N -Methyl-2-pyrrolidone-promoted crystallization of MEL zeolite and its acceleration mechanism

Article abstract of DOI:10.1039/d0ta08107h

ZSM-11 zeolite is an important microporous material that has wide application prospects in various petrochemical and chemical processes. Herein, we reported a simple method to significantly reduce the crystallization time of ZSM-11 zeolite by adding N-methyl-2-pyrrolidone (NMP) as the 'promoter'. The effects of various synthetic parameters on the NMP-assisted synthesis of ZSM-11 zeolite were also investigated. Subsequently, characterization techniques such as SEM, XRD, XRF, FT-IR, TEM-EDS, TG and 13C NMR were used to unveil the acceleration mechanism of NMP. It was revealed that NMP can facilitate the bonding of TBA+ with aluminosilicate by breaking the hydration layer around them, thus reducing the apparent nucleation activation energy and accelerating the nucleation process. Meanwhile, we used N-ethyl-2-pyrrolidone (NEP) instead of NMP as the promoter to verify the acceleration mechanism. In addition, the fast-synthesized ZSM-11 zeolite exhibited comparable composition, hydrothermal stability, and catalytic performance to conventionally synthesized counterparts. This journal is

Full text of DOI:10.1039/d0ta08107h

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N-Methyl-2-pyrrolidone-promoted crystallization
of MEL zeolite and its acceleration mechanism†
Cite this: DOI: 10.1039/d0ta08107h
ab
ab
a
a
a
Chuanyu Yang,‡ Dongpu Zhao,‡ Weifeng Chu, Yanan Wang, Xiujie Li,
Sujuan Xie, Wenjie Xin, Xiangxue Zhu, * Shenglin Liu* and Longya Xu *a
a
a
a
a
ZSM-11 zeolite is an important microporous material that has wide application prospects in various
petrochemical and chemical processes. Herein, we reported a simple method to significantly reduce the
crystallization time of ZSM-11 zeolite by adding N-methyl-2-pyrrolidone (NMP) as the ‘promoter’. The
effects of various synthetic parameters on the NMP-assisted synthesis of ZSM-11 zeolite were also
investigated. Subsequently, characterization techniques such as SEM, XRD, XRF, FT-IR, TEM-EDS, TG and
13
C NMR were used to unveil the acceleration mechanism of NMP. It was revealed that NMP can
+
facilitate the bonding of TBA with aluminosilicate by breaking the hydration layer around them, thus
reducing the apparent nucleation activation energy and accelerating the nucleation process. Meanwhile,
we used N-ethyl-2-pyrrolidone (NEP) instead of NMP as the promoter to verify the acceleration
mechanism. In addition, the fast-synthesized ZSM-11 zeolite exhibited comparable composition,
hydrothermal stability, and catalytic performance to conventionally synthesized counterparts.
Received 18th August 2020
Accepted 15th November 2020
DOI: 10.1039/d0ta08107h
rsc.li/materials-a
researchers have been devoted to reducing the cost and envi-
ronmental hazards of zeolite synthesis. Notably, Liang et al.
Introduction
14
ZSM-11 zeolite (structural type MEL) was rst obtained by proposed a green synthesis process with addition of minimal
1
Socony Mobil in 1973, which shows a similar pore size and amounts of water and structure directing agents and shortened
2,3
framework density to ZSM-5 zeolite. However, unlike ZSM-5 steps to prepare nano-sized SSZ-13 zeolite (0.12 mm) using the
zeolite that has intersecting sinusoidal and straight channels, trans-crystallization strategy. Meanwhile, high crystallization
ZSM-11 zeolite possesses two crossing straight channels (5.3 ꢀ temperature tends to produce the ZSM-5 phase. However,
2
˚
reducing the content of organic templates and crystallization
5
.4 A). Thus, the diffusion resistance in the channel of ZSM-11
15,16
zeolite is supposed to be weaker than that within ZSM-5 temperature will greatly increase the synthesis time.
zeolite. As a consequence, ZSM-11 exhibits outstanding cata- exploring a simple strategy to accelerate the crystallization of
lytic performance in the ethene to propene and butene reac- ZSM-11 zeolite under low temperature and organic template
tion, dehydration of glycerol to acrolein, ethanol to propylene concentration is of great interest from both academic and
reaction, and alkylation of benzene with dimethyl ether/ industrial points of view.
Thus,
4,5
6
4,7
8
9
ethanol processes.
Nowadays, several strategies for the rapid synthesis of
Currently, zeolite ZSM-11 is synthesized primarily under zeolites have been developed, including the microwave-assisted
1
7,18
hydrothermal conditions, with tetrabutylammonium bromide approach,
the seeding (including homogeneous and
19
(
TBABr) or tetrabutylammonium hydroxide (TBAOH) as the heterogeneous seeds) method and the introduction of crys-
7,10
organic structure-directing agent (OSDA).
However, unlike tallization promoters. Among them, the introduction of partic-
11–13
ZSM-5 zeolite that could be synthesized without OSDAs,
synthesis of ZSM-11 is usually performed under high OSDA Yu et al. reported that hydroxyl free radicals (cOH) could
the ular promoters has been widely considered recently. Notably,
20
21
content and high temperature. The use of high OSDA content accelerate the crystallization processes of zeolites. Hu et al.
not only increases the cost of production, but also leads to found that the incorporation of citric acid (CA) into the
severe environmental pollution. Especially in recent years, synthesis gels of silicoaluminophosphate (SAPO-n) could
signicantly reduce the crystallization time too. Lately, our
group also discovered that N-methyl-2-pyrrolidone (NMP) as an
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
organic promoter could increase the crystallization rate of beta
Academy of Sciences, Dalian 116023, Liaoning, PR China. E-mail: zhuxx@dicp.ac.
