Generating Assembled MFI Nanocrystals with Reduced b-Axis through Structure-Directing Agent Exchange Induced Recrystallization

Article abstract of DOI:10.1002/anie.202017031

Controlling crystal size and shape of zeolitic materials is an effective way to promote their mass transport and catalytic properties. Herein, we report a single step, Na⁺- and porogen- free crystallization of MFI hierarchical architecture made up of aligned nanocrystals with reduced b-axis thickness (5–23 nm) and adjustable Si/Al ratios between 35 to 120, employing the commonly used tetrapropylammonium hydroxide (TPAOH) and tetrabutylammonium hydroxide (TBAOH) as structure-directing agents (SDAs). Homogeneous nucleation driven by both SDAs and subsequent SDA-exchange induced dissolution-recrystallization are responsible for the formation of such structure. The enhanced textural and diffusion properties account for a notable exaggeration of propene selectivity and catalyst lifetime in dimethyl ether-to-olefins (DTO) conversion. This protocol is extendable to the rational synthesis of other hierarchical zeolites through crystallization process control.

Full text of DOI:10.1002/anie.202017031

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
www.angewandte.org
Accepted Article
Title: Generating assembled MFI nanocrystals with reduced b-
axis through structure-directing agent exchange induced
recrystallization
Authors: Xiaoling Zhao, Shu Zeng, Xueliang Zhang, Quanzheng Deng,
Xiujie Li, Wenguang Yu, Kake Zhu, Shutao Xu, Jichang Liu,
and Lu Han
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Generating assembled MFI nanocrystals with reduced b-axis
through structure-directing agent exchange induced
recrystallization
Xiaoling Zhao# [a], Shu Zeng# [b,d], Xueliang Zhang[c,e], Quanzheng Deng , Xiujie Li , Wenguang Yu ,
[c]
[b]
[b]
[
a]
[b]
[a]
[c]
Kake Zhu* , Shutao Xu* , Jichang Liu and Lu Han*
[
[
[
a]
b]
c]
X. Zhao, Prof. K. Zhu, Prof. J. Liu
UNILAB, State Key Laboratory of Chemical Engineering
East China University of Science and Technology
Shanghai 200237, P. R. China
E-mail: kakezhu@ecust.edu.cn
S. Zeng, Prof. X. Li, Prof. W. Yu, Prof. S. Xu
National Engineering Laboratory for Methanol to Olefins, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian 116023, P. R. China
E-mail: xushutao@dicp.ac.cn
X. Zhang, Q. Deng, Prof. L. Han
School of Chemical Science and Engineering
Tongji University
Shanghai, 200092, P. R. China
E-mail: luhan@tongji.edu.cn
[
[
d]
e]
S. Zeng
University of Chinese Academy of Sciences, Beijing 100049, China
X. Zhang
School of Chemical and Chemical Engineering
Shanghai Jiao Tong University
Shanghai, 200024, China
#
These authors contributed equally to this work
Supporting Information for this article is given via a link at the end of the document.
Abstract: Controlling crystal size and shape of zeolitic materials is an
ring (10-MR, 5.1×5.5 Å) channels along the a-axis, interconnected
by straight 10-MR channels (5.3×5.6 Å) along the b-axis.[
Miniaturization of ZSM-5 crystals down to nanometer scale and
generation of hierarchically porous structure have been found to
be effective to enhance the transport property, acid site
accessibility and catalytic performances.[4] Especially, as
molecules diffuse more rapidly along the straight channels than
within the sinusoidal channels,[5] for the large pore dimension and
low tortuosity, decrease of zeolitic domain size preferentially along
b-axis is more influential to boost transport and catalytic
properties. For instances, significant enhancement of catalyst
3]
effective way to promote their mass transport and catalytic properties.
+
Herein, we report a single step, Na - and porogen- free crystallization
of MFI hierarchical architecture made up of aligned nanocrystals with
reduced b-axis thickness (5-23 nm) and adjustable Si/Al ratios
between 35 to 120, employing the commonly used
tetrapropylammonium hydroxide (TPAOH) and tetrabutylammonium
hydroxide (TBAOH) as structure-directing agents (SDAs).
Homogeneous nucleation driven by both SDAs and subsequent SDA-
exchange induced dissolution-recrystallization are responsible for the
formation of such structure. The enhanced textural and diffusion
properties account for a notable exaggeration of propene selectivity
and catalyst lifetime in dimethyl ether-to-olefins (DTO) conversion.
This protocol is extendable to the rational synthesis of other
hierarchical zeolites through crystallization process control.
[
6]
lifetime and selectivity in methanol-to-gasoline (MTG) and
methanol-to-olefin (MTO)[ conversions have been demonstrated
when b-axis was reduced to a single unit cell thickness.
