Effect of H2SiF6modification of IM-5 on catalytic performance in benzene alkylation with ethylene

Article abstract of DOI:10.1039/d1ra02427b

Ethylbenzene (EB) is an important bulk chemical intermediate. The vapor-phase process is considered to be more efficient than the liquid-phase process when using dilute ethylene (e.g. FCC or DCC off-gas) as the feed due to its high ethylene space velocity. However, realizing a balance between reducing the xylene formation and enhancing the EB selectivity is still a challenge due to the poor performance of ZSM-5 at low reaction temperature. This study concerns an IM-5 zeolite (IMF topology) modified by H2SiF6, with 89% ethylbenzene selectivity, 98.6% total EB + DEB selectivity and only 540 ppm of xylene at 330 °C. IM-5 zeolites with different Si/Al2 ratios (40-170) were prepared by H2SiF6 modification and their catalytic performance in vapor phase alkylation of benzene with ethylene was investigated. There was an obvious decrease in the acid sites and acid strength of IM-5 in the H2SiF6 treatment process, which led to a slight decrease in ethylbenzene selectivity and a significant decline in xylene yield. Under the conditions of complete ethylene conversion, the selectivity to EB + DEB increased from 96.1% to 98.6% in the parent I-40 and modified IM-5. Compared with ZSM-5 that has a similar acidity, the slightly bigger channel opening makes IM-5 more conductive to the formation and diffusion of DEB while xylene may present adverse effects. The 120 hour-lifetime test showed that IM-5 (I-110) has superior activity, equivalent stability, higher DEB selectivity and a much lower xylene selectivity in comparison with ZSM-5. The catalytic performance of the IM-5 zeolite in the vapor phase process provides a new choice for the production of ethylbenzene. This journal is

Full text of DOI:10.1039/d1ra02427b

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Effect of H2SiF6 modification of IM-5 on catalytic
performance in benzene alkylation with ethylene†
Cite this: RSC Adv., 2021, 11, 18288
*
Yunping Zhai,
Junwen Chen, Yongrui Wang, Yibin Luo
and Xingtian Shu
Ethylbenzene (EB) is an important bulk chemical intermediate. The vapor-phase process is considered to be
more efficient than the liquid-phase process when using dilute ethylene (e.g. FCC or DCC off-gas) as the
feed due to its high ethylene space velocity. However, realizing a balance between reducing the xylene
formation and enhancing the EB selectivity is still a challenge due to the poor performance of ZSM-5 at
low reaction temperature. This study concerns an IM-5 zeolite (IMF topology) modified by H2SiF6, with
89% ethylbenzene selectivity, 98.6% total EB + DEB selectivity and only 540 ppm of xylene at 330 ꢀC.
IM-5 zeolites with different Si/Al2 ratios (40–170) were prepared by H2SiF6 modification and their
catalytic performance in vapor phase alkylation of benzene with ethylene was investigated. There was an
obvious decrease in the acid sites and acid strength of IM-5 in the H2SiF6 treatment process, which led
to a slight decrease in ethylbenzene selectivity and a significant decline in xylene yield. Under the
conditions of complete ethylene conversion, the selectivity to EB + DEB increased from 96.1% to 98.6%
in the parent I-40 and modified IM-5. Compared with ZSM-5 that has a similar acidity, the slightly bigger
channel opening makes IM-5 more conductive to the formation and diffusion of DEB while xylene may
present adverse effects. The 120 hour-lifetime test showed that IM-5 (I-110) has superior activity,
equivalent stability, higher DEB selectivity and a much lower xylene selectivity in comparison with ZSM-5.
The catalytic performance of the IM-5 zeolite in the vapor phase process provides a new choice for the
production of ethylbenzene.
Received 27th March 2021
Accepted 10th May 2021
DOI: 10.1039/d1ra02427b
rsc.li/rsc-advances
diffusion process, limiting the mass transport to and from the acid
sites.7 Thus, a relatively higher reaction temperature (e.g. 360–420
ꢀC) is usually adopted in the vapor-phase process.8 Under the given
1. Introduction
As one of the most important intermediates in the petrochemical
industry, ethylbenzene (EB) is mainly used to produce styrene.1–3
Ethylbenzene is mainly produced by the alkylation of benzene with
ethylene. The process was mainly catalyzed by the corrosive Frie-
del–Cras catalyst (mainly referring to AlCl3–HCl) in the past.
