Construction of a tandem HZSM-5 with CuZnAl catalyst for alkylation of benzene with syngas

Article abstract of DOI:10.1039/c9nj05273a

Aiming at the problem of over-production of benzene and the demand for aromatics in industry, a CuO/ZnO/Al₂O₃ catalyst was prepared by a coprecipitation method, and mixed with ZSM-5 to form a bifunctional composite catalyst. The prepared catalyst is used in an alkylation reaction of benzene with synthesis gas in a liquid phase in a slurry bed reactor. A series of analytical methods such as ammonia gas temperature programmed desorption (NH₃-TPD), X-ray diffraction (XRD), nitrogen adsorption-desorption, scanning electron microscopy (SEM) and inductively coupled plasma emission spectrometry (ICP) were conducted to characterize the catalyst. The results show that the structure and acidity of the modified ZSM-5 have changed during the modification process, and ZSM-5 produces a more mesoporous structure after desiliconization. Therefore, the specific surface area of the modified ZSM-5 zeolite increases and the number of acid sites become smaller than those of the unmodified zeolite. The results show that the acid-modified ZSM-5 improves the conversion of CO, while the selectivity of CO to aromatics decreases. In contrast, only alkali modification and acid-alkali co-modification on ZSM-5 contributed to increasing the selectivity to aromatics, especially acid-alkali co-modified ZSM-5. The reason for this can be attributed to the fact that the modified zeolite produces a mesoporous structure, enhancing the mass transfer ability further. Moreover, the amount of strong acid sites on the modified ZSM-5 is reduced, which reduces the side reaction of methanol and olefin, thereby improving the selectivity of CO to aromatics.

Full text of DOI:10.1039/c9nj05273a

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Construction of a tandem HZSM-5 with CuZnAl
catalyst for alkylation of benzene with syngas†
Cite this:DOI: 10.1039/c9nj05273a
a
a
a
a
Bize Gao, Chuanmin Ding, * Junwen Wang,
Guangyue Ding,
a
b
b
Jinxiang Dong,
Hui Ge* and Xuekuan Li
Aiming at the problem of over-production of benzene and the demand for aromatics in industry, a
CuO/ZnO/Al catalyst was prepared by a coprecipitation method, and mixed with ZSM-5 to form a
2 3
O
bifunctional composite catalyst. The prepared catalyst is used in an alkylation reaction of benzene with
synthesis gas in a liquid phase in a slurry bed reactor. A series of analytical methods such as ammonia
gas temperature programmed desorption (NH
desorption, scanning electron microscopy (SEM) and inductively coupled plasma emission spectrometry
ICP) were conducted to characterize the catalyst. The results show that the structure and acidity of the
modified ZSM-5 have changed during the modification process, and ZSM-5 produces more
3
-TPD), X-ray diffraction (XRD), nitrogen adsorption–
(
a
mesoporous structure after desiliconization. Therefore, the specific surface area of the modified ZSM-5
zeolite increases and the number of acid sites become smaller than those of the unmodified zeolite.
The results show that the acid-modified ZSM-5 improves the conversion of CO, while the selectivity of
CO to aromatics decreases. In contrast, only alkali modification and acid–alkali co-modification on
ZSM-5 contributed to increasing the selectivity to aromatics, especially acid–alkali co-modified ZSM-5.
The reason for this can be attributed to the fact that the modified zeolite produces a mesoporous
structure, enhancing the mass transfer ability further. Moreover, the amount of strong acid sites on the
modified ZSM-5 is reduced, which reduces the side reaction of methanol and olefin, thereby improving
the selectivity of CO to aromatics.
Received 21st October 2019,
Accepted 12th December 2019
DOI: 10.1039/c9nj05273a
rsc.li/njc
utilization of benzene has become an urgent problem to be
solved. Traditional preparation methods of aromatic hydro-
1
. Introduction
Aromatic hydrocarbons are the most important basic organic carbons include methanol to aromatic hydrocarbons, toluene
compounds in the petrochemical industry. Among aromatic alkylation to produce xylene and so on, but the scale in actual
hydrocarbons, para-xylene (PX) is the most valuable xylene isomer, production is small. Syngas composed of hydrogen and carbon
which can be converted into PET (polyethylene terephthalate), monoxide can be produced from non-petroleum carbon resources
fibers, films, resins, PBT (polybutylene terephthalate) and other such as coal, natural gas and biomass, which are cheap and easy
1–3
chemical compounds. Although there are many reports on the to obtain, and has been widely used in industrial production, and
preparation of aromatic hydrocarbons by alkylation of benzene, has attracted great attention. Therefore, it is more economical to
they are still limited to small-scale production. The source of prepare aromatic hydrocarbons by alkylation of excess produced
6,7
benzene is very extensive and can be obtained by petroleum benzene with syngas than by traditional methods.