7]
Towards this goal, decreasing the b-axis length of MFI
crystals to less than 100 nm have been pursued by using
designed SDAs such as bi-functional SDA that has a head group
with hydrophilic diquaternary ammonium cations,[6] diquaternary
ammonium cations,[8] tetrabutylphosphonium (TBPOH) induced
Introduction
repetitive branching (producing intergrown MFI/MEL house-of-
Zeolites are inorganic solids comprising of crystalline
framework and penetrating micropores whose size is close to that
of molecules,[ showing broad applications as commercial
adsorbents, ion-exchangers or catalysts.[2] As solid acid catalyst,
their catalytic properties are not only determined by the acid
properties or micropore size and geometry, but also by the
morphology-dependent diffusion properties. ZSM-5 (MFI, zeolite
framework code according to International Zeolite Association) is
among the most widely used zeolites in industrial applications, in
processes such as propene-selective catalytic cracking,
methanol-to-hydrocarbon (MTH) conversion, alkylation of
aromatics, isomerization, etc. MFI contains sinusoidal 10-member
[9]
[10]
cards), multi-quaternary ammonium, by taking advantage of
the mismatch of these SDAs with the MFI framework that impedes
crystal growth along b-axis. However, these methods are
dependent on the use of designed, unconventional SDAs that are
economically prohibitive for practical use. It is also noteworthy that
the preparation of unit cell thickness zeolite often interrupts the
inherent crystalline structure of zeolites, and compromises their
1]
(
hydro)thermal stability and acidity. The obtained zeolites are
+
often Na -form, which entails an ion-exchange process to afford
the proton form, hence, producing voluminous waste water. On
the other hand, fabrication of hierarchical zeolite by solely control
of crystallization process in the absence of porogens often lead to
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
+
the formation of MFI crystals with non-mesoporous core and an
epitaxially grown, hierarchically structured shell.[11] The
anisotropic growth of hierarchical ZSM-5 crystal with simple,
commonly used starting materials through regulation of
nucleation and crystal growth remains elusive, in particular, for
the generation of MFI crystals with shortened b-axis thickness.
intersections as that of TPA . Therefore, a precise control of
morphology could be achieved by tailoring nucleation and growth
conditions in the co-presence of both SDAs.
+
+
The influence of TPA /TBA ratio was first inspected for
samples with Si/Al ratios of 50 and 100, respectively, with the
reaction composition of 100 SiO
2
: 0.5 (or 1.0) Al
2 3
O : (36-x)
+
Herein, we report a Na -free crystallization of hierarchical
TBAOH : x TPAOH : 400 EtOH : 1140 H
2
O (x = 0, 0.5, 1, 3, 4 and
MFI crystals, comprising of b-axis reduced domains throughout
the entire architecture, in the presence of commonly available
6). The corresponding X-ray diffraction (XRD) analyses (Figure
S1, Supporting Information, SI) showed that the MFI phase could
only be obtained when TPA /TBA ratio was above 2:34, and
below this value MEL was synthesized instead. The
2
corresponding SEM micrographs and N physisorption data
+
+
tetrapropylammonium
tetrabutylammonium hydroxide (TBAOH) through homogeneous
nucleation and following-up SDA exchange induced
hydroxide
(TPAOH)
and
a
reconstruction pathway. Such a synthesis avoids the use of
expensive porogens and the ion-exchange process. Mesopores
between zeolite domains has been constructed in a single
crystallization step.
(Figure S2-S3 and Table S1, SI) demonstrated that the sizes of
these agglomerates decreased with reducing TPAOH content and
+
+
the sample derived from a TPA /TBA ratio of 2:34 was found to
possess the smallest crystal size and highest surface area. In
regard of TPA+ occluded as-synthesized MFI, |((C
H
7
)
4
-NOH)
3
4
|
[
Si96-xAl O192], this ratio ensures that both SDAs participate
x
crystallization despite that TPA is preferred over TBA+ for the
stronger interaction with inorganic framework.[14] The
crystallization temperature was found suitable at 423 K to ensure
proper crystallization time and high crystallinity (Figure S4-S5, SI).
+
Results and Discussion
Synthesis and Characterizations
MFI zeolite often crystallizes as micrometer-sized crystals
+
+
O
n
t
h
e
o
t
h
e
r
h
a
n
d
,
p
r
e
s
e
n
c
e
o
f
N
a
s
h
o
u
l
d
b
e
a
v
o
i
d
e
d
a
s
N
a
+
when using solely tetrapropylammonium cation (TPA ) as SDA,
could compete with other SDAs and is detrimental to the formation
of hierarchical architecture[6] (Figure S6, SI). In addition, extensive
+
while tetrabutylammonium cation (TBA ) alone often leads to the
formation of bulky ZSM-11 (MEL). Theoretical calculations
implied that accommodation of all 4 intersections in one MFI unit
trials further demonstrated that monomeric silica (i. e.,
tetraethylorthosilicate, TEOS or tetramethylorthosilicate, TMOS)
and Al (i. e., Al(i-PrO) , AlCl , Al (SO ) ) precursors were required,
3 3 2 4 3
implying that homogeneous nucleation from clear suspension
was indispensable for the formation of hierarchical structure
cell only by 4 TBA+ molecules would give rise to space
overcrowding and was therefore disfavored.[
12]
Previous co-
+
+
+
templating synthesis using both TPA and TBA (and also Na )
only produces bulky, densely packed intergrown MFI/MEL
crystals.[13] It is postulated that TBA , behaves like that of
TBPOH,[9] is not well accommodated inside MFI channel
(
Figure S7-S8, SI). After narrowing down the above synthetic
+
parameters, a series of ZSM-5-CT-x samples (x stands for the
Si/Al ratio in the starting materials, x = 35-200) were generated.
