Effect of the Si/Al ratios of nanocrystalline HZSM-5 zeolite on the performance in catalytic conversion of ethanol to propylene

Article abstract of DOI:10.1166/jnn.2017.13992

Four ZSM-5 zeolite samples, with similar crystal sizes (about 100 nm) but different Si/Al molar ratios (nominal ratios from the compositions of the synthesis mixtures: 50, 75, 125, or 200; actual ratios determined by ICP: 50, 70, 131, 189), were synthesized hydrothermally and characterized by ICP-OES, SEM, XRD, N₂ adsorption-desorption, NH₃-TPD, Py-IR, and TG. The characterization results showed that the structural and textural properties of these samples are identical to each other, but their acid properties are critically influenced by the Si/Al molar ratios. The amounts of Br?nsted and Lewis acid sites increase when the Si/Al ratio decreases. The catalytic performance of these catalysts in the conversion of ethanol to propylene was examined in a fixed-bed microreactor at atmospheric pressure, 500°C, and a WHSV of 1.58 h^-1. HZSM-5 nanocatalysts with relatively low Si/Al ratios are favorable for the formation of propylene and butylene, whereas those with higher Si/Al ratios are favorable for the formation of ethylene. Possible reasons for the difference in selectivity of these catalysts are discussed.

Full text of DOI:10.1166/jnn.2017.13992

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 3779–3785, 2017
Copyright © 2017 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Effect of the Si/Al Ratios of Nanocrystalline HZSM-5
Zeolite on the Performance in Catalytic
Conversion of Ethanol to Propylene
1
1ꢀ∗
1
2ꢀ∗
Tao Meng , Dongsen Mao , Qiangsheng Guo , and Zhen Ma
1Research Institute of Applied Catalysis, School of Chemical and Environmental Engineering,
Shanghai Institute of Technology, Shanghai 201418, P. R. China
Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, P. R. China
2
Four ZSM-5 zeolite samples, with similar crystal sizes (about 100 nm) but different Si/Al molar
ratios (nominal ratios from the compositions of the synthesis mixtures: 50, 75, 125, or 200; actual
ratios determined by ICP: 50, 70, 131, 189), were synthesized hydrothermally and characterized by
ICP-OES, SEM, XRD, N2 adsorption–desorption, NH -TPD, Py-IR, and TG. The characterization
3
results showed that the structural and textural properties of these samples are identical to each
other, but their acid properties are critically influenced by the Si/Al molar ratios. The amounts of
Brönsted and Lewis acid sites increase when the Si/Al ratio decreases. The catalytic performance
of these catalysts in the conversion of ethanol to propylene was examined in a fixed-bed micro-
IP: 178.159.97.1
ꢀ25 On: Sun, 29 Dec 201 9− 119:29:55
reactor at atmospheric pressure, 500 C, and a WHSV of 1.58 h . HZSM-5 nanocatalysts with
Copyright: American Scientific Publishers
relatively low Si/Al ratios are favorable for the formation of propylene and butylene, whereas those
Delivered by Ingenta
with higher Si/Al ratios are favorable for the formation of ethylene. Possible reasons for the difference
in selectivity of these catalysts are discussed.
Keywords: HZSM-5, Nanoscale, Si/Al Ratio, Ethanol, Propylene.
1
. INTRODUCTION
of a series of nanocrystalline ZSM-5 zeolites in the liq-
uid phase acylation of anisole with acetic anhydride was
found to increase with the decrease of ZSM-5’s particle
Zeolites are crystalline aluminosilicates with ordered
micropores and well-defined structures. In particular,
ZSM-5 is useful in catalysis because it has abundant
acid sites and excellent shape selectivity. ZSM-5-based
catalysts have diversified functionalities, which makes
them useful in the conversion of fossil fuels, the abla-
tion of environmental pollutants, and the synthesis of fine
8
size. Nanocrystalline Y and beta zeolites showed better
performance in catalytic conversion of used palm oil for
the production of biofuel than microcrystalline Y and beta
7
zeolites did.