processing, coal tar processing, alkane aromatization, catalytic ZSM-5 zeolite is widely used in the alkylation reaction of
reforming, and aromatics-separation. With the continuous benzene with methanol due to its special surface area, adjust-
8
–11
improvement of industrial systems, the production capacity of able acidity and unique pore structure.
benzene has continued to increase in recent years with a to its high selectivity, activity and stability, HZSM-5 has been
Furthermore, due
4
,5
12–14
situation of overproduction. Therefore, the comprehensive widely used for the aromatization of methanol.
ZnO/Al catalyst is well-acknowledged for syngas to methanol
The CuO/
2 3
O
1
5,16
reaction.
The reaction of benzene with syngas could be
a
College of Chemistry & Chemical Engineering, Taiyuan University of Technology,
Taiyuan 030024, P. R. China. E-mail: dingchuanmin@tyut.edu.cn,
wangjunwen@tyut.edu.cn; Fax: +86-0351-6014498
considered as coupling of syngas conversion to methanol and
reaction of benzene with the methanol to form toluene. In recent
years, the idea of constructing a multifunctional composite catalyst
b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry of CAS,
Taiyuan 030001, China. E-mail: gehui@sxicc.ac.cn
1
7,18
for continuous reaction has been extensively studied.
For
1
8
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj05273a instance, Jiao et al. used ZnCrOx + MSAPO as a bifunctional
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composite catalyst for the conversion of synthesis gas to light was conducted using a slurry bed instead of the traditional fixed
olefins. If ZSM-5 and CuO/ZnO/Al are mixed, a bifunctional bed. This study focuses on the construction of a tandem catalyst
composite catalyst having catalytic hydrogenation and alkylation and the effects of changing morphology and acid property of the
2 3
O
1
9–21
sites can be prepared.
Previous research on alkylation of tandem catalyst on the alkylation of benzene in homogeneous
benzene with syngas using bifunctional composite catalysts has reaction systems.
been very rare. Therefore, the design and construction of high-
efficiency catalysts for the alkylation of benzene with syngas is of
great significance.
2. Exprimental
Mass transfer capacity is a very important property of the
catalyst, because the mobility of the molecule in the catalyst
will ultimately determine the rate of desorption and catalytic
2.1 Preparation of CuO/ZnO/Al O
2 3
The CuO/ZnO/Al
method. 14.86 g Cu(NO
2
O
3
catalyst was prepared by a co-precipitation
3 2
, 8.92 g Zn(NO and 3.75 g Al(NO )
2
2–24
3
)
2
3
)
2
reaction.
However, the inherent microporous structure
were weighed and dissolved in 200 ml distilled water to make a
mixed nitrate solution of copper, zinc and aluminum (n(Cu) :
n(Zn) : n(Al) = 6 : 3 : 1). Then, 11.50 g Na
in conventional ZSM-5 inhibits the mass transfer capacity of
ZSM-5, which results in a very low selectivity to toluene and
2
5
CO was dissolved in
2 3
xylene for in the benzene alkylation. In order to solve the
problem of catalyst mass transfer and deactivation in the
catalytic reaction, larger size pores, like mesopores, are usually
introduced into the zeolite to improve the mass transfer ability
of the catalyst, and thus improve the contact between the active
ꢀ
1
200 ml of distilled water, and the concentration was set to 1 mol L
to prepare an alkali solution; 100 ml of distilled water was added to
a beaker having a capacity of 500 ml, and the beaker was placed in a
water bath which had been raised to 80 1C; the mixed solution and
the alkali solution were slowly dropped into the beaker. The
precipitate was heated to 80 1C in a water bath; the precipitate
was aged for 2 h, while maintaining a water bath temperature of
80 1C. After the aging was completed, the precipitate was suction
filtered and washed with distilled water and ethanol. After
drying at 110 1C for 12 hours, a precursor of the catalyst was
obtained. The precursor was placed in a muffle furnace and
2
4,26
sites of the catalyst and the reactant molecules.
