Confining Pore-mouth: an Efficient Way to Increase the Selectivity to Ethylene in the MTO Reaction

Article abstract of DOI:10.1002/cctc.202001241

The chemical liquid deposition modification can decorate the pore aperture size by external surface deposition, which is a very efficient way to further improve the shape-selective performance of zeolites. Here, SAPO-34 zeolite was modified with silica by chemical liquid deposition, and then was applied in MTO reaction. It was found the Br?nsted acid of SiO₂-modified SAPO-34 zeolite increased, mainly due to the transformation from Al-OH to Si-OH-Al, which can promote the selectivity to light olefins in MTO reaction. After the chemical liquid deposition, the silica located on the external surface of SAPO-34 zeolite, reduces the apparent diffusion coefficient of propylene, which will restrain the formation and diffusion of propylene. So the selectivity to ethylene and the ratio of ethylene to propylene were significantly increased over the SiO₂-modified SAPO-34 in the MTO reaction. Furthermore, the increase of the reactive sites can prolong the lifetime of catalyst effectively.

Full text of DOI:10.1002/cctc.202001241

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Confining Pore-mouth: an Efficient Way to Increase the
Selectivity to Ethylene in the MTO Reaction
Authors: Jiajia Ding, Xuefeng Shen, Jian Zhou, Yingchun Ye, Hongxing
Liu, and Zaiku Xie
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Confining Pore-mouth: an Efficient Way to Increase the
Selectivity to Ethylene in the MTO Reaction
Jiajia Ding+,[a] Xuefeng Shen+,[a] Jian Zhou,[a] Yingchun Ye,[a] Hongxing Liu*[a] and Zaiku Xie*[b]
[a]
Dr. J. Ding, X. Shen, J. Zhou, Y. Ye, Prof. H. Liu,
State Key Laboratory of Green Chemical Engineering and Industrial Catalysis
SINOPEC Shanghai Research Institute of Petrochemical Technology,
Shanghai 201208, (P.R. China)
E-mail: liuhx.sshy@sinopec.com,
[b]
[+]
Prof. Z. Xie
China Petrochemical Corporation (SINOPEC Group)
Beijing 100728, (P.R. China)
E-mail: xzk@sinopec.com
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: The chemical liquid deposition modification can decorate
the pore aperture size by external surface deposition, which is a very
efficient way to further improve the shape-selective performance of
zeolites. Here, SAPO-34 zeolite was modified with silica by chemical
liquid deposition, and then was applied in MTO reaction. It was found
the Brønsted acid of SiO2-modified SAPO-34 zeolite increased,
mainly due to the transformation from Al-OH to Si-OH-Al, which can
promote the selectivity to light olefins in MTO reaction. After the
chemical liquid deposition, the silica located on the external surface
of SAPO-34 zeolite, reduces the apparent diffusion coefficient of
propylene, which will restrain the formation and diffusion of propylene.
So the selectivity to ethylene and the ratio of ethylene to propylene
were significantly increased over the SiO2-modified SAPO-34 in the
MTO reaction. Furthermore, the increase of the reactive sites can
prolong the lifetime of catalyst effectively.
thermal cracking of naphtha.[7] Due to the limited reserve of
petroleum and increasing demand for light olefins, more and more
attention have been focused on methanol-to-olefin (MTO)
conversion processes, the reactant of which can be produced
from coal, biomass and natural gas.[7-8] SAPO-34 exhibits
excellent catalytic performance in the MTO reaction, due to its 8-
ring pore opening, medium to strong acidity and good thermal
stability.[9] For economical value, ethylene is more valuable than
propylene. The selectivity of ethylene and ratio of ethylene to
propylene increase with time on stream, due to the increasing
diffusion barriers introduced by coke formation, inspired by which,
some strategies have been proposed to achieve higher selectivity
to ethylene, such as increasing the diffusion restriction by the
cation exchange,[9c,10] changing the experimental conditions
(temperature, pressure or the weight hourly space velocity). To
the best of our knowledge, the CLD modification has not been
used in the SAPO zeolites to improve their catalytic activity.
