Efficient synthesis of aluminosilicate RTH zeolite with good catalytic performances in NH3-SCR and MTO reactions

Article abstract of DOI:10.1039/c8ta01734d

2,6-methyl-N-methylpyridinium, as a novel organic template, is employed for the synthesis of RTH aluminosilicate zeolite with a SiO₂/Al₂O₃ ratio of 17.6. The amount of 2,6-methyl-N-methylpyridinium template, the Na₂O/SiO₂ ratio, the SiO₂/Al₂O₃ ratio, and the H₂O/SiO₂ ratio in the starting gel significantly influence the crystallization of RTH zeolite. Several analytical methods such as XRD, SEM, N₂ sorption, TG-DTA, DRIFT and NMR were employed for the characterisation of the obtained RTH zeolites and to understand the crystallization process with the new template. Very interestingly, the crystallization of RTH zeolite with the new template takes a very short time (12 h at 130 °C and 50 min at 240 °C) compared with conventional RTH zeolite synthesis reported in the literature (72 h at 130 °C). Theoretical calculations show that this novel organic template has lower interaction energies for zeolite cage space filling than those of the organic templates previously reported in the literature, which lead to stronger structural directing. Kinetic results show that the activation energy of this novel organic template is much lower than that of the traditional one. Catalytic tests show that copper exchanged RTH zeolite (Cu-RTH) exhibits good catalytic properties in the NH₃-SCR reaction and the H-RTH zeolite catalyst has excellent selectivities for ethylene and propylene in MTO reactions.

Full text of DOI:10.1039/c8ta01734d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Efficient synthesis of aluminosilicate RTH zeolite with good
catalytic performances in NH3-SCR and MTO reactions
Hao Xu,a Qinming Wu,*,a Yueying Chu,b Jingang Jiang,c Ling Zhang,a Shuxiang Pan,a Changsheng
Zhang,a Longfeng Zhu,a,d Feng Deng,b Xiangju Meng,a Stefan Maurer,e Robert McGuire,f Andrei-
Nicolae Parvulescu,f Ulrich Müllerf and Feng-Shou Xiao*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
2,6-methyl-N-methylpyridinium, as a novel organic template, is employed for synthesis of RTH aluminosilicate zeolite with
a SiO2/Al2O3 ratio of 17.6. The amount of 2,6-methyl-N-methylpyridinium template, the Na2O/SiO2 ratio, the SiO2/Al2O3
ratio, and the H2O/SiO2 ratio in the starting gel significantly influence the crystallization of RTH zeolite. Several analytical
methods such as XRD, SEM, N2 sorption, TG-DTA, DRIFT and NMR were employed for characterisation of the obtianed RTH
zeolites and to understand the crystallization process with the new template. Very interestingly, the crystallization of RTH
zeolite with the new template takes very short time (12 h at 130 °C and 50 min at 240 °C) compared with conventional RTH
zeolites synthesis reported in the literature (72 h at 130 °C). Theoretical calculations show that this novel organic template
has lower interaction energies for the zeolite cage space filling than that of the previous organic templates in the
literatures, which lead to the stronger structural directing. Kinetic results show that the activation energy of this novel
organic template is much lower than the traditional one. Catalytic tests show that copper exchanged RTH zeolite (Cu-RTH)
exhibits good catalytic properties in the NH3-SCR reaction and H-RTH zeolite catalyst has excellent selectivities for ethylene
and propylene in MTO reaction.