Figure 1. FE-SEM, TEM micrographs, the corresponding SAED patterns and schematic for ZSM-5-CT-35 (a-e), ZSM-5-CT-50 (f-j) and ZSM-5-CT-100 (k-o).
2
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RESEARCH ARTICLE
MFI phase for these samples was validated by collected
XRD patterns (Figure S9a, SI). No product was recovered with
Si/Al ratio of 25, possibly because of mismatch between low
charge density, bulky SDAs and high charge density, low Si/Al
ratio inorganic components.[15] At higher Si/Al ratio of 150 and
micrograph (Figure 1a) revealed that ZSM-5-CT-35 comprised of
uniform particles with size ranges from 200 to 470 nm (average
ca. 330 nm) in diameter. The enlarged SEM image shows that
each particle consists of small plate-like primary units with a
house-of-cards morphology. From the selected area electron
diffraction (SAED) pattern, the four-fold symmetry can be
observed, indicating the overlap of the a* and b* axes of the MFI
structure from the common c axis with a 90° rotational epitaxial
relationship (Figure 1b). The transmission electron microscopy
(TEM) and high-resolution TEM (HRTEM) images also indicate
the house-of-cards structure. The SAED pattern and TEM image
taken from the common a/b axis of one particle are shown in
Figure S12, SI. The HRTEM image and the corresponding
Fourier diffractogram perpendicular to the primary plate show the
typical contrast and the characteristic diffraction pattern of the b
axis of MFI crystals. Therefore, each primary plate is grown
along the a-c plane and the thickness of the b-axis is greatly
reduced to several pentasil layers (5-14 nm), as indicated in the
schematic representation (Figure 1e). This arrangement
generates abundant amount of mesoporosity, similar to the
report by Tsapatsis et al.[9] The overlapping feature of the small
nanosheets makes it difficult to determine the detailed
connectivity if the MEL structure is presented.
200, the recipe would afford bulky crystals with smooth surfaces
and insignificant amount of mesopores (Figure S9g-j, SI). The
formation of large crystal may result from inconspicuous
+
inhibiting effect of TBA over crystal growth at high Si/Al ratios.
As the inorganic framework becomes less negatively charged as
a result of increased Si/Al ratio, weakened attraction of TBA by
growing crystal surface can be anticipated. Collectively, a typical
crystallization of hierarchical MFI crystals was found to be
feasible between Si/Al ratio of 35 and 120.
The physicochemical properties of three representative
samples, i.e., ZSM-5-CT-35, ZSM-5-CT-50 and ZSM-5-CT-100
were characterized, and comparisons were made with three
control samples denoted as ZSM-5-C-x (with C standing for
conventional synthesis and x representing Si/Al ratio in the
starting materials, x = 35, 50, 100). XRD pattern (Figure S10, SI)
revealed that pure phase MFI zeolite was obtained, while
scanning electron microscopy (SEM) images revealed their
typical coffin-like shape (Figure S11, SI). Elemental analyses
showed that the Si/Al ratio of final products were slightly lower
than that of staring materials (Table 1). The panoramic SEM
+
On the other hand, the average particle diameter of ZSM-5-
CT-50 was increased to ca. 420 nm. The enlarged SEM image
Figure 2. TEM images (ZSM-5-CT-35 (a), ZSM-5-CT-50 (e) and ZSM-5-CT-100 (i)) and corresponding 3D tomography images of ZSM-5-CT-35 (b-d), ZSM-5-CT-
50 (f-h) and ZSM-5-CT-100 (j-l) crystals showing cross-sectional snapshots from Videos S1, S3 and S5, SI, respectively.
3
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RESEARCH ARTICLE
shows that the particle is formed by the ordered stacking of plate-
like primary units (Figure 1f), recommending a non-classic
oriented attachment growth history.[16] The alignment of primary
crystallites is also evidenced in SAED and the corresponding
TEM micrographs (Figure 1g-i). The SAED pattern taken from
the c axis of ZSM-5-CT-50 shows the typical single crystal
feature with nice crystallinity, indicating each particle is basically
a single crystal composed of primary MFI plates. Each primary
MFI plate is grown along the a-c plane and the b-axis is
increased to 8-23 nm. Extra spots with very weak diffraction
intensity can be also observed, as indicated by red arrows in the
SAED pattern, are corresponding to the same MFI structure with
that of the external ones. Between crystalline domains, porous
regions could be clearly visualized.
Notably, the reduction of MFI crystal thickness for these
sample implies a significant enhancement of transport properties
with respect to other shapes. Overall, b-axis reduced hierarchical
MFI crystals have been generated within Si/Al of 35 to 120, and
the lower the Si/Al ratio, the more uniform the structure becomes.