Common
methods for the preparation of mesopores include limiting
2
7
crystal growth, introduction of a carbon template or polymeric
2
8,29
mesoporous template
and post-synthesis dealumination or
3
0
desiliconization. The first two types of methods are very
3
1
expensive and can be harmful to the environment. However,
dealuminization or desiliconization of zeolite by acid or alkali
modification has proved to be a simple and low-cost pathway
2 3
calcined at 550 1C for 4 h to obtain the final CuO/ZnO/Al O catalyst.
3
1–34
and can significantly increase the production of aromatics.
2.2 Preparation of HZSM-5 and modified HZSM-5
3
5
Bjørgen et al. studied the effect of desiliconization of ZSM-5 on
the MTG reaction and discovered that the alkali modified ZSM-5
improved the selectivity to the product in the MTG reaction.
The HZSM-5 and modified HZSM-5 precursors were prepared
using a modification of the method reported in the literature.
Commercial ZSM-5 (Nankai Chemical Plant, NH form) with an
Si/Al ratio of 150 was used in this experiment. Before use, the
4
3
3
6
Li et al. studied the effect of desiliconization of ZSM-5 on its
catalytic performance in the catalytic pyrolysis of lignocellulosic
biomass and found that the desiliconized ZSM-5 formed many
mesopores, and the diffusion properties of ZSM-5 were also
improved, and more aromatic hydrocarbons are also produced
in the reaction product.
4
zeolite was calcined in air at 550 1C for 7 hours to obtain ZSM-5.
ꢀ
1
1
0 g of the zeolite and 100 ml of a 1 mol L oxalic acid
solution were placed in a round bottom flask, and stirred at
63 K for 2 hours under stirring and reflux. Then, the solid was
3
filtered and washed repeatedly with distilled water until the
acid was completely removed. Finally, the sample was dried at
In addition to improving the mass transfer capacity, acid
and alkali modification can remove the aluminum element and
the silicon element in the ZSM-5, regulating acid strength and
383 K and calcined at 823 K for 7 hours to obtain an acid-
3
5,37–40 modified zeolite, designated ZSM-5-H.
density and adjusting the catalytic performance further.
ꢀ1
4
1
In the same manner, ZSM-5 was suspended in a 0.5 mol L
Meng et al. modified ZSM-5 with HF and used it in the MTG
reaction, indicating that the acid content of the zeolite treated
with HF changed significantly and many mesoporous structures
were formed at the same time, and the conversion rate of methanol
NaOH solution at 353 K for 2 hours under stirring and reflux to
produce a mixture. The mixture was then immediately cooled
and then washed with deionized water. Finally, the prepared
zeolite sample was dried overnight at 383 K, then exchanged
42
was also significantly improved. Mochizuki et al. used NaOH to
desiliconize HZSM-5 zeolite for the hexane cracking reaction, and
studied the influence of the outer surface and acidity on its catalytic
performance in hexane cracking, and the results showed that
NaOH modification regulated the acid content of HZSM-5 and
reduced the production of coke.
ꢀ
1
4
three times in 0.5 mol L NH Cl solution, and calcined in air
at 823 K for 7 hours to produce an alkali-modified zeolite,
labeled as ZSM-5-OH. Acid-alkali co-modified zeolite is represented
by ZSM-5-H-OH.
2.3 Catalyst characterization
In this study, a series of ZSM-5 molecular sieves were
prepared by desilicication and dealumination of ZSM-5 molecular X-ray diffraction (XRD) of the catalyst was carried out using a
sieves by sodium hydroxide and oxalic acid, respectively, and then SHIMADZU-6000 X-ray powder diffractometer (Shimadzu,
2 3
combined with Cu/ZnO/Al O to form a bifunctional composite Japan). The crystal form and crystallinity of the catalyst were
catalyst for alkylation of benzene with syngas. Besides, this study analyzed. The copper target Ka radiation was 2 kW and the
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ꢀ1
scanning range was 5–851. The scanning speed is 81 min
.