However, zeolite-based catalysts are now commonly applied,
including ZSM-5 (for vapor-phase alkylation), beta (for-liquid-
phase alkylation) and MCM-22 (for liquid phase alkylation).3
Compared to the liquid-phase process, the ZSM-5 catalyzed
vapor-phase process has better exibility in the selection of
ethylene resources, including the dilute ethylene from FCC,
DCC and ethane cracking units in reneries.4,5 Moreover, the
medium pore ZSM-5-based catalysts have been proven to
suppress the formation of bulkier products due to steric
hindrance, resulting in better EB selectivity and catalyst
stability.5,6 However, the two types of 10-member ring channel
systems (0.51 nm ꢁ 0.55 nm and 0.53 nm ꢁ 0.56 nm) in the
ZSM-5 structure exhibits stronger resistance for the pore
temperature, more xylenes, residues and other undesired by-
products are generated through side reactions, such as cracking,
isomerization or disproportionation.9 It is believed that most part
of xylene is generated by the side reactions, e.g. EB isomerization,
on the strong acid sites of catalysts. Furthermore, the formation of
xylene can also be accelerated when raising the reaction temper-
ature.10 Xylene is one of the most important by-products and their
boiling points are very close to that of ethylbenzene. There are no
economic ways to separate xylene from ethylbenzene.11 As a result,
the purity of EB is affected. Therefore, reducing the reaction
temperature and decreasing the strength of zeolite acid sites are
both effective ways to enhance the selectivity in benzene alkylation
with ethylene. However, it is difficult for ZSM-5 to realize the goal
due to its narrow 3D 10-ring pore system. The strong diffusion
resistance to aromatics and quick ethylene oligomerization at
a relative low reaction temperature will lead to a rapid deactivation
of ZSM-5.12,13 Therefore, developing an alternative catalyst is crucial
for the synthesis of ethylbenzene. Taking into consideration of the
steric hindrance and mass transport, the IM-5 (framework type:
IMF) zeolite may be an ideal candidate for vapor phase alkylation
of benzene with ethylene as its slightly bigger channel opening and
adjustable acidity.
State Key Laboratory of Catalytic Material and Reaction Engineering, Research
Institute of Petroleum Processing, Sinopec, Beijing 100083, China. E-mail: wangyr.
ripp@sinopec.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra02427b
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IM-5 was rstly synthesized by Benazzi in 1998 with 1,5- increase the Si/Al2 ratio of IM-5. The ethylated benzene (EB +
bis(N-methyl-pyrrolidinium) pentane as a structure-directing DEB) selectivity can be further increased and the formation of
agent14 and its crystal structure was resolved by an enhanced coke can be inhabited at an optimum Si/Al2 ratio. Moreover,
charge ipping structure-solution algorithm method in 2007.15 reducing the reaction temperature can increase the total EB +
In contrast to ZSM-5, which channel structure consists of DEB selectivity and reduce xylene selectivity. The unique pore
intersectional straight and sinusoidal ten-membered ring structure of IM-5 played a key role in contributing to a higher EB
˚
˚
channels (5.1 ꢁ 5.5 A, 5.3 ꢁ 5.6 A) and an intersection cavity of + DEB selectivity and a much lower xylene yield compared with
˚
about 8.6 A, IM-5 consists of two 2D channel systems. One 2D ZSM-5 with a similar Si/Al2 ratio under equivalent stability.
˚
˚
system has channel diameters of 5.5 ꢁ 5.6 A and 5.3 ꢁ 5.4 A,
˚
while the other has channel diameters of 4.8 ꢁ 5.4 A and 5.1 ꢁ 5.3
2. Experimental
2.1 Synthesis and preparation
˚
A. These two 2D systems are connected by a channel with
˚
a diameter of 5.3 ꢁ 5.9 A from each other and generate a bigger
16
˚
cavity (10.4 A) in the channel intersections. With similar acidic
IM-5 zeolite was synthesized by traditional hydrothermal
synthesis as reported in the literature method.14 The as-synthesized
IM-5 zeolite was rstly calcined at 823 K for 5 h to remove the
organic templates and then ion exchanged 3 times with an NH4Cl
solutions at 353 K for 3 h to obtain the parent H-IM-5 via subsequent
calcination at 823 K for 4 h. IM-5 zeolites with different SiO2/Al2O3
ratio was prepared by treating the parent H-IM-5 (SiO2/Al2O3 ¼ 40,
denoted as I-40) with H2SiF6 aqueous solution at 353 K for 60 min.
