Three-Stage Crystallization: an Effective Way to Reduce the Crystal Size and Improve the Catalytic Performance of SAPO-34 for MTO

Article abstract of DOI:10.1002/ejic.201800393

SAPO-34 crystals were prepared via a high-low-high temperature crystallization procedure with triethylamine (TEA) as template. Three-stage crystallization is beneficial for the nucleation while the growth of grains is inhibited and small size crystals are formed. The lifetime of three-stage samples was prolonged for the conversion of methanol to light olefins (MTO).

Full text of DOI:10.1002/ejic.201800393

A Journal of
Accepted Article
Title: Three-stage crystallization: an effective way to reduce the crystal
size and improves the catalytic performance of SAPO-34 for MTO
Authors: MIngjian Luo, Dakang Wang, Yadong Fu, Guoliang Mao, and
Baohui Wang
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10.1002/ejic.201800393
European Journal of Inorganic Chemistry
COMMUNICATION
Three-stage crystallization: an effective way to reduce the crystal
size and improves the catalytic performance of SAPO-34 for MTO
Mingjian Luo,* Dakang Wang, Yadong Fu, Guoliang Mao, and Baohui Wang
SAPO-34 crystals were prepared via a high-low-high temperature
crystallization procedure with triethylamine (TEA) as template.
Three-stage crystalization benefits the nucleation while inhibits the
growth of grains, which favors the formation of small size crystals.
The lifetime of three-stage samples were prolonged in the MTO
process.
after short high temperature duration, then raise the
crystallization temperature again for the further nucleation and
crystal growth. This simple high-low-high temperature
crystallization procedure is proved to be effectively in reducing
the size of SAPO-34 crystals. Contrast to samples obtained from
conventional one-stage or two-stage methods, three-stage
SAPO-34s show longer catalytic lifetime in MTO process.
A series of SAPO-34 samples were prepared with gel of a
same composition 1Al2O3: 1.1 P2O5: 0.6 SiO2: 4.0 TEA: 60 H2O.
The one-stage sample was crystallized at 200 C for 24 h and
Due to its unique CHA structure and moderate acidity, SAPO-
34 zeolite exhibits good performance in catalytic conversion of
methanol to light olefins (MTO). SAPO-34 based catalyst has
been successfully applied in the commercialized MTO unit since
2010.[1] However, the conversion of methanol and formation of
olefins is accompanied with the rapid formation of confined
aromatics in SAPO-34 cages. These aromatics, especially the
large scale phenanthrene and pyrene, reduce the diffusion of
o
labelled as S-200. The two-stage one was first crystallized at
o
o
100 C for 2 h, then 200 C for 24 h. This sample was labelled
as S-100/200. Three-stage samples were first crystallized at
200 oC for 0.5, 1, or 2 h, then 40, 70, 100 or 130 oC for 2 h, and
at last 200 oC for 24 h. Three-stage samples were labelled as S-
200-x/y/200, where x indicated the first stage time and y
indicated the second stage temperature.
methanol and olefins and even fully block the channels which
[2]
leads the deactivation of the catalyst.
Catalyst has to be
regenerated continuously in circulating fluidized bed to maintain
its reactivity. The continuous burning of coke increases the costs
and reduces the economic benefits of MTO process. [1, 3]
S-200-1/130/200
S-200-1/70/200
Analysis reveals that the coke compounds, especially
phenanthrene and pyrene, tend to locate at the near-surface
[2c, 2d, 4]
region of SAPO-34 crystal.
The near-core region of
SAPO-34 crystal, though still active for methanol conversion, is
inaccessible for reactant. Preparation of small size or
hierarchical SAPO-34 has been proved to be effective way to
increase the accessibility of near-core cages and to prolong the
catalytic lifetime. Many methods, such as optimizing the raw
material, microwave assisted,[5] ultrasonic assisted,[6] seed
assisted,[7] the using of crystal growth inhibitor,[8] and two-stage
crystallization,[9] have been proposed to reduce the crystal size
of SAPO-34. These methods are effective on the synthesis of
small or nano-scale SAPO-34. However, these methods lead the
