High yield synthesis of nanoscale high-silica ZSM-5 zeolites: Via interzeolite transformation with a new strategy

Article abstract of DOI:10.1039/d0cy01345e

A novel interzeolite transformation strategy using internally confined organic structure directing agents (OSDAs), called the "OSDA-confined"strategy, is proposed for highly crystalline nanosized high-silica ZSM-5 zeolite synthesis, by which high yield and fast synthesis of ZSM-5 is achieved using BEA zeolite as a raw material. The transformation process was investigated by XRD, SEM, ICP, FTIR and UV Raman spectroscopy, which revealed that the decomposition of a parent zeolite and the crystallization process of products were well modulated by the confined OSDA. The transformation process could be divided into three stages consisting of raw material deconstruction, small size product formation and product growth. The strong etching power of NaOH contributes to the fast decomposition of BEA zeolites. In addition, tetrapropylammonium hydroxide (TPAOH) as an OSDA was confined in the parent zeolite instead of being added into the mother liquid, achieving a high local concentration of TPAOH, which could reduce the decomposition rate of the parent BEA zeolite and promote the efficient organization of the building units generated by the decomposition of the BEA zeolite. Thereof, a high yield was achieved by the synergistic effect of NaOH and occluded TPAOH. During the interzeolite transformation process, the six-membered rings (6Rs), five-membered rings (5Rs) and four-membered rings (4Rs) in the BEA zeolite framework were disassembled and combined with the help of 5Rs to produce the ZSM-5 zeolite structure. Moreover, due to the nanoscale and large surface area, the obtained ZSM-5 zeolite catalyst showed extremely high catalytic activity in the self-etherification reaction of benzyl alcohol (BA) to dibenzyl ether (DE). This study provides insights into the interzeolite transformation process, which would be helpful for achieving size control and high-efficiency synthesis.

Full text of DOI:10.1039/d0cy01345e

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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
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DOI: 10.1039/D0CY01345E
ARTICLE
High yield synthesis of nanoscale high-silica ZSM-5 zeolites via
interzeolite transformation with a new strategy†
Lanjian Xu, ab Yangyang Yuan,* Qiao Han, Lei Dong, Lei Chen, Xiaomin Zhang and Lei Xu*
a
ab
ab
ab
a
a
Received 00th January 20xx,
Accepted 00th January 20xx
A novel interzeolite transformation strategy using internally confined organic structure directing agents (OSDA), called
OSDA-Confined” strategy, is proposed for highly crystalline nanosized high-silica ZSM-5 zeolite synthesis, by which the high
DOI: 10.1039/x0xx00000x
“
yield and fast synthesize of ZSM-5 is achieved using BEA zeolite as raw material. The transformation process was investigated
by XRD, SEM, ICP, FTIR and UV Raman spectra, which revealed that the decomposition of parent zeolite and crystallization
process of products were well modulated by the confined OSDA. The transformation process could be divided into three
stages consisting of raw material deconstruction, small size product formation and product growth. The strong etching
power of NaOH contributes to the fast decomposition of BEA zeolite. In addition, the tetrapropylammonium hydroxide
(TPAOH) as OSDA was confined in the parent zeolite instead of being added into the mother liquid, achieving a high local
concentration of TPAOH, which could reduce the decomposing rate of parent BEA zeolite and promote the efficient
organization of the building units generated by the decomposition of BEA zeolite. Thereof, a high yield was achieved by the
synergistic effect of NaOH and occluded TPAOH. During the interzeolite transformation process, the six-membered rings
(6Rs), five-membered rings (5Rs) and four-membered rings (4Rs) in the BEA zeolite framework were disassembled and
combined with the help of 5Rs to produce the ZSM-5 zeolite structure. Moreover, due to the nanoscale and large surface
area, the obtained ZSM-5 zeolite catalyst showed extremely high catalytic activity in the self-etherification reaction of benzyl
alcohol (BA) to dibenzyl ether (DE). This study provides insights into the interzeolite transformation process, which would
be helpful for the achieving size control and high-efficiency synthesis.
nucleation and crystal growth could be well controlled.6-9 To
date, a number of methods, involving adding organic structure-
Introduction
1
0
11
directing agents (OSDA) or seed solutions , changing the
Zeolites, as microporous crystalline materials with molecular-
sized channel systems, large surface area and specific acidity,
have been widely applied in catalysis, adsorption, and
separation process. However, the micropores could bring
diffusion limitation, hinder the intracrystalline mass transport
and further decrease the catalytic efficiency, especially in
reactions involving bulky molecules. Reducing the crystal size to
nanometer scale is regarded as an effective way to provide
possible new application in conventional and new fields due to
the shortened diffusion path, larger external area and other
ingredients or crystallization process,13,14 crystallization with
12
growth inhibitors (e.g. organosilanes,15 surfactants,
alcohols, polymers ) and lower temperature synthesis,
16
17
18
19
have been adopted to fabricate nanosized zeolites. However,
despite these tremendous efforts made by researchers,
methods still suffer from complex organic additives, long
crystallization time and low yield.