22
cn; slliu@dicp.ac.cn; lyxu@dicp.ac.cn
b
zeolite.
Indeed, for the hydrothermal synthesis of ZSM-11 zeolite, the
University of Chinese Academy of Sciences, Beijing 100049, PR China
†
1
‡
Electronic supplementary information (ESI) available. See DOI: induction period is pretty long, but the growth period is very
0.1039/d0ta08107h
10
short. And the induction period accounts for a large part of the
Equally contributed to this work.
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ꢁ
whole synthesis time. So, it is of great signicance to shorten
The synthesized zeolite samples were calcined at 550 C for
+
the induction period for the fast synthesis of ZSM-11 zeolite. 6 h to remove the organic species, and the Na -ZSM-11 zeolites
23,24
ꢂ1
Sandra Burkett and Davis
investigated the nucleation were ion-exchanged three times with 0.8 mol L
NH NO3
4
ꢁ
mechanism of OSDA-mediated synthesis of pure-silica zeolites. solution (80 C, 2 h), washed, and dried. The NH -form zeolites
They found that the rate of nucleation slowed down when using were calcined at 530 C for 4 h to transform into the H-forms,
4
ꢁ
D
2
O instead of H
hydrogen bonding network of D
O, and thus the reconstruction of the network to allow the
2
O during synthesis. It was reported that the and the corresponding H-form samples were denoted as
2
O is more strong than that of HZ11-xNMP-y h.
H
2
organic and silicate components to come into contact during Synthesis of ZSM-5 zeolite
nucleation may be slowed in D O. Therefore, we proposed the
2
Typically, at room temperature, 0.94 g of aluminium sulfate,
.78 g of tetrapropylammonium hydroxide (TPAOH, 25 wt%)
and 0.69 g of sodium hydroxide were dissolved in 6.70 mL H
concept of introducing a strongly polar and aprotic chemical to
6
+
weaken the hydration layer around TBA and aluminosilicate
2
O
for rapidly establishing the interaction between them thereby
accelerating nucleation.
with fast stirring, and 0.00 or 12.49 g of NMP and 9.83 g of
chromatography silica gel (100 wt%) were added sequentially.
The nal gel with a molar composition of 1.0 SiO /0.10 Na O/
Herein, we demonstrated a novel approach for rapidly
synthesizing ZSM-11 zeolite by adding low-toxic, cost-effective
2
2
0
.01667 Al
2
O
3
/0.1 TBABr/8 H
2
O/x NMP (x ¼ 0.0 and 1.5) was
25
and biodegradable NMP. Meanwhile, the effects of other
synthesis parameters on the acceleration effect of NMP were
also investigated in depth. In our previous work, the NMP-
assisted synthesis of beta zeolite needed high OSDA concen-
tration and the added NMP partially underwent ring-opening
reaction in a highly alkaline hydrothermal environment to
introduced into a 75 mL Teon-lined stainless-steel autoclave
rotated at 60 rpm, and crystallized under autogenous pressure
ꢁ
at 120 C. Aer crystallization, the solid product was separated,
ꢁ
washed and dried overnight at 120 C.
Synthesis of MOR zeolite
+
22
promote the formation of activated TEA . However, in this
work, with the aid of NMP, pure ZSM-11 zeolite could even be
rapidly synthesized under an ultra-low content of organic
Typically, at room temperature, 0.36 g of aluminium sulfate,
0.70 g of tetraethylammonium bromide (TEABr, 98 wt%) and
+
0.58 g of sodium hydroxide were dissolved in 21.85 mL H O with
2
templates (TBA /SiO2 ¼ 0.02). More importantly, no ring-
fast stirring, and 0.00 or 7.51 g of NMP and 9.84 g of silica sol
opening product of NMP was detected throughout the hydro-
thermal synthesis of ZSM-11 zeolite and the acceleration
(30 wt%) were added sequentially. The nal gel with a molar
22
composition of 1.0 SiO /0.196 Na O/0.035 Al /0.065 TEABr/32
2
2
2 3
O
mechanism was much different from that of beta zeolite.
Meanwhile, it was also found that the NMP content, crystalli-
zation temperature and Si/Al ratio all signicantly affected the
acceleration effect of NMP. In addition, the as-synthesized ZSM-
1 catalyst exhibited similar hydrothermal stability to the
H
2
O/x NMP (x ¼ 0.0 and 1.5) was introduced into a 75 mL
Teon-lined stainless-steel autoclave rotated at 40 rpm, and
crystallized under autogenous pressure at 150 C. Aer crys-
2
ꢁ
tallization, the solid product was separated, washed and dried
1
ꢁ
overnight at 120 C.
conventional ZSM-11 sample and excellent reaction perfor-
mance in the alkylation of ethanol with benzene.
Characterization
X-ray diffraction (XRD) data used for the phase identication
were collected on a PANalytical B. V. X-ray diffractometer using
Experimental section
Synthesis of ZSM-11 zeolite
˚
Cu Ka radiation (l ¼ 1.54059 A), operating at 40 kV and 40 mA,
26
ꢁ ꢁ
The samples were synthesized according to our previous work.
in the range of 2q from 5 to 50 . The relative crystallinity (RC)
Typically, at room temperature, 0.54 g of aluminium sulfate was calculated according to the ratio of the integrated height of
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(
Al
bromide (TBABr, 99 wt%) and 0.58 g of sodium hydroxide and 45.0 ꢃ 0.1 ) of the sample and the well crystallized sample.