Propylene, a useful chemical stock, is mainly pro-
duced as a byproduct in the catalytic cracking of naphtha
and crude oil in petrochemical industry. However, these
catalytic processes involve and generate a lot of environ-
mentally unfriendly chemicals/materials/wastes. Consider-
ing the shortage of petroleum in the long run and the
growing demand for propylene combined with environ-
mental concerns, it would be desirable to seek alternative
“cleaner” routes to propylene.
1
–4
chemicals.
Conventional zeolites synthesized have crystal sizes on
the order of micrometers. In recent years, nanoscale zeo-
lites have attracted much interest because they may show
better catalytic performance than conventional zeolites.
For example, nanosized ZSM-5 showed higher catalytic
activity than micronsized ZSM-5 did in the transalkyla-
5
–7
5
tion of benzene with 1,2,4-trimethylbenzene. Nanosized
H-ZSM-5 showed higher activity, stability, and propy-
lene selectivity in the conversion of methanol to propy-
lene, compared with microsized H-ZSM-5. The activity
Ethanol can be widely and easily produced by the
fermentation of renewable biomass feed-stocks such as
corn, sugarcane, and cellulose. Bioethanol is used directly
to fuel vehicles in Brazil, and it can be blended with
gasoline to fuel vehicles in some countries such as the
6
∗
Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 6
1533-4880/2017/17/3779/007
doi:10.1166/jnn.2017.13992
3779

Effect of the Si/Al Ratios of Nanocrystalline HZSM-5 Zeolite
Meng et al.
United States. Besides, ethanol, either from fermentation
or chemical processes, can be used to produce chemicals
was stirred at room temperature overnight to ensure the
complete hydrolysis of TEOS. The prepared mother gel
was heated at 80 C to remove alcohols and some water.
9
–27
ꢀ
such as propylene in a cleaner fashion.
Ethanol can be converted to propylene over HZSM-
The concentrated solution was crystallized in Teflon-lined
stainless-steel autoclaves under autogenous pressure. The
1
0–20ꢀ22–25ꢀ27
5
-based catalysts.
High propylene selectivity
ꢀ
could be achieved when the reaction is conducted at a suit-
able temperature and a low weight hourly space velocity
crystallization process was first carried out at 100 C for
24 h and then at 170 C for 30 h.
ꢀ
1
1ꢀ13ꢀ14
14ꢀ15
(
WHSV),
or if HZSM-5 is modified with La,
The solid product was recovered after several cycles of
centrifuging and washing with deionized water, dried at
1
1
13
12ꢀ18–20ꢀ25
Zr, Sr, or P.
However, the propylene selectiv-
ꢀ
ꢀ
ity and reaction stability of HZSM-5 catalysts must be
110 C overnight, and calcined in air at 600 C for 6 h.
The obtained ZSM-5 samples were turned into the H-form
by three consecutive ion exchanges in a large excess of
improved for their industrial applications.2
6
Recently, we tested the catalytic performance of a
series of HZSM-5 zeolites with the same composition
ꢀ ꢀ
1 M solutions of NH NO at 90 C, and calcined at 550 C
4 3
(
5
Si/Al = 75) but with different crystal sizes (100, 250,
for 6 h. The obtained HZSM-5 samples are denoted as
HZ-x, where x is the nominal Si/Al molar ratio (50, 75,
2
8
00 nm) in the conversion of ethanol to propylene.
1
25, or 200).
HZSM-5 with the smallest size (100 nm) in that study
showed the highest selectivities to propylene and butylene
as well as the lowest selectivities to ethylene and aromat-
ics. The selectivity to propylene on this catalyst was more
stable on stream than those on HZSM-5 catalysts with
bigger crystal sizes (250, 500 nm). The relatively stable
generation of more propylene on nanosized HZSM-5 was
ascribed to the zeolite’s larger external surface area, more
secondary pores, and shorter diffusion channels. Xia and
co-workers also studied the influence of particle size of
H-ZSM-5 on the catalytic conversion of ethanol to propy-
lene, and found that smaller H-ZSM-5 is preferable for the
2.2. Catalyst Characterization
X-ray diffraction (XRD) experiments were performed
using a PANalytical X’Pert diffractometer operating with
Ni ꢁ-filtered Cu Kꢂ radiation at 40 kV and 40 mA. Two
ꢀ
ꢀ
ꢀ
theta angles ranged from 5 to 50 with a speed of 4 per
minute.
