Improved selectivity toward light olefins in the reaction of toluene with methanol over the modified HZSM-5 catalyst

Article abstract of DOI:10.1002/cctc.201301072

Light olefins and para-xylene were produced with high selectivity in the reaction of toluene with methanol over a modified HZSM-5 catalyst. Ordered mesoporous structure and weaker acidic sites led to significant contribution to the selectivity toward para-xylene. By means of a single-pulse reaction and the temperature-programmed surface reaction, it was confirmed that the presence of toluene and as-produced xylene shortened the reaction induction period and reduced the formation temperature of ethylene and propylene, which was beneficial to improve the selectivity toward light olefins, particularly ethylene. Time is shortening: Light olefins and para-xylene are produced with high selectivity in the reaction of toluene with methanol over a modified HZSM-5 catalyst. The presence of toluene and as-produced xylene shortens the reaction induction period and reduces the formation temperature of ethylene and propylene, which is beneficial to improve the selectivity toward light olefins, particularly ethylene.

Full text of DOI:10.1002/cctc.201301072

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DOI: 10.1002/cctc.201301072
Improved Selectivity toward Light Olefins in the Reaction
of Toluene with Methanol Over the Modified HZSM-5
Catalyst
Yi Bi,[a, b] Yingli Wang,[a] Yingxu Wei,[a] Yanli He,[a] Zhengxi Yu,[a] Zhongmin Liu,*[a] and
Lei Xu*[a]
Light olefins and para-xylene were produced with high selec-
tivity in the reaction of toluene with methanol over a modified
HZSM-5 catalyst. Ordered mesoporous structure and weaker
acidic sites led to significant contribution to the selectivity
toward para-xylene. By means of a single-pulse reaction and
the temperature-programmed surface reaction, it was con-
firmed that the presence of toluene and as-produced xylene
shortened the reaction induction period and reduced the for-
mation temperature of ethylene and propylene, which was
beneficial to improve the selectivity toward light olefins, partic-
ularly ethylene.
lene and polypropylene, and ethylene was also a major com-
ponent for the production of polyethylene terephathalate.
Therefore, it would be meaningful to improve toluene conver-
sion by using an optimum feed ratio of toluene and methanol,
to restrain the formation of undesired aromatics with a de-
signed catalyst, as well as to transform the excess methanol
(unreacted with toluene) to important chemicals under the
same conditions. Hence, we proposed a new mechanism to
produce PX with use of extremely high-purity and high-con-
centration ethylene and propylene in one chemical reaction
through the transformation of toluene and methanol
(Scheme 1) over the modified HZSM-5 catalyst. It was also of
great significance that the two materials used for the produc-
tion of polyesters—PX and ethylene—could be obtained in
one chemical reaction.
para-Xylene (PX) is one of the important organic materials
used for the synthesis of pure terephthalic acid, which is used
to make polyesters, particularly polyethylene terephathalate. In
industrial processes, PX is produced from the aromatics of
a naphtha catalytic reformer through complicated crystalliza-
tion or adsorption separation; this method is very costly to
obtain the desired purity of more than 99%. The selective alky-
lation of toluene with methanol over the modified HZSM-5 cat-
alyst has been studied extensively for decades. In general,
steam treatment,[1] poison treatment of large molecules,[2]
modification of Mg,[3,4] B,[5] P,[6] La,[7] Pt,[8] and so forth, and sily-
lation method[9,10] were used to improve porous and acidic
properties of the HZSM-5 catalyst to produce PX in concentra-
tions greater than equilibrium concentrations. In addition,
methanol reacted partially with toluene to produce alkylben-
zenes and the rest was dehydrated to form hydrocarbons by
using the methanol-to-hydrocarbon (MTH) process.[11] Some
studies focused on improving the alkylation reaction as well as
restraining side reactions. Thus, a high toluene/methanol
molar feed ratio was often used, which would result in low
conversion of toluene. However, light olefins, particularly ethyl-
ene and propylene, among hydrocarbons were not useless.
They were important chemicals for the production of polyethy-
Scheme 1. A new mechanism for the transformation of toluene and
methanol.