NaOH, 96 wt%) were dissolved in 28.8 mL H O with fast stir- X-ray uorescence (XRF) experiments were performed using
ring, and 0.00 or 7.51 g of N-methyl-2-pyrrolidone (NMP, a PANalytical zetium X-ray uorescence spectrometer.
2
(SO
4
)
3
$18H
2
O, 99 wt%), 3.22 g of tetrabutylammonium the peaks (2q ¼ 7.9 ꢃ 0.1 , 8.8 ꢃ 0.1 , 23.0 ꢃ 0.1 , 23.8 ꢃ 0.1
ꢁ
ꢁ
(
2
9
9 wt%) and 9.83 g of silica sol (30 wt%) were added sequen-
N adsorption and desorption experiments were performed
2
ꢁ
tially. The nal gel with a molar composition of 1.0 SiO /0.15 at ꢂ196 C on a Micromeritics ASAP-2020 analyzer. Each sample
2
ꢁ
ꢂ3
Na
2
2
O/0.016 Al
2
O
3
/0.2 TBABr/40 H
2
O/x NMP (x ¼ 0.0, 0.5, 1.0, was degassed at 350 C for 5 h under a vacuum of 10 Pa before
.0, 2.5 and 3.0) was introduced into a 75 mL Teon-lined analysis. Fourier transform infrared (FT-IR) spectroscopy was
stainless-steel autoclave rotated at 40 rpm, and crystallized performed using a VERTEX 70 FT-IR spectrometer (Bruker
ꢁ
under autogenous pressure at 130 C. Aer crystallization, the Company). Thermogravimetric analysis (TGA) was performed
ꢂ1
solid product was separated, washed and dried overnight at on a Pyris diamond TG/DTA analyser in air ow (60 mL min
)
ꢁ
ꢁ
ꢂ1
120 C. All zeolite samples were named Z11-xNMP-y h, where x and a ramp rate of 10 C min .
was the NMP/SiO ratio and y was the crystallization time. In
The scanning electron microscopy (SEM) observation was
2
addition, the synthesis process of ZSM-11 zeolite with N-ethyl-2- carried out on a HITACHI SU1510 microscope. Transmission
pyrrolidone (NEP, 99 wt%) as the promoter was the same as that electron microscopy (TEM) and selected area electron diffrac-
of NMP.
tion (SED) were performed using a JEM-2100 transmission
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electron microscope (JEOL Company). The acidic properties of
the H-form ZSM-11 zeolites were analyzed using temperature-
Ea 1
ln V ¼ ln C ꢂ
R T
1
3
programmed desorption of ammonia (NH -TPD). The C MAS
3
where V represents the nucleation rate or growth rate, Ea
represents E or E , C is a constant, R is the gas constant (8.314 J
g n
NMR spectra were recorded on a Bruker AV-400 spectrometer.
ꢂ1 ꢂ1
mol
K ), and T is the crystallization temperature.
Crystallization kinetics
The crystallization curve of crystallinity with crystallization time
at different temperatures was measured and drawn here. The
corresponding crystallization time is the crystal nucleation
Catalytic tests
The alkylation of ethanol with benzene was performed in
a stainless xed bed reactor with 0.5 g catalyst. First, the catalyst
was pretreated in N (40 mL min ) at 400 C for 2 h. The
induction period t
/t is used as the nucleation rate V
rising segment of the crystallization curve is the growth rate Vg.
0
when the crystallinity is 5%. The reciprocal
ꢂ1
ꢁ
2
1
0
n
. The slope of the rapidly
reaction was performed under the conditions of 1.5 MPa, 380 or
ꢁ
4
00 C, and benzene/ethanol molar ratio of 6.0, and the weight
The apparent growth activation energy E and apparent nucle-
g
ꢂ1
hourly space rate (WHSV) of ethanol was 1 h . Liquid phase
products were analyzed using an Agilent 7890A GC equipped
with a PONA capillary column (50 m ꢀ 0.2 mm ꢀ 0.5 mm) and
a FID.
ation activation energy E follow the Arrhenius equation:
n
Results and discussion
(I) The acceleration effect of NMP
To investigate the effect of the cost-effective and low-toxic lac-
tam NMP (Scheme 1) on ZSM-11 zeolite crystallization, the
samples with initial gel compositions of 1.0 SiO /0.016 Al O /
2
2 3
0
.15 Na O/0.2 TBABr/40 H O/x NMP (x ¼ 0.0 and 1.5) were
2
2
ꢁ
hydrothermally treated at 130 C for 72 h. In the synthesis
system containing NMP, the XRD pattern and SEM image of the
as-synthesized product Z11-1.5NMP-72 h are shown in Fig. 1A
Scheme 1 Chemical structural formula of NMP and NEP.
Fig. 1 XRD patterns of Z11-1.5NMP-72 h (A) and Z11-0NMP-72 h (B), SEM images of Z11-1.5NMP-72 h (C) and Z11-0NMP-72 h (D).