The textural properties were measured by Ar physisorption
isotherms, and the inferred data were outlined in Table 1 and
Figure S13, SI. The control samples, ZSM-5-C-35, ZSM-5-C-50
and ZSM-5-C-100, exhibited a steep uptake curve at relative
−
3
pressure P/P
35, ZSM-5-CT-50 and ZSM-5-CT-100, jump of Ar uptakes at
P/P > 0.80 was also observed, owing to capillary condensation
0
< 10 ascribed to micropore filling. In ZSM-5-CT-
90° rotational relationship along the common c axis, suggesting
a small number of plates are perpendicularly intergrown in the
0
particle, which could contribute to the formation of domain
inside the secondary meso- or macro- pores between interlayer
space formed by the stacking of primary plates. Complementary
mercury intrusion measurements were also employed to
estimate porosity. The deduced data were displayed in Figure
S14, SI. The distributions of pore-size verified the presence of
62, 40 and 40 nm macropores on ZSM-5-CT-35, ZSM-5-CT-50
and ZSM-CT-100, respectively. It is noteworthy that mercury
intrusion measures penetrating meso- and macro- pores that are
accessible through external surface that are beneficial to
boundaries.[
17]
The structural feature is further confirmed in
HRTEM image (Figure 1i). A large amount of interlayer space
meso- and macro- porosity) has been created in this sample due
(
to the hierarchical structure. ZSM-5-CT-100 shows a similar
structural feature (Figure 1k-o), except that the diameter of the
individual particle generally reduced to ca. 210 nm and the size
of the primary plates were generally increased. The single
crystalline feature is also observed for this sample. It is worthy to
note that the sample ZSM-5-CT-100 has the highest orientational
order of the primary units. The schematic model is displayed in
Figure 1o, to illustrate the architecture.
[
18]
transport properties. The measured BET surface areas are
2
-1
426, 532 and 451 m g , for ZSM-5-CT-35, ZSM-5-CT-50 and
ZSM-5-CT-100, respectively, which are larger with respect to
their microporous counterparts (292, 323 and 352 m2 g-1 for
ZSM-5-C-35, ZSM-5-C-50 and ZSM-5-C-100, respectively). The
increase in surface area, in line with SEM and TEM observations,
arising from increased external surface area and decrease of
crystal size (Table 1). Micropore volumes were nearly unchanged,
verifying that the intrinsic microporosity of MFI crystal structure
was largely preserved, which agreed with XRD results.
Three-dimensional
(3D)
electron
tomographic
reconstruction of ZSM-5-CT-x (Figure 2 and Videos S1-S6, SI)
further confirmed that the interior of particle is constructed by b-
axis shortened MFI primary units, which is different from
hierarchical structures reported recently.[11] TEM tomograms of
ZSM-5-CT-x (Figure 2b-d, 2f-h and 2j-l for ZSM-5-CT-35, ZSM-
5-CT-50 and ZSM-5-CT-100, respectively, at the location from
27Al MAS NMR spectra for calcined samples (Figure S15a,
SI) showed one major resonance peak sitting at ca. 56 ppm and
top to bottom near the center of the particles) clearly show that
the internal structure of the stacking domains is consistent with
Figure 3. Time-dependent XRD patterns (a), crystallization curve (b), and time-dependent TEM micrographs (c-h) (c: 6 h; d: 8 h; e: 9 h; f-g: 24 h; h: 48 h) for ZSM-
-CT-50 during crystallization.
5
4
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Figure 4. Solid state 13C MAS NMR spectra (a), peak fitting method of 13C MAS NMR spectra (b), and the change curve of TBA /TPA+ ratio (c) for as-synthesized
+
ZSM-5-CT-50 in the course of crystallization. The relative value of TBA /TPA+ was calculated by using the formula ((S
+
2 3 4 6 7 n
+S +S )/3)/((S +S )/2), where S represents
the peak area of the corresponding peak of n, n represents the number noted in Figure 4b.
negligible quantities ca. -2.0 and 1.0 ppm associated with extra-
framework, octahedrally coordinated Al moieties, manifesting
abnormally, that the 8 h collected sample possessed a high
relative crystallinity (RC, calculated by integrating the peak areas
at 2θ range from 7° to 9°, 22.5° to 25° and referred to the final
-
that most Al was incorporated as tetrahedral AlO
4
into the
29
[22]
framework of MFI. Si MAS NMR spectra (Figure S15b, SI)
could be deconvoluted into four peaks centered at ca. -116, -113,
product, presetting as 100%) of 89.1%, which declined to 61.2%
at 9 h before growing again with prolonged time. The temporal
evolution of morphology recorded by SEM (Figure S17, SI) and
TEM micrographs (Figure 3c-h) demonstrated that the 6 h
sample was made of uniform, loosely packed particulates of ca.