Interzeolite transformation has been reported as an
attractive strategy to synthesize zeolite. By introducing a parent
zeolite as raw material, the nucleation and crystallization
process could be well modulated, achieving high crystallization
rates and convenient size control as well as providing
alternative routes for the preparation of some special
1
-5
exceptional properties.
On account of the great value for fundamental studies and
practical applications, a substantial number of attention is
devoted to developing facile synthetic methods for nanosized
zeolites. Pioneer works have demonstrated that nanoscale
zeolites could be synthesized through precise regulation of the
gel composition or the crystallization parameters by which the
2
0-22
zeolites.
To date, a variety of zeolites has been obtained by
23
24
25
26
27
28
this strategy, including MFI, BEA, CHA, LEV, AEI, MTW
and so on. Mechanism investigation on interzeolite
transformation has shown that the parent zeolites undergo
decomposition/dissolution to release building units, which
could be further organized for the construction of new zeolite
a.National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian 116023, People’s Republic of China.
E-mail: yuanyangyang@dicp.ac.cn, leixu@dicp.ac.cn; Fax: +86 411 82463032;
Tel: +86 411 82463032
21,29
with different topology.
Generally, high alkalinity is a
prerequisite for efficient decomposition of the parent zeolite,
which in turn would increase the silicon concentration in the
liquid, leading to a low yield of the target products. Thus, for
high efficient synthesis, the modulation of the transformation
b.
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China.
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
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process to achieve the balance of decomposition, solution and Kermel Reagent Co., Ltd), sodium hydroxide (NaOH, >96wt%,
View Article Online
DOI: 10.1039/D0CY01345E
crystallization based on fundamental understanding of China
National
Pharmaceutical
Group
Co.,
Ltd),
crystallization process, would be of vital importance. Recently, teraethylammonium hydroxide (TEAOH, 25wt% in water,
Corma et al. achieved 85% yield in USY-CHA transformation with Runjing Chemical Co., Ltd), tetrapropylammonium hydroxide
the lowest content of TEAOH and a moderate amount of (TPAOH, 25wt% in water, Runjing Chemical Co., Ltd),
mobilizing agent (NaOH) in the synthesis media which hydrofluoric acid (HF, 40wt% in water, Tianjin Kemiou Reagent
demonstrates the significance of these two key parameters Co., Ltd), Benzyl alcohol (BA, >98wt%, Tianjin Kemiou Reagent
2
5
control in yield promotion. Xiao and co-workers reported an Co., Ltd). All reagents were utilized as purchased without any
amazing 90% yield of CHA from interzeolite transformation of further purification.
high silica FAU zeolite adopting the solvent-free synthesis, Typical synthesis procedure
where the dissolution of aluminosilicate species in aqueous
Synthesis of parent zeolite BEA. According to the previous
literature, procedures of preparing parent zeolite BEA are as
follows. Molar composition of the initial gel mixture for was
media was avoided due to the absence of water solvent, but the
3
1
30
process still took 1 day. However, the solid yield for BEA-MFI
transformation was still limited. Iglesia et al. synthesized ZSM-5
TEOS : 1/800 Al
would be treated at 140 C for 4 days. Then, the products
needed to be washed by deionized water until the pH was
2
(SO
4
)
3
: 0.55 TEAOH : 0.55 HF : 6 H
2
O, which
2
3
zeolite with the yield of about 50%. Sano et al. achieved the
interzeolite conversion from BEA to MFI with the yield below
o
29
2
0%. The regulation of the alkalinity, which is responsible for
o
below 8 and dried at 80 C for 8 h. At last, before used as the
the decomposition, solution and crystallization, would be of
high importance for efficient interzeolite synthesis.
raw material of interzeolite transformation, zeolite BEA was
o
calcined at 550 C for 6 hours to remove organic molecules.
Herein, we report an “OSDA-Confined” interzeolite
transformation strategy for nanosized zeolite synthesis, where
the OSDA were confined in the parent zeolite and facilitated the
reassemble of the building units, and the alkali in the aqueous
solution could assist decomposition of the parent zeolites. By
this strategy, the released building units could be reorganized
efficiently and nanoscale high-silica ZSM-5 zeolite with a 73.8%
solid yield for 3 h from hydrothermal conversion of Beta zeolite
was obtained. The critical role of occluded TPAOH and
transformation process were investigated in detail. Also, the
catalytic performance of obtained nanosized ZSM-5 was
evaluated. Based on this investigation, we provide an insight
into the process control and key factors for obtaining a high
yield in the interzeolite transformation process.