catalyst, which was employed in the alkylation reaction of
3
NH temperature programmed desorption (TPD) of the catalysts benzene with syngas in a slurry bed reactor. This study focuses
was carried out using a Tianjin XQ TP-5076 chemisorption on the effects of acid-treatment, base-treatment, and acid–
instrument equipped with a thermal conductivity detector alkali co-treatment on the catalytic performance of alkylation
(
TCD). Transmission electron microscopy (TEM) images were of benzene with syngas.
taken on FEI F30 transmission electron microscopes. The over- The powder XRD patterns of these four samples are shown
all composition of the catalyst (Si/Al and Si/Zn) was analyzed in Fig. 6. All the samples demonstrated characteristic peaks of
4
4,45
using an ICP-AES Autoscan 16 inductively coupled plasma an MFI crystal structure between 7–101 and 22–251,
and all
optical emission spectrometer (TJA, USA). The specific surface of the samples show the skeleton structure of ZSM-5 and have
area, pore volume and pore size of the molecular sieve catalyst good crystallinity. However, the intensity of the peaks has
were analyzed using a BELSORP-max type surface adsorber changed significantly. The crystallinity of the acid-modified
(
(
McKeker). The catalyst was first treated under high vacuum ZSM-5 is slightly increased, while the crystallinity of the
ꢀ4
o10 Torr) and 400 1C for 6 h before testing to remove the alkali-modified ZSM-5 is slightly reduced due to the formation
4
6
impurities on the surface of the molecular sieve and the of an amorphous phase. This suggests that desiliconization
adsorption in the pores. Specifically, the specific surface area occurred under alkali treatment. At the time of aging of the
was calculated by the BET method; the micropore volume was gel, the degree of crystallinity slightly increased (ZSM-5-H),
measured by the t-plot method, and the mesoporous volume suggesting that dealumination in the zeolite framework was
was measured by the BJH method.
negligible. Fig. S1 (ESI†) is an XRD spectrum of a different
catalyst before mixing with ZSM-5. It can be found from Fig. S1
2.4 Catalyst testing
(ESI†) that the diffraction peaks of CuO and ZnO alone are
The prepared CuO/ZnO/Al O and ZSM-5 were ground and more obvious, and the crystallinity is better. The diffraction
2
3
mixed in a ratio of 1 : 1. Prior to the reaction, the catalyst was peaks of CuO and ZnO also can be seen in the graph of CuO/
reduced with hydrogen at 250 1C for 4 h. 2 g of the catalyst and ZnO/Al , but they are more diffuse than a and b. It shows that
0 ml of benzene were added to the slurry bed reactor, and as the composition increases, the crystallinity deteriorates and
syngas was introduced at a certain temperature and pressure the dispersibility is better.
2 3
O
2
for 8 hours. The product was cooled by condensed water and
analyzed by gas chromatography.
As shown in Fig. 2, the ZSM-5 structure changed during the
modification process. The peak with a desorption temperature
The conversion of CO (C), selectivity of CO to toluene (ST), between 200 and 250 1C represents a weak acid level; the peak
selectivity of CO to ethylbenzene (SEB), selectivity of CO to with a desorption temperature between 300 and 450 1C represents
xylene (S ) and selectivity of carbon monoxide to para-xylene a medium acid level; the peak with desorption temperature
X
(S
PX) are defined as follows, respectively:
between 600 and 800 1C represents a strong acid level. It is mainly
caused by the desorption of NH adsorbed on the strong acid sites
3
CO in feed ꢀ CO in product
Cð%Þ ¼
of molecular sieves. In general, the area of the NH desorption
3
CO in feed
peak can be used to measure the amount of acidity of the
4
7
molecular sieve. Table 1 summarizes the acidity of these
zeolites. The acidity of ZSM-5 is an important factor in determining
the catalytic activity and product selectivity of the alkylation
reaction of benzene. It is well known that the amount of ammonia
desorbed from the catalyst surface can be estimated at the peak
