High-efficient hierarchical [B]-ZSM-5 catalyst by simultaneously using of CTAB surfactant and boron promoter for methanol to olefins reaction

Article abstract of DOI:10.1007/s11164-021-04465-z

In this work, a one-pot hydrothermal route was applied to prepare the hierarchical high-silica ZSM-5 catalyst (Si/Al = 200), including boron promoter in the structure and CTAB surfactant. XRD, FE-SEM, BET, NH₃-TPD, and FT-IR techniques were applied to evaluate the physical and chemical properties. The effect of different amounts of secondary template (CTAB) and different operating conditions was studied on the ZSM-5 catalyst preparation and performance in methanol to olefins reaction. The results showed the high surface area (391.8?m²g^?1), mesoporous structure, high crystallinity, and well-adjusted acidity for the modified catalyst. The prepared catalyst with CTAB/TPABr molar ratio of 1 led to the highest propylene selectivity (48.56%), the highest P/E ratio of 6.3, and the highest light olefin selectivity (71%). This improved performance can be assigned to the high surface area, short diffusion path length by mesopore structure, and optimum acidity. It was found that the optimum temperature and weight hourly space velocity were 480?°C and 7.2?h^?1, respectively.

Full text of DOI:10.1007/s11164-021-04465-z

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Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04465-z
High‑efcient hierarchical [B]‑ZSM‑5 catalyst
by simultaneously using of CTAB surfactant and boron
promoter for methanol to olefns reaction
Fereshte Dalirian1,2 · Mohammad Rostamizadeh1,2 · Reza Alizadeh1,2
Received: 12 December 2020 / Accepted: 10 April 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
In this work, a one-pot hydrothermal route was applied to prepare the hierarchical
high-silica ZSM-5 catalyst (Si/Al=200), including boron promoter in the struc-
ture and CTAB surfactant. XRD, FE-SEM, BET, NH3-TPD, and FT-IR techniques
were applied to evaluate the physical and chemical properties. The efect of diferent
amounts of secondary template (CTAB) and diferent operating conditions was stud-
ied on the ZSM-5 catalyst preparation and performance in methanol to olefns reac-
tion. The results showed the high surface area (391.8 m2g−1), mesoporous structure,
high crystallinity, and well-adjusted acidity for the modifed catalyst. The prepared
catalyst with CTAB/TPABr molar ratio of 1 led to the highest propylene selectivity
(48.56%), the highest P/E ratio of 6.3, and the highest light olefn selectivity (71%).
This improved performance can be assigned to the high surface area, short difu-
sion path length by mesopore structure, and optimum acidity. It was found that the
optimum temperature and weight hourly space velocity were 480 °C and 7.2 h−1,
respectively.
Keywords Methanol to olefns · Catalyst · Secondary template · Promoter · ZSM-5
Introduction
The high oil price and environmental issues have ignited interest in develop-
ing methanol to olefns (MTO) process, which can be a dominant process for the
olefns industry [1, 2]. The development of a catalyst with high performance is a
crucial point for enhancing the efciency and selectivity of the MTO reaction. The
*
Mohammad Rostamizadeh
Rostamizadeh.m@gmail.com
1
2
Faculty of Chemical Engineering, Sahand University of Technology, Sahand New Town, P.O.
Box: 51335-1996, Tabriz, Iran
Research Center of Environmental Engineering, Sahand University of Technology, Sahand New
Town, P.O. Box: 51335-1996, Tabriz, Iran
Vol.:(0123456789)
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F. Dalirian et al.
performance of a variety of catalysts like ZSM-5 [3, 4], SAPO-34 [5], MOR [6],
and HZSM-22 [7] has been evaluated. Among them, ZSM-5 with microsize pores
showed excellent performance due to its high thermal stability, resistance against
coke formation, and surface area [8]. In addition, acidity and specifc shape selectiv-
ity are the other important parameters, which can be controlled easily for ZSM-5
[9]. Despite ZSM-5 promising features, its microporous structure accelerates cata-
lyst deactivation due to blocking microspore channels by large hydrocarbon mol-
ecules. One strategy to overcome this drawback of ZSM-5 is modifcation of the
microporous structure of ZSM-5 by adding mesopores into the porous structure of
the catalyst [10]. The mesopore ZSM-5 can be synthesized by either chemical leach-
ing or templating routes. Unfortunately, the chemical leaching method sufers from
reducing the high amount of acid sites [11]. On the other hand, the amount of acid
sites can be controlled by synthesis conditions (e.g., template type) in a templat-
ing synthesis route (TSR) [12]. In this regard, a secondary template such as CTAB
[13] has been developed to prepare mesoporous catalysts. It is added as a structure-
directing agent (SDA) and its hydrophobic tail is responsible for mesopores [14].
The formed mesopores increase the difusion rate of reactants, surface area, and the
number of accessible acidic sites compared with conventional ZSM-5 [15, 16]. The
acidity of catalyst is the other parameter that plays a vital role in MTO reaction. The
high density and strong acid sites accelerate coke formation and so catalyst deactiva-
tion [17]. In addition, the selectivity of products can be controlled by tuning acid-
ity of catalyst. Therefore, promoter incorporation and metal doping routs have been
used to change the selectivity of products by controlling acidity of the catalyst [18].
Incorporation of boron promoter in the structure of the ZSM-5 catalyst can adjust