The concentration of H2SiF6 in the aqueous solution was varied
from 0.05 to 0.20 mol Lꢂ1. All samples were ltrated and the solids
were washed with deionized water, dried at 393 K for 12 hours and
calcined at 823 K for 3 h to obtain H-IM-5 samples with different
SiO2/Al2O3 ratio. The samples were designated as I-80, I-110, I-130
and I-170 according to their SiO2/Al2O3 ratio, respectively. The H-
ZSM-5 zeolite (SiO2/Al2O3 ¼ 110, denoted as Z-110) was obtained
from the Research Institute of Petroleum Processing in order to
investigate the effect of zeolite topology on product distribution in
benzene alkylation with ethylene over IM-5 and ZSM-5.
properties (e.g. similar acid strength and Brønsted to Lewis acid
site ratio),17 a better thermal and hydrothermal stability18,19 as well
as higher adsorption capacity of alkyl aromatic hydrocarbon20
compared to ZSM-5, IM-5 is expected to replace ZSM-5 in many
petrochemical processes. The catalytic performance of IM-5 zeolite
has been investigated in alkane hydro-isomerization,21 methyla-
tion of toluene,22,23 disproportionation of toluene24 and alkylation
of benzene with ethanol25 as well as alkylation with methanol.26
However, the application of IM-5 zeolite in the vapor phase alkyl-
ation of benzene with ethylene has not been reported.
The catalytic performance of the zeolites depends upon the
acidity controlled by the framework Si/Al2 ratio, which can be
regulated either during synthesis or by post-treatment deal-
umination methods. Steam treatments, SiCl4 treatment, reac-
tion with chelating agents such as ammonium
hexauorosilicate (NH4SiF6), oxalic acid, etc. and leaching with
mineral acid are some of the common post-synthesis methods
used to control the acidity of the zeolites. Due to the crystalli-
zation of pure IM-5 is possible only from synthesis mixtures
with a narrow range of SiO2/Al2O3,27 thus, it is an effective way to
2.2 Catalyst characterization
regulate the Si/Al2 ratio of IM-5 by post-treatment method. The elemental analysis of all samples were conducted with an X-
Compared with other methods, the dealumination with hexa- ray uorescence (XRF) spectrometer. X-ray diffraction (XRD)
uorosilicic acid (H2SiF6) is an effective way to increase Si/Al2 patterns of all samples were recorded on a Siemens D5000
ratio while preserving the structural and textural properties of diffractometer using nickel-ltered Cu Ka radiation. The rela-
the zeolite.28 Moreover, the effect of H2SiF6 treatment on the tive crystallinity (R.C.) of IM-5 zeolite was calculated based on
physical and chemical properties of IM-5 has not been the sum of relative peak area of the reection at 2q range of
systematically studied, even though it is of great signicance for 22.1–25.4 of the prepared sample with that of the parent IM-5
establishing the correlation between the properties of zeolites (whose R.C. was assumed as 100%). Physical adsorption/
and its catalytic performance on vapor-phase alkylation of desorption of nitrogen on the samples were performed with
benzene with ethylene. Herein, we report an IM-5 zeolite (IMF an adsorption analyzer. The total surface area of each sample
topology) modied by H2SiF6, with an 89% ethylbenzene was determined with the Brunauer–Emmett–Teller (BET)
selectivity, 98.6% total EB + DEB selectivity and with only method, and the micropore volume and external surface area
540 ppm of xylene at 330 ꢀC. A series of IM-5 zeolites with were determined with the t-plot method. The mesopore size
different Si/Al2 ratios (40–170) were prepared by H2SiF6 modi- distributions prole and the mesopores volume of the samples
cation and their catalytic performance in vapor phase alkyl- were calculated with the Barrett–Joyner–Halenda (BJH)
ation of benzene with ethylene was explored. The structural and desorption branch of the isotherms. Chemical adsorption of
acidity properties of IM-5 before and aer modication by NH3 on samples were carried out with an adsorption analyzer
H2SiF6 were investigated. Characterization techniques such as (Autochem 2920, Micromeritics, USA). Pyridine infrared (Py-IR)
X-ray diffraction, X-ray uorescence, N2 adsorption–desorption spectra of the samples in the range of 1700–1300 cmꢂ1 were
isotherms measurement, scanning electron microscopy, magic recorded on a Fourier transform infrared FT-IR spectrometer
angle spinning nuclear magnetic resonance, and pyridine- with a resolution of 0.35 cmꢂ1. The morphologies and particle
adsorption IR measurement were adopted to characterize the sizes of the samples were measured by using a scanning elec-
changes in structural and acidities for the modied materials. tron microscope (SEM: Quanta200F, FEI, USA) at an acceler-
The results suggest that H2SiF6 treatment is an effective way to ating voltage of 20 kV. In a typical GC-MS analysis, 200 mg of the
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2.3 Catalytic evaluation
The vapor-phase alkylation of benzene with ethylene was
carried out in a xed-bed reactor with an internal diameter of 12
mm. Analytical grade (99 wt%) benzene and ethylene were used.