tetrahedral coordination, have received noticeable attention
due to their outstanding performance in selective catalytic
reduction of NOx with NH3 (NH3-SCR) and methanol-to-olefin
Introduction
Zeolites, microporous silica-based and aluminophosphate-
based crystalline materials, have been widely used as catalysts
in the industrial processes of oil refining and chemical
production due to their high surface area, large pore volume,
uniform accessible microporous channels, and excellent
thermal and hydrothermal stabilities.1 For example,
aluminosilicate Y zeolite is applied for fluid catalytic cracking,
and aluminosilicate ZSM-5 zeolite is successful used for shape-
selective catalytic synthesis of p-xylene.2 Recently, small-pore
(MTO) reactions. Examples include SAPO-34 and SSZ-13, which
show excellent MTO and NH3-SCR performances, respectively.3
The RTH-type zeolite discovered in 1995, are promising
candidates for the aforementioned catalytic applications due
to the particular two-dimensional
8 MR pore structure
(aperture size of 4.1×3.8 Å and 5.6 2.5 Å). RTH zeolite was first
×
synthesized as borosilicate in the presence of 1,2,2,6,6-
pentamethylpiperidine as organic template, and denoted as
RUB-13.4 Later, the aluminosilicate RTH designated as SSZ-50
was successfully prepared by using N-ethyl-N-methyl-5,7,7-
trimethyl-azonium bicyclo [4.1.1] octane cation.5 It is worth
noting that the synthesis of above organic templates is very
complex due to the multistep synthesis methods, which
strongly hinders its large scale synthesis. Recently, a facile
method for synthesizing aluminosilicate RTH in the presence of
a series of novel imidazolium cations has been reported.6
Despite the encouraging progress achieved in RTH zeolite
synthesis in recent years, the crystallization of RTH zeolite is still
very slow, which impacts its further application.4-7 Therefore, it is
strongly desirable to shorten the crystallization time of RTH zeolite.
Our group previously reported that the high silica Y zeolite with
excellent properties could be successfully synthesized in the
presence of N-methylpyridinium iodide.8 Furthermore, we have
zeolites with pore openings limited by 8-membered rings in
a.Key Laboratory of Applied Chemistry of Zhejiang Province, Zhejiang University,
Hangzhou 310028, China. E-mail: qinmingwu@zju.edu.cn, fsxiao@zju.edu.cn.
b.State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,
Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and
Mathematics, Wuhan 430071, China..
c. Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, Shanghai
200062, China.
d.College of Biological, Chemical Sciences and Engineering, Jiaxing University,
Jiaxing 314001, China.
e. BASF Catalysts (Shanghai) Co., Ltd., 239 Luqiao Road, Jinqiao Export Process Zone
Pudong New District, Shanghai 201206, China.
f. BASF SE, GCC/PZ -M311, Ludwigshafen 67056, Germany.
Electronic Supplementary Information (ESI) available: Extended characterization
details, TG-DTA Curves, XRD patterns, SEM images, DRIFT spectra, NMR spectra,
NH3-TPD curves, MTO reaction curves, porosity data and the dependence of the
crystallinity on crystallization time. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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made theoretical calculations to check the pyridyl cation for Aldrich Co.), tetrapropylammonium hydroxide solution (40%
synthesizing RTH zeolite. Very interestingly, the interaction energy TPAOH in water, Sinopharm Chemical Reagent Co., Ltd.),
between 2,6-methyl-N-methylpyridinium cation and RTH zeolite is anion-exchange resin (Tianjin Fuchen Chemical Reagents
lower than the energies of the conventional organic template Factory), and ammonium nitrate (NH4NO3, AR, 99%, Beijing
(polymethylated [4.1.1] octanes) and the newly developed organic Chemical Reagent Co., Ltd.) were used without further
template (1,2,3-trimethylimidazolium) reported in the literature.5,6c purification. The deionized water was made in our laboratory.
Therefore, 2,6-methyl-N-methylpyridinium cation as organic
template could be suitable for synthesizing RTH zeolite.
Synthesis
It is consistent with the expectation that the observed
experimental results agree well with the results of the predicted
interaction energies. More importantly, the aluminosilicate RTH
zeolite would be obtained at very short crystallization time (12 h at
130 °C and 50 min at 240 °C). In addition, Cu-RTH zeolite exhibits
good catalytic property in NH3-SCR reaction, and H-RTH zeolite
displays excellent selectivity for light olefins in MTO reaction.
Synthesis of OSDAs. 2,6-methyl-N-methylpyridinium iodide was
synthesized by dissolving 0.1 mol of 2,6-dimethylpyridine and 0.12
mol of iodomethane in 20 g of ethanol, then heating the mixture to
80 °C under stirring and keeping it under these conditions for 12 h
in a dark place. The solvent and excess iodomethane were removed
under rotary evaporation and the product was washed with ether.
The molecular structure was verified using 1H and 13C nuclear
magnetic resonace (NMR). The product was converted from the
iodide to the hydroxide form (denoted as ROH) using an anion
exchange resin.