Here, we used CLD to reduce the effective pore mouth size on
the outer surface of SAPO-34 zeolites. Polydimethylsiloxane
(PDMS, CH3-(Si(CH3)2-O)n-CH3) is the SiO2-CLD modifier, which
firstly covers on the external surface of SAPO-34 by physical and
chemical adsorption, then transforms to silica after calcined.[5]
The silica modified the acidity and pore mouth of SAPO-34 zeolite,
which increased the selectivity to ethylene and introduced extra
diffusion limitation for bulky hydrocarbons in the MTO reaction.[11]
Introduction
The shape selective properties of zeolite show greatest interest
for catalysis, which is the greatest advantage compared with other
inorganic catalysts.[1] Adjusting the aperture is a very efficient way
to alter the diffusion restriction of zeolites, then further improves
their performance in catalysis and separation. The pore size of
zeolites and their adsorptive properties can be tailored by
changing the crystal structure and modifying the external
surface.[2] There are two ways to modify the external surface of
zeolite: chemical vapor deposition (CVD)[3] and chemical liquid
deposition (CLD)[2c]. The separation performance of siliceous
CHA membranes was improved after CVD process.[4] Li et al.
found the catalyst shown high catalytic selectivity after SiO2-CLD
modification with polysiloxane.[5] Yang et al. using CLD to modify
4A zeolite as a size selective sorbent at 0.01 nm resolution for
gas separation.[2f] Wei et al. increased the para-xylene selectivity
from the usual 23-24% to 89.6% by modifying ZSM-5 based
catalyst with SiO2.[6] The external surface modification is a very
efficient way to improve the performance of zeolites, no matter as
catalysts or sorbents, so this method for modifying the zeolites or
other catalysts may be never outdated.
Results and Discussion
The powder XRD patterns show these two samples have CHA
framework topology, without other phases. These diffraction
peaks at 2θ = 9.5, 12.9, 16.1, 20.6, 26.2 and 30.8° are indexed to
(101), (110), (021), (121), (220) and (401) planes of SAPO-34,
respectively;[12] The crystallinity of M-SAPO-34 was maintained
well, indicating the integrity of the crystallinity after the CLD
process.
Light olefins (ethylene and propylene) are basic chemicals in the
petrochemical industry. A mass of light olefins are yielded by
1
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shows the CLD process has negligible impact on the crystallinity
of SAPO-34 zeolite.
Figure 1. Powder XRD patterns of P-SAPO-34 and M-SAPO-34.
Figure 3. IR spectra of P-SAPO-34 and M-SAPO-34.
The P-SAPO-34 crystals show cubic morphologies with the
diameter of 0.5-2 μm (Fig. 2a). From these SEM images, there is
no obvious difference between samples before and after CLD
process (Figure 2a and 2b). The same result can also be obtained
from these TEM images (Figure 2c and 2d). No amorphous silica
particle can be found from the TEM image of M-SAPO-34 (Figure
2d).
Table 1. Textual properties of P-SAPO-34 and M-SAPO-34.
SBET
(m2/g)a
Smicro
(m2/g)
Sext
(m2/g)
Vtotal
(cm3/g)b
Vmicro
(cm3/g)c
Sample
P-SAPO-34
M-SAPO-34
633.9
610.7
630.5
607.9
3.4
2.8
0.25
0.24
0.23
0.22
aBET surface area calculated from the adsorption data obtained at P/P0 between
0.05 and 0.25. bTotal pore volume via the single point method. cMicropore
volume obtained through t-plot method.
The N2 adsorption-desorption isotherms of P-SAPO-34 and M-
SAPO-34 are shown in Figure S1, and the corresponding textual
properties are list in Table 1. It can be found that micropore
specific surface area (Smicro) and micropore volume (Vmicro) of M-
SAPO-34 are slightly smaller than that of P-SAPO-34, due to that
bits of small cracking fragments of PDMS deposited in the
micropore.[5]
Table 2. The composition and element distribution in P-SAPO-34 and M-
SAPO-34 zeolites.
c
Sample
Bulka
Surfaceb
RSi
P-SAPO-34
M-SAPO-34
Si0.061Al0.536P0.403
Si0.082Al0.532P0.386
Si0.079Al0.472P0.449
Si0.335Al0.355P0.310
1.295
4.085
aDetermined by XRF. bDetermined by XPS. cSurface Si enrichment index de-
fined as [Si/(Si+P+Al)] surface/[Si/(Si+P+Al)]bulk.