SEM micrographs were obtained using a Hitachi S-4000
instrument. The bulk Si/Al molar ratios of the sam-
ples were determined by ICP-OES (PerkinElmer Optima
7000DV).
2
7
ꢀ
formation of propylene.
IP: 178.159.97.125 On: Sun, 2 N9 i tDr oe gc e n2 0 a1 d 9s o1 r 9p :t 2i o 9n : /5d 5e sorption isotherms at −196 C
Copyright: American Scientific Publishers
It is recognized that the acidity of HZSM-5 is another
were obtained after outgassing the sample under vacuum at
ꢀ
Delivered by Ingenta
important factor that affects the propylene selectivity and
200 C for 12 h, using a Micromeritics ASAP2020 M+C
1
1ꢀ14ꢀ18ꢀ22ꢀ29
reaction stability,
and the acidity of HZSM-5
adsorption apparatus.
1
1ꢀ13–15ꢀ29
depends greatly on the Si/Al ratio.
However, to
Temperature programmed desorption of ammonia
the best of our knowledge, the influence of the Si/Al
ratio of nanoscale HZSM-5 on the performance in ethanol
to propylene process has not been reported. Xia et al.
studied catalytic conversion of ethanol to propylene over
micrometer-sized H-ZSM-5 samples with different Si/Al
ratios, and found that the production distribution is inde-
(NH -TPD) was carried out on a conventional flow appara-
3
tus equipped with a thermal conductivity detector (TCD).
A 0.1 g sample of 40–60 mesh was pretreated in flow-
ꢀ
ꢀ
ing N at 500 C for 1 h, cooled to 100 C, and then
2
exposed to 10% NH /N (30 mL/min) for 1 h. The sam-
3
2
ple was purged with N (30 mL/min) to remove physically
2
2
9
adsorbed NH . The measurements were conducted in flow-
pendent of the Si/Al ratio. In our current work, four
nanoscale HZSM-5 samples with similar crystal sizes
3
ꢀ
ꢀ
ing N (30 mL/min) from 100 C to 550 C at a heating
rate of 10 C/min.
2
ꢀ
(
100 nm) but different Si/Al ratios were synthesized, and
The Py-IR spectra were recorded using a Nicolet 6700
the effect of Si/Al molar ratios on the catalytic perfor-
mance for the selective conversion of ethanol to propylene
was investigated and discussed.
−1
infrared spectrometer with a resolution of 4 cm . A self-
supported 12 mm diameter circular wafer was placed in
an infrared cell with CaF windows and connected to a
2
ꢀ
vacuum system. The wafer was dehydrated at 400 C and
at P < 10 mbar for 150 min. After cooling to room
2
. EXPERIMENTAL DETAILS
−4
2
.1. Catalyst Preparation
temperature, pyridine vapors were adsorbed for 30 min.
Finally, excess pyridine was desorbed by evacuation of
the samples at the desired temperature for 30 min, and
pyridine-adsorbed IR spectra were recorded.
All ZSM-5 samples were synthesized from clear gel solu-
tions with molar composition of 36 TPAOH:1 Al O :2x
2
3
2
8
SiO :2300 H O. A required amount of aluminum iso-
2
2
propoxide (AIP) and deionized water were added to a
Coke amount was determined by a thermogravimetric
analyzer (Shimadzu DTG-60H), with spent sample in air
5
0 wt.% aqueous solution of tetraproplammonium hydrox-
ꢀ
ꢀ ꢀ ꢀ
from 30 C to 850 C at a heating rate of 10 C/min. The
ide (TPAOH). The mixture was stirred at 0 C to obtain
a clear solution. Tetraethylorthosilicate (TEOS) was added
dropwise under stirring at room temperature. The solution
ꢀ
ꢀ
weight loss between 400 C and 700 C in each profile
was defined as the content of coke on the used catalyst.