Experimental
Calculational Method
1,2,3-trimethylimidazolium was synthesized by dissolving
0.1 mol of 1,2-dimethylimidazole and 0.1 mol of iodomethane
in 20 g of ethanol, stirring at room temperature for 48 h in a
dark place. The solvent and excess iodomethane were
removed using rotary evaporation and the product was
The location and amount of the organic structure directing
agent (OSDA) within the RTH framework were studied by
molecular mechanics simulations using the sorption program
in Materials Studio 7.0 software. The CVFF force field was
selected for the calculation, and the most stable locations for
the OSDA molecules were obtained by sorption program. The
zeolite framework was assumed to be pure silicate, due to the
distribution of the Al atoms in the framework is not clear. The
geometry of the OSDA molecules and the RTH zeolite was first
optimized using the CVFF force field and allowed to change for
the locate calculation.
washed with ether. The structure was verified using 1H and 13
C
nuclear magnetic resonace (NMR). The product was converted
from the iodide to the hydroxide form using an anion
exchange resin.
Synthesis of RTH zeolites. A synthetic mixture with a molar
composition of 0~0.36 ROH : 0~0.18 Na2O : 1.0 SiO2 : 0.04
Al2O3 : 18~85 H2O was prepared by mixing Y zeolite, ROH,
sodium hydroxide, and deionized water. In a typical synthesis,
1 g of Y zeolite was mixed with 5.8 g of ROH solution (0.6 mol/L
ROH) and stirred at room temperature for 2 h. Then, 0.15 g of
NaOH was added. The synthesis mixture was stirred at room
temperature for 2 h, transferred in a Teflon-lined autoclave
oven and crystallized at 130 °C for 12 h. After filtrating,
Based on the simulation structures, the OSDAs in RTH
zeolite are further optimized by Dmol3 program, with the
generalized gradient approximation (GGA) proposed by
Perdew-Burke-Ernzerhof (PBE) functional. The ultrasoft
pseudopotential, medium planewave cut-off energy and a
default medium level Monkhorst-Pack K-point (1×1×1) were
adopted to sample the Brillouin zone. DFT-D method was used
in the structure optimization to accurately describe the weak
interaction in the zeolite. During the calculation, the OSDAs
are allowed to relax while the unit cell and the framework
atoms were fixed. On the basis of the DFT calculational result,
the interaction energy was calculated as the energy difference
between the zeolite adsorption complex and the sum of the
washing, drying, and calcining at 550 °C for 4 h, the product
was finally obtained. The H-form of the sample (H-RTH) was
prepared by ion-exchange with 1 M NH4NO3 solution and
calcination at 550 °C for 4 h. This ion-exchange procedure was
repeated once. The Cu-form of the sample (Cu-RTH) was
prepared by ion-exchange of the H-form RTH zeolite with 1 M
CuAc2 solution and calcination at 550 °C for 4 h.
By replacing OSDA to 1,2,3-trimethylimidazolium hydroxide,
OSDAs and zeolite. i.e., Eint = Ezeolite-OSDAs -Ezeolite-EOSDAs
.
the comparative RTH zeolites were synthesized at 130
0~144 h.
°C for
Materials
Synthesis of ZSM-5 zeolite. A synthetic mixture with a molar
composition of 0.30 TPAOH : 0.05 Na2O : 1.0 SiO2 : 0.03 Al2O3 :
20 H2O was prepared by mixing sodium metaaluminate,
TPAOH solution (40% in water), collodial silica (40% in water),
and deionized water. The synthetic mixture was transferred in
a Teflon-lined autoclave oven and crystallized at 180 °C for 72
Y
zeolite (Si/Al = 6, 12) provided by BASF, sodium
metaaluminate (NaAlO2, AR, 99%, Sinopharm Chemical
Reagent Co., Ltd.), sodium hydroxide (NaOH, AR, 96%,
Sinopharm Chemical Reagent Co., Ltd.), 2,6-dimethylpyridine
(C7H9N, 98%, Aladdin Chemistry Co., Ltd.), 1,2-
dimethylimidazole (C5H8N2, 98%, Aladdin Chemistry Co., Ltd.),
iodomethane (CH3I, 99.5%, Sinopharm Chemical Reagent Co.,
Ltd.), ethanol (C2H6O, 99.7%, Sinopharm Chemical Reagent Co.,
Ltd.), diethyl ether (C4H10O, 99.5%, Sinopharm Chemical
Reagent Co., Ltd.), collodial silica (40 wt% SiO2 in water, Sigma-
h. After filtrating, washing, drying, and calcining at 550 °C for 4
h, the product was finally obtained. The H-form of the sample
(H-ZSM-5) was prepared by ion-exchange with 1 M NH4NO3
2 | J. Name., 2012, 00, 1-3
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solution and calcination at 550
procedure was repeated once.