ꢀ
Samples were loaded and pretreated with N2 ow at 400 C for
2 h prior to the reaction. Benzene was rstly introduced into the
reactor and then the ethylene was introduced when benzene
was detected in the outlet of the condenser. The reaction
temperature varied from 300 ꢀC to 360 ꢀC and the reaction
pressure was 0.8 MPa. The ethylene weight hourly space velocity
(WHSV) was 0.5 hꢂ1 and the molar ratio of benzene to ethylene
was 8.5 : 1. The products were analyzed by an on-line Agilent
7890A gas chromatograph, equipped with a ame ionization
detector and 60 m cross-linked methyl silicone capillary column
innowax (0.32 mm). The ethylene conversion and product
selectivity were calculated as follows:
Fig. 1 XRD patterns of IM-5 and ZSM-5 samples.
used catalyst was completely dissolved in 2 mL of 40% HF
solutions and neutralized with NaOH. CH2Cl2 (Aldrich, 99.9%)
was employed to extract the organic species from the resulting
solutions. The GC-MS total ion chromatograms of extracted
organic phases were recorded on a Varian CP 3800 gas chro-
matograph equipped with a Varian 320-MSD mass-selective
detector, using electron impact ionization at 70 eV.
Ethylene conversion (%) ¼ (Xethylene before reaction
ꢂ Xethylene after reaction
)
/Xethylene before reaction ꢁ 100%
Fig. 2 SEM images and particle size distribution of IM-5 and ZSM-5: I-40 (a), I-80 (b), I-110 (c), I-140 (d), I-170 (e) and Z-110 (f).
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Fig. 3 N2 adsorption–desorption isotherms (a) and BJH pore size distribution curves (b) for IM-5 and ZSM-5 samples.
samples. Which would be further improved by the N2 adsorp-
tion–desorption isotherms. Fig. 3(a) displays the N2 adsorption–
desorption isotherms of IM-5 and ZSM-5 samples. The corre-
sponding data are summarized in Table 1. The curves clearly
indicate that the combination characteristic of type I and type
IV isotherms with a closed hysteresis loop at high relative
pressure of 0.45–0.99 of both IM-5 and ZSM-5 samples. This
suggest that the mesopores are mainly intergranular pores
generated by the accumulation of nanosized zeolite crystals.30–32
This is further veried in the mesopore size distributions from
BJH method (Fig. 3(b)).
The mesopores are centered around 35 nm and 19 nm for
IM-5 and ZSM-5, respectively. The parent I-40 sample has a large
specic surface area of 343 cm2 gꢂ1 and micropore surface area
of 295 cm2 gꢂ1. With the increase of SiO2/Al2O3 ratio, the specic
surface area, micropore surface area and micropore volume of IM-
5 are all decrease, which might be associated with dealumination
during the H2SiF6 treatment process. However, the mesopore
volume of IM-5 rstly increase and then decrease with increasing
SiO2/Al2O3 ratio. I-110 has the largest mesopore volume of 0.271
cm3 gꢂ1. Z-110 has a specic surface area of 370 cm2 gꢂ1 and
micropore volume of 0.158 cm3 gꢂ1, larger than 264 cm2 gꢂ1 and
0.121 cm3 gꢂ1 of I-110. However, the mesopore volume of Z-110 is
only about 0.08 cm3 gꢂ1. The hierarchical factor (HF), dened as
(VMicro/Vtotal) ꢁ (Smeso/SBET), which is one of the very important
factor in describing structural properties of zeolites, was calculated
for the tested samples (Table 1). The HF (ꢁ100) values increased
with Si/Al2 ratio increasing from 80 to 140. Further increasing Si/
Al2 ratio to 170, the HF began to decrease.
Selectivity (%) ¼ (amount of i% in the products)
P
/( amounts of i% in the products) ꢁ 100%
where i represented one of the products, such as EB, DEB and so on.
3 Results and discussion
3.1 Textural and physicochemical characterization
The crystal structure of IM-5 zeolites and the reference ZSM-5
are shown in Fig. 1. The diffraction peaks observed in the 2q
range of 22.1–25.4 of all IM-5 samples indicate the typical IMF
structure29 without detection of any other phases. Moreover, there
is no obvious decline in the crystallinity of post modied IM-5
zeolites compared with I-40, which illustrates that the structure
of IM-5 was not destroyed during H2SiF6 treatment. In addition,
ZSM-5 (Z-110) exhibits typical characteristic patterns of the MFI
topology without other impurity crystal phase observed.
SEM was employed to analyse the changes of microscopic
morphology of IM-5 zeolites before and aer H2SiF6 treatment.