The modified HZSM-5 catalyst for this process is expected to
function in two transformations: one is the alkylation of tolu-
ene with methanol to produce PX with high selectivity and the
other is the conversion of methanol to high-concentration eth-
ylene and propylene. Therefore, the Si/PLaHZSM-5 catalyst was
designed by using a progressive modification method to im-
prove the acidity and channel character. The XRD patterns of
HZSM-5 and Si/PLaHZSM-5 catalysts, in which peak positions
matched the characteristics of the MFI structure, are shown in
Figure 1a. The diffraction peak intensity of the Si/PLaHZSM-5
catalyst decreased at 2q=23.0–24.38, which indicated that the
crystal structure corresponding to that lattice spacing changed
after the modification. A new signal at 2q=7.28 appeared,
which was possibly due to the phase transformation of the
HZSM-5 catalyst.[12] N2 physisorption isotherms of HZSM-5 and
Si/PLaHZSM-5 catalysts are shown in Figure 1b. For the HZSM-
5 catalyst, the adsorption and desorption branches of the iso-
therm coincided with each other and no hysteresis loop was
observed; this finding was attributed to a type I isotherm typi-
cal of microporous materials. For the Si/PLaHZSM-5 catalyst,
the adsorption feature in microporous materials was well main-
tained and the desorption branch of the isotherm demonstrat-
[a] Dr. Y. Bi, Y. Wang, Prof. Dr. Y. Wei, Y. He, Z. Yu, Prof. Dr. Z. Liu, Prof. Dr. L. Xu
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics
Chinese Academy of Sciences Institution
457 Zhongshan Road, Dalian 116023 (P.R. China)
Fax: (+86)411-8437-9318
E-mail: zml@dicp.ac.cn
leixu@dicp.ac.cn
[b] Dr. Y. Bi
Graduate University of the Chinese Academy of Sciences
Beijing 100049 (P.R. China)
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sieve.[13] In addition, the im-
provement in the thermal stabili-
ty of the HZSM-5 catalyst during
the P/La modification prevented
the removal of framework Al
atoms in steaming treatment.[14]
Therefore, many weaker acidic
sites were left even after silaniza-
tion modification, and owing to
the coverage and migration of
the SiO2 layer, the acidic strength
was reduced. The 27Al and
29Si MAS NMR spectra of HZSM-5
and Si/PLaHZSM-5 catalysts are
shown in Figure 1e and f. The
chemical shifts at 54.94 and
À0.95 ppm in the 27Al MAS NMR
spectrum of the HZSM-5 catalyst
were ascribed to tetrahedral
framework Al and six-coordinat-
ed extra-framework Al species,
respectively. The chemical shifts
at 113.6 and À107.6 ppm in the
29Si MAS NMR spectrum of the
HZSM-5 catalyst were ascribed
to Si(0Al) and Si(1Al) species, re-
spectively. After the modifica-
tion, the intensity of resonance
peaks of both tetrahedral frame-
work Al and six-coordinated
extra-framework Al species was
weakened and the broadening
and shifting of resonance peaks
to high field were observed.
However, the intensity of reso-
nance peaks of Si(0Al) and
Si(1Al) species was improved
and the narrowing and shifting
of resonance peaks to high field
Figure 1. a) XRD patterns, b) N2 physisorption isotherms, c) pore size distribution, d) NH3-temperature pro-
grammed desorption profiles, and e) 27Al and f) 29Si MAS NMR spectra of HZSM-5 and Si/PLaHZSM-5 catalysts.
ed two hysteresis loops in the relative pressure range of 0.15–
0.35 and 0.50–0.90, respectively, which was similar to the type
IV isotherm observed for the mesoporous structure of MCM-
41. This fact could also be observed in the pore size distribu-
tion of HZSM-5 and Si/PLaHZSM-5 catalysts (Figure 1c). After
the modification, the emergence of an ordered mesoporous
structure with a diameter of 2.5–3.5 nm was observed, instead
of the packing pore with a diameter of 1.0–4.0 nm for the
HZSM-5 catalyst. The NH3-temperature programmed desorp-
tion profiles of HZSM-5 and Si/PLaHZSM-5 catalysts are shown
in Figure 1d. The HZSM-5 catalyst demonstrated weaker and
stronger acidic sites, with the desorption peaks centered at
235 and 4678C, respectively. For the Si/PLaHZSM-5 catalyst, the
stronger acidic sites disappeared completely and part of the
weaker acidic sites remained and shifted to 1968C, which
could be attributed to the fact that the formation of P hydroxyl
(Scheme 2) during the P/La modification decreased the interac-
tion of the silane agent with surface Al atoms in the molecular
were observed. These results illustrated that dealuminated sili-
con-enriched zeolite was obtained,[15] and ordered lattice ar-
rangement and regular framework structure were confirmed
further.[16]
Scheme 2. The formation of P hydroxyl.