3780
J. Nanosci. Nanotechnol. 17, 3779–3785, 2017

Meng et al.
.3. Catalytic Testing
Catalytic reaction tests were conducted at 500 C under
atmospheric pressure in a continuous flow fixed-bed reac-
tor made of stainless steel (inner diameter 6 mm). Prior
Effect of the Si/Al Ratios of Nanocrystalline HZSM-5 Zeolite
2
ꢀ
HZ-50
to the catalytic testing, the fresh catalyst was activated
ꢀ
HZ-75
at 500 C for 1 h in N flow. Ethanol (purity > 99ꢃ5%)
2
was fed at the rate of 0.6 mL/h (controlled with a piston
pump) through a 0.3 g catalyst (40–60 mesh) placed in
the central zone of the reactor. The weight hourly space
velocity (WHSV) of ethanol was 1.58 h . The effluent
products were analyzed online via an Agilent 6820 gas
chromatograph with a flame ionization detector (FID) and
a HP-Plot-Q capillary column. The product lines were
heated electrically to avoid unwanted condensation of the
products.
HZ-125
−1
HZ-200
1
0
20
30
40
50
2
Theta/º
Figure 1. XRD patterns of HZSM-5 zeolites with different Si/Al ratios.
show typical NH -TPD profiles with two maximum peaks
3
ꢀ
3
. RESULTS AND DISCUSSION
below and above 300 C, corresponding to NH desorbed
3
3
.1. Structural and Textural Characterization
from weak and strong acid sites, respectively. The amounts
of both weak and strong acid sites decrease as the Si/Al
ratio of HZSM-5 increases, in agreement with previous
The Si/Al molar ratio of each HZSM-5 sample, as mea-
sured by ICP-OES (50, 70, 131, 189, Table I), is fairly sim-
ilar to that of the clear gel solution (50, 75, 125, 200). For
convenience, the HZSM-5 samples are denoted as HZ-x,
where x is the nominal Si/Al molar ratio (50, 75, 125,
or 200).
As shown in Figure 1, HZSM-5 samples with differ-
ent Si/Al ratios exhibit the typical XRD patterns corre-
sponding to the MFI-type structure, and no other peak can
be observed. The very low backgrou nC do ps i yg rni ga lh ta: n Ad ms eh ar ircp an S tci vi ee nl yt .i f iTc h Pe us bi gl ins ah le ar ts 1490 cm is assigned to pyridine on
reflections indicate their excellent crystallinity. Delivered byb Iont hg e Bn t aa nd L acid sites. The intensities of the bands at
Figure 2 shows the SEM micrographs of these samples.
It can be seen that all the samples have almost the same
spherical morphology, and their average particle sizes are
all about 100 nm.
2
9–32
studies.
Figure 4 shows the IR spectra of pyridine adsorbed
on HZSM-5 samples. The spectra of adsorbed pyridine
−1
exhibit characteristic bands at 1546 and 1448 cm ,
attributed to pyridinium ions (pyridine adsorbed on
Brönsted (B) acid sites) and coordinatively bound pyridine
(pyridine interacting with Lewis (L) acid sites), respec-
IP: 178.159.97.125 On: Sun, 29 Dec 2019 19:29:55
−1
−1
1546, 1490, and 1448 cm all increase when the Si/Al
ratio of HZSM-5 decreases, being the highest when the
Si/Al ratio is 50.
−1
The intensities of the bands at 1546 and 1448 cm
The results of N adsorption and desorption measure-
were used to calculate the concentrations of B and L
acid sites of HZSM-5 and the results are summarized in
Table II. The values of B/L site ratios were calculated by
the formula of 1ꢃ88A /1ꢃ42A , taking into account the
coefficients of molar extinction. The data show that the
decrease in Si/Al ratio of HZSM-5 leads to a significant
increase of the concentrations of both B and L acid sites at
the same desorption temperature, consistent with the NH3-
TPD results described above. On the other hand, the B/L
ratio of HZSM-5 with different Si/Al ratios increases with
the increase of pyridine desorption temperature, indicating
that the B acid sites are stronger than the L acid sites.