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and C, respectively. The diffraction peaks with strong intensity poorly crystalline ZSM-11 zeolite appeared when the NMP/SiO
2
accorded with the typical topological characteristics of the MEL was increased to 0.5. Furthermore, the relative crystallinity of
3
framework, while, in the NMP-free synthesis system, the the products increased gradually with the increase of NMP/SiO2
product was still amorphous nanoparticles, as shown in Fig. 1B ratio from 0.5 to 3.0. This indicated that the crystallization rate
and D. This indicated that the ZSM-11 zeolite with high crys- increased with the amount of NMP in the gel.
tallinity was rapidly synthesized by employing NMP as
a promoter and the crystallization rate of ZSM-11 zeolite was of NMP was also investigated with the initial gel molar
truly increased by NMP. composition of 1.0 SiO /y Al /0.15 Na O/0.2 TBABr/40 H O/x
2
Meanwhile, the effect of Si/Al ratio on the acceleration effect
2
2
O
3
2
2
Furthermore, we tracked the crystallization process of ZSM- NMP (x ¼ 0.0 and 1.5; y ¼ 0.025, 0.016, 0.0025 and 0.0004) at
ꢁ
1
1 zeolite to further investigate the acceleration effect of NMP. 130 C. As shown in Fig. 3 and S5,† the required crystallization
In the absence of NMP, as conrmed by XRD (Fig. S1B†), weak time decreased with the increase of Si/Al molar ratio for both
2
diffraction peaks assigned to poorly crystalline ZSM-11 zeolite synthesis systems with and without NMP. This indicated that
emerged at 116 h. Highly crystalline zeolite ZSM-11 was devel- the higher the Si/Al molar ratio was, the faster the crystalliza-
2
oped until 123 h, while in the presence of NMP, primary ZSM-11 tion rate was. A similar phenomenon had also been reported in
crystals were already visible at 54 h as shown in SEM images the synthesis of ZSM-5 and SSZ-13 zeolite by Toru Wakihara
27,28
(Fig. S3†), and the XRD patterns (Fig. S1A†) showed that highly et al.
They thought that aluminosilicate species were more
crystalline ZSM-11 zeolite was obtained at 68 h. In comparison stable than silicate species and were not easily converted into
with the NMP-free synthesis system, the induction period in the crystals, resulting in a long crystallization period. However,
presence of NMP was obviously much shorter, while the growth under the same Si/Al ratio, the required crystallization time of
2
period was similar, as conrmed by the corresponding crystal- ZSM-11 zeolite with NMP was shorter than that without NMP. In
lization curves (Fig. 2). This indicated that NMP plays a crucial fact, with the aid of NMP, the required crystallization time was
role in promoting the nucleation process of ZSM-11 zeolite. In reduced from 204, 123, 58 and 46 h to 98, 72, 48 and 42 h for the
addition, the synthesis system with only NMP as the sole Si/Al
2
molar ratios of 40, 60, 400 and 2500 of the gel, respec-
organic additive was also investigated (Fig. S6†), and the tively. Especially, the crystallization curves with and without
product was still amorphous even aer hydrothermal treatment NMP were almost identical for the synthesis system with a Si/Al
at 130 C for 130 h. This indicated that NMP itself cannot direct ratio of 2500. We dened the promote factor (PF) as the ratio of
2
ꢁ
the crystallization of ZSM-11, but acts as a promoter in the crystallization time needed without promoter and with
+
presence of TBA .
promoter, which is used to reect the acceleration ability of
a promoter. Interestingly, with the increase of Si/Al molar ratio
2
from 40 to 2500, the PF decreased from 2.08 to 1.10. The results
(
II) Effects of synthetic parameters on the acceleration effect
suggested that the acceleration effect of NMP decreased with
of NMP
the increase of Si/Al molar ratio.
2
In order to obtain ZSM-11 zeolite more economically, we also
studied the effect of OSDA concentration on ZSM-11 zeolite
synthesis in the presence of NMP. The initial gel with molar
compositions of 1.0 SiO /0.0166 Al O /0.15 Na O/m TBABr/40
2 2 3 2
To investigate the effects of synthetic parameters on the accel-
eration effect of NMP, rst, the synthesis was performed under
different NMP contents (NMP/SiO ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 2.5
2
ꢁ
and 3.0) at 130 C for 68 h. As shown in Fig. S4,† the product was
H
2
O/1.5 NMP (m ¼ 0.2, 0.1, 0.02 and 0.01) was hydrothermally
amorphous in the absence of NMP. Weak peaks assigned to
ꢁ
treated at 130 C for 72 h. As shown in Fig. S6 and S7,† pure and
Fig. 2 Crystallization curves of ZSM-11 zeolite synthesized with NMP
ꢁ
ꢁ
and without NMP under TBABr/SiO
2
¼ 0.2 and 0.02 at 130 C.
Fig. 3 Si/Al
2
dependence of acceleration effect of NMP at 130 C.
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highly crystalline ZSM-11 zeolite was observed under the
In addition, we used TBAOH instead of TBABr as OSDAs, and
condition of TBABr/SiO ¼ 0.02–0.2. However, when the TBABr/ studied the effect of NMP on the synthesis of ZSM-11 zeolites
2
SiO ratio was further reduced to 0.01, the product was ZSM-5/ with molar compositions of 1.0 SiO /0.0166 Al O /0.15 Na O/0.2
2
2
2
3
2
ꢁ
ZSM-11 co-crystalline zeolite.
TBAOH/40 H O/x NMP (x ¼ 0.0 and 1.5) at 130 C. As shown in
2
+
Furthermore, under the ultra-low content of OSDAs (TBA / Fig. S11 and S12,† in comparison with the case of TBABr as
SiO
¼ 0.02), the crystallization curves of ZSM-11 zeolite are OSDAs (Fig. 2 and 3), the full crystallinity time of ZSM-11 zeolite
shown in Fig. 2. In the synthesis system without NMP, the was obviously longer. Similarly, adding NMP into the synthesis
nucleation period in the case of TPABr/SiO
¼ 0.02 was obvi- system could also accelerate the nucleation. Furthermore, the
ously prolonged in comparison with the case of TPABr/SiO PF was 2.70 and the acceleration effect was more obvious.