crystallization apparatus more complex, extend the length of
synthetic route or increase the use of additional reagent.
Moreover, most of these methods employ the costly
tetraethylammonium hydroxide (TEAOH) as template agent. The
development of simple, convenient and low cost method for
small size SAPO-34 synthesis is still of great significance.
S-200-1/40/200
*
*
*
S-200-2/100/200
S-200-1/100/200
S-200-0.5/100/200
S-100-2/200
S-200
Index lines of SAPO-34
30 40
Triethylamine and morpholine (MOR) are much cheaper than
TEAOH. However, gels with them as template have higher
crystal growth speed than ones with TEAOH do, which leads to
the formation of large scale SAPO-34 crystal.[10] In order to
overcome the rapid crystal growth and increase the amount of
initial crystalline grain, we lower the crystallization temperature
10
20
2 theta, degree
Figure 1. XRD patterns of samples. Symbol * indicates the AFI
type SAPO-5.
Powder XRD patterns of samples and index lines for SAPO-
34 are showed in Figure 1. Typical diffraction peaks of CHA
structure SAPO-34 at 9.6o, 13.2o, 16.2o, 20.8o, 26.0o and 31.1o
were observed on all samples. Very weak peaks for AFI-type
SAPO-5 at 7.7o, 19.9o, and 22.6o were observed on the sample
S-200-2/100/200. Crystallinities of samples were listed in Table
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of
Chemistry & Chemical Engineering
Northeast Petroleum University
Daqing 163318, Heilongjiang, P.R. China
E-mail: luomingjian@nepu.edu.cn
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10.1002/ejic.201800393
European Journal of Inorganic Chemistry
COMMUNICATION
1. Except for the low value of S-200-1/40/200, crystallinities of
other samples were similar.
that the one-stage S-200 had the lowest P/Al and the highest
Si/Al, while the two-stage S-100/200 was the opposite. Values of
three-stage samples were similar and between those of S-200
The yields, prosities, sizes and framwork (P(Si)/Al of samples
are listed in Table 1. All samples had similar yields except S-200. and S-100/200. High initial temperature (one- and three-stage
All samples had type I isotherms typically for pure microporous
material (Figure S1). Although there were no remarkable
differences, the specific surface areas and micro pore volumes
of three-stage samples were a little higher than those of S-
100/200 and S-200 (Table 1). EDX results (Table 1) indicated
samples), especially the persistent high crystallization
temperature (S-200), favored the substitution of Si for P and Al
in the framework. Conversely, low initial temperature (two-stage,
S-100/200) inhibited the reactivity of Si.
Table 1. Yield, porosity, crystallinity, size and framework P(Si)/Al of samples
b
c
d
c
Yield a
%
45
65
69
66
67
61
65
Vmicro
m3/g
0.23
0.24
0.27
0.24
0.26
0.25
0.26
0.26
SBET
Smicro
m2/g
484
500
571
508
550
515
544
544
Sext
Crystallinity Average size e
Samples
P/Al f
Si/Al f
(Si+P)/Al f
m2/g
495
512
581
525
565
531
559
557
m2/g
11
12
10
17
15
16
15
13
%
89
94
93
90
94
80
92
92
m
10.7
4.6
3.5
3.1
4.3
4.5
2.9
3.9
S-200
S-100/200
0.807
0.840
0.827
0.828
0.822
0.830
0.835
0.815
0.284
0.229
0.258
0.251
0.251
0.263
0.243
0.247
1.091
1.069
1.085
1.079
1.073
1.093
1.079
1.062
S-200-0.5/100/200
S-200-1/100/200
S-200-2/100/200
S-200-1/40/200
S-200-1/70/200
S-200-1/130/200
67
a Product yield is calculated by the mass ratio of calcined solid product to SiO2, Al2O3 and P2O5 in the synthesis gel; b SBET (total surface area),
calculated by BET equation with data 0.05<P/Po<0.30; c Smicro (Micropore area) and Vmicro (Micropore surface volume) calculated by t-plot
method; d Sext (external surface area) calculated by SBET -Smicro e number average size; and f Measured by EDX.
;
SEM images of samples are showed in Figure 2 and S2.
Crystal size distributions are showed in Figure S3. The
crystal size of S-200 (Figure 2 A) distributed in 6~20
range with a average value 10.7 m, which was much
larger than other samples. The crystal size of S-100/200
(Figure 2 B) distributed in 2~9 m, mainly 3~6 m with a
average value 4.6 m. Three-stage samples had small
crystal sizes. Particles were mostly in 2~5 m range. A
m