As shown in Fig. 2, all IM-5 samples exhibit rectangular shape
with crystal size of 200–250 nm in length and about 80 nm in
width, which is in good agreement with that of ref. 27. In
addition, all IM-5 samples have a smooth surface and dense
structure, with no amorphous material found aer H2SiF6
treatment. The results further conrm that there was no serious
damage to the IM-5 structure during H2SiF6 treatment. The
crystal size of ZSM-5 is in the range of 400–800 nm, which is
incomparable to that of IM-5 zeolite. In addition, an aggrega-
tion of small crystals was observed in both IM-5 and ZSM-5
Table 1 Textural properties and chemical composition of IM-5 and ZSM-5 samples
Textural properties
Si/
HFc
(ꢁ100)
Samples C(H2SiF6) mol Lꢂ1 R.C.a (%) Al2
Stotal (m2 gꢂ1) Smicro (m2 gꢂ1) Sext (m2 gꢂ1) Vtotal (cm3 gꢂ1) Vmicro (cm3 gꢂ1) Vmeso (cm3 gꢂ1
)
b
I-40
I-80
I-110
I-140
I-170
Z-110
0
100
40
84
110
132
170
110
5.06
4.33
4.83
6.89
5.50
5.20
343
317
313
316
264
370
295
277
264
259
223
341
48
40
49
57
41
29
0.373
0.379
0.392
0.309
0.291
0.238
0.135
0.130
0.121
0.118
0.103
0.158
0.238
0.249
0.271
0.191
0.188
0.080
0.05
0.10
0.15
0.20
99.8
99.6
99.4
98.2
a
b
Relative crystallinity. Si/Al2: molar ratio detected by XRF. c HF: hierarchical factor, dened as (Vmicro/Vtotal) ꢁ (Smeso/SBET).
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Fig. 4 27Al (a) and 29Si (b) MAS NMR spectra of IM-5 and ZSM-5 samples.
Fig. 4(a) shows the 27Al MAS NMR spectra of IM-5 samples different NH3 desorption peaks at low and high temperatures,
before and aer H2SiF6 treatment and ZSM-5 sample. Two types corresponding to weak and strong acid sites, respectively. Both
of Al coordinate centered at around 55 ppm and ꢂ2 ppm are the low and high temperature peaks shi to lower temperature
observed in the I-40, for which tetrahedral-coordinated frame- with the increase of SiO2/Al2O3. The results demonstrate that
work Al and octahedral-coordinated extra-framework Al are the H2SiF6 treatment reduced both weak and strong acid
assigned respectively.27 The extra-framework Al of IM-5 samples strength of IM-5.
aer the H2SiF6 treatment disappeared, indicating that the
The amount of acid obtained from NH3-TPD proles are
extra-framework Al of I-40 was removed out and no additional summarized in Table 2. It is clear that H2SiF6 treatment has
extra-framework Al was formed during acid treatment. The a signicant effect on the acid amount of IM-5 sample. Both the
dealumination of parent IM-5 can be further conrmed by the strong and the weak acid decrease with the increase of H2SiF6
information obtained from the 29Si MAS NMR spectra. As concentration. The concentrations of the weak acid sites
shown in Fig. 4(b), the two signals at about ꢂ107 and ꢂ113 ppm decreased to a larger extent. It is generally accepted that the
are assigned Si (3Si, 1Al) and Si (4Si, 0Al) respectively. Aer the strong acid sites enhance the side reactions such as EB isom-
H2SiF6 treatment, the positions of peaks in the spectra of all IM- erization and cracking, leading to the formation of xylene and
5 samples are not change, but the intensity of the Si (3Si, 1Al) coke. Hence, the decreased concentration of strong acid sites in
signal is remarkably reduced with the increases of H2SiF6 H2SiF6 treated IM-5 samples is expected to improve the catalytic
concentration. The results indicate that the dealumination rst performance in the alkylation of benzene with ethylene reac-
occurred in the Si (3Si, 1Al) opposition.
tion. The acid concentration between I-110 and Z-110 is
The SiO2/Al2O3 ratios of the parent and post modied IM-5 comparable. The acidity of IM-5 and ZSM-5 was also investi-
samples are listed in Table 1. Clearly, the SiO2/Al2O3 of IM-5 gated using FTIR spectroscopy with pyridine as a probe mole-
samples increased greatly with the increase of H2SiF6 concen- cule. The results obtained according to the corresponding
tration. There are no distinct differences in the NH3-TPD curves extinction coefficient of Brønsted and Lewis acid sites are listed
between I-110 and Z-110, indicating that the strength of acid in Table 2. It was reported that the aqueous uorosilicate
sites between them is similar. It is in good accordance with the treatment led to the substitution of an Al atom by an Si atom in
17
˚
results obtained by Martin Kubu et al.