.2. However, in the presence of NMP, ZSM-11 zeolite could still Interestingly, in the case of TBABr as OSDAs, their primary
2
2
2
¼
0
be synthesized rapidly when the TPABr/SiO ratio was reduced particle size was similar (Fig. S13†) and the granular aggregates
2
to 0.02 and the crystallization curve was roughly coincident with of ZSM-11 zeolite obtained by adding NMP were larger than
a TBABr/SiO ratio of 0.2. With the decrease of TBABr/SiO ratio those in the absence of NMP (Fig. S2† (72 h) vs. Fig. S3† (123 h)).
2 2
from 0.2 to 0.02, the PF increased from 1.71 to 1.84. The above However, in the case of TBAOH as OSDAs, the nal granular
results suggested that the NMP exhibited a more signicant aggregates of ZSM-11 zeolite had a similar size no matter that
acceleration effect under the lower OSDA concentration. It NMP was added or not (Fig. S14†). The results showed that NMP
should be noted that NMP is much more low-toxic and cost- could not only promote nucleation, but also promote particle
effective compared with TBABr. More importantly, the added aggregation in the presence of bromine ions.
NMP did not decompose throughout the hydrothermal
In a word, the NMP content, OSDA content, Si/Al2 ratio,
synthesis (see below). Meanwhile, NMP remained in the mother synthesis temperature, and type of OSDAs collectively affect the
liquor and was not detected in the solid product (Fig. 5D). This acceleration effect of NMP. Among them, NMP exhibited a more
indicated that NMP may act as a recyclable promoter. Thus, this signicant acceleration effect under low Si/Al
low-OSDA rapid synthesis approach of ZSM-11 zeolite can content and low synthesis temperature.
signicantly reduce production costs and is highly efficient and
2
ratios, low OSDA
environment-friendly.
(III) Investigation of the acceleration mechanism
Furthermore, we compared the production cost of the NMP-
assisted rapid synthesis process with that of the conventional To unveil the hydrothermal evolution process of ZSM-11 zeolite
synthesis process. The costs of raw materials, electricity, and assisted by NMP, the crystallization processes of ZSM-11
labor are shown in Table S1.† Herein, we calculated the theo- zeolites with molar compositions of 1.0 SiO /0.016 Al O /0.15
2
2 3
ꢁ
retical production cost of 500 kg ZSM-11 zeolite as shown in Na O/0.2 TBABr/40 H O/x NMP (x ¼ 0.0 and 1.50) at 130 C were
2
2
Table S2.† Under the synthesis parameters of TBABr/SiO
2
¼ 0.2 investigated in detail. The evolution processes of structural
and 0.02, the production costs of the NMP-assisted rapid subunits recorded by FT-IR spectroscopy are shown in Fig. 4.
synthesis process were lower than those of the traditional For the synthesis system with NMP, only three FT-IR bands
ꢂ1
ꢂ1
ꢂ1
synthesis process, 91 254 U per ton vs. 94 532 U per ton and centered at 470 cm , 800 cm and 1113 cm were observed
ꢁ
64 134 U per ton vs. 75 012 U per ton, respectively.
in the samples synthesized at 130 C for 0 and 50 h, belonging to
If the synthetic mother liquor is recycled, the cost of NMP the bending vibration, symmetric stretching vibration and
can be ignored, since NMP does not decompose throughout the antisymmetric stretching vibration absorption peak of the
29
hydrothermal synthesis and maintains its intrinsic properties T–O–T bond of the gel, respectively. At 64 h, bands at 1300–
+
ꢂ1
ꢂ1
+
in the mother liquor. Moreover, the TBA consumption of the 1500 cm assigned to TBA were observed and the band at
+
ꢂ1
2
two methods are the same (TBA /SiO z 0.02) based on the 1113 cm began to split into two bands at 1222 and 1098 cm ,
thermogravimetry results (Fig. S16†). Thus, the theoretical which belong to the antisymmetric stretching vibration
production costs for the NMP-assisted rapid synthesis process absorption peaks of the external TO tetrahedron and internal
4
2
9
and traditional synthesis process are 47 012 U per ton and TO tetrahedron, respectively. Meanwhile, a weak absorption
4
ꢂ1
7
5 012 U per ton. In a word, the production cost of the NMP- peak appeared at 545 cm , which belongs to the double ve-
assisted rapid synthesis process is signicantly lower than membered rings (D5Rs) of crystalline ZSM-11. This indicated
+
that of the traditional synthesis process.
that aluminosilicates were combined with TBA and the ZSM-11
The acceleration effect of NMP on the synthesis of ZSM-11 phase began to appear in the product. With the prolongation of
ꢂ1
zeolite was also investigated at different temperatures with crystallization time from 64 to 90 h, the band at 545 cm of the
2 2 3
the initial gel molar compositions of 1.0 SiO /0.0166 Al O /0.15 product became sharper and narrower gradually, and corre-
Na O/0.2 TBABr/40 H O/x NMP (x ¼ 0.0 and 1.5). As can be seen spondingly, the crystallinity of the product increased. This was
2
2
from Fig. S8 and S9,† the required crystallization time for fully consistent with the results of SEM and XRD. Notably, the band
crystalline ZSM-11 decreased with the increase of temperature, of NMP molecules was not observed throughout the crystalli-
2
7
ꢁ
due to the temperature effect. At 150, 140 and 130 C, the PFs zation process.