2
ments are summarized in Table I. Although the Si/Al ratios
of HZSM-5 samples are different, there is only a marginal
difference in the textural properties among these sam-
ples, probably owing to the similar crystal sizes of these
samples.30 The average pore diameters of these samples
are large, due to the formation of many secondary pores
B
L
3
3
2
8
by aggregation of nanoparticles.
3
.2. Acidity Characterization
NH -TPD was performed to characterize the acidity of
3
the catalysts. As shown in Figure 3, HZSM-5 catalysts
Table I. Composition and textural properties of HZSM-5 zeolites with different Si/Al ratios.
2
−1
3
−1
)
BET surface area (m ·g
)
Pore volume (cm ·g
a
Sample
Si/Al molar ratio
Total
Micropore
External
Total
Micropore
Mesopore
Average pore diameter (nm)
HZ-50
HZ-75
HZ-125
HZ-200
50ꢃ3
70ꢃ0
131ꢃ4
189ꢃ2
376
387
369
366
145
154
142
151
231
233
227
215
0ꢃ59
0ꢃ53
0ꢃ57
0ꢃ51
0ꢃ065
0ꢃ070
0ꢃ068
0ꢃ073
0ꢃ525
0ꢃ460
0ꢃ502
0ꢃ437
6ꢃ28
5ꢃ48
6ꢃ18
5ꢃ57
Note: aDetermined by ICP-OES.
J. Nanosci. Nanotechnol. 17, 3779–3785, 2017
3781

Effect of the Si/Al Ratios of Nanocrystalline HZSM-5 Zeolite
Meng et al.
Figure 2. SEM images of HZSM-5 zeolites with different Si/Al ratios.
3
.3. Catalytic Performance
then falls below 12% after 96 h on steam. A similar trend
is observed for HZ-75; the highest propylene selectiv-
The catalytic performance of HZSM-5 catalysts with dif-
ferent Si/Al ratios in the conversion of ethanol into propy-
lene (and other products such as ethylene, butyl eDn ee ls i ,v ea nr ed d bybI en l go we n 1t a2 % after 54 h on stream. However, the propylene
IP: 178.159.97.125 On: Sun, 29 Dec 2019 19:29:55
ity is 23.8% at 7.5 h, and the propylene selectivity falls
Copyright: American Scientific Publishers
aromatics) was studied. For all the samples, the ethanol
conversion is 100% during the whole operation. How-
ever, the initial selectivitities to various products and their
change patterns with respect to time on stream (TOS) are
remarkably different for different catalysts.
selectivities on HZ-125 and HZ-200 decrease rapidly with
TOS. The selectivities to propylene on catalysts in the
long run follow the sequence of HZ-50 > HZ-75 > HZ-
125 > HZ-200. Apparently, HZ-50 is the most useful for
the stable production of propylene. Previous investigations
showed that a moderate surface acidity favors propylene
production; here we found that nanoscale HZ-50 with
the largest amount of acid sites among the catalysts stud-
ied here is the best for propylene production. Our result
Figure 5 shows the effects of TOS on the selectivi-
ties to light olefins (C –C ꢄ and aromatics. As presented
1
1
2
4
in Figure 5(a), for HZ-50 with the lowest Si/Al ratio in
this study, the propylene selectivity initially increases with
TOS, reaching a maximum value (22.4%) at 30 h, and
HZ-50
HZ-50
HZ-75
HZ-75
HZ-125
HZ-125
HZ-200
HZ-200
1
00
200
300
400
500
1420 1440 1460 1480 1500 1520 1540 1560
Wavenumber/cm–1
Temperature/ºC
Figure 3. NH -TPD curves of HZSM-5 zeolites with different Si/Al
ratios.
Figure 4. IR spectra of pyridine adsorbed on the different HZSM-5
zeolites after desorption at 100 C.
3
ꢀ
3782
J. Nanosci. Nanotechnol. 17, 3779–3785, 2017

Meng et al.
Effect of the Si/Al Ratios of Nanocrystalline HZSM-5 Zeolite
Table II. Results of IR spectra of Py adsorbed on the different HZSM-5
zeolites after desorption at different temperatures.