temperature, by the TPD peak area and acid strength properties. It
was observed that the modified sample exhibited varying degrees
of change in the number and intensity of acid sites compared to
unmodified ZSM-5. The alkali-modified ZSM-5 had greatly reduced
strong acid sites, while the weak acid and medium acid sites were
nðthe amount of CO converted to tolueneÞ
nðconversion of COÞ
STð%Þ ¼
SXð%Þ ¼
nðthe amount of CO converted to xyleneÞ
nðconversion of COÞ
nðthe amount of CO converted to EBÞ
SEBð%Þ ¼
nðconversion of COÞ
nðthe amount of CO converted to para-xyleneÞ
nðconversion of COÞ
SPXð%Þ ¼
Table 1 Acid content and distribution of different ZSM-5
b
ꢀ1
Acid sites /(mmol g
)
3. Results and discussion
a
Samples
Weak acidity Medium acidity Strong acidity Total acidity
3
.1 Catalyst characterization
ZSM-5
0.037
0.069
0.108
0.110
0.123
0.098
0.223
0.284
0.003
0.032
0.368
0.219
0.463
0.179
The parent ZSM-5 is treated with acid and alkali for 2 h, thereby ZSM-5-H
removing aluminum and silicon atoms on the molecular sieve
framework, and introducing a mesoporous structure into
ZSM-5-OH 0.093
ZSM-5-H-
OH
0.049
the ZSM-5 zeolite. The obtained ZSM-5 and Cu/ZnO/Al
2
O
3
were
a
b
m
cat = 0.1 g. Density of acid sites, determined by NH
3
-TPD (total:
mixed at a ratio of 1 : 1 to form a bifunctional composite NH
3
desorbed at 200–800 1C; strong: NH
3
desorbed at 600–800 1C).
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Table 2 Silicon to aluminum ratio of different ZSM-5
Composition (ICP-AES)
ꢀ1
ꢀ1
Samples
Si/mg L
Al/mg L
2 2 3
SiO /Al O
ZSM-5
40.69
42.32
40.29
38.79
0.54
0.51
0.88
0.55
150.70
165.96
91.57
ZSM-5-H
ZSM-5-OH
ZSM-5-H-OH
141.05
significantly increased. The number of weak acid sites on the acid-
modified ZSM-5 increased slightly, the medium acid sites were
almost unchanged, and the strong acid sites increased slightly.
The ZSM-5 modified by acid and alkali also showed a significant
decrease in the strong acid sites, the weak acid sites increased
slightly, and the medium and strong acid sites decreased slightly.
This indicates that the alkali modification has a significant effect
on the strong acid sites of ZSM-5, and the acid modification has a
significant effect on the weak acid sites of ZSM-5. From the above
studies we conclude that alkali or acid modification further
modulates the acid properties of the ZSM-5 zeolite.
3
Fig. 2 NH -TPD profiles of parent and modified ZSM-5.
the high crystallinity of ZSM-5 (Fig. 1). The acid-modified
ZSM-5 morphology has undergone some changes, no longer a
regular rectangular structure, and the pores are slightly increased
Table 2 shows the results of elemental analysis of different
ZSM-5 ICP-AES measurements. We can find that the unmodi-
(
Fig. 3b). Although the alkali-modified ZSM-5 still retains the
rectangular shape, many faults and voids have appeared inside.
This is consistent with what other researchers have observed.
fied ZSM-5 has a ratio of SiO /Al O of 150 (nominal SiO /Al O
49,50
2
2
3
2
2
3
ratio of 150); the acid-modified ZSM-5 is increased to 165.96
ZSM-5-H), the alkali-modified ZSM-5 is reduced to 91.57 (ZSM-
The acid–base co-modified ZSM-5 not only changed its morphology,
but also showed many pores inside. The acid–alkali co-modified
ZSM-5 has changed its appearance in addition to having more
pores. This indicates that the modification by acid and alkali
not only causes a mesoporous structure in ZSM-5, but also
changes the morphology of ZSM-5.
(
5
1
-OH), and the acid–alkali co-modified ZSM-5 is reduced to
50.7 (ZSM-5-H-OH). This change indicates that part of the
framework of the acid and base modification-processed ZSM-5
dissolves, thereby achieving the purpose of desiliconization
4
8
and dealumination. Desiliconization or dealumination will
result in a larger pore structure, as shown in Fig. 5. Based on
the results, it can be concluded that the acid modification
succeeded in removing the Al element in ZSM-5, while the
alkali modification successfully removed the Si element in
ZSM-5.