were 1.44, 1.56 and 1.71, respectively. The above results showed In contrast, in the absence of NMP, the bands assigned to
+
that the promotion effect of NMP increased with the decrease of D5Rs and TBA were observed until 116 h, suggesting that the
crystallization temperature and NMP was an excellent low- introduction of NMP could prominently promote incorporation
+
temperature crystallization promoter.
of
aluminosilicates
with
TBA ,
polymerization
of
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Fig. 4 FT-IR spectra of samples crystallized with ((a) NMP/SiO
2
¼ 1.5) and without ((b) NMP/SiO
2
¼ 0.0) NMP for different periods of time at
ꢁ
1
30 C.
22
aluminosilicates and the formation of the D5Rs secondary decomposed to TEA during hydrothermal treatment, in this
+
structure units of ZSM-11 zeolite.
work, only peaks associated with aqueous TBA and NMP
The thermal analysis results are shown in Fig. S16.† All (marked as 1, 2, 3, 4 and 5) species were detected in the
ꢁ
samples showed weak weight loss peaks before 150 C, which supernatant. Namely, NMP was relatively stable in our synthesis
corresponded to the removal of adsorbed water in the samples. system and no ring-opening reaction of NMP occurred. This is
ꢁ
ꢁ
The weight loss at 340–455 C and 455–670 C was related to the probably due to the lower alkalinity and crystallization
+
oxidation and decomposition of TBA occluded in ZSM-11 temperature of the synthesis system of ZSM-11 than that of beta
zeolite. The synthesis system with (Fig. S16a and b†) and zeolite. These results suggest that the NMP-mediated rapid
without (Fig. S16c and d†) NMP showed a similar variation synthesis of ZSM-11 zeolite was much different from that re-
tendency, i.e., the quantity of the occluded TBA species rst ported on beta zeolite.
increased with crystallization time, and then kept almost
constant when the crystallization was complete. The weight organic TBA species existed in the washed products (as also
losses assigned to TBA of well crystalline products were suggested by the IR spectra, see Fig. 4A), implying that only
similar, about 11.5–12.3%. However, the combination rate of TBA species were indeed occluded in ZSM-11 zeolite and the
TBA with aluminosilicates in the NMP-containing system was added NMP only remained in the mother liquor and kept intact.
22
+
22
1
3
Solid-state C NMR spectra (Fig. 5D) revealed that only
+
+
+
+
notably faster than that in the NMP-free system. Since NMP was
The evolution of inorganic elemental compositions of solid
not detectable in the solid product, it may be assumed that the products with crystallization time was surveyed by XRF and is
+
NMP molecules promoted the combination of organic TBA
2
summarized in Table S3.† The Si/Al ratio of the amorphous
with inorganic aluminosilicates to form organic–inorganic product at 0 h was about 46, which was lower than feeding. This
composite species (key precursor for nucleation), and thus the indicated that at rst more silicon dissolved into the liquid. For
nucleation process was promoted.
the synthesis system with NMP and without NMP, with the
The liquid-state C NMR spectra of the supernatant are prolongation of crystallization time, the ratios of Si/Al all
shown in Fig. 5. In the absence of NMP, the spectra only increased gradually at rst, and then remained basically
1
3
2
+
2
exhibited peaks associated with aqueous TBA (Fig. 5A, marked unchanged. The Si/Al ratios of the nal product synthesized by
as a, b, c and d). In the presence of NMP, differently from NMP- adding NMP were slightly higher than that of without NMP (52.1
mediated rapid synthesis of beta zeolite, in which partial NMP vs. 49.2). Accordingly, the yield of products in the NMP-added
underwent ring-opening and a large amount of TEAOH system was slightly higher than that of non-NMP-added
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1
3
13
Fig. 5 (A) C liquid NMR spectra of the supernatants synthesized without NMP at various crystallization times; (B) C liquid NMR spectra of the
13
+
13
supernatants synthesized with NMP at various crystallization times; (C) C liquid NMR spectra of TBA and NMP; (D) solid-state C NMR spectra
of the solid product synthesized with NMP.
(
87 wt% vs. 82 wt%). In addition, we also investigated the Si/Al2 follows a similar nucleation mechanism. As shown in Scheme 2,
+
ratio of ZSM-11 zeolite synthesized under the initial gel with rst, ordered hydration spheres around the TBA cation and
different Si/Al ratios. The results are shown in Table S4,† and soluble aluminosilicate species formed due to the hydration
2
2
the Si/Al ratios of the nal products synthesized by adding effect, and then they collided which led to overlapping of
NMP all are higher than that of without NMP too. This may be hydration spheres. The water molecules surrounding the OSDAs
attributable to the decrease of saturated solubility of water due were replaced by aluminosilicate species as the key step in the
to the addition of NMP.
nucleation process. Subsequently, favorable electrostatic and
+
Fig. S17† displays TEM-EDS results of samples synthesized van der Waals contacts between the alkyl chains of TBA and the
with NMP and without NMP during the hydrothermal synthesis. aluminosilicate species were established and generated the
During the hydrothermal crystallization process of ZSM-11 inorganic–organic composite species. These species provide the
zeolite, whether NMP was added or not, the silicon and means by which the geometry of the OSDAs was translated into
aluminum species formed a primary amorphous aluminosili- the zeolite pore architecture.