Synthesis of zeolite MFI through interzeolite
transformation. In a typical procedure, zeolite BEA was added
into a TPAOH solution (25wt% in water) in a Teflon lining fitted
with 100 ml stainless-steel autoclave. For every 1 g BEA zeolite,
1
2.5 g that kind of TPAOH solution was used. Then, the BEA
zeolite and TPAOH solution in the lining were stirred at ambient
o
temperature for 2.5 h and then dried at 30 C for 24 h. After
that, NaOH aqueous solution (1 M) and deionized water were
added. The detailed initial feed molar composition of the
synthesis gel based on the original aqueous solution was BEA :
0
.92 TPAOH : 0.6 NaOH : 166.7 H
2
O. When calculating the molar
approximately,
quantity of BEA zeolite, it was regarded as SiO
2
due to the high Si/Al molar ratio (364) which was determined by
ICP. Next, the lining was transferred into a 100 ml stainless-steel
autoclave to crystallize under static conditions. Typically,
o
synthesis system would be heated for 3 h at 150 C. The
Experimental Section
Chemical and reagents
resulting product was collected by centrifugation and washed
by deionized water for several times until the pH of supernatant
o
reached 7 - 8. Finally, it was dried at 80 C overnight. This
Tetraethoxysilane (TEOS, >98wt%, Tianjin Kemiou Reagent Co.,
Ltd), aluminum sulfate (Al (SO ) ·18H O, >99wt%, Tianjin
2 4 3 2
synthesis strategy is called “OSDA-Confined” strategy (Strategy
Scheme 1 Scheme illustration for different interzeolite transformation strategys to synthesize ZSM-5 zeolite
2
| J. Name., 2012, 00, 1-3
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A in Scheme 1). For comparison, another synthesis strategy, in Laboratory of Catalysis at Dalian Institute of CVhieewmAritcicalel Oannlinde
which the TPAOH and NaOH were added directly without the Physics, The Third Generation UV-visibDleOIR: 1e0s.1o0n3a9n/Dc0eCYR0a1m34a5nE
impregnating and drying process before crystallization but Spectra, China). The UV Raman spectrometer used above
keeping the same amount of TPAOH, NaOH, H
2
O and zeolite equipped with a 224.0 nm line, which was generated by
BEA as “OSDA-Confined” strategy used, is called “OSDA-Added” frequency doubling the 448 nm line. The power of laser was
-
1
strategy (Strategy B in Scheme 1). The specific synthesis below 4.0 mW and the resolution of spectrometer was 2 cm .
27
condition mentioned above was listed in Table 1 below. The Solid–state
Al magic-angle-spinning nuclear-magnetic-
solid yields of samples were defined as
resonance (MAS NMR; Bruker, AvanceIII 500 MHz, Germany)
spectra was recorded to study the framework structure of
obtained zeolites. Thermogravimetric analysis (TGA; STA 449
product (g)
yield (%) =
×100 (1-1)
parent zeolite (g)
o
-1
F3, Netzsch,Germany) with a heating rate of 10 C·min from 40
The as-made product samples ZSM-5 zeolites was calcined
o
to 1000 C was conducted to evaluate the amount of TPAOH in
o
at 550 C for 6 h to remove the organic molecules in the
the channel systems of zeolites.
channels in the atmosphere of air before being applied in the
equation (1-1) above. Before the samples were applied in the
catalytic reactions, the Na-form was transformed into H-form
by ion exchange method. The details were as follows. The as-
synthesized zeolite (1 g) after calcination was mixed with a
Catalytic tests
The Self-etherification reaction of benzyl alcohol (BA) occurred
in a 10 ml one-necked round bottom flask which was equipped
with a reflux condenser at the atmospheric pressure and an oil
bath with magnetic stirring function. Typically, 0.8 ml BA and
certain amount of aqueous solution NH
they were stirred at 80 C for 2 h and next the solid was obtained
by centrifugalization. This procedure was repeated for 3 times.
4
NO
3
(20 ml, 1 M), then
0
.20 g zeolite catalyst were mixed in the flask and they were
o
o
heated to 110 C for various time with stir. When the reaction
was over, the liquid product was obtained by filtering and it was
injected into a gas chromatograph (Bruker 450) with a HP-5
column (30 m × 0.320 mm × 0.25 μm) and flame ionization
detector (FID) to analyze components. The temperature control
program in the oven was set as follows. First, the temperature
o
Finally, the solid sample was calcined at 550 C for 4 h to
become into the H-form.