°C for 4 h. This ion-exchange
Characterizations
X-ray powder diffraction (XRD) patterns were measured with a
Rigaku Ultimate VI X-ray diffractometer (40 kV, 40 mA) using
CuKα (λ=1.5406 Å) radiation. The N2 sorption isotherms at the
temperature of liquid nitrogen were measured using
Micromeritics ASAP 2020M and Tristar system. The sample
composition was determined by inductively coupled plasma
(ICP) with a Perkin-Elmer 3300DV emission spectrometer.
Scanning electron microscopy (SEM) experiments were
performed on Hitachi SU-1510 and SU-8010 electron
microscopes. 27Al, 29Si, and 13C MAS NMR spectra were
Figure 1 Positions of OSDAs within the RTH framework for (a)
N-ethyl-N-methyl-5,7,7-trimethyl-azonium bicyclo
[4.1.1]
octane cations, (b) 1,2,3-trimethylimidazolium cations and (c)
2,6-methyl-N-methylpyridinium cations.
recorded on a Varian Infinity Plus 400 spectrometer, and the
3+
chemical
shifts
were
referenced
to
Al(H2O)6 ,
tetramethylsilane and adamantine, respectively. The acidity of
the catalysts was measured by the temperature-programmed-
desorption of ammonia (NH3-TPD). The catalyst was prepared
at 450 °C in a He flow for 1 h, followed by the adsorption of NH3 at
100 °C for 1 h. After saturation, the catalyst was purged by He flow
for 3 h to remove the physically adsorbed ammonia on the sample.
Then, desorption of NH3 was carried out from 100 to 600 °C with a
heating rate of 10 °C/min. The amount of NH3 desorbed from the
sample was detected by a thermal conductivity detector. The
diffuse reflectance infrared Fourier transform (DRIFT) spectra were
recorded on a Bruker Tensor 27 in situ infrared spectrometer.
5,7,7-trimethyl-azonium bicyclo [4.1.1] octane cations were fitting
per unit cell. In contrast, as for 1,2,3-trimethylimidazolium cations
and 2,6-methyl-N-methylpyridinium cations, four of these OSDA
molecules were fitting per unit cell based on energy optimizations
for the framework pore space filling.
The interaction energy between 2,6-methyl-N-methylpyridinium
cation is lower than that of the other two OSDAs. This result
indicates that 2,6-methyl-N-methylpyridinium cation is more
suitable for synthesizing zeolite RTH.
Sample characterization
Catalytic tests
Figure 2A shows XRD pattern of RTH zeolite synthesized in the
presence of 2,6-methyl-N-methylpyridinium cation under
static condition for 12 h at 130 °C, giving characteristic peaks
associated with RTH structure. Figure 2B-a shows N2 sorption
isotherms of as-synthesized RTH zeolite. Clearly, the sample
has not any microporous adsorption, suggesting that the
microporosity is fully filled with the OSDA. Thermal analysis
(TG-DTA) of the as-synthesized RTH zeolite exhibits major
Catalytic activities in selective catalytic reduction of ammonia
(NH3-SCR) were measured in a fixed-bed quartz reactor in a
gaseous mixture containing 500 ppm of NO, 500 ppm of NH3,
10% of O2, and N2 as a balance gas. The gas hourly space
velocity (GHSV) was 80 000 h-1.
Methanol-to-olefin (MTO) reactions were carried out in a
fixed-bed reactor at atmospheric pressure. A total of 0.5 g of
catalyst (20-40 mesh) was loaded into the reactor. The sample
was pretreated in flowing nitrogen at 550 °C for 1 h, and then
the temperature of the reactor was decreased to 480 °C or
400 °C. CH3OH was pumped into the reactor under nitrogen.
The weight hourly space velocity (WHSV) was 1.0 h-1 or 2.0 h-1.