cate species with a certain proportion, and silicon and
According to the experimental results (Fig. 2), NMP mainly
aluminum species were incorporated into the zeolite framework played the role of accelerating the nucleation. Burkett and
2
3,24
synchronously. This is different from NMP-assisted conversion Davis
found that the rate of nucleation slowed down when
O instead of H O, due to the stronger hydrogen
a high Si/Al molar ratio was rstly formed. This may be an bonding network of D O than that of H O. Namely, strong
of USY into hollow beta zeolite, in which the beta crystal with using D
2
2
30
2
2
2
important reason why NMP assisted synthesis of ZSM-11 zeolite hydrogen bonding networks hamper the reconstruction of the
30,31
can't form a hollow structure like beta zeolite.
networks to allow the organic and silicate components to come
The above-described analyses provide valuable insights into into contact during nucleation. NMP is a strongly polar and
25
the accelerated formation mechanism of ZSM-11 zeolite by the aprotic lactam. As an ideal strong hydrogen-bond-acceptor
23,24
assistance of NMP. Burkett and Davis
investigated the (HBA) solvent, NMP can interact with water molecules
nucleation mechanism of pure-silicon zeolites with a structure through hydrogen bonding. Molecular dynamic simulation
directing agent, and we speculate that ZSM-11 zeolite synthesis results showed that NMP has a strong structural effect on water
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Scheme 2 Proposed mechanism of the NMP-assisted rapid synthesis of ZSM-11 zeolite.
32,33
molecules around it.
And thermodynamic analysis of NMP– species, thus reducing the apparent nucleation activation
H
2
O mixtures showed that the three-dimensional network of energy as shown in Fig. 6.
hydrogen bonds in H O underwent transformations and was The signicant variation of the acceleration effect of NMP
broken with the addition of NMP. Thus, we speculated on the under different Si/Al ratios may be due to the fact that the
2
34
2
accelerating nucleation mechanism of NMP. As shown in hydration of aluminosilicate is enhanced due to the charge
Scheme 2, in the suitable aluminosilicate synthesis system density increase with the increase of aluminum species. That is,
+
containing NMP, when NMP molecules were close to the TBA
the hydration layer around aluminosilicate intensies with the
and aluminosilicate species, NMP could break the hydration decrease in Si/Al ratio. Breaking the barrier of the hydration
2
+
layer around them. Due to the lack of the barrier of the hydra- layer and establishing the interaction between TBA and
tion layer, it was more easy and rapid to establish interaction aluminosilicate were more difficult, and hence the nucleation
+
between TBA and aluminosilicate species to generate the process was restricted, while, in the presence of NMP, the
+
inorganic–organic composite species that serve as the key hydration layer around TBA and aluminosilicate was weak-
components in nucleation and crystal growth. Thus, NMP could ened, and therefore, the nucleation was greatly accelerated.
accelerate the nucleation and promote the synthesis of ZSM-11 However, with the increase of Si/Al
zeolite. At present, the accelerated nucleation mechanism of
NMP lacks direct experimental evidence. In future work, direct
experimental evidence to prove the mechanism will be followed
up.
2
ratio, the hydration around
In addition, we calculated the apparent nucleation activation
energy and apparent growth activation energy of ZSM-11 zeolite
synthesized with and without NMP based on the crystallization
curves in Fig. S8.† As shown in Fig. S10 and Table S5,† the
apparent growth activation energies with and without NMP are
ꢂ1
similar, 136.4 and 134.1 kJ mol , respectively. However, the
apparent nucleation activation energies with and without NMP
ꢂ1
are 62.6 and 91.8 kJ mol , respectively. Clearly, the addition of
NMP to the synthesis system could reduce the apparent nucle-
ation activation energy. We speculated that NMP broke the
+
hydration layer around TBA and aluminosilicate, enabling the
+
water molecules around the TBA to be replaced more easily by
Fig. 6 Diagram of reducing the nucleation activation energy of ZSM-
aluminosilicate to generate the inorganic–organic composite 11 zeolite with NMP as the promoter.
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ꢁ
aluminosilicate was weakened too. It was easier to establish at 700 C for 6 h are shown in Fig. 7. The crystal structure of the
+
interaction between TBA and aluminosilicate. Thus, the sample could be well maintained, and the RCs of HZ11-
ꢁ
ꢁ
nucleation was fast under a high Si/Al ratio, and the accelera- 1.5NMP-72 h (700 C, 6 h) and HZ11-0NMP-123 h (700 C, 6 h)
2
tion effect of NMP was not as obvious as that under a low Si/Al2 remain at 98% (where the RC of HZ11-1.5NMP-72 h was
ratio.
assumed as 100%) and 97% (where the RC of HZ11-0NMP-
Furthermore, we used NEP instead of NMP as the promoter 123 h was assumed as 100%), respectively, aer the steam
to carry out the verication experiment under the same gel treatment. According to N
2
adsorption–desorption measure-
ꢁ
composition and crystallization conditions. NEP can also form ment results (Table S6†), HZ11-1.5NMP-72 h (700 C, 6 h) and
ꢁ
hydrogen bonds with H
2
O molecules, but it is weaker than HZ11-0NMP-123 h (700 C, 6 h) still had an abundant micro-
NMP. Namely, the ability to break the hydrated layer of NEP is porous structure although the microporous volume slightly
weaker than that of NMP. Thus, the crystallization time of the decreased. As shown in Fig. S20,† HZ11-1.5NMP-72 h and
synthesis system with NEP should be shorter than that without HZ11-0NMP-123 h possessed a similar acid amount. And aer
ꢁ
promoter, but longer than that with NMP, as demonstrated in the steam treatment with 700 C for 6 h, although the acid
Fig. S18.†
amount decreased, they still showed a nearly equal acid
Thermodynamic analysis results showed that an increasing amount. The above results demonstrated that the zeolite ZSM-
NMP concentration led to breakage of a three-dimensional 11 synthesized by NMP assistance exhibited similar hydro-
hydrogen bonding network of waters in NMP & water thermal stability to that synthesized without NMP.