Characterization
Powder X-ray diffraction (XRD; X'pert Pro-1, PANAlytical,
Holland) was used to determine the crystalline of obtained
zeolites. The crystallinity of zeolite MFI was estimated by
comparing its height of characteristic quintet diffraction peaks
at 2θ of 23 ° - 25 ° with others. The material (Sample no. 8 in
Table 1) which was under crystallization for 9 hours, was
assumed to have a crystallinity of 100% for MFI-type zeolites.
Parent zeolite BEA was taken as a reference, which was
o
o
was held at 50 C for 5 min, next heated up to 200 C at a rate
o
-1
o
of 20 C·min , finally held at 200 C for 10 min. In the column,
-1
the flow rate of N
zeolite catalysts, almost all of the reaction products were
dibenzyl ether (DE) molecules. Other by-products barely formed.
2
carrier gas was 2.4 ml·min . For all the tested
assumed that it had a crystallinity of 100% for that type Result and discussion
material. The relative crystallinity of other samples was
Characterization of product nano-zeolite ZSM-5
calculated by comparing their observed diffraction peak height
at 2θ = 22.4 ° with the reference. Information of morphology,
geometry and crystal size of obtained zeolites were observed by
scanning electron microscopy (SEM; JSM-7800F, JEOL, Japan).
The BET surface area and pore characteristics of obtained
The proposed “OSDA-Confined” interzeolite transformation
strategy, in which the OSDA was firstly introduced into the
parent zeolite to achieve a high local concentration, followed by
2
zeolites after calcination were measured by N absorption-
desorption (BET; Autosorb iQ2, QuantaChrome, USA). Before
o
measurement, a sample was kept at 300 C for 5 h under the
high vacuum to remove impurities in the pores. The analysis of
acidity was performed by temperature-programed desorption
of ammonia (NH -TPD; Micromeritics, AutoChem 2920, USA).
3
Inductively coupled plasma mass spectrometer (ICP-OES;
PerkinElmer, NexION 300D, USA) was used to measure the
relative content of Si and Al. The vibration frequency of zeolite
framework was collected under room temperature on the
Fourier transform infrared spectra (FTIR; Thermofisher
-1
scientific, Nicolet iS50, USA) whose resolution was 4 cm to
investigate zeolite formation mechanism. Before measurement,
the sample mixed with KBr needed to be pressed into a self-
o
supporting pellet and kept at 200 C for 1 h to dehydrate. The
content of zeolite in the mixture pellet was kept about 1wt%.
The formation mechanism of zeolite framework was also
analyzed by UV Raman spectra (UV Raman; State Key
2
Fig. 1 XRD patterns (A), SEM images (B), N adsorption-desorption isothermals (C) and
27Al MAS NMR spectra (D) of the product zeolite ZSM-5.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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the addition of aqueous alkali and hydrothermal crystallization synthesis mixture but the sample also underwent tVhieewpArrtoiccleeOsnslionef
were illustrated by Strategy A in scheme 1. Nanosized ZSM-5 impregnating and drying TPAOH, and keepDiOnIg: 1t0h.1e0s3a9m/De0CaYm01o3u4n5Et
was synthesized by employing TPAOH as OSDA and Beta zeolite of TPAOH, H
as a starting source at 150 C for 3 h, with a yield of 73.8% used. Typical truncated bipyramid morphology of Beta zeolites
2
O and zeolite BEA as “OSDA-Confined” strategy
o
(
Sample no. 6 in Table 1), much higher than that reported in accompanied with tiny cuboid particles are observed, indicating
2
3, 29
previous the Beta-MFI interconversion
and the “OSDA- that the parent Beta zeolite could hardly be decomposed with
Added” strategy in this work.
the TPAOH only. In contrast, no crystal diffraction peaks are
In order to confirm the successful preparation of ZSM-5 observed when NaOH was used alone (no the process of
nano-zeolites, a series of characterization techniques were impregnating and drying TPAOH but keeping the amount of
2
7
2
used, such as XRD, SEM, N physical absorption, ICP and Al others the same as “OSDA-Confined” strategy used) although
MAS NMR spectra. The parent Beta zeolites is well-crystallized, that partially destroyed Beta zeolite profile was remained in
exhibiting a typical shape of truncated bipyramid with a SEM image, suggesting the strong etching power and weak
diameter of about 13 µm with clear crystal edges and faces (Fig. structure directing ability of NaOH. When TPAOH and NaOH
S1). Fig. 1A shows the XRD patterns of the as-synthesized were added to the synthesis solution simultaneously before
products, exhibiting typical peaks at 2θ of 8.0°, 8.9° and quintet crystallization instead of undergoing the pre-soaked process of
at 23 ° - 25 ° associated with MFI framework. The SEM image TPAOH, namely, by using the “OSDA-Added” strategy, the
reveals that the products possessing spheroidal morphology products exhibit typical diffraction peaks assigned to MFI
with a size of about 60 nm are quite uniform (Fig. 1B). The N
sorption isotherms in Fig. 1C exhibit characteristic type and differentiated distribution than that of the “OSDA-Confined”
type properties with a hysteresis loop at a relative pressure synthesis. Besides, the solid yield decreased to 5.3% (Sample no.