The products were analyzed online by an Agilent 6890 gas
chromatograph equipped with a FID detector and a HP-PONA
methyl siloxane capillary column.
exothermic peaks at 200-800
at about 22.4 %, which is related to the decomposition of the
OSDA (Figure S1). After calcination at 550 C for 4 h and ion-
°C accompanied by a weight loss
°
exchanging to H-form, N2 sorption isotherms of the sample
(Figure 2B-b) shows typical Langmuir-type curve. A steep
adsorption increase occurs in the curve at a relative pressure
10-6<P/P0<0.01, which is due to the filling of micropores by N2.
Correspondingly, a BET surface area and micropore volume of
576 m2/g and 0.26 cm3/g were measured.
Table 1 The calculated interaction energies (kcal/mol) between
Results and discussion
OSDAs and RTH zeolite.
Theoretical calculations
OSDAs
1 OSDA
/cell
2 OSDAs
/cell
3 OSDAs
/cell
4 OSDAs
/cell
Results of the interaction energies interaction between the OSDA
and the RTH structure are shown in Figure 1 and the calculated
values are presented in Table 1. The conventional OSDA of
polymethylated [4.1.1] octane cations5 and the newly developed
OSDA of 1,2,3-trimethylimidazolium cation6c were compared to the
new designed organic template of 2,6-methyl-N-methylpyridinium
cation. In these calculations, only two of the N-ethyl-N-methyl-
a
b
c
-95.5
-72.4
-77.1
-205.3
-165.5
-168.3
-
-
-249.9
-259.4
-353.9
-360.7
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Si(3Si)OH species is of 9.3% at the synthesis temperature of 130 °C,
while this signal intensity is of 6.3% and 4.2% at the synthesis
temperature of 180 °C and 240 °C, respectively. Considering the
same Si/Al ratios in the products, the lower intensity of Si(3Si)OH
species means lower concentration of structural defects (Si-OH).
Figure 3B shows the 27Al MAS NMR spectra of the as-synthesized
RTH zeolites, showing a sharp band at 59 ppm associated with
tetrahedrally coordinated aluminum species in the framework. The
absence of a signal around zero ppm indicates that there is no extra
framework Al species in the samples. Figure 3C compares the 13C
MAS NMR spectra of the as-synthesized RTH zeolites with the liquid
13C NMR spectrum of 2,6-methyl-N-methylpyridinium iodide.
Clearly, 2,6-methyl-N-methylpyridinium cations mostly exist in the
channel of RTH zeolite obtained at the three crystallization
temperatures. Figure S3 shows the DRIFT spectra of H-RTHs
synthesized at 130 °C, 180 °C and 240 °C, respectively. Obviously,
C
A
a
a
5
10
20
25
35
40
125 Theta(degree)30
300
250
200
150
100
50
D
B
b
a
a
0
0.0
0.2
0.6
0.8
1.0
0.4 P/P
o
Figure 2 (A) XRD pattern, (B) N2 sorption isotherms, (C) SEM image the H-RTH zeolite synthesized at higher temperature has lower
(scale bar 2 μm), and (D) SEM image (scale bar 500 nm) of (a) as- concentration of structural defects (Si-OH). The BET surface areas
synthesized RTH zeolite, and (b) H-RTH zeolite.
and micropore volume of H-RTHs synthesized at 130 °C, 180 °C and
240 °C are presented in Table S1.
Table 2 presents effects of the synthesis conditions variation
Figure 2C shows low magnification SEM image of as-synthesized
RTH sample, giving very uniform crystal morphology, which on the crystallization of the RTH zeolite. When NaOH is absent,
indicates the high crystallinity of the product obtained. The high the product is Y zeolite (Run 1, Table 2). When the Na2O/SiO2
magnification SEM image (Figure 2D) of as-synthesized RTH sample ratio is higher than 0.18, the product is mixed RTH zeolite with
exhibits that the particles are blocky (100-300 nm). ICP analysis Y zeolite phase (Run 4, Table 2). When the organic template is
shows that the SiO2/Al2O3 ratio in the product is about 17.6. RTH absent, the product is amorphous (Run 5, Table 2). When the
zeolite synthesized in the presence of 2,6-methyl-N- ROH/SiO2 ratio is 0.07, the sample is a mixture of RTH with Y
methylpyridinium cation at the temperature of 150 °C, 180 °C, and zeolite (Run 6, Table 2). Increasing the ROH/SiO2 ratio to 0.14
240 °C are also well characterized by XRD patterns and SEM images results in a complete crystallization of RTH zeolite (Run 7,
(Figure S2).