the zeolite framework, leading to a sharp decrease in the
concentration of Brønsted acid sites ascribed to the extraction
of framework Al atom.33 As shown in Table 2, both the Brønsted
and Lewis acid concentration dropped with the increase of
H2SiF6 concentration. The B/L ratio increased in the lower
3.2 Acidity characterization
The NH3-TPD curves of IM-5 and ZSM-5 (Z-110) are shown in
Fig. 4. The spectrum of all samples can be divided into two
Table 2 The acidity properties of IM-5 and ZSM-5 samples
473 K
623 K
Brønsted
Lewis acid
Brønsted
Lewis acid
Sample
mmol NH3 per g
acid (mmol gꢂ1
)
(mmol gꢂ1
)
B/L
acid (mmol gꢂ1
)
(mmol gꢂ1
)
B/L
I-40
I-80
I-110
I-140
I-170
Z-110
1.30
0.82
0.66
0.58
0.28
0.69
180.0
114.2
85.2
63.1
33.2
86.1
51.4
31.7
23.0
22.3
13.4
26.2
3.5
3.6
3.7
2.8
2.5
3.3
94.4
88.8
79.7
48.6
22.4
80.2
36.5
31.7
24.2
23.3
12.6
23.6
2.6
2.8
3.3
2.1
1.8
3.4
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from the substitution of a Si atom by an Al atom in the framework
of zeolite,34 while the Lewis acid site is a coordinative unsaturated
Al3+. Thus, the coordinative of Al atom in zeolite framework varied
with the increase of H2SiF6 concentration, which was consistent
with the 27Al NMR results. In short, increasing H2SiF6 concentra-
tion caused the total acid sites to decrease. Furthermore, there
were no obvious difference in the total acid sites and B/L ratio of I-
110 and Z-110 samples (Fig. 5 and Table 2).
3.3 Catalytic performance in benzene alkylation with
ethylene
3.3.1 Effect of reaction temperature. This section investi-
gates the ethylene conversion, EB and EB + DEB selectivity and
xylene content of IM-5(I-40) as a function of reaction tempera-
ture to prove the feasibility of using IM-5 zeolite as a catalyst in
benzene alkylation with ethylene reaction at a relative low
temperature. It can be seen from Fig. 6(a) that the ethylene
Fig. 5 NH3-TPD curves of IM-5 and ZSM-5 samples.
degree of dealumination. However, the values declined consider-
ably at a higher degree of extraction of Al atom. It is common
knowledge that the production of a Brønsted acid site originated
Fig. 6 The effect of reaction temperature on ethylene conversion (a), EB and EB + DEB selectivity (b), toluene (MB) and xylene (c) selectivity, by-
products selectivity (d) as well as GC-MS total ion chromatogram of the heavier product (e). (The structures annotated above the chromatogram
are peak identifications made by comparing the mass spectra with those in the NIST database). Reaction conditions: pressure ¼ 0.8 MPa, WHSV ¼
0.5 hꢂ1, benzene/ethylene ¼ 8.5 mol molꢂ1
.
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Fig. 7 TGA (a) and DTG (b) curves of the used samples after 12 h on stream.
conversion is always higher than 98.5% at all reaction temper- reaction occurred in 420 ꢀC, the second weight loss event arises
atures. Moreover, the ethylene conversion decreases slightly at signicantly higher temperatures of 500–670 ꢀC. This is
with the reaction temperature due to the dealkylation reaction associated with the oxidation of “harder” coke deposited on the
at a higher temperature.35 Both the EB and (EB + DEB) selectivity catalyst during the alkylation reaction.43 Moreover, the second
are enhanced with the decreasing temperature, reaching weight loss event arises at a signicantly lower and broader
a highest value at 330 ꢀC (Fig. 6(b)). While the by-products temperature range of 200–500 ꢀC when the reaction temperature
selectivity, such as C9 aromatics (n- and iso-propyl, methyl- decreased. Such low and broad decomposition temperatures is
ethylbenzene), C10 aromatics (butylbenzene), and heavier likely due to the desorption or oxidation of “soer” coke (poly-
products deceased greatly (Fig. 6(d)) as the cracking and alkyl merization of ethylene). These polymerized olens could be
transfer reaction is deeply inhibited.36 If the reaction tempera- adsorbed strongly onto the acid sites of the zeolite,44 thus inhib-
ture drops further, both the selectivity to EB and (EB + DEB) will iting the benzene alkylation reaction and leading a lower EB and
decrease due to the oligomerization of ethylene and the EB + DEB selectivity (e.g. 300 ꢀC in Fig. 6(b)). However, the yielding
subsequent alkylation reaction at lower temperatures.37,38 The of xylene and toluene, which are mainly originated from isomeri-
formation of bulky products such as diphenyl methane and di- zation or cracking of EB, is signicantly inhibited at low reaction
phenyl-ethane are accelerated in low reaction temperatures temperatures (Fig. 6(c)). Reducing the xylene formation is gaining
(Fig. 6(e)). It is hard for bulky products to diffuse out of the more and more attentions as it is one of the most important aims
zeolite channel, and subsequently block the zeolite pore chan- for the practical use of catalyst. The optimum reaction temperature
nels. Consequently, the activity and stability of the catalyst are of IM-5 is 330 ꢀC in the gas phase alkylation of benzene with
reduced.39–42 This can also be further evidenced through the ethylene. A detailed reaction network was proposed based on the
TGA and DTG analysis of the used zeolites. Fig. 7(a) shows that product distribution, a (Scheme 1).