Fig. 3 shows SEM images of ZSM-5, ZSM-5-H, ZSM-5-OH and
ZSM-5-H-OH zeolites. We can observe that the unmodified
ZSM-5 has a smooth morphological surface and a highly ordered
structure with only a small number of pores, demonstrating
2
Fig. 4 shows the N adsorption isotherms and pore size
distribution of ZSM-5 and modified ZSM-5. As shown in Fig. 4A,
unmodified ZSM-5 and acid-modified ZSM-5 exhibited a type I
adsorption isotherm, indicating a microporous structure.
Comparing the two adsorption isotherms, we can find that
the adsorption isotherm of acid-modified ZSM-5 has no significant
change compared to the adsorption isotherm of unmodified
ZSM-5, indicating that the modification by acid had no signifi-
cant effect on the pore structure of ZSM-5. The alkali-modified
ZSM-5 and the acid–alkali co-modified ZSM-5 showed a type IV
adsorption isotherm, and a hysteresis loop appeared, which was
typically associated with the emptying and the filling of meso-
pores, indicating that a mesoporous structure appeared in the
pores of ZSM-5. This is consistent with the observations of TEM.
The process of alkali modification is shown in Fig. 5, in which a
larger pore structure and a larger specific surface area are
obtained by removing the silicon element in ZSM-5. As shown
in Fig. 4B, it can be found that the mesoporous pore size
distribution of all the ZSM-5 samples is mainly concentrated
at 3.5 nm, and the 3.5 nm mesoporous distributions of the
alkali-modified ZSM-5 and the acid–alkali co-modified ZSM-5
are more visible. As shown in Table 4, the BET surface area,
external surface area, and pore volume of the ZSM-5 and the
acid–alkali co-modified ZSM-5 were significantly increased,
while the microporous area was significantly reduced, which
was attributable to the ZSM-5 part of the microporous structure
in the channel being converted to a mesoporous structure.
Fig. 1 XRD patterns of HZSM-5 zeolites before and after treatment.
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Fig. 3 TEM images before and after modification of ZSM-5: (a) ZSM-5, (b) ZSM-5-H, (c) ZSM-5-OH and (d) ZSM-5-H-OH.
Fig. 5 ZSM-5 modification process diagram.
The acid-modified ZSM-5 did not change significantly compared
to the unmodified ZSM-5.
3.2 Catalytic performance
ZSM-5 and modified ZSM-5 samples were evaluated in the
alkylation reaction of benzene with synthesis gas, and the
catalytic performance is shown in Table 4 and Fig. 6. The data
in Table 4 and Fig. 6 are the average of multiple experiments,
where the data for xylene refer to the sum of all isomers of
xylene. We have found that although acid modification can
increase the conversion of CO in the reaction, the selectivity to
5
1
various aromatics is almost reduced. In this reaction, the
reactants CO and H produced methanol under the catalysis
of Cu/ZnO/Al , and then the methanol entered the pores of
Fig. 4 (A) N
distributions for different ZSM-5 samples.
2
adsorption and desorption isotherms and (B) BJH pore size
2
2 3
O
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Table 3 The porosity properties for ZSM-5 before and after modification
Selectivity/%
Catalysts
Conversion/% Toluene Ethylbenzene Xylene p-Xylene
ZSM-5
ZSM-5-H
ZSM-5-OH
85.8
93.6
84.4
45.1
40.1
48.3
51.9
0.3
0.3
0.3
0.3
13.8
11.0
13.5
18.7
4.6
3.1
6.1
6.6
ZSM-5-H-OH 88.1
mesoporous structure. Therefore, the selectivity to aromatic
hydrocarbons has decreased. It can be seen from Table S1
(
ESI†) that Cu/ZnO/Al O only acts on the reaction of syngas to
2 3
methanol, and has no effect on the reaction of methanol to
aromatics, while ZSM-5 only acts on the reaction of methanol
to aromatics, and has no effect on the reaction of syngas to
methanol.
Fig. 6 Catalytic performance of different zeolites in benzene alkylation
with methanol.