Figure 2. SEM and TEM images of P-SAPO-34 (a, c) and M-SAPO-34 (b, d).
The ratio of Si atom in P-SAPO-34 is very low (6.1 mol %, Table
2), and the SiO2-CLD modifier deposited on the surface of SAPO-
34 by generating new Si-O-Si bonding. So there is a small
quantity of SiO2 deposited on the surface of M-SAPO-34, leading
to that it is difficult to observe the silica layer by SEM and TEM.
To clarify that bits of silica was deposited on the surface of M-
SAPO-34, X-ray fluorescence (XRF) and X-ray photoelectron
spectroscopy (XPS) measurements were carried out (Table 2).
The results obtained by XRF are bulk-content based, while those
by XPS are surface-content based, ≤ 10 layers of atoms in depth
of surface. After modified with PDMS, the Si-content of the bulk
Infrared spectroscopy (IR) is used to identify the properties of
zeolite hydroxyls of P-SAPO-34 and M-SAPO-34 (Figure 3). The
peak absorbing at 1640 cm-1 is ascribed to the bending mode of
water adsorbed in the CHA cages.[13] The peak at about 1100 cm-
1
ascribed to the O-P-O antisymmetric stretching vibration, and
the characteristic peaks of the SAPO-34 appeared at 630, 580,
530, 480 cm-1, which belong to the vibration of D-6 rings, T-O (T
= P, Al, Si) bend.[14] The differences of the IR spectra of these two
samples are mainly reflected in the intensity of the peaks, which
2
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10.1002/cctc.202001241
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slightly increased, while the Si-content on the edge of crystal
significantly increased, which resulted in the RSi of M-SAPO-34 is
3.2 times of P-SAPO-34. SEM coupled with energy dispersive X-
ray spectroscopy (EDX) was used to prove that the silica is evenly
distributed over the surface of M-SAPO-34 (Figure S2).
acid content of Brønsted acid sites in P-SAPO-34 was obviously
lower than M-SAPO-34, especially for Si-OH-Al bridging hydroxyl
groups. The acid content of Al-OH is decreased. The increase of
the Si-OH-Al bridging hydroxyl group can be attributed to the
transformation from Al-OH to Si-OH-Al with the small cracking
fragments of PDMS during the CLD process. The decrease of the
structural defect Al-OH groups leads to less ammonia adsorb on
the weak acid sites, namely the smaller peak area of the low-
temperature peak in NH3-TPD curve, which is corresponding to
the result of NH3-TPD.
Figure 5. In-phase (
curves of ethane in P-SAPO-34 and M-SAPO-34 by the single diffusion process
with surface resistance model.
■
,
) and out-of-phase (□, ▽) characteristic function
Figure 4. (a) NH3-TPD curves of P-SAPO-34 and M-SAPO-34; (b) Fitted FTIR
spectra of CD3CN adsorption over calcined P-SAPO-34 and M-SAPO-34.
To investigate the influence of CLD on the pore size of micropores
of M-SAPO-34, frequency-response (FR) technique was
employed. The pore size of eight-membered ring in SAPO-34
zeolite is about 0.38 nm, which is comparative with the kinetic
diameter of the ethane molecule (0.44 nm).[17] Therefore, the
mass transfer of ethane molecular through micropores of SAPO-
34 zeolite is restricted by the pore mouth. The FR spectra of
ethane in P-SAPO-34 and M-SAPO-34 zeolite at 273 K, 133 Pa
was shown in Figure 5. The out-of-phase characteristic function
of ethane in P-SAPO-34 gives a single peak response located at
2Hz, which can be ascribed to a pure single diffusion process of
ethane molecular through the pore mouth of SAPO-34 zeolite.