Table 2). Furthermore, the ratios of H2O/SiO2 in the synthesis
Figure 3 shows 29Si, 27Al, and 13C MAS NMR spectra of the as- are carefully studied, and it is found that relatively high
synthesized RTH zeolites at the temperature of 130 °C, 180 °C, and H2O/SiO2 ratios (e.g. 85) result in incomplete crystallization
240 °C. Figure 3A shows the 29Si MAS NMR spectra of the as- (Run 11, Table 2). When the Y zeolite with Si/Al ratio at 6 was
synthesized RTH zeolites giving the peaks at about -112.2, -107.7 used, we cannot obtain RTH zeolite (Run 12-14, Table 2).
and -102.1 ppm. The peaks at -112.2 and -107.7 ppm are assigned
to Si(4Si) species, while the peaks at -102.1 ppm are assigned to synthesis of RTH zeolite takes only 12 h at 130
Si(3Si)OH species. Very interestingly, the signal intensity of which is very short compared with RTH zeolite reported in the
It is worth mentioning that the crystallization time for
°C in this work,
A
C
B
d
c
c
b
a
c
b
b
a
a
200 160 120 80
ppm
40
0
-80 -90 -100 -110 -120 -130 -140
ppm
100 80 60 40 20
ppm
0
-20
Figure 3 (A) 29Si, (B) 27Al, and (C) 13C MAS NMR spectra of as-synthesized RTH zeolites at the temperature of (a) 130 °C, (b) 180 °C and (c)
240 °C. (d) 13C NMR spectrum in D2O solution of 2,6-methyl-N-methylpyridinium cation.
4 | J. Name., 2012, 00, 1-3
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0.5
Table 2 Impact of synthesis conditions on RTH zeolite crystallization
a
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
Runa
Na2O /SiO2 ROH/SiO2
H2O/SiO2 Productsb
1
0
0.21
0.21
0.21
0.21
0
18
18
18
18
18
18
18
18
4.5
45
85
18
18
18
18
18
Y
2
0.04
0.14
0.18
0.11
0.11
0.11
0.11
0.11
0.11
0.11
0.11
0.11
0.11
0.11
0.11
RTH
RTH
Y+RTH
Amor
Y+RTH
RTH
RTH
RTH
RTH
Y+RTH
Y
3
4
5
0.0019 0.0020 0.0021 0.0022 0.0023 0.0024 0.0025
6
0.07
0.14
0.36
0.21
0.21
0.21
0.07
0.11
0.14
0.21
0.21
1/T
7
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
b
8
9
10
11
12
13
14
15c
16c
Y
Y
0.0022
0.0023
0.0024
0.0025
1/T
Figure 5 Arrhenius plots for synthesizing RTH zeolite using (a) 2,6-
methyl-N-methylpyridinium cation and (b) 1,2,3-
RTH
RTH
a Using Y zeolite (Si/Al = 12, run 1-11, 15-16; Si/Al = 6, run 12-14) as
raw material.
trimethylimidazolium cation as organic templates.
b The phase appearing first is dominant.
6c
literature. For example, Hong et al
use 1,2,3-
c The samples were synthesized at 130 °C for 12 h (run 1-14), 180 °C
for 3 h (run 15) and 240 °C for 50 min (run 16).
trimethylimidazolium as organic template to synthesize RTH zeolite,
giving the crystallization time for 16 days; Zones et al 5 employ
polymethylated [4.1.1] octanes to synthesize zeolite SSZ-50
with the crystallization time of 5-6 days. When increasing the
crystallization temperature to 180 °C and even to 240 °C, we
have still managed to produce RTH zeolites while significantly
shortening the crystallization time (3 h at 180 °C and 50 min at 240
°C, Run 15-16, Table 2, Figure 4).
Figure S4 shows the changes of the crystalline phases during the
crystallization of RTH zeolite at 130 °C. After 6 h crystallization time,
the sample still shows peaks associated with Y zeolite (Figure S4A-b).