As shown in Fig. 6, the alkali-modified ZSM-5 has a higher
ZSM-5 and was adsorbed on the acidic sites, and alkylated selectivity than the unmodified ZSM-5 catalyst. We can see from
benzene to form aromatic hydrocarbons (Fig. 7). The step of Fig. 2 and Table 1 that the alkali-modified ZSM-5 is signifi-
methanol to aromatics is the most critical. In this step, the MTO cantly reduced compared with ZSM-5 which is modified by acid
reaction is one of the main side reactions in which methanol is and base, and the unmodified ZSM-5, so that the strong acid
first dehydrated to dimethyl ether (DME), and the equilibrium sites are greatly reduced, thus effectively inhibiting the side
mixture formed includes methanol, dimethyl ether and water, reaction of MTO. From the SEM image, we can see that this is
and then converted into light olefins. The olefins are converted due to the channels in the alkali-modified ZSM-5 catalyst,
to alkanes, aromatics, naphthenes and higher olefins by hydro- which provide a larger reaction area for the reactants to enter
gen transfer, alkylation and polycondensation. According to the active site. Furthermore, due to the effects of acid leaching,
relevant literature reports, strong acid sites are conducive to ZSM-5-H-OH showed better activity than ZSM-5-OH, resulting
4
9,52
the occurrence of MTO reactions.
We can find from Fig. 2 in more active site exposure and improved accessibility of the
and Table 1 that the acid-modified ZSM-5 has increased acid zeolite pores. From Table 3 and Fig. 3, we can also find that the
sites compared to the unmodified ZSM-5, thereby increasing specific surface area and pore volume of ZSM-5 modified by
the MTO side reaction. At the same time, we can find from alkali modification and acid–base increase, while increasing
Table 3 and Fig. 3 that the pores of the acid-modified ZSM-5 the mesoporous structure. On the other hand, the selectivity to
did not increase significantly and did not have an increased xylene has also been significantly improved, indicating that the
Fig. 7 Process for syngas and benzene to form aromatic hydrocarbons.
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Table 4 Catalytic performance of different zeolites in benzene alkylation The acid–alkali co-modified ZSM-5 not only has an improved
with methanol
conversion rate, but also a significant increase in the selectivity
2
ꢀ1
3
ꢀ1
to aromatic hydrocarbons, and the amount of industrially
Surface area (m g
)
Pore volume (cm g )
5
4
required para-xylene is also increased. Compared to Zhao and
Samples
S
BET
S
ext/mes
S
micro
V
mesopore
V
micro
5
5
Bai’s research, this catalyst not only has higher conversion
and selectivity, but also the reaction can be carried out at lower
pressures and temperatures. The acid–alkali co-modified ZSM-5
has formed many mesoporous structures, which improved its
mass transfer ability and is the main reason for the improvement of
selectivity to aromatics.
ZSM-5
433.7
431
487.8
489.9
140.8
135.9
240.5
247.8
292.9
295.1
247.2
242.1
0.119
0.12
0.103
0.103
0.229
0.233
0.433
0.52
ZSM-5-H
ZSM-5-OH
ZSM-5-H-OH
presence of mesopores promotes the diffusion of large amounts
of aromatics in the reaction.
1
8,52
Ethylbenzene in the product is formed by alkylation of
ethylene and benzene. Ethylene is produced by the Fischer–
Conflicts of interest
Tropsch synthesis of ethanol, and the composite catalyst of The authors declare no competing financial interest.
Cu/ZnO/Al and ZSM-5 is not suitable as a Fischer–Tropsch
2 3
O
5
3
synthesis catalyst. Therefore, it is difficult for methanol to
form ethylene, and therefore, only a small amount of toluene is Acknowledgements
produced in the product. This indicates that this experiment
This research was funded by the Natural Science Foundation of
can be detrimental to the occurrence of the Fischer–Tropsch
synthesis reaction, thereby reducing many side reactions
related to olefins. The selectivity of ZSM-5, to toluene, xylene
and p-xylene, is greatly improved by modification by acid and
alkali. From the TEM image, we can see that ZSM-5-H-OH,
compared with ZSM-5-OH, has more pores, which is more
conducive to the diffusion of toluene and xylene. In summary,
acid-alkali co-modified ZSM-5 significantly improves selectivity
increase the catalytic activity and product selectivity of alkylation of
benzene with methanol.
China, grant number 21706178, and 21978189; Natural Science
Foundation of Shanxi Province, grant number 201801D221074,
and 201801D121058; China Postdoctoral Science Foundation
2019M651079. The authors also gratefully acknowledge the
support and guidance of teachers of the 508 groups from
Institute of Coal Chemistry, Chinese Academy of Sciences.
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. Conclusions
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