HZSM-5 catalysts follow the sequence of HZ-50 > HZ-
7
5 > HZ-125 > HZ-200, and they all decrease with TOS
1
1
16
ꢀ
ꢀ
c
1
00 C
300 C
B/L
(Fig. 5(d)). Song et al. and Inoue et al. found that lower
surface acidity leads to the formation of more ethylene,
while aromatics are promoted at higher acidity. Indeed,
here we found that HZ-200 with the lowest surface acidity
is more favorable for the formation of ethylene than HZ-
a
B
b
L
ꢀ
ꢀ
Sample
A
A
AB
AL
100 C
300 C
HZ-50
HZ-75
HZ-125
HZ-200
1ꢃ41
0ꢃ41
0ꢃ31
0ꢃ20
1ꢃ42
0ꢃ61
0ꢃ26
0ꢃ17
0ꢃ76
0ꢃ32
0ꢃ22
0ꢃ13
0ꢃ46
0ꢃ10
0ꢃ09
0ꢃ08
1ꢃ32
0ꢃ89
1ꢃ58
1ꢃ56
2ꢃ19
4ꢃ24
3ꢃ24
2ꢃ15
1
25, HZ-75, and HZ-50, whereas HZ-50 with the highest
acidity is more favorable for the formation of aromatics
than HZ-75, HZ-125, and HZ-200. In addition, the changes
of the selectivities of ethylene and aromatics with TOS in
Figures 5(c and d) can be explained by the deactivation of
Note: aA —Integrated area of peak at 1546 cm ; bA —Integrated area of peak
−1
B
L
−1 c
at 1448 cm
;
B/L—Calculated by the formula of 1ꢃ88A /1ꢃ42A .
B
L
2
8
is different from Goto et al.’s finding that HZSM-5 with
a SiO /Al O ratio of 184 is better that HZSM-5 with a
SiO /Al O ratio of 52 in terms of propylene production.
The variation trend of butylene selectivity of each
HZSM-5 sample is similar to propylene selectivity
Fig. 5(b)), implying that propylene and butylene might be
the strong acidic sites due to the coke deposition.
2
2
3
1
3
3.4. Correlation Between Acidity and
Catalytic Performance
2
2
3
The reaction pathway of ethanol to propylene over
1
4ꢀ35
(
HZSM-5 can be assumed as follows.
Ethanol is first
produced through parallel pathways and common interme-
converted to ethylene on acid sites. Higher hydrocar-
bons such as propylene are produced from ethylene via
oligomerization-cracking. These conversions proceed over
different acid sites: ethanol dehydration to ethylene can
occur on weak acid sites, but the subsequent reaction
2
0ꢀ34
diate on the catalyst.
In addition, the selectivities to
butylene on catalysts in the long run follow the sequence
of HZ-50 > HZ-75 > HZ-125 > HZ-200.
As presented in Figure 5(c), the selectivities to ethy-
lene on HZSM-5 catalysts follow the sequence of HZ-50 <
HZ-75 < HZ-125 < HZ-200, and they all increase with
TOS. On the other hand, the selectivities to aromatics on
3
4ꢀ36
steps require stronger acid sites.
Generally, the cat-
alyst with high acid density and strong acidity convert
intermediate hydrocarbons such as ethylene into higher
IP: 178.159.97.125 On: Sun, 29 Dec 2019 19:29:55
3
2
1
1
0
4
8
2
6
0
Copyright: American Scientific Publishers
HZ-50
HZ-75
HZ-125
HZ-200
a
HZ-50
HZ-75
20
b
Delivered by Ingenta
HZ-125
HZ-200
16
1
2
8
4
0
0
20
40
60
80
100
0
20
40
60
80
100
Time/h
Time/h
1
00
HZ-50
HZ-75
HZ-125
HZ-200
c
HZ-50
HZ-75
HZ-125
HZ-200
30
d
8
6
4
2
0
0
0
0
0
2
1
4
8
12
6
0
0
20
40
60
80
100
0
20
40
60
80
100
Time/h
Time/h
Figure 5. Effect of time on stream on selectivity of C –C olefins and aromatics over HZSM-5 zeolites with different Si/Al ratios at atmospheric
2
4
ꢀ
−1
pressure, 500 C and WHSV = 1ꢃ58 h : (a) propylene; (b) butylene; (c) ethylene; (d) aromatics.