The typical morphology assigned to Y zeolite is clearly observed via
SEM image (Figure S4B-b). After 9 h crystallization time, the sample
(Figure S4A-c) at 25° (2 theta) shows peaks associated with RTH
zeolite structure. At the same time, block-like crystals appear in the
product zeolites (Figure S4B-c), indicating the formation of RTH
zeolite. Increasing the crystallization time from 10 to 11 h, the
intensity of XRD peaks associated to RTH phase gradually increases
(Figure S4A-d-e). Correspondingly, more RTH zeolite crystals are
100
80
60
a
c
b
d
40
20
0
0
2
4
6
10
12
14
16
Tim8e (h)
Figure
4
The dependences of RTH zeolite crystallinity on
crystallization time using 2,6-methyl-N-methylpyridinium cation as
organic template at (a) 240 °C, (b) 180 °C, (c) 150 °C and (d) 130 °C.
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formed in the products (Figure S4B-d-e). RTH zeolite with high
crystallinity can be obtained after a crystallization of 12 h (Figure
S4A-f). Further increasing the crystallization time to 432 h, the
intensity of XRD peaks show no change, which means the formed
RTH phase is stable under these synthesis conditions.
100
80
60
40
20
0
a
Figure 4 and Figure S5 show the dependences of RTH zeolite
crystallinity on crystallization time using 2,6-methyl-N-
methylpyridinium cation and 1,2,3-trimethylimidazolium cation as
OSDAs under different crystallization temperatures, respectively.
From Arrhenius plots depicted in Figure 5, the activation energy
using 2,6-methyl-N-methylpyridinium cation and 1,2,3-
trimethylimidazolium cation is 42.0 kJ/mol and 70.8 kJ/mol,
respectively. Obviously, lower activation energy is more favorable
for rapid synthesis of RTH zeolite. In addition, the same zeolite
building units of four-membered rings (4MRs) in both Y and RTH
zeolite frameworks and higher framework density of RTH than that
of Y zeolite might be helpful for the successful transformation from
Y zeolite to RTH zeolite.9-11
b
Cu-RTH
Cu-SSZ-13
100
200
300
400
500
600
(o
Reaction Temperature
C)
Figure 6 NOx conversion as a function of temperature in NH3-SCR
over (a) Cu-RTH synthesized at 130 °C and (b) Cu-SSZ-13 obtained
from BASF.
Figure S6 shows the XRD patterns of as-synthesized RTH zeolite
and RTH zeolites calcined at 550 °C, 700 °C and 850 °C for 4 h,
respectively. When calcination temperature is below 700 °C, the
RTH zeolite has good stability. When RTH zeolite is calcined at 850
°C, the crystallinity of the RTH zeolite reduced remarkably. Figure
S7A and S7B show the 29Si and 27Al MAS NMR spectra of H-RTH
zeolites calcined at 550 °C, 700 °C and 850 °C, respectively. The
peak at -105.3 ppm in Figure S7A is assigned to Si(3Si)OH species,
giving at 2.7%, 3.1% and 3.8% in the total 29Si peak area of the
samples calcined at 550 °C, 700 °C and 850 °C, respectively. These
results suggest that the RTH zeolites calcined at higher temperature
have higher concentration of structural defects (Si-OH), which
should decrease the thermal stability of the RTH zeolite structure.
The signals around 0 ppm in Figure S7B is associated with
extraframework Al species, showing positive effect to the
calcination temperature. These results indicate severe
dealumination of RTH zeolite at high calcination temperature.
the NH3-SCR over the Cu-RTH catalyst synthesized at 130 °C and a
Cu-SSZ-13 catalyst provided by BASF. Notably, the Cu-RTH catalyst
exhibits promising catalytic performance (above 90% of NO
conversion in the temperature range of 200-450 °C, Figure 6a),
which is completely comparable with those of Cu-SSZ-13 catalyst
(Figure 6b).
Catalytic tests
Figure 7 shows catalytic activities in MTO reactions over H-RTH and
H-ZSM-5 catalysts as function of time on stream at 480 °C. The
weight hourly space velocity (WHSV) is 1.0 h-1. Table 3 presents the
results of the MTO reactions for a reaction time of 90 min at 480 °C.
Obviously, compared with the H-ZSM-5 catalyst, H-RTH catalyst
gives very high selectivity for ethylene and propylene (> 75%),
which is very important for selective production of light olefins in
the industrial processes. Figure S9 shows catalytic data in MTO
reactions over H-RTH and H-ZSM-5 catalyst under different reaction
temperature and weight hourly space velocity (WHSV). The results
show that decreasing the reaction temperature could enhance the
catalyst life, while decrease the selectivity for ethylene and
propylene. In contrast, increasing the WHSV results in reduction of
catalyst life and the selectivity for ethylene and propylene,
compared with those in Figure 7.