Comparatively, the out-of-phase characteristic function of M-
SAPO-34 moved to the lower frequency zone, with the value
lower than 0.2 Hz. The fitted diffusion data (Table 3, calculated by
the equation in supporting information) indicated that the diffusion
rate was declined obviously which can be ascribed to the pore
mouth contract by the deposition of silica. Moreover, the
intersection of the in-phase with the out-of-phase functions can
be found in the FR spectra of both SAPO-34 zeolite samples,
which indicates the transport resistance for the “skin” effect in the
system.[18]
NH3-TPD was used to measure the acidity of P-SAPO-34 and M-
SAPO-34 (Figure 4a). The acid strength and content are
represented by desorption temperature and peak area,
respectively. Both samples show two desorption peaks. The low-
temperature peak is ascribed to the ammonia adsorbed on the
weak acid sites originated from the structural defect T-OH groups
(T=P, Al, Si). The high-temperature desorption peak is attributed
to the ammonia adsorbed on Brønsted acid sites in the form of
bridging hydroxyl groups.[9c,15] After CLD process, the strength
and content of weak acid decreased, meanwhile the strength stay
the same and the content increased for the strong acid.
The CD3CN-FTIR spectra (Figure 4b) shows that there is no shift
in peak positions ascribed to Lewis acid and Brønsted acid sites
after CLD process. From the fitting results, it can be found that
there are four kinds of Brønsted acid sites: Si-OH-Al bridging
hydroxyl groups (2294 cm−1), Al-OH (2287 cm-1), P-OH (2283 cm-
1) and Si-OH (2273 cm−1),[16] and the acid strength of Al-OH, P-
OH and Si-OH is obviously weaker than that of Si-OH-Al bridging
hydroxyl groups. According to the quantified results (Table S1),
the acid strength of P-SAPO-34 is equal to M-SAPO-34, but the
3
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Table 3. The diffusion properties of P-SAPO-34 and M-SAPO-34.
apparent diffusion coefficient for ethane, the kinetic diameter of
which is obvious smaller than propylene, then the diffusion
limitation for propylene is more remarkable. So the selectivity of
propylene attained over M-SAPO-34 obviously decreases. For
further revealing the effect of the diffusion restriction caused by
modified silicon, the coke amount over P-SAPO-34 and M-SAPO-
34 catalysts after methanol reaction for 30 min (P-SAPO-34-
30min and M-SAPO-34-30min) were analyzed by TGA. The
weight loss below 225 °C corresponds to the volatilization of
adsorbed water, while the weight loss between 225 and 1000 °C
corresponds to trapped organic species (Figure S3). The coke
amount of M-SAPO-34-30min (2.6%) is obviously less than that
of P-SAPO-34-30min (3.4%). Liu et al found that increasing the
coke amount and introducing extra diffusion hindrance for the
generated products resulted in the enhanced selectivity to
ethylene in the MTO reaction.[9c,19] M-SAPO-34-30min with less
coke amount shows higher selectivity of ethylene and higher ratio
of ethylene to propylene compared with P-SAPO-34-30min, which
is attributed to the restricted pore mouth of micropore of M-SAPO-
34 by the deposition of silica.
Sample
ξ(a.u.)
ka
D (m2/s)
P-SAPO-34
M-SAPO-34
0.8
15
0.14
0.19
4.37 X 10-16
3.17 X 10-17
Another reason is the changes of the acidity of M-SAPO-34.
According to the hydrocarbon pool mechanism,[20,11c] ethylene is
predominantly formed via the polymethylbenzene cycle, whereas
propylene is mainly produced via the alkene cycles.[11d,21] The
weaker acid sites are favorable for enhancing the contribution of
the alkene cycle to form more propylene.[11d] The strength and
content of weak acid of P-SAPO-34 is higher than that of M-
SAPO-34, so the selectivity to propylene of P-SAPO-34 is higher
than that of M-SAPO-34.[22] The increase of the acid content of
strong Brønsted acid of SAPO-34 is mainly due to the increase of
Si-OH-Al. The FT-IR spectra of pivalonitrile adsorption on zeolite
was used to detect the acid sites on the external surface of P-
SAPO-34 and M-SAPO-34 (Figure S4). The kinetic diameter of
pivalonitrile is about 0.6 nm, which is much larger than the pore-
mouth of SAPO-34 (0.38 nm). The pivalonitrile molecule could
only adsorb on the surface of SAPO-34. No band of Brønsted acid
can be found in the FT-IR spectras of pivalonitrile adsorption on
P-SAPO-34 and M-SAPO-34 zeolite, which means no Brønsted
acid on the external surface of M-SAPO-34. Bits of small cracking
fragments of PDMS deposited in the micropore as mention above,
so the transformation from Al-OH to Si-OH-Al might mainly take
place in the cage of SAPO-34. The number of reactive sites
increases, which promotes the selectivity to light olefins in MTO.