170 nm with rough surfaces. SEM tracking disclosed that surface
roughening corresponding to the primary plates started from 8 h
on, and became clearly visible at 9 h, accompanied by a distinct
increase of mean particle size to 300 nm. Thereafter, crystals
gradually grew from 300 nm to 400 nm and porous regions were
developed in a recrystallization process. The corresponding
temporal TEM micrographs (Figure 3c-h) demonstrated that
recrystallization started from 8 h on, the thickness contrast
indicating the formation of primary plates (inset of Figure 3d), and
further growth was escorted by core area dissolution. From 24 h
on, the growth of primary crystalline plates in the marginal
regions was identified, associated with the further crystallization
process. No morphological characteristics of early particles could
be observed, indicating recrystallization extended into the interior
parts of crystals.
-
1
(
107, and -103 ppm, respectively. ZSM-5-CT-35 (70.5 and
7.9%), ZSM-5-CT-50 (67.8 and 14.0%) and ZSM-5-CT-100
63.9 and 22.0%) exhibited major broad signal at
a
approximately -113 and -116 ppm, which could be attributed to
Q =[Si(OSi)
compared with control samples except for ZSM-5-CT-35 (ZSM-
5
4
],[19] suggesting that they were less condensed
4
-C-35: 85.8% and 2.6%; ZSM-5-C-50: 75.6% and 14.9%; ZSM-
-C-100: 84.8% and 8.9%, Table S2 and Table S3, SI). The
5
heterogeneous broadening was ascribed to Si locating on the 12
T-sites.[20] Besides, the small resonance peak centered at ca. -
1
and 11.3%), ZSM-5-CT-50 (8.5 and 9.7%) and ZSM-5-CT-100
(
from surface Q =[Si(OSi)
moieties, respectively, by referring to 27Al MAS NMR data, 29Si
CP/MAS NMR (Figure S16, SI) and reported ones.[21] Hence, the
NMR data evidenced that the short range order in the obtained
materials was comparable to that in the control samples except
the presence of large external surface areas and mesopore
volumes.
03 and -107 ppm could be distinguished in ZSM-5-CT-35 (0.3
9.4 and 4.7%). The peak is assigned here to the contribution
3
3
(OH)] and framework [Si(OSi)
3
(OAl)]
Solid state MAS NMR spectra for the crystallization series
were recorded to monitor the changes in both occluded SDAs
and framework elements, to gain molecular level understanding
of formation process. 13C MAS NMR spectra (Figure 4)
Formation mechanism
+
Since the hierarchical architecture was formed during the
crystallization in the absence of porogens, it is important to
understand the formation mechanism and controlling synthesis
parameters. To this end, the crystallization processes of ZSM-5-
CT-50 and ZSM-5-CT-100 were monitored using combined ex
situ XRD, SEM, TEM and MAS NMR characterizations. It was
assumed that quenching of crystallization would freeze structure
evolution in hydrothermal synthesis. No solid product could be
recovered from the growth solution before 6 h for ZSM-5-CT-50.
Time-dependent XRD patterns and the corresponding
crystallization curve of ZSM-5-CT-50 (Figure 3a-b) showed,
manifested that TBA was found to be the mainly occluded SDA
in the separated gel (collected via rotary evaporation at 353 K) in
the early stage of nucleation (5 h), as shown by the chemical
shifts at 60.9, 24.4, 20.3 and 14.4/12.8 ppm, which originated
+
+
from C1, C2, C3 and C4 in the TBA . A small amount of TPA
could also be detected, indicating resonance peaks at 63.2, 16.5
and 11.4/10.4 ppm, associated with C5, C6 and C7 in the
molecule. Split of resonance peaks can be explained by the
position of SDA locating at the micropore intersection between
straight and sinusoidal positions.[19] The broadening of
resonance peaks for SDAs could be caused by the co-presence
5
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RESEARCH ARTICLE
of encapsulated and surface adsorbed SDAs in TPA-TEOS
gradually and was completely consumed, as only the ones of
[
23]
crystallization system.
Deconvolution and quantification of
crystalline MFI phase could be detectable. With the evolution of
+
+
4
TBA /TPA ratio change with crystallization time was plotted in
Figure 4c, indicating a high ratio of 8.1 before condensation at 5
h. The TBA /TPA ratio at 6 h dropped to 1.4 when crystalline
structure of MFI could be observed, and a minimum of 0.7 was
detected at 7 h, which progressively increased to 1.2 at 24 h and
maintained constant afterwards. Considering the nominal
order, the chemical shift for Q remained virtually the same, while
3
the signal for Q moieties shifted from -101 to -103 ppm from 9 h
+
+
till the end of crystallization. As the shift was escorted by
+
+
TPA /TBA exchange, which could be tentatively ascribed to a
stronger Coulomb attraction of negatively charged [Si(OSi)
moieties by neighboring, charge balancing TPA . This
3
(OAl)]
+
+
+
TBA /TPA ratio of 17 in the growth solution, obviously, a
preferential encapsulation of TPA+ over TBA+ during both
nucleation and recrystallization processes was observed, as the
former could better stabilize MFI framework.[14] 27Al MAS NMR
spectra (Figure S18a, SI) displayed one resonance signal at 56.2
assignment is reasonable, since [Si(OSi)
3
(OH)] contribution to
3
Q tends to decrease with degree of condensation and prolonged
3
4
crystallization time. The overall Q /Q ratio versus crystallization
time variations were plotted in Figure S18c, SI. The maximum
3
percentage of Q moieties appears in recrystallization (8 h), then
ppm assignable to tetrahedral AlO -
units in amorphous silica
reached the lowest in the final crystallization sample. The
crystallization tracking for the formation of ZSM-CT-100 (Figure
S19-S21, SI), likewise, conformed to the same rule. Conversely,
the formation of ZSM-5-C-50 (Figure S22, SI) or ZSM-5-C-100
(Figure S23, SI) was found to follow another route. Together, by
correlating with aforementioned characterizations, SDA
exchange induced recrystallization (SEIR) is responsible for the
formation of such architectures.