all TGA curves display two main weight loss events. The rst
3.3.2 Effect of Si/Al2 ratio. Apart from decreasing the xylene
weight loss event before ꢃ200 ꢀC is mostly due to desorption of yield, the other aim of benzene alkylation with ethylene is to
water and/or light aromatics adsorbed in the spent catalysts. increase the ethyl (EB + DEB) yield. The goal can be achieved by
However, the second weight loss event at higher temperatures adjusting the acidity of the zeolite.5,45 Fig. 8 shows the catalytic
(above 200 ꢀC) is more complex. The complexꢀity can be evi- performance of the IM-5 with different Si/Al2 ratios in benzene
ꢀ
denced by the weight loss peaks at around 393 C, 473 C and alkylation with ethylene. It can be seen that the increase of Si/Al2
558 ꢀC in Fig. 7(b), which belong to different coke species. When ratio from 40 to 110 did not have a signicant effect on the
Scheme 1 Reaction network of benzene alkylation with ethylene T: reactions kinetically favored at higher temperatures. ꢂT: reactions kinetically
favored at lower temperatures.
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Fig. 8 The influence of Si/Al2 ratio on ethylene conversion (a), EB, EB + DEB selectivity (b), content of xylenes (c) and by-products selectivity (d).
Temperature ¼ 330 ꢀC, pressure ¼ 0.8 MPa, WHSV ¼ 0.5 hꢂ1, benzene/ethylene ¼ 8.5 : 1 mol molꢂ1
.
conversion of ethylene (Fig. 8(a)), with a slight decrease from 99.82% generated by consecutive alkylation of EB with ethylene and can
to 99.78%. However, there was a visible rise in EB + DEB selectivity be used to yield EB via trans-alkylation with benzene.49 The DEB
(Fig. 8(b)) from 96.06% to 98.60%. Especially, the xylene content selectivity was enhanced with the increase of Si/Al2 ratio and the
decreased from 2089 to 539 ppm, indicating that the zeolite with EB selectivity decreased as a result. This is due to the higher
a Si/Al2 ratio of 110 can effectively suppress the formation of side intrinsic reactivity of ethylbenzene compared with benzene. It is
products. This is mainly due to the decrease in density and strength reported that the reactivity of alkyl benzene molecules increases
of Brønsted acid sites of IM-5 during the H2SiF6 treatment,46 which with increasing number of alkyl groups on the benzene ring.50
suppressed the ethylene oligomerization and coke formation.47,48 Thus, the subsequent alkylation of EB with ethylene to DEB was
This can also be explained with the signicant increase of mesopore accelerated at IM-5 with a lower amount of Brønsted acid sites.
volume of I-110. With a shorter diffusion path length, the product is
more likely to transport out of the zeolite channel, decreasing the aer 12 h on stream. With an increase in the Si/Al2 ratio, the
further alkylation or oligomerization reaction.7
second weight loss of the IM-5 showed a more uniform loss rate.
Fig. 9 shows the TGA and DTG curves of the used catalysts
The zeolite activity declined from 99.78% to 98.39%, while The result suggests that the weight loss is not associated with
the EB + DEB selectivity and xylene content kept almost the formation of harder coke, but likely due to the desorption or
remained almost unchanged aer the Si/Al2 ratio was increased oxidation of polymerization of ethylene.51 The amount ofꢀcoke
to 170. Thus, 110 was considered as the optimum Si/Al2 ratio for deposits was calculated in the temperature range of 200 C to
IM-5 in catalyzing benzene alkylation with ethylene. It should 700 ꢀC. Obviously, the coke amount formed on the IM-5 zeolite
be noted that DEB is an exceptive by-product, which is is decreased owing to the reducing of amount and strength of
Fig. 9 TGA (a) and DTG (b) curves of the used samples with different Si/Al2 ratio after 12 h on stream.