The ratio of Brønsted to Lewis acid sites is increased after CLD
process (Table S1), which is also beneficial to improve the
selectivity of ethylene.[22,24] Combined the increase of the diffusion
hindrance of propylene and the changes of the acidity of SAPO-
34 after CLD process, the selectivity to ethylene and ratio of
ethylene to propylene are enhanced over M-SAPO-34 in the MTO
reaction.
Figure 6. The selectivity to light olefins, ethylene and propylene of methanol
conversion over P-SAPO-34 and M-SAPO-34. Experimental conditions: WHSV
= 2.4 h-1, T = 733 K, catalyst weight = 2 g.
The MTO reaction was conducted in a fixed bed reactor over P-
SAPO-34 and M-SAPO-34 at 733K with WHSV of 2.4 h-1. In the
whole catalytic process, the conversion of methanol is higher than
99.9% over these two catalysts. The selectivity to light olefins,
ethylene and propylene versus time-on-stream (TOS) are shown
in Figure 6, and the detailed MTO results are summarized in Table
S2. The selectivity of ethylene over M-SAPO-34 catalyst is
obviously higher than that over P-SAPO-34 after methanol
reaction for 30 min. The highest selectivity of ethylene attained
over M-SAPO-34 is 55.5%, which is 3.4% higher than that over P-
SAPO-34. Likewise, higher ratio of ethylene to propylene is
obtained over the M-SAPO-34 catalysts (Table S2). With the
predominant generation of ethylene, the selectivity to light olefins
For SAPO-34, the typical density of acid sites is correspond to
one site per cavity.[11c] The number of isolated reactive sites
increases and the strength of the Brønsted acid keeps unchanged,
so the hydrogen transfer pathway, leading to alkanes, aromatics
and coke in MTO,[11c,25] is deferred over M-SAPO-34, namely
enhancing the catalyst lifetime of M-SAPO-34.
=
increase, but the selectivity to C3 , C4, and C5+ decreases. Not
only the selectivity to light olefins, but also the catalyst lifetime is
obviously increased over SiO2-modified SAPO-34 catalyst.
The difference in catalytic activity between these two samples is
mainly due to the increasing of the apparent diffusion coefficient
by CLD modification. The results of FR technique indicate the
silica located on the external surface of M-SAPO-34 reduces the
Conclusion
4
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Prior to the measurements, the sample was degassed at 350 °C until a
stable vacuum of about 0.67 Pa was reached. FE-SEM (Field Emission
Scanning Electron Microscopy) analysis was performed on a Hitachi
S4800 electron microscope with an accelerating voltage of 2.0 kV. HRTEM
was performed using JEOL JEM-2100LaB6 operating at 200 kV (Cs = 1.0
mm, point resolution of 2.3 Å). Images were recorded with a Keen View
CCD camera (resolution of 1376 pixels × 1032 pixels, pixel size of 6.45 μm
× 6.45 μm) at 50000-120000× magnification under low-dose conditions. IR
were obtained on Bruker IFS88, and Samples were dried at 300 °C before
testing and mixed with KBr powder and pressed to produce a transparent
disk. Temperature-programmed desorption of ammonia (NH3-TPD)
experiments were performed on a chemisorption analyzer (FINETEC
FINSORB-3010). In situ CD3CN and pivalonitrile spectra were recorded
using a FTIR spectrometer (Bruker Vertex 70V) equipped with a stainless
steel high vacuum transmission infrared cell. After dehydrated at 350-
400 °C in vacuum (<2.0 × 10-8 mbar) for 2.5 h, the samples were cooled
down to room temperature, and then the background spectrum was
collected. Subsequently, CD3CN or pivalonitrile was introduced into the
cell, remaining for 1 h. Finally, CD3CN or pivalonitrile was evacuated and
the corresponding spectra were collected. The frequency-response (FR)
spectra, characteristic functions against frequency, were derived from the
In conclusion, the MTO reaction using SiO2 modified SAPO-34 as
the catalyst showed enhanced ethylene selectivity and higher
ratio of ethylene to propylene than conventional SAPO-34. SiO2-
CLD modification does not destroy the framework structure of
SAPO-34, which mainly occurs on the external surface of SAPO-
34. The Si-OH-Al bridging hydroxyl group increased after SiO2-
CLD modification, originating from the transformation from Al-OH
to Si-OH-Al. The increase of the content of Brønsted acid of M-
SAPO-34 promotes the selectivity to light olefins in MTO. The
decrease of weak acid of zeolite is unfavorable for the alkene
cycle to forming propylene. The confined micropore introduces
extra diffusion limitation for propylene. So the selectivity to
propylene of M-SAPO-34 is lower than that of P-SAPO-34.