2
zeolite. But it possesses a larger crystal size and more
Ⅰ
Ⅱ
over 0.4, indicating the presence of meso/micropores structure 12 in Table 1). The results suggest that the synergetic effect of
arised from nanoparticles accumulation. The nanoscale ZSM-5 TPAOH and NaOH give rise to nano-zeolite ZSM-5. And, the solid
2
-1
zeolite exhibits a large BET surface area (478.7 m ·g ), external yield could be regulated by the distribution of TPAOH. In
2
-1
3
-1
surface area (110.8 m ·g ) and pore volume (0.51 cm ·g ). Fig. addition, the molar ratio of Si/Al also decreased to 192
1
27
D illustrates the Al NMR spectra tested after the zeolite was measured by ICP.
protonized, showing only one resonance peak located at 53
ppm, which is signed to tetrahedral aluminum. This indicates
that all the aluminum are located in the zeolite framework and
no extra-framework aluminum formed. The molar ratio of Si/Al
for the zeolite was 243 tested by ICP. The above observations
demonstrate successful preparation of high crystalline
nanosized ZSM-5 zeolite by the interzeolite transformation of
Beta zeolite.
To shed light on this high efficient “OSDA-Confined”
interzeolite transformation mechanism, the roles of confined
TPAOH and added NaOH were investigated. The results were
shown in Fig. 2. Only diffraction peaks associated with zeolite
BEA are detected when NaOH was not introduced in the
Table 1 Synthesis conditions and characteristics of product zeolite ZSM-5
Sam.
no.
time
phase
Relative
crystallinity
(%)
Silica
Solid
yieldg
(%)
95.2
83.3
79.8
f
species
-
1
(mg·L )
a
0 hc
d
1
BEA
BEA
BEA+
MFI
BEA+
MFI
MFI
MFI
MFI
MFI
BEA
MFI
MFI
MFI
98.1
a
d
2
3
20 min
40 min
66.3
36.6 /
39.8
a
d
e
10.2
4a
1 h
23.1 /
d
919
272
80.1
53.1e
a
e
5
6
7
2 h
3 h
5 h
85.8
96.2e
98.5
100e
23.1
36.2e
37.8
43.5e
77.4
73.8
a
a
e
a
8
9
9 h
b
d
20 min
40 min
1 h
4702
b
1
1
1
0
1
2
b
b
e
5669
5847
3 h
5.3
a
b
c
the sample was synthesized via Strategy A
the sample was synthesized via Strategy B
the sample underwent the same synthesis process with others but no
heating for crystallization
d
the relative crystallinity was assigned to zeolite BEA
e
the relative crystallinity was assigned to zeolite MFI
f
“Silica species” refers to the concentration of silicon species in mother
liquor for zeolites synthesis measured by ICP
g
Solid yield was calculated by the formula (1-1) mentioned in the section of
“
Synthesis of zeolite MFI through interzeolite transformation”
Fig. 2 XRD patterns (A) and SEM images of zeolites obtained with TPAOH (B), NaOH (C),
TPAOH and NaOH (D)
4
| J. Name., 2012, 00, 1-3
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Based on the above results, it is inferred that the TPAOH
confined in the parent zeolites plays a critical role in the
transformation process.