o
relative short first stage (200 C) time (Figure 2 C) favored
the formation of small size crystal. Prolonging the first stage
time to 2 h (Figure 2 E) led to dual-scale distribution: ones
were in 8~12
m, which were similar to one-stage product;
the others were about 2~3
m, the same as other three-
stage samples. The temperature of second stage also
affected crystal size. Low temperature sample was relative
large (40 oC, Figure 2 F, average 4.5
m), moderate
o
temperatures favored small size products (70 C, Figure 2
G, average 2.9 m), and grain size increased again at a
relative high temperature (100 and 130 C, Figure 2 D and
H, 3.1 and 3.9 m, respectively).

o

All SAPO-34 samples show similar NH3-TPD profiles
(Figure 3). Two peaks at about 175 and 400 oC are
corresponded to weak and strong acidic sites.[7a, 11] Amount
of acidic sites are summarized in Table S1. Except S-200-
2/100/200, other three-stage samples had some more
strong acidic sites than S-200 and S-100/200. Perhaps the
first two hour at 200 oC is the decisive factor of the acidity of
S-200-2/100/200. Though remarkable difference in silicon
content (Table 1), no apparent correlation between Si
content and amount or strength of acidic site was observed.
Therefore, rather than the amount of Si, how Si cooperated
with Al and P should be the main factor that affected the
acidity of sample.
Figure 2. SEM image of samples. A: S-200; B: S-100/200; C: S-200-
0.5/100/200; D: S-200-1/100/200; E: S-200-2/100/200; F: S-200-
1/40/200; G: S-200-1/70/200; H: S-200-1/130/200.
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10.1002/ejic.201800393
European Journal of Inorganic Chemistry
COMMUNICATION
duration. The increase in nucleus led to more small size
crystal, as it was observed in all three stage samples.
Additionally, EDX results indicated one-stage high
temperature sample had the highest silicon content, the
two-stage low-high temperature sample had the lowest
silicon content, while the silicon contents of three-stage
high-low-high temperature samples were between one- and
two- stage ones. It is supposed that Al-O-Si participates in
the formation of aggregates (nucleus) and the growth of
crystal. High crystallization temperature promotes the
formation Al-O-Si which favors the formation of large
aggregates (nucleus) and the growth of crystal. Therefore,
crystals obtained under high temperature condition (S-200)
tended to big size and high Si content. Conversely, low
temperature reduces the formation of Al-O-Si. Aggregates
grow in a low speed but also less amount. As a result,
crystals obtained under low-high temperature condition (S-
100/200) tended to relative small size and high Si content.
The high-low-high temperature process, however, favors
the formation of massive Al-O-Si at high temperature, then
temperature is low down to prevent the formation of large
aggregates and favors the formation of massive small ones,
at last these small aggregates are grow into SAPO-34
crystal at high temperature. Therefore, three-stage samples
were small size with moderate Si contents. However, more
evidences are needed for confirming and further revealing
the role of silicon.
S-200-1/40/200
S-200-1/70/200
S-200-1/100/200
S-200-1/130/200
S-200-0.5/100/200
S-200-1/100/200
S-200-2/100/200
S-200
S-100/200
S-200-1/100/200
100
200
300
400
500
600
Temperature, oC
Figure 3. NH3-TPD profiles of samples.
100
80
60
40
20
0
100
80
60
40
20
0
S-200
S-100/200
S-200-0.5/100/200
S-200-1/100/200
S-200-2/100/200
S-200
S-100/200
S-200-1/40/200
S-200-1/70/200
S-200-1/100/200
S-200-1/130/200
Scheme 1. Mechanism of one-stage and three-stage crystallization
0
60
120 180 240 300 360
0
60
120 180 240 300 360
procedure.
Time on stream, min
Time on stream, min
80
60
40
20
0
80
60
40
20
0
The crystallization of microporous aluminophosphates
was supposed to compose of four general steps: [12] (1) the
initial formation of a reactant gel, comprising (Me)AlO3/5-O-
PO3 type nanometer-sized primary species, (2) the growth
of such species into secondary larger macromolecularly
disordered aggregates containing interdispersed template
cations, (3) the onset of crystallization (nucleation), and (4)
crystallite growth. Accordingly, a probable mechanism of
three-stage crystallization of SAPO-34 is illustrated in
scheme 1. Primary species and disordered aggregates
participated in both nucleation and the growth of grain. The
nucleation and the grain growth competed with each other.
A persistent high temperature (one-stage) favored the
growth of grain which led to large scale crystal. For
example, the average crystal size of S-200 was about 10.7
S-200
S-100/200
S-200-1/40/200
S-200-1/70/200
S-200-1/100/200
S-200-1/130/200
S-200
S-100/200
S-200-0.5/100/200
S-200-1/100/200
S-200-2/100/200
0
60
120 180 240 300 360
0
60
120 180 240 300 360
Time on stream, min
Time on stream, min
Figure 4. Conversion of methanol and selectivity of ethene +
propene at 425 oC and 2 h-1.
The catalytic performances of samples on methanol to
olefin reaction are showed in Figure 4. All three stage
samples, except the S-200-1/40/200, had longer life time
and high ethene + propene selectivity than the one- and
two-stage samples. Since samples were similar in
crystalline phase (XRD), porosities (N2 adsorption/
desorption) and acidity (NH3-TPD), this improvement should

m. And the S-200-2/100/200, which was first hold on 200
oC for two hours, had certain amount of big crystals. On the
contrary, nucleation dominated the crystallization procedure
if the temperature was lowered down after a short high
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10.1002/ejic.201800393
European Journal of Inorganic Chemistry
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smaller particle size, the better catalytic performance.
However, though the two-stage S-100/200 sample had
much smaller size than that of one-stage S-200, it showed
a shorter life time than the latter. This anti size-performance
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[3]
[4]
In summary, we have demonstrated a high-low-high
temperature three-stage crystallization process for reducing
the crystal size of SAPO-34. Lowering the crystallization
temperature after short high temperature duration benefited
the nucleation while inhibited the growth of grains, and
consequently favored the formation of small size SAPO-34.
Crystallization process also affected Si content in final
product. The crystallization process had little effect on
crystalline phase, porosities and acidity of samples. The
lifetime of three-stage samples were prolonged in the MTO
process, which was mainly ascribed to the decrease in
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Acknowledgements
We thank the financially supported from Heilongjiang
Youth Science Foundation (QC2015013) and NEPU
cultivate Science Foundation (2017PYZL-03). We also
thank the support of Analysis and Test Center of Northeast
Petroleum University for XRD and SEM analysis.
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[12]
Keywords: three-stage crystallization; small size; SAPO-34;
methanol to olefin; triethylamine
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