylene, m-xylene, 1,3,5-triisopropylbenzene and
diethylbenzene was got from Sigma-Aldrich reagent Co. ZSM-5
zeolites (Si/Al = 50) were purchased from Nankai University
Catalyst Factory. All reagents were used directly without any
purification.
spectrometry
(ICP-OES).
The
ammonia
temperature-
programmed desorption (NH3-TPD) profiles were measured on a
Micromeritics Chemisorb 2750. N2 absorption/desorption
isotherms were conducted on Micromeritics ASAP 3000 porosity
analyzer at 77K. The specific surface areas were calculated by
the Brunauer–Emmett–Teller (BET) method and the pore size
distributions were calculated by the Barrett–Joyner–Halenda
(BJH) method. External surface areas and micropore volume
were calculated by the t-plot method. The 27Al NMR spectrums
were recorded on a Bruker Avance III spectrometer. The
gravimetric adsorption measurements were conducted with an
intelligent gravimetric analyzer (IGA100, Hiden Analytical Ltd.,
Warrington, UK) at 25 ℃ using m-X and EB as sorbates.
Materials preparation
Catalytic reactions
Synthesis of HSZ: According to the previously reported
procedures [12a], 10.41 g TEOS, 0.2042 g Al(iPrO)3 and 18.0 g H2O
was mixed under room temperature for 0.5 h at first. Then, 4.1 g
TPAOH (25 wt% aqueous solution) was added dropwise and then
aged at 40 ℃ for 48 h. Subsequently, the resultant gel was
Isomerization of o-X: this catalytic reaction was conducted on
a continuous flow microreactor equipping a quartz fixed-bed (i.d,
8 mm). 0.2 g catalyst particles (diameter of catalyst particles:
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10.1002/cctc.201800044
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Acknowledgements
This work was sponsored by the National Natural Science
Foundation of China (21776297, U1510107, 21403303) and
National Basic Research Program of China (2013CB933200).
Keywords: Hierarchically structured zeolites • surface
passivation • shape-selectivity
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10.1002/cctc.201800044
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FULL PAPER
Entry for the Table of Contents
FULL PAPER
1. Hierarchically structured ZSM-5
zeolites (HSZ) were synthesized
and passivated by dealumination or
CLD of TEOS.
Jian Lv, Zile Hua,* Jian Zhou,
Zhicheng Liu, Hangle Guo and Jianlin
Shi. *
Page No. – Page No.
2.Surface-passivated HSZ showed
Surface-passivated hierarchically
structured ZSM-5 zeolites: high-
remarkably
enhanced
shape-
selectivity (p-X selectivity) and
improved catalytic activity than the
microporous ZSM-5 counterparts in
the model reaction of o-X
isomerization.
performance
catalysts
shape-selective
for
para-xylene
production
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