J. Nanosci. Nanotechnol. 17, 3779–3785, 2017
3783

Effect of the Si/Al Ratios of Nanocrystalline HZSM-5 Zeolite
hydrocarbons (C aliphatic and aromatic) species and
Meng et al.
3
+
1
00
98
96
coke.35
HZ-200
For fresh HZSM-5 with lower Si/Al ratios and larger
amounts of strong acid sites, more aromatics are expected
HZ-125
HZ-75
to be produced by aromatization.1
1ꢀ16
Thus, HZ-50 and
HZ-75 exhibit high initial aromatics selectivity (Fig. 5(d))
and low initial C –C olefins selectivity (Figs. 5(a and b)).
9
4
3
4
On the other hand, initial ethylene selectivity increases
with the Si/Al ratio (Fig. 5(c)), indicating that weaker acid
sites are beneficial for the formation of ethylene.
92
HZ-50
800
9
0
The catalyst surface is covered by coke gradually with
TOS, thus inducing the decrease in the number of strong
acid sites. Therefore, the selectivities of C –C olefins
0
200
400
600
Temperature/ºC
2
4
Figure 6. TG curves of HZSM-5 zeolites with different Si/Al ratios
after the duration test of ethanol to propylene catalytic reaction.
increase but the selectivity to aromatics decreases with
TOS. It can be concluded that decreasing the amount
of strong acid sites is beneficial for raising propylene
2
2
4
. CONCLUSIONS
selectivity. As the reaction over HZSM-5 prolongs, more
and more coke is formed and covers the catalyst sur-
face. Accordingly, the number of acid sites with medium
ZSM-5 zeolites with different Si/Al ratios (50, 75,
115, 200) but similar crystal sizes (100 nm) were suc-
cessfully synthesized. These HZSM-5 catalysts had simi-
lar physicochemical properties except for the acidity. The
decreased Si/Al molar ratio of ZSM-5 zeolite led to an
increase in the amounts of the strong acid sites and weak
acid sites. HZSM-5 with different Si/Al molar ratios all
exhibited high activity for ethanol conversion. In partic-
ular, HZSM-5 with a Si/Al ratio of 50 was found to be
the most favorable for the stable production of propylene.
strength, responsible for the formation of propylene and
butylenes,1
1ꢀ16ꢀ22
also decreases with TOS. As a result, the
selectivity to C –C olefins decreases, whereas ethylene
3
4
selectivity keeps increasing.
The Si/Al ratio of HZSM-5 plays an important role in
the time length of the induction period. HZSM-5 samples
of low Si/Al ratios (50, 75) have larger numbers of strong
acid sites, so more coke is needed to cover the strong
acid sites, which makes the induction period longer. On
IP: 178.159.97.125 On: Sun, 29 Dec 2019 19:29:55
Further attempts may be made to modify the catalyst in
order to improve the propylene yield and stability.
Copyright: American Scientific Publishers
the contrary, the numbers of strong acid sites on HZSM-5
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samples with high Si/Al ratios (125, 200) are fewer, so
these strong acid sites can be covered readily by smaller
amounts of coke. Therefore, the induction period is much
shorter for high Si/Al ratio samples. For the same rea-
son, the order of the reduction rate of propylene selectiv-
ity over different HZSM-5 catalysts (HZ-200 > HZ-125 >
HZ-75 > HZ-50) is opposite to that of the time length
of the induction period. Our data indicate that high Si/Al
ratio HZSM-5 zeolites are not suitable for catalytic reac-
tion of ethanol to propylene, which is different from the
Acknowledgment: The authors thank Shanghai Munic-
ipal Education Commission (J51503) and Graduate Inno-
vation Foundation of Shanghai Institute of Technology
(yjscx2010005) for financial support.
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Received: 11 April 2016. Accepted: 8 May 2016.
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3785