The reaction of toluene with methanol was performed over
HZSM-5 and Si/PLaHZSM-5 catalysts in a fixed-bed reactor,
with the toluene/methanol feed molar ratio ranging from 0:1
to 2:1. All effluents were kept warm and analyzed online with
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This observation could be attributed to the fact that
ordered mesoporous structure (Figure 1b and f) and
narrower average micropore opening (Figure 1c)
were favorable for the diffusion of product molecules
with the smallest size among xylene isomers and the
deactivation of acidic sites (Figure 1d) was conducive
to decrease the secondary reactions of product mole-
cules on the external surface, which led to significant
contribution to the formation of PX.
Table 1. Performance of the alkylation of toluene with methanol.[a]
Variable
HZSM-5
Si/PLaHZSM-5
toluene/methanol ratio [mol]
reaction temperature [8C]
toluene conversion [wt%]
methanol conversion [wt%]
2:1
420
37.48
100.00
0:1
420
–
0:1
550
–
1:1
420
38.11
99.03
2:1
420
24.19
99.57
85.59
99.77
product distribution [wt%]
C1–C5 hydrocarbons
benzene
toluene
ethylbenzene
para-xylene
7.31
20.76
–
89.95
0.16
1.11
0.34
5.95
0.00
0.03
2.46
100.00
84.26
0.61
4.03
0.52
9.60
0.05
0.08
0.84
100.00
16.66
0.17
–
0.29
75.66
1.66
1.05
4.51
100.00
11.35
0.17
–
0.21
82.54
1.23
0.83
3.67
100.00
In addition, if the toluene/methanol feed molar
ratio was 1:1 and 2:1, methanol conversion was more
than 99% at 4208C over the Si/PLaHZSM-5 catalyst.
However, if neat methanol was fed, methanol conver-
sion was only 85.59% at the same temperature. How-
ever, if the temperature was increased to 5508C,
methanol conversion could reach 99.71%. These re-
sults indicated that methanol conversion increased
significantly at lower temperatures with the co-feed-
ing of toluene and methanol.
0.90
12.35
27.48
11.70
19.50
100.00
meta-xylene
ortho-xylene
>C9 hydrocarbons
total
xylene composition [wt%]
para
meta
ortho
23.97
53.33
22.70
99.46
0.00
0.54
98.66
0.51
0.82
96.54
2.12
1.34
97.57
1.45
0.98
In the MTH reaction, if neat methanol was fed, the
selectivity toward ethylene and propylene among the
C1–C5 hydrocarbons was only 58.37 wt% at 4208C
and increased to 60.30 wt% at 5508C. However, if tol-
uene and methanol were co-fed in a molar ratio of
1:1 and 2:1, the selectivity toward ethylene and pro-
pylene reached 83.98 and 85.62 wt%, which was
comparable with the performance of the SAPO-34
catalyst.[17] The selectivity toward ethylene was ap-
proximately 55.79 wt% if toluene and methanol were
co-fed in a molar ratio of 2:1, which was quite differ-
ent from the selectivity obtained if only methanol
was fed (26.81 wt%) at the same temperature. These
results indicated that the presence of toluene favored
C1–C5 composition [wt%]
CH4
C2H4
C2H6
C3H6
C3H8
C4
3.74
3.50
5.04
2.08
71.46
11.24
2.89
5.69
26.81
0.44
31.56
1.67
21.33
12.50
100.00
8.90
22.45
0.55
37.85
2.42
19.41
8.43
2.55
51.77
0.24
32.21
3.16
8.66
2.75
55.79
0.28
29.83
2.59
7.41
C5
total
1.41
100.00
1.35
100.00
100.00
100.00
C2H4 +C3H6
5.58
58.37
60.30
83.98
85.62
[a] Weight hourly space velocity (WHSV)=2 hÀ1
stream=10 min.
; atmospheric pressure; time on
a gas chromatograph equipped with a flame ionization detec-
tor. The gas phase products were collected and analyzed by
another gas chromatograph to determine C1–C5 distribution.
The reaction results are summarized in Table 1.
the production of ethylene and propylene, particularly of ethyl-
ene, over the Si/PLaHZSM-5 catalyst.