the acidity, leading to the better performance of the ZSM-5 catalyst in the MTO
reaction. It is reported that strong acid sites are responsible for the side reactions and
coke formation [14, 19, 20]. It is accepted that the high adsorption of hydrocarbons
on the MTO catalyst surface results in the high catalyst activity and more cracking
reactions. Sacchetto et al. [21] studied the adsorption of gaseous hydrocarbons on
high-silica ZSM-5. They found that high Si/Al ratio favored hydrocarbon adsorp-
tion on ZSM-5 surface. Furthermore, it is reported that chemical and thermal sta-
bility, as well as the yield of synthesis of the high-silica ZSM-5, is more than the
low-silica ZSM-5 [22, 23], favoring the economy of the catalyst preparation for the
methanol conversion. On the other hand, promoter is necessary to achieve appropri-
ate acidity properties as well as the high catalytic performance. It is accepted that
acid sites are responsible for the main and side reactions in the MTO process. There-
fore, the well-adjusted acidity and structural properties of the catalyst play a crucial
role for developing of the improved catalyst for the MTO reaction. The minimiz-
ing of the synthesize steps, including post-treatment steps such as addition of pro-
moter and modifcation of catalyst structure, leads to the one-pot synthesize of the
optimum catalyst as a cost-efective and fast route. To the best of our knowledge,
among all researches in this feld, there is no report about the modifying chemistry
and structure of high-silica ZSM-5 by simultaneously using CTAB and B promoter.
Therefore, we focused on the efect of CTAB as a SDA agent and B promoter on the
physic-chemical properties as well as the performance of the catalyst in the MTO
reaction.
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High‑efcient hierarchical [B]‑ZSM‑5 catalyst by…
Materials and method
Materials
Silicic acid (SiO2.xH2O, 99 wt.%), sodium aluminate (NaAlO2, Al2O3 55 wt.%).
sulfuric acid (H2SO4, 98 wt.%), ammonium nitrate (NH4NO3, 99 wt.%), sodium
hydroxide (NaOH, 99.6 wt.%), boric acid (H3BO3, 99.8 wt.%), tetrapropyl ammo-
nium bromide (TPABr), and cetyl-trimethylammonium bromide (CTAB) were
applied for the catalyst synthesis. All the chemicals were used as received from
Merck Company (Germany).
Catalyst synthesis
Firstly, NaAlO2, H3BO3, and TPABr were dissolved in NaOH solution for 30 min
(solution A). Solution B was prepared by adding silicic acid and CTAB to NaOH
solution. The solution A was gradually added to the solution B under stirring. After
adjusting pH at 10.5 by concentrated sulfuric acid, the solution was stirred for 2 h.
Table 1 shows the molar composition of the fnal solutions. The crystallization was
carried out at 180 °C for 48 h in an autoclave. The sample was dried at 110 °C for
12 h, followed by calcination at 540 °C for 24 h with 3°Cmin−1 heating rate. In the
fnal step, the catalyst was ion-exchanged with 1 M NH4NO3 solution for 10 h at
90 °C, dried at 110 °C for 12 h, and calcined at 550 °C for 12 h. The catalysts were
denoted by CTZM-y where y was the CTAB/TPABr molar ratio.
The details of the characterization methods were provided in the supplementary
information.
Experiments
The MTO reaction was carried out in a fxed bed reactor with a 450 mm length and
11 mm inner diameter. The reactor was connected to a gas chromatograph (GC),
which contained a fame ionization detector (FID) and HP-plot Q column with 30 m
length and 0.53 mm inner diameter. The applied catalyst had 16–25 mesh particle
size. The reaction was carried out at diferent temperatures (450, 480, and 500 °C)
at atmospheric pressure. The feed was a 50/50 wt.% methanol/water solution, which
was charged into the reactor with determined weight hourly space velocity (WHSV).
Table 1 Molar composition of
Catalysts
CTZM-0
CTZM-0.75 0.05
CTZM-1 0.05
CTZM-1.5 0.05
Compositions
the to synthesize solutions
Al2O3 SiO2 TPABr Na2O B2O3 CTAB H2O
0.05
20
20
20
20
1
1
1
1
1.511
1.511 0.05 0.75
1.511 0.05
1.511 0.05 1.5
0
0
209
209
209
209
1
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F. Dalirian et al.
The methanol conversion and product selectivity were defned by the following
equations:
Ni
− No
MeOH
MeOH
xMeOH
=
∗ 100
(1)
Ni
MeOH
x ∗ No
CxHy
SC H
=
∗ 100
(2)
x
y
Ni
− No
MeOH
MeOH
where N is the mole number of components. The superscript i and o stand for com-
ponents at the inlet and outlet of the reactor, respectively.
Results and discussion
Physic‑chemical properties
The X-ray difraction (XRD) pattern for the synthesized CTZM-y samples is shown
in Fig. 1. The results confrm an MFI-structure (JCPDS NO. 00-042-0023) [24] for
the prepared samples. The relative crystallinity of the modifed catalysts is deter-
mined by the height of the peak at 2θ=23° based on the CTZM-0 catalyst, and the
crystallinity change is determined by Eq. (3):
(
)
I
ΔC% =
∗ 100 − 100
(3)
I0
where I and I0 are the height of the peak at 2ꢀ=23° for the modifed and parent cata-
lysts, respectively [25]. As shown in Table 2, the CTZM-0.75 and CTZM-1 cata-
lysts show positive crystallinity change while the CTZM-1.5 catalyst shows a nega-
tive crystallinity change. The crystallinity is improved in the limited range of the
added CTAB because structure-directing characteristic of CTAB molecules assists
the growth of crystal. On the other hand, if the amount of the added CTAB exceeds
from the certain value, the overloaded CTAB forms some amorphous phases and
Fig. 1 XRD pattern of the
catalysts
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