View Article Online
DOI: 10.1039/D0CY01345E
Investigation of the crystallization process of nano-zeolite ZSM-5
To further investigate the mechanism for this high-efficient
transformation, samples synthesized for different time were
investigated by XRD, SEM, FTIR and UV Raman technique. The
diffraction patterns and relative crystallinity of as-made
products were represented in Table 1 and Fig.3. To relatively
crystallinity, the red lines stand for the decomposition process
of parent zeolites and the black lines stand for the construction
of products zeolites both in Fig. 3B and Fig. 3D. For the “OSDA-
Confined” strategy, the sample (0 h) with confined TPAOH but
no further crystalization, exhibits only typical diffraction peaks
assigned to zeolite BEA and a slightly lower crystallinity than
that of parent BEA zeolite. When extending the crystallization
time to 20 min, 40 min and 1 h, the relative crystallinity of BEA
structure decreased to 66.3%, 36.6% and 23.1%, respectively. structure reduced greatly, reaching a very low relative
As Fig. 3B shown, deconstruction speed of the part zeolite BEA crystallinity of 23.1% after 20 min of crystallization, and
was very fast. The characteristic peaks associated with BEA disappeared totally after 40 min, showing a higher rate of the
structure disappeared after 1 h of crystalization, which indicates parent zeolite decomposition than the “OSDA-Confined”
the total dissolution of the parent zeolite at this time. Typical strategy. Meanwhile the concentration of the silicon species in
-1
peaks assigned to MFI structure appeared at 40 min and the mother liquid increased from 4702 to 5847 mg·L , suggesting
height of them increased over the crystallization time, reaching that most of the silicon species remained in the liquid. The much
a relative crystallinity of 53.1%, 85.8% and 96.2% at 1 h, 2 h and lower concentration of the silicon species in “OSDA-Confined”
3
h, respectively. And with the crystallization time prolonged strategy declares that confined TPAOH could greatly promote
further, the relative crystallinity increased in a very small range. the reconstruction of building units, by which the path of
It demonstrates the great efficiency of interzeolite interzeolite transformation may be regulated. As shown in Fig.
transformation. The concentration of silicon species in mother 3D, the relative crystallinity of the products in “OSDA-Added”
-
1
liquid was very low (39.8 - 919 mg L ) during the transformation strategy increased slowly at a low level. Besides, the relative
process, indicating the efficient reconstruction of the release crystallinity of the 3 h sample in that strategy was much lower
building units by the surrounded organic templates. For this than that of 3 h one in “OSDA-Confined” strategy.
strategy, the XRD patterns of all the samples synthesized for
The content of OSDA in the solid obtained prepared via
various time were shown in Fig. S2. In contrast, for the “OSDA- these two strategies at 20 min were investigated by TG
Added” strategy, the characteristic peaks height of BEA experiment. As the TG curves shown in Fig. 4, the content of
TPAOH was about 15.5 wt% with the pre-occlusion process,
much higher than the 6.0 wt%, where the TPAOH was added in
the gel directly. Combining the previous conclusions, it could be
deduced that the confined TPAOH alleviated the corrosion from
alkali in the mother liquid and facilitated the zeolite transition
with a higher yield and crystallinity.
SEM images of the samples obtained at different period are
shown in Fig. 5. The 0 h sample shows typically truncated
bipyramid morphology of zeolite BEA with a diameter about 13
µm. After 20 min, the surface became rough, in consistent with
the decreased crystallinity. As the crystallization time was
prolonged to 40 min, a lot of particles with regular morphology
and some amorphous species could be seen. They would be
precursors of zeolite ZSM-5 with a diameter about 250 nm and
deconstructed species resulting from zeolite Beta, respectively.
It also suggests that parent zeolite BEA was disintegrating and
new zeolite ZSM-5 was forming. Upon crystallizing for 1 h, those
amorphous species disappeared basically and the most of
particles associated with typical ZSM-5 morphology remained.
Fig. 3 XRD diffraction patterns and the relative crystallinity of solid products crystallized
for varied time via “OSDA-Confined” strategy (A and B) and “OSDA-Added” strategy (C
and D)
Besides, some tiny granules are also observed carefully. From
then on to 3 h, it is interesting that the size of overall zeolite
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Journal Name
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DOI: 10.1039/D0CY01345E
particles decreased with the increase of time. The distribution
Based on the above results, the process of “OSDA-Confined”
of product zeolite size is considerably uniform and its diameter interzeolite transformation from BEA to ZSM-5 could be briefly
reaches to about 60 nm, on the average. Although the size divided into three phases on account of the evolution of
became smaller and smaller duration this process, the relative framework and morphology resulting from XRD and SEM as the
crystallinity assigned to MFI structure increased rapidly (Fig. 3). crystallization time increased,
With further prolonging the crystallinity time to 9 h, the size of
obtained zeolite particles became bigger. The size of solid
products increased to about 160 nm and particles were tightly
coupled to each other.
(1) Raw material deconstruction phase, i.e., parent
zeolite BEA was broken gradually with the
crystallization time from 0 to 40 min;
(2) Small size product formation phase, i.e., the size of
product zeolite ZSM-5 decreased with the time from
4
0 min to 3 h;
(3) Product growth phase, i.e., the product size increased
in the crystallization process from 3 to 9 h.
In the following, attention would be paid to the first two
phases to investigate the formation process in detail.
Fourier transform infrared and UV Raman spectra are two
important and complement vibrational spectroscopic data. To
understand this structural evolution at micro-level during the
whole crystallization process, FTIR and UV Raman spectra of the
samples were exhibited in Fig. 6 - 8, respectively.