Therefore, if toluene and methanol were co-fed, high selec-
tivity toward PX among xylene isomers and high conversion of
methanol were well maintained and, at the same time, high se-
lectivity toward ethylene and propylene in gas phase products
was obtained in methanol transformation over the Si/
PLaHZSM-5 catalyst. An interesting observation was that the
selectivity toward ethylene increased significantly and was
higher than that of propylene.
When the alkylation of toluene with methanol performed
over the HZSM-5 catalyst with a toluene/methanol feed molar
ratio of 2:1, toluene conversion and methanol conversion was
37.48 and 100.00%, respectively. The selectivity toward PX
among xylene isomers was only 23.97 wt%, which was close
to the thermodynamic equilibrium concentration. At the same
time, the selectivity toward propane largely formed among C1–
C5 hydrocarbons was 71.46 wt%, and an amount of ethylene
and propylene was also observed. The results indicated that
the secondary isomerization of PX and hydrogen transfer reac-
tion occurred simultaneously in addition to the primary alkyla-
tion reaction over the HZSM-5 catalyst.
To identify the reason why the co-feeding of toluene and
methanol was conducive to methanol conversion, the per-
formance of the single-pulse reaction was investigated by
using different feeding methods: feeding with methanol only
and co-feeding with toluene and methanol. The contact time
could be altered by changing the bed height and partial pres-
sure of feed components. Methanol conversion as a function
of contact time is plotted in Figure 2. The curve slope is used
to represent the consumption rate of methanol.[18] The inter-
section point between the curve and the x axis is the maxi-
mum value of contact time when methanol conversion is zero,
which is considered as the reaction induction period in metha-
nol transformation. The performance of the single-pulse reac-
tion indicated that methanol conversion increased linearly with
Over the Si/PLaHZSM-5 catalyst, the proportion of PX
among xylene isomers was approximately 96.54 and
97.57 wt% with the toluene/methanol feed molar ratio of 1:1
and 2:1, respectively. If neat methanol was fed (toluene/metha-
nol=0:1), the selectivity toward PX among xylene isomers was
approximately 99.46 wt%. These results indicated that the Si/
PLaHZSM-5 catalyst possessed an excellent shape selectivity in
both toluene alkylation and methanol-to-aromatics processes.
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Figure 2. Methanol conversion as a function of contact time over the Si/
PLaHZSM-5 catalyst in different feeding methods.
contact time in both feeding methods. With the increase in
contact time, the methanol consumption rate remains un-
changed. The intersection point is 0.000684 s if the curve for
the feeding with methanol only is extended to the x axis,
which suggests that the reaction induction period existed in
methanol transformation over the Si/PLaHZSM-5 catalyst. How-
ever, the intersection point is 0.0000126 s if the curve for the
co-feeding with methanol and toluene is extended to the
x axis. This result indicated that the reaction induction period
in methanol transformation shortened in the presence of tolu-
ene. It was also possible that methanol conversion in the MTH
process increased in the presence of toluene or as-produced
aromatics. This observation was in accordance with the hydro-
carbon pool mechanism,[19,20] in which large aromatics, such as
multi-methylbenzene, was considered as the catalytic engine
to promote methanol conversion. In the alkylation of toluene
with methanol, these lower methylbenzenes that were largely
fed (toluene) or produced (xylene) improved methanol trans-
formation correspondingly.
Figure 3. Product distribution as a function of reaction temperature in the
TPSR by using different preadsorption methods: a) preadsorption with meth-
anol only and b) preadsorption with toluene and methanol. The insets show
clearly the desorption peaks of products when the signal intensity is
amplified.
methane was lowered to 134.78C, suggesting the shortening
of the reaction induction period in the presence of toluene,
which was in accordance with the result of the single-pulse re-
action (Figure 2). The desorption peaks of ethylene and propyl-
ene were observed at 147.8 and 148.38C, respectively, which
suggested that the formation temperature of ethylene and
propylene, particularly of ethylene, decreased in the presence
of toluene and methanol co-feed. Considering that the as-pro-
duced xylene could also work as the reaction center for ethyl-
ene formation,[20,21] the presence of toluene and xylene was
beneficial to the formation of ethylene in methanol transfor-
mation, which was in accordance with the result of alkylation
of toluene with methanol (Table 1). However, in both pread-
sorption methods, two desorption peaks of methane were ob-
served at approximately 140 and 2508C. These results indicat-
ed that methane production occurred in two ways: one was
associated with the carbon deposition at the initial stage of
the reaction and the other was accompanied by the genera-
tion of hydrocarbons in methanol transformation.