3
4
mixtures. Therefore, the higher the NMP content (NMP/SiO ¼
The reaction performance of selected samples was evaluated
.0–2.5) in the hydrothermal synthesis system of ZSM-11, the in gas phase alkylation of ethanol with benzene. Table 1
more obvious the acceleration effect. But by adding excessive displays the reaction performances of HZ11-1.5NMP-72 h
2
0
ꢁ
ꢁ
NMP (NMP/SiO $ 2.5), the crystallization rate would not (700 C, 6 h) and HZ11-0NMP-123 h (700 C, 6 h). The selec-
2
increase due to the dilution effect. Obviously, the increase of tivities of liqueed petroleum gas and dry gas over the two
crystallization temperature itself would weaken the hydration catalysts were very low, and the selectivity of the liquid hydro-
effect around the template and aluminosilicate in the hydro- carbon products was as high as ca. 98% for whole reaction
thermal synthesis system. Thus, when NMP was added to the processes. As shown in Table 1, the two samples showed similar
synthesis system at a higher temperature, the promotion effect reaction activity (conversion of ethanol was 100%). The benzene
of NMP was weakened, and NMP had good low temperature conversions of the two samples were 14.57% and 15.16%
acceleration performance.
respectively (the highest conversion of benzene was 16.67% in
Overall, NMP itself does not decompose during the synthesis theory). Ethylbenzene and diethylbenzene were the main liquid
process, as the promoter, and NMP accelerated the nucleation hydrocarbon products. The selectivities of ethylbenzene were
+
of ZSM-11 zeolite mainly by promoting the binding of TBA and 83.30% and 83.86%, and the values of ethylation products
aluminosilicate that are surrounded by the hydrated layer. (ethylbenzene and diethylbenzene) were 97.95% and 97.83%,
Meanwhile, the NMP content, Si/Al
temperature all affect the binding rate.
Based on our proposed promotion mechanism, the crystal- catalyzed by acid. HZ11-0NMP-123 h (700 C, 6 h) and HZ11-
lization of MFI and MOR zeolites was also accelerated 1.5NMP-72 h (700 C, 6 h) samples also possessed similar acid
2
ratio, and synthesis respectively.
The alkylation of ethanol with benzene is a typical reaction
35
ꢁ
ꢁ
successfully by adding NMP to the synthesis systems. As shown amounts (Fig. S20†), besides the superior hydrothermal
in Fig. S19,† in the synthesis systems containing NMP, MFI and
ꢁ
MOR zeolites could be obtained by crystallization at 120 C for
ꢁ
2
4 h and 150 C for 94 h respectively. However, in the NMP-free
synthesis system, the corresponding products were still amor-
phous. This result suggested that the present strategy may be
generalized for accelerating the crystallization of zeolites
synthesized with cationic OSDAs.
(
IV) Hydrothermal stability and reaction performance
In general, the hydrothermal stability of the zeolite catalyst is
a crucial factor affecting its practical application, especially for
reactions containing water (such as the alkylation of benzene
with ethanol/methanol). Therefore, we compared the hydro-
thermal stability of ZSM-11 zeolites with a similar Si/Al ratio
2
prepared with and without NMP (Table S6†). In general, the
change of crystallinity before and aer high temperature
steam treatment is an index to measure the hydrothermal
stability of zeolite. The XRD patterns of HZ11-1.5NMP-72 h
and HZ11-0NMP-123 h before and aer 100% steam treatment Fig. 7 XRD patterns of samples before and after steaming treatment.
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ꢁ
ꢁ
Table 1 Reaction performance over HZ11-0NMP-123 h (700 C, 6 h) and HZ11-1.5NMP-72 h (700 C, 6 h) catalysts for alkylation of ethanol with
benzene
a
Conversion/%
Product selectivity/%
Non-aromatic
Benzene Ethanol Ethylbenzene Ethylation Methylbenzene C3 benzene* Xylene hydrocarbon Others
Catalyst
ꢁ
ꢁ
HZ11-0NMP-123 h (700 C, 6 h) 14.57
100
100
83.30
83.86
97.95
97.83
0.22
0.22
0.67
0.72
0.04
0.05
0.16
0.15
0.96
1.03
HZ11-1.5NMP-72 h (700 C, 6 h) 15.16
a
Reaction conditions: m(catalyst) ¼ 0.5 g; 400 C; 1.5 MPa; WHSV(ethanol) ¼ 1.0 hꢂ1; n(benzene)/n(ethanol) ¼ 6/1; time-on-stream ¼ 46 h. *:
ꢁ
propylbenzene and ethyltoluene.
stability (Fig. 7). Hence the conversion of reactants and the
selectivity of products over the two catalysts were similar. In
Notes and references
addition, the HZ11-1.5NMP-72 h catalyst possessed good reac-
tion performance. As shown in Fig. S21,† during the 500 h life
test, the conversion of ethanol and benzene remained at 100%
and over 15%, respectively. The selectivities of ethylbenzene
and ethylation products nearly kept at about 90% and 96%,
respectively.
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8
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Foundation of China (No. 21773233), Dalian Science and
Technology Innovation fund project (2019J12GX027), DICP
(Grant: DICP ZZBS201806), LiaoNing Revitalization Talents
Program (XLYC1901006 and XLYC1908010), and CNPC-DICP
Joint Research Center, Strategic Priority Research Program of
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