The yields of solids, their respective Si/Al ratios and SDA
contents were also tracked, as outlined in Figure S24, SI. The
solid yields in for both ZSM-5-CT-50 and ZSM-5-CT-100 were
found to increase with prolonged hydrothermal heating time.
Meanwhile, a continuous decrease of Si/Al ratio from ca. 140 to
approximately the final products composition was observed. The
early time yields (6-9 h) were low because it was only the nuclei
that were collected. It is no surprise that solid yield would
increase thereafter as these nuclei exerted a seeding effect and
dissolution of nuclei and recrystallization occurred
simultaneously. The Si/Al ratios of solids in the early heating time
were much higher than that of the initial gel, indicating a
preferential formation of high Si/Al ratio nuclei. Both the Si/Al
ratio and solid yield became flattened at a value close to
4
matrix at 5 h, which, gradually shifted towards 54 ppm, coinciding
with SDA exchange and recrystallization process, hence,
+
suggesting a stronger SDA-framework interaction for TPA than
that of TBA . The concurrent 29Si MAS NMR spectrum (Figure
S18b, Table S4 and Table S5, SI) of 5 h sample before
condensation contained three broad, partially overlapping
resonance signals at -88.4 (11.6%), -98.3 (56.8%), and -109.1
+
2
3
4 29
(
12.0%) ppm, associated with amorphous Q , Q and Q Si in
[
21b, 24]
2
polysilicate units, respectively.
The high contents of Q and
3
Q in 5 h sample indicated poor degree of long-range order and
less cross-linking at early stage of nucleation, i.e., the sample
consisted mainly of partially condensed oligomers of silica
together with incorporated Al in tetrahedral coordination. At 6 h,
when solid powder could be collected, the resonance intensity of
peaks corresponding to amorphous species decreased
considerably to insignificant amounts (2.9% in normalized
integral area). Meanwhile, 29Si signals associating with Q3 (-
4
1
01.0 ppm) and Q (-112.7 ppm, -116.1 ppm) moieties in
crystalline phase increased rapidly to 16.4% and 67.2%,
suggesting quick homogeneous nucleation from clear
solution.[24] Subsequently, the amorphous phase decreased
a
Scheme 1. The proposed formation mechanism of ZSM-5-CT-x: hydrolysis of monomeric precursors, homogeneous nucleation that incorporates both SDAs and
produces semi-crystalline particulates, SDA exchange induced dissolution-recrystallization and hierarchical order development in ripening process.
6
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Table 1. Si/Al ratio, textural, diffusion properties and acidity of ZSM-5 samples derived from the results of elemental analysis, Ar physisorption results, diffusivity
measurements of 2,2-dimethylbutane and Py-IR measurements, respectively.
Brønsted acid
density
Lewis acid
density
(μmol g-1)[e]
S
BET
S
ext
V
micro
V
meso
-1 [d]
(cm3 g )
Sample
Si/Al[a]
D
eff/R2[f]
TON[g]
-1 [c]
(cm3 g )
-1 [c]
(
m2 g-1)[b]
(m2 g )
-
1
[e]
(
μmol g )
3.47×10-6
1.67×10-4
2.84×10-6
9.89×10-5
3.00×10-6
7.47×10-6
1.21×103
6.45×103
1.92×103
1.13×104
4.32×103
4.48×104
ZSM-5-C-35
ZSM-5-CT-35
ZSM-5-C-50
33
34
46
42
83
83
292
426
323
532
352
451
35
164
45
0.11
0.11
0.08
0.09
0.13
0.12
0.11
0.48
0.12
0.64
0.10
0.40
233
121
206
218
126
132
54
200
35
ZSM-5-CT-50
ZSM-5-C-100
ZSM-5-CT-100
293
61
107
19
280
50
[
a] Determined by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES); [b] Calculated by the Brunauer-Emmett-Teller (BET) method; [c] Deduced
by the t-plot method; [d] Calculated from Barret-Joyner-Halenda (BJH) method. [e] Calculated corresponding to Py-IR spectra desorbed at 573 K; [f] Calculated by
the diffusivity measurements corresponding to Figure 5a; [g] The Turnover number (TON) considered as the accumulated number of dimethyl ether (DME) molecules
converted per Al site.
compositions of products after 24 h of hydrothermal treating. By
developed. This also explains why the synthesis of hierarchical
structure failed in the presence of Na or at very high Si/Al ratios
(i.e., 150). Crystalline, hierarchical structure is evolved during the
ripening process.