Combined confining aperture and adjusted acidity, high ratio of
ethylene to propylene is obtained. The increase of the reactive
sites restrain the formation of alkanes, aromatics and coke, then
the lifetime of catalyst enhances. Therefore, SiO2-CLD
modification of SAPO-34 is still a good strategy to alter the
diffusion hindrance and product selectivity in MTO reaction, which
shows high potential to modulate products for the SAPO-34
catalysts in the industrial MTO process.
equivalent fundamental sine-wave perturbation by
a
Fourier
transformation of the volume and pressure square-wave forms. The
pressure change response to the volume perturbations was measured by
a
high-accuracy differential Baratron pressure transducer (MKS
698A11TRC). The experimental data was fitted by using the Yasuda
sorption model or the single pure diffusion process model.
Experimental Section
Catalytic tests
Materials
The reaction products were analyzed using an on-line gas chromatograph
(Agilent GC 7890N), equipped with a flame ionization detector (FID) and
Plot-Q column (Agilent J&W GC Columns, HP-PLOT/Q 19091-Q04, 30 m
× 320 μm × 10 μm). The conversion and selectivity were calculated on CH2
basis and dimethyl ether (DME) was considered as reactant in the
calculation.
Orthophosphoric acid (H3PO4, 85 wt%), triethylamine (TEA, 99 wt%),
tetraethylammonium hydroxide (TEAOH, 25 wt% in H2O), cyclohexane (99
wt%), polydimethylsiloxane (99 wt%), colloidal silica (SiO2, 40 wt%) and
pseudoboehmite (Al2O3, 70 wt%) were purchased from Sinopharm
Chemical Reagent Co., Ltd. All chemicals were used as received without
further purification.
Synthesis of SAPO-34
Acknowledgements
A typical SAPO-34 was synthesized based on the hydrothermal method.
H3PO4, pseudoboehmite and silica sol were added to the aqueous solution
of template (TEA and TEAOH) to from the initial gel with the molar
The authors acknowledge Prof. Yucai Qin and Prof. Lijuan Song's
group from Liaoning Shihua University for the help of testing and
data analysis of FR technique. This work was supported by the
National Key Research and Development Program of China
(Grant No. 2016YFB0701100).
compositions of 1.0 P2O5 : 1.0Al2O3 : 1.8TEA : 0.2TEAOH : 0.4SiO2
:
60H2O. Then the obtained gel was transferred into a stainless steel
autoclave and heated rotationally (15 rpm/min) at 200 oC for 24h under
autogenous pressure. The products were recovered by filtration, washed
with deionized water and dried in air. The template was removed by
calcination at 823 K for 6 h in air. The sample is referred to as P-SAPO-
34.
Keywords: Confining Pore-mouth • chemical liquid deposition •
ethylene • SAPO-34 zeolite
CLD Modification of SAPO-34 Zeolite
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Typically, PDMS (3 g) was dispersed in cyclohexane (65 mL). Then SAPO-
34 zeolite powder (10 g) was introduced into the mixture of PDMS and
cyclohexane. The CLD modification process was carried out for 4 h under
stirring at ambient temperature. The mixture was dried at 373 K for 8h,
then was calcined at 823 K for 6 h in air. The sample is referred to as M-
SAPO-34.
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Characterization
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10.1002/cctc.202001241
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Using amphipathic molecules as co-structure-directing agent, a tri-level hierarchically
porous SAPO-34 with flower-like morphology was synthesized. The co-structure-
directing agent could promote the Si atoms incorporating into the AlPO4 framework,
change the morphology of particles and decrease the acidity of prepared SAPO-34
zeolite, leading to higher selectivity of C2H4 and C3H6 than conventional SAPO-34. The
morphology and microstructure of samples vary with the carbon chain length of co-
structure-directing agent.
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