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Fig. 10 Ethylene conversion and xylene content versus time on stream (a), selectivity (b) and by-products selectivity (c) in 120 hour-lifetime test
on the IM-5 and the ZSM-5 samples. Reaction conditions: temperature ¼ 330 ꢀC, pressure ¼ 0.8 MPa, WHSV ¼ 0.5 hꢂ1, benzene/ethylene ¼
8.5 : 1 mol molꢂ1
.
acid sites at rst. However, I-170 sample exhibits a similar coke the effect of pore topology on catalytic performance in benzene
amount with that of I-110. This may be because the low amount alkylation with ethylene, the detailed product distributions of
of acid sites is insufficient to alkylate the benzene with ethylene. IM-5 and ZSM-5 are given in Fig. 10(b) and (c).
The ethylene react with each other to form ethylene oligomer-
The main products of IM-5 and ZSM-5 were ethylbenzene and
ization. This is also evident with the incomplete ethylene di-ethylbenzene. IM-5 and ZSM-5 showed a similar ethylbenzene
conversion (Fig. 8(a)) and higher butyl selectivity (Fig. 8(d)). In selectivity. However, the EB + DEB selectivity (98.4%) for IM-5 is
addition, it was found that there was a good correspondence higher that of ZSM-5 (96.6%). The higher DEB selectivity on IM-5
between the HF factor and the carbon deposition rate over IM-5 contributed to this increase. Considering the two samples have
samples. As shown in Fig. S1,† on the whole, the carbon a similar amount of acid sites as well as B/L ratio, we speculate that
˚
deposition rate decreases with the increase of HF (ꢁ100). Apart the pore structure of IM-5 (pore diameter: 5.4–5.9 A) is more
from the rst peak corresponding to the physical adsorption of conductive to the formation and diffusion of DEB (d: m-DEB z 5.5
˚
˚
˚
water/light aromatics, it can be seen from the DTG curves A nm, p-DEB z 4.3 A and o-DEB z 7.8 A) than ZSM-5 (pore
˚
shown in Fig. 9(b) that another two weight loss peaks are diameter: 5.3–5.6 A). Compared to ZSM-5 the diffusion properties
ꢀ
ꢀ
observed at 420 C and 560 C, which belong to the different of aromatics, such as toluene and m-xylene, in IM-5 proved to be
coke species formed on various acid sites with different better in the adsorption experiment.20,21
strengths. Moreover, the decomposition peaks shi to lower
temperature at both high and low amount of acid sites, indi- between IM-5 and ZSM-5. The xylene content is at about 1600 ppm
cating that the coke species were easier to eliminate. on ZSM-5 compared to 540 ppm on IM-5. The difference can be
In addition, there was a large difference in xylene content
3.3.3 Comparison of IM-5 and ZSM-5. In order to evaluate attributed to the different pore structures of IM-5 and ZSM-5. In
the stability of IM-5 120 hour-lifetime tests were carried out. terms of reducing of xylene yield, IM-5 zeolite exhibited a better
ZSM-5 with a similar acid property was comparatively investi- catalytic performance than ZSM-5 in practical use.
gated to clarify the effect of zeolite topology on catalytic
C9 and C10 aromatics were the major by-products for both
performance in vapor-phase alkylation of benzene with IM-5 and ZSM-5 samples (Fig. 9(c)). Notably, the selectivity of C9
ꢀ
ethylene. In this section, the reaction temperature was 330 C and C10 aromatics on ZSM-5 was higher than IM-5. In the
which was optimized in Section 3.3.1. Fig. 10(a) shows the alkylation of benzene with ethylene reaction, the active ethylene
ethylene conversion and xylene content of IM-5(I-110) and ZSM- species formed on a Brønsted acid site of 10-ring member
5(Z-110). The ethylene conversion of both samples is nearly zeolite can follow two major routes: (1) it can alkylate with
complete aer 120 h on stream no obvious deactivation is found benzene to produce EB which can later undergo subsequent
during the period of reaction. Compared with ZSM-5, a slightly reactions to produce mainly di-ethylbenzene. (2) It can react
higher ethylene conversion is found on IM-5. To further clarify with another one or more ethylene molecules to produce a C4,
Fig. 11 TGA curves (a) and GC-MS analysis of retained species (b) of the reacted IM-5 and ZSM-5 samples after 120 hour-lifetime test.
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C6 species which can further transform via alkylation, oligo-
merization, isomerization or cracking yielding other alkylben-
zenes and olens (Scheme 1).49 Compared to the free diameter
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