-
1
-1
In FTIR spectra (Fig. 6), the peaks at 1236 cm , 1223 cm
-1
and 1095 cm , whether the source sample is zeolite BEA or
ZSM-5, are attributed to the T-O-T asymmetric stretching
As for zeolite BEA, the bands at 624 cm ,
73 cm and 524 cm are assigned to typical vibration of six-
32-34
-1
vibration mode.
-
1
-1
5
membered rings (6Rs), five-membered rings (5Rs) and four-
35-37
membered rings (4Rs) in zeolite framework,
respectively.
Fig. 6 FTIR of raw material zeolite BEA and product samples zeolite ZSM-5 obtained for
varieties of time from 0 h to 3 h (Sample no. 1-6 in Table 1)
6
| J. Name., 2012, 00, 1-3
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Journal Name
Through the whole stage of raw material deconstruction, there
ARTICLE
View Article Online
DOI: 10.1039/D0CY01345E
-
1
is no adsorption at around 940 cm , assigned to the vibration
of Si-O in the zeolite structure defects produced in
38,39
dealumination.
This absence indicates no obvious
3
h (Fig. S4B) which suggests that the as-made zeolite possesses
dealuminationin in the course of deconstruction. It was also
4
5
27
a good crystallinity.
confirmed by the results of the Al MAS NMR spectra in which
all of the three samples (0 h, 20 min and 40 min) showed no
resonance peaks at 0 ppm (Fig. S3). Combining this
consequence with the Si/Al ratio of the parent and product
zeolite mentioned above, it could be deduced that only siliceous
species dissolved in the synthetic solution containing TPAOH
and NaOH.
For a complementary perception of this transformation
process from the perspective of skeleton structure change, UV
Raman spectra was also used. As shown in Fig. 7, the bands near
38 cm , 397 cm and 815 cm correspond to the bending
mode of 6Rs, 5Rs and T-O-T symmetric stretching vibration,
-1
-1
-1
3
-
1
-1
respectively and the peaks at 430 cm and 465 cm are both
assigned to 4Rs vibration in zeolite EBA framework.4
6, 47
The
The height strength ratio between peaks assigned to the
vibration of specific member rings was used, as the method in
peak height of sample 0 h was basic invariant which suggests
that the framework of BEA zeolite was not destroyed after being
soaked and dried. From 0 to 40 min, the intensities of bands
assigned to 6Rs, 5Rs, 4Rs as well as T-O-T symmetric stretching
vibration decreased, demonstrating the occurrence of structure
decomposition. With regard to zeolite ZSM-5, the peaks at 294
3
7, 40
previous literature.
The relative change in ratio of 6Rs to
5
Rs, 4Rs to 5Rs could be investigated by this method (Fig. S4A).
The relative quantity of both 6Rs and 4Rs declined through the
whole 40 min suggesting that 6Rs and 4Rs decomposed faster
than 5Rs. And it could be also inferred that a large number of
-
1
-1
-1
-1
cm , 380cm , 472 cm and 811 cm are attributed to the
bending mode of 6Rs, 5Rs, 4Rs and T-O-T symmetric stretching
5
Rs, some 6Rs and very few 4Rs from the starting material
composed the framework of product zeolite.
In the second stage, in Fig. 6, the adsorption peaks near 796
cm-1 belongs to the symmetric stretching of T-O-T in ZSM-5
4
6, 47
vibration mode.
Only obvious peak associated with 5Rs was
observed at the initial period of phase transformation,
indicating that the new framework was built by the
intercombination of 5Rs from the old structure, inconsistent
with the FT-IR. Afterwards, a wide peak at 380 m belonging to
the vibration of 5Rs building units in framework was
3
2, 34, 41
-1
-
framework.
1
The adsorption bands near 621 cm , 545 cm
and 450 cm-1 as assigned to the vibration of isolated 5Rs,
condense 5Rs and the T-O-T bending vibration mode.
-1
4
1-43
The
-1
decreased intensity of the band near 621 cm from 1 h to 3 h
demonstrates that during the decomposition stage, some
isolated 5Rs were produced by parent zeolite and transformed
directly into the structure of zeolite ZSM-5. It could be also
speculated that 5Rs are the key point in phase conversion. The
latter two bands near 545 cm and 450 cm are the
characteristic of zeolite ZSM-5 framework and the ratio
between them could be evidence to assess the crystallinity of a
4
7-50
observed.
With extending crystallization time, this
characteristic band become stronger. The strongest band
associated with the product of 3 h manifests the good
crystallinity of the sample which is also in line with information
4
7
-
1
-1
resulted from XRD and FTIR previously.