+
+
+
correlating TBA to TPA content variations with the concomitant
changes of Si/Al ratio, one may find that the initial gel has a
higher Si/Al ratio because the nucleation is driven by a mixture
+
+
with high TBA and low TPA content, and the low charge density
Acid and diffusion properties & Catalytic performance in
DTO
+
of TBA favors high Si/Al ratio of framework. Along with
recrystallization process, the Si/Al ratio of the framework
+
+
decreases as more TPA replaces TBA and the overall content
of encapsulated SDAs remains virtually constant.
Before catalytic tests, the acid and diffusion properties were
measured by standard pyridine adsorbed Infrared spectroscopy
(Py-IR) and gravimetric method, respectively. The results are
compiled in Table 1, Table S6, Figure S25, SI and Figure 5. ZSM-
On the ground of the above experiments, and also referring
to crystallization in TPA-TEOS clear solution,[24-25] a putative
crystallization mechanism can be proposed as sketched in
Scheme 1 for ZSM-5-CT-x. In the early stage, monomeric
-1
5-CT-35 (121 μmol g ) showed decreased medium-to-strong
Brønsted acid site density (measured at 573 K) with respect to
+
-1
inorganic precursors hydrolyze and encapsulate both TBA and
ZSM-5-C-35 (233 μmol g ), while the values for ZSM-5-CT-50
TPA+ to produce amorphous, embryonic zeolite suspensions
-1
-1
(218 μmol g ) and ZSM-5-C-50 (206 μmol g ), ZSM-5-CT-100
-1
-1
(
apparent clear solution), which quickly condenses into loosely
(132 μmol g ) and ZSM-5-C-100 (126 μmol g ) were
comparable. More Lewis sites and less Brønsted sites were
measured on ZSM-5-CT-35 with respect to their conventional
counterparts. Formation of Lewis acid site out of dihydroxylation
and dealumination of Brønsted acid site sitting at the external
surface is likely the reason,[26] and increment of external surface
area often induces formation of more Lewis acid sites in
packed, semi-crystalline particulates. Since nucleation is driven
+
+
mainly by TBA (because of insufficient TPA in the gel), and also
as a requirement of charge balance, more TBA+ has been
encapsulated in the nucleated high Si/Al ratio particulates. Since
+
+
TBA -MFI interaction is relatively weak with respect to TPA -MFI
system,[ the loosely packed and less ordered particulates tend
12]
+
[27]
to dissolve and recrystallize, induced by TPA substitution of
hierarchical zeolites. 2,2-dimethylbutane was used as probe
+
+
TBA . As the amount of available TPA in the mother liquor could
molecule in the gravimetric measurements for its kinetic diameter
(6.3 Å) is close to micropore dimension of MFI structure.[28] The
diffusion properties were obtained quantitatively by Fick’s
second law. The initial adsorption was often used to calculate
diffusivity because it was not sensitive to the heat of
+
not fulfill all the intersections in MFI lattice. TBA therefore also
serves as co-SDA in this recrystallization process. With the
progress of recrystallization and ripening, the predominant
+
+
surface adsorbed species becomes TBA as the majority of TPA
has been consumed and incorporated into the crystal lattices.
adsorption.[28]
q
(
t
)
/ q
(

)
showed a good linear relationship with
+
TBA has a better fit to the pore openings of straight channels
2
+
t
as shown in Figure 5a. The values of
D
eff / R (Table 1)
along b-axis, and the decoration of crystal surface by TBA could
suppress the crystal growth along b-axis by stabilizing the a-c
deduced from the slope of the adsorption profiles have increased
48.1, 34.8 and 2.5 times for ZSM-5-CT-35, ZSM-5-CT-50 and
ZSM-5-CT-100, respectively, with respect to control samples.
plane.[24-25] Since external surface adsorption of SDAs originates
-
+
-
+
4
from either strong AlO -TBA or weak surface -SiO -TBA pairs,
2
and the respective contribution is dependent on framework Si/Al
ratios. At low Si/Al ratios, the former could contribute more to a
stronger inhibition effect and the plate-like morphology is well
The increase of Deff / R represents the shortening of effective
diffusion lengths (R) as a consequence of the reduction of b-axis
thickness. The diffusion property improvements coincide with
7
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RESEARCH ARTICLE
TEM observations of structure, i.e., more uniform samples with
low Si/Al ratio exhibit better transport performance.
for ZSM-5-CT-35 (36 h), ZSM-5-CT-50 (51 h) and ZSM-5-CT-100
(103 h) with respect to ZSM-5-C-35 (6 h), ZSM-5-C-50 (8 h) and
ZSM-5-C-100 (10 h), were observed, respectively. Polyaromatic
compounds derived from aromatic intermediates are the cause
of catalyst deactivation. The expansion in catalyst lifetime
reflects that less aromatics were formed from hydride transfer
The catalytic properties for ZSM-5-CT-35, ZSM-5-CT-50 and
ZSM-5-CT-100 were evaluated in catalytic conversion of
dimethyl ether-to-olefin (DTO), and were compared with the
microporous control samples. The DTO reaction was carried out
on an atmospheric fixed-bed reactor at 723 K with a space
reaction on hierarchical catalysts, which is more prominent at low
-
1
-1
[29]
velocity of 4 h
g
-DME
g
-cat . The conversion versus time-on-
acid site density.