Figure S8 shows NH3-TPD curves of H-RTHs calcined at 550 °C,
700 °C and 850 °C, respectively. Obviously, the sample calcined at
lower temperature has more acidic sites than that of calcined at
higher temperature. The BET surface areas and micropore volume
of these samples are presented in Table S1. It is clear that the
surface areas and micropore volume of the samples calcined at 550
°C and 700 °C are very similar. However, when increasing the
calcination temperature to 850°C, both BET surface area and
micropore volume decrease greatly, in according with those
obtained from XRD patterns.
Compared to catalytic data in the literatures, the selectivity for
ethylene and propylene in the H-RTH catalyst (76.8%, Si/Al ratio at
8.8) in this work is higher than that of the H-RTH with Si/Al at 17
(64%)6a and the H-RTH catalyst with B species (73.3%).7 The
Figure 6 shows the dependence of the NOx conversion as a
function of the reaction temperatures ranged from 100 to 600 °C in
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Table 3 The results of MTO reactions for a reaction time of 90
100
90
80
70
60
50
40
30
20
10
0
A
minutes at 753 K.
a
SiO2 / Conv.
Selectivities (%)
Al2O3
[%]
C2H4
40.5
26.0
C3H6 C2H4 + C3H6 ≥ C4
b
H-RTH
17.6
100
100
36.3
19.9
76.8
45.9
12.2
38.3
H-ZSM-5 20.8
c
d
difference in the catalytic selectivity might be resulted from their
distinguishable composition in the RTH zeolite.
e
0
50
100
150
Time on stream (min)
Conclusions
100
90
80
70
60
50
40
30
20
10
0
a
B
In summary, we have employed a novel organic template (2,6-
methyl-N-methylpyridinium) for synthesizing aluminosilicate
zeolite RTH. The crystallization of RTH is very fast (12 h at 130
°C and 50 min at 240 °C). Theoretical calculations have
confirmed that this template is more suitably matched with
the RTH pore geometry than other conventional organic
templates. More importantly, catalytic tests in NH3-SCR and
MTO reactions show that the Cu-RTH and H-RTH are promising
catalysts for these transformation. The combination of rapid
crystallization and excellent catalytic properties might offer a
good opportunity for wide applications of RTH zeolites in the
near future.
b
e
c
d
0
50
100
150
Time on stream (min)
Figure 7 (a) Methanol conversion (
of (b) ethylene with propylene (
C2H4), (d) propylene ( C3H6) and (e) hydrocarbons higher
than C4 ▼≥C4) in MTO over (A) H-RTH and (B) H-ZSM-5
catalysts.
◆
) and product selectivities
▲C2H4 + C3H6), (c) ethylene
Author contribution
(
■
●
X. Hao and S. X. Pan performed the zeolite catalysts. L. Zhang
and C. S. Zhang performed catalytic tests. Y. Y. Chu performed
the theoretical calculations. R. McGuire, A. N. Parvulescu and
U. Müller partially discussed and analyzed results for synthesis
and catalysis. J. G. Jiang and F. Deng partially discussed and
analyzed the results of NMR data. L. F. Zhu, X. J. Meng and S.
Maurer partially discussed and analyzed the catalytic results. Q.
M. Wu and F.-S. Xiao designed this study, analyzed the data,
and wrote the paper.
(
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work is supported by National Key Research and
Development Program of China (2017YFC0211101), National
Natural Science Foundation of China (21333009, 21703203 and
21673205), China Postdoctoral Science Foundation
(2017M611978).
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Table of contents
Efficient synthesis of aluminosilicate RTH zeolite with good
catalytic performances in NH3-SCR and MTO reactions
Hao Xu, Qinming Wu,* Yueying Chu, Jingang Jiang, Ling Zhang, Shuxiang Pan,
Changsheng Zhang, Longfeng Zhu, Feng Deng, Xiangju Meng, Stefan Maurer,
Robert McGuire, Andrei-Nicolae Parvulescu, Ulrich Müller and Feng-Shou Xiao*
50 min
Y
RTH
The synthesis of aluminosilicate RTH zeolite is very fast in the presence of
2,6-methyl-N-methylpyridinium cation.