Catalytic conversion Benzyl alcohol (BA) on zeolite
3
2, 44
zeolite approximately.
The ratio of their two peak heights
To evaluate the catalytic performances of obtained nanosized
ZSM-5 zeolites (the sample for 3 h crystallization by “OSDA-
confined” strategy) after being protonated as acid catalysts,
continues to rise with the increase of time, reaching to 0.80 at
Fig. 7 UV Raman of parent zeolite BEA and solid product zeolite ZSM-5 synthesized for
varied time (Sample no. 1-6 in Table 1)
Fig. 8 The conversion curves of self-etherification of BA over H-nano-ZSM-5 (A) and com-
ZSM-5 (B)
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Page 8 of 10
ARTICLE
Journal Name
Table
etherification of BA
2
Catalytic performances of H-nano-ZSM-5 and com-ZSM-5 on self-
zeolite and crystallization process were well modulated. The
parent BEA zeolite was decomposed at the function of NaOH,
View Article Online
DOI: 10.1039/D0CY01345E
a
releasing building units, while the high local concentration of
TPAOH could protect the parent BEA zeolite from decomposing
too fast as well as facilitate the efficient organization of building
units produced to construct a new framework. Thereof,
nanoscale high silica ZSM-5 zeolite with a 73.8% solid yield was
obtained after 3 h of interzeolite transformation. Detailed
-
1 -1
Sample
Reaction time
Conversion of BA
TOF (moBAmolAl h )
(
h)
1
(%)
38.2
87.4
6.2
H-nano-ZSM-5
com-ZSM-5
212.7
162.8
30.3
3
1
3
15.3
25.0
a
Reaction conditions: benzyl alcohol = 0.8 ml (7.6 mmol), catalyst = 0.20g, investigation on the transformation process revealed that the
o
temperature = 110 C.
whole crystallization process could be divided into three
phrases, raw material deconstruction, small size product
named H-nanoZSM-5, a reaction of bulky molecules, the self-
formation and product growth. The interzeolite conversion
etherification reaction of benzyl alcohol (BA) to dibenzyl ether
achieved by means of the deconstruction of 6Rs, 5Rs and 4Rs in
(
5
DE) as shown in Scheme 2, was applied. The commercial ZSM-
zeolites catalyst bought from Nankai University Catalyst Co.,
the BEA zeolite framework and the combination of 5Rs to
produce the ZSM-5 zeolite structure. The obtained nanosized
ZSM-5 zeolite catalyst showed extremely high catalytic activity
in the self-etherification reaction of benzyl alcohol (BA) to
dibenzyl ether (DE) compared with the commercial one because
of the relatively small crystal size.
Ltd, named com-ZSM-5, was used as a comparison sample
which possesses a similar molar ratio of Si/Al, 213 measured by
ICP, to that of H-nanoZSM-5 as mentioned above.
The acidic properties of com-ZSM-5 and H-nanoZSM-5 were
tested by NH
3
-TPD and the results were shown in Table S1. The
results declare the two catalysts possess similar acidity. The big
differences of the two were the size of the particles and some Conflicts of interest
other structure properties. The zeolite particles of com-ZSM-5
There are no conflicts to declare.
was about 1 µm and that of H-nanoZSM-5 was only about 60
nm. As shown Table S2, although they possessed similar SBET and
S
micro and Vmicro, H-nanoZSM-5 obtained a higher external
Acknowledgements
This study was supported by the National Natural Science
Foundation of China (21902154)
surface area and a larger mesoporous volume than that of com-
ZSM-5. It would be expected that bulk molecules could
complete catalytic transformation more easily on H-nanoZSM-5
because a higher external surface provide more convenient
reactive sites. Besides, a larger mesoporous volume could also
bring enhanced diffusion properties.
Notes and references
As shown in Fig. 8, H-nanoZSM-5 exhibited higher catalytic
activity in comparison with the commercial one. The conversion
of them both increased gradually as time extended but the H-
nanoZSM-5 possesses a higher growth rate of BA conversion
than the commercial one. Besides, it could be seen in Table 2
that TOF value over H-nanoZSM-5 was about seven time of that
over com-ZSM-5. As previous literature reported, the self-
etherification reaction principally occurred on the external
surface of the ZSM-5 zeolite with ten-membered rings (10Rs)
channel system due to the large molecular size of raw material
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Catalysis Science & Technology
Page 10 of 10
View Article Online
DOI: 10.1039/D0CY01345E
High Yield Synthesis of Nanoscale High-silica ZSM-5 Zeolites via
“OSDA-Confined” Interzeolite Transformation
An “OSDA-Confined” interzeolite transformation strategy was employed
successfully to synthesized nanoscale high-silica ZSM-5 with high yield.