The deduced turnover numbers (TONs),
stream were depicted in Figure 5b, prolonged catalyst lifetime,
defined as the accumulated number of dimethyl ether (DME)
molecules converted per Al site, of ZSM-5 samples are shown in
Table 1, which can be used to accurately evaluate their catalytic
activity.[
30]
The TONs of ZSM-5-CT-x (x=35, 50 and 100
respectively) are 5.33, 5.88 and 10.37 times that of the control
samples, which agrees well with their enhanced diffusion
properties. The corresponding integral product slates for light
hydrocarbons were shown in histograms in Figure 5c. Within the
product slates, more light olefins (mainly, ethene and propene),
in particular, propene was produced by the hierarchical catalysts,
ZSM-5-CT-35 (49.4% vs. 43.9%), ZSM-5-CT-50 (51.8% vs.
43.1%) and ZSM-5-CT-100 (53.8% vs. 47.5%), with respect to
the microporous counterparts. It is also observed that less
ethene is produced in hierarchical samples with respect to the
control samples, as reflected by ethene/propene ratio variations.
From mechanistic knowledge, ethene is produced solely by
aromatic- mediated cycle, while propene can be generated by
both olefinic and aromatic cycles (with the former more efficient
than that of the latter),[31] hence, the ratio variations, as a
measure of cycle dominance, disclose that aromatic cycle-based
conversion is disfavored by the shortening of retention time as a
result of b- axis reduction.[7] In addition, aromatic cycle
dominates at high acid site density explains the impact of Si/Al
ratios.[ Propane is regarded as a hydride transfer product from
propene, hence, the variations of propane/propene ratios among
samples reflect the impact of hydride transfer over selectivity.
The higher the ratio, the more olefins are transformed to
paraffins.[ Likewise, much lower fraction of paraffins, such as
32]
33]
CH
4
(as a product of hydride transfer from CH
=, which is considered as product or
3
-Z),[34] is produced
on ZSM-5-CT-x. Besides, C
4
intermediate of olefin-based cycle, is shown to increase hand-in-
hand with that of propene selectivity, lending a further evidence
for the exaggeration of olefinic cycle by the presence of
hierarchical architecture.
Overall, high propene selectivity and resistance to coking,
which are the two most important parameters in DTO catalysts,
have been attained on b-axis reduced MFI crystal, as a
consequence of suppression of the aromatic cycle stemming
from enhanced transport property and attenuated Brønsted acid
site density.[
32]
Conclusions
To sum up, the co-presence of both TPAOH and TBAOH
permitted the synthesis of self-assembled hierarchical MFI
zeolite with reduced b-axis without using mesoporogens.
Homogeneous nucleation from clear solution and the use of
monomeric Al and Si sources have been identified as key factors
influencing the structure of final products. The disclosed SDA
exchange induced anisotropic recrystallization is found to be
Si/Al ratio- dependent, hence, allowing tailoring of crystal
morphology between Si/Al ratio between 35 and 120, and b-axis
Figure 5. Adsorption rates of 2,2-dimethylbutane in the calcined ZSM-5
zeolites (a), DME conversion with TOS (b) and overall product slates (c) when
the DME conversion exceeds 95% in the reaction system of DTO on various
ZSM-5 samples. Reaction temperature: catalyst loading, 0.10 g, 723 K,
WHSV=4 h-1
-cat-1.
g
-DME
g
8
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RESEARCH ARTICLE
reduction down to tens of nanometers could be effective
achieved preferably at 35 to 100. Hierarchical MFI zeolite
showed an increased propene selectivity and catalyst lifetime in
DTO conversion, owing to the enhanced transport efficiency and
suppressed side reactions. As many zeolites can be generated
by more than one SDAs, and each SDA exerts distinct
stabilization effect on the same framework, this synthetic
strategy is expected to be extendable to the control of zeolite
morphology. Moreover, the catalytic advantage resulting from
reduced b-axis deserves future explorations.
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RESEARCH ARTICLE
Entry for the Table of Contents
Aligned plate-shaped MFI crystals with reduced b-axis thickness was synthesized through homogeneous nucleation followed by
structure-directing agent (SDA) exchange induced recrystallization, using tetrapropylammonium hydroxide (TPAOH) and
tetrabutylammonium hydroxide (TBAOH). The obtained materials markedly exaggerated propene selectivity and showed prolonged
catalyst lifetime in the conversion of dimethyl ether-to-olefins.
1
0
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