To understand why the co-feeding of toluene and methanol
was conducive to selectivity toward ethylene and propylene,
the performance of the temperature-programmed surface reac-
tion (TPSR) was investigated by using different preadsorption
methods: preadsorption with methanol only and preadsorp-
tion with toluene and methanol. Product distribution as a func-
tion of reaction temperature in the TPSR is plotted in Figure 3.
The products of the TPSR of toluene with methanol were de-
tected as methane, ethylene, and propylene. Owing to the
presence of a large amount of toluene covered with the raw
material in MS detection, no xylene was detected in the prod-
uct distribution of the TPSR.
By using the method of preadsorption with methanol only,
a weak desorption peak of propylene at 186.98C and two de-
sorption peaks of methane at 147.0 and 249.88C were ob-
served. A negligible desorption peak of ethylene appeared at
188.48C. These results indicated the existence of the reaction
induction period in methanol transformation: Methane produc-
tion was associated with carbon deposition. With the increase
in the carbon content up to a certain amount, the reaction in-
duction period ended and light olefins, particularly propylene,
were produced quickly. By using the method of preadsorption
with toluene and methanol, the desorption temperature of
Coke species were also examined in different feeding meth-
ods (feeding with methanol only and co-feeding with toluene
and methanol) over the Si/PLaHZSM-5 catalyst. The zeolite
frameworks of the used samples were dissolved in a 25%
aqueous solution of hydrofluoric acid. After solvent extraction,
coke species were identified by using GC–MS and further ana-
lyzed quantitatively by using GC, and the results are illustrated
in Figure 4. Coke species were mainly composed of methyl-
substituted benzene derivatives, naphthalene, dimethylnaph-
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bility could be due to the fact that the reduction in the forma-
tion of the condensed ring aromatic hydrocarbons confirmed
in the examination of coke species would help retard coke and
prolong catalyst life. In addition, greater than 97 wt% selectivi-
ty toward PX among xylene isomers and greater than 85 wt%
selectivity toward ethylene and propylene among C1–C5 hydro-
carbons were observed. A life test demonstrated that high ac-
tivity and selectivity were still well maintained, which con-
firmed the high hydrothermal stability and sustainable durabili-
ty of the Si/PLaHZSM-5 catalyst. Hence, the Si/PLaHZSM-5 cata-
lyst not only demonstrated high activity and selectivity in the
reaction of toluene with methanol but was also sufficiently
stable under practical conditions.
Figure 4. Product distribution and relative composition of coke species over
the Si/PLaHZSM-5 catalyst in different feeding methods. Methanol transfor-
mation with a toluene/methanol/water (molar ratio) of 2:1:2 for 4 h and
a toluene WHSV of 2 hÀ1. The values in percent denote the relative composi-
tion of the corresponding coke species.
In conclusion, ethylene, propylene, and PX could be pro-
duced in one chemical reaction over the Si/PLaHZSM-5 cata-
lyst. The high selectivity toward PX could be well maintained,
and light olefins were formed with high efficiency in the trans-
formation of toluene and methanol. A substantial improve-
ment in selectivity toward light olefins, particularly selectivity
toward ethylene among C1–C5 hydrocarbons, which was differ-
ent from the classical process of methanol-to-olefin conversion,
may stem from the presence of toluene and as-produced
xylene. This study gives a potential way of providing two reac-
tants for polyester production in one chemical reaction.
thalene, multi-methylnaphthalene, and condensed ring aro-
matic hydrocarbons such as phenanthrene and pyrene over
the Si/PLaHZSM-5 catalyst in both feeding methods. However,
less coke was formed and the contents of naphthalene
(19.57%) and multi-methyl-substituted derivatives (30.80%) in-
troduce higher intensity in the reaction with toluene and
methanol co-feed than do coke species of the reaction with
methanol-only feed, in which a larger amount of coke species
was detected and the content of polycyclic aromatic hydrocar-
bons was high (47.87%). These results indicated that coke spe-
cies were similar in both feeding methods and the formation
of the condensed ring aromatic hydrocarbons could be inhibit-
ed owing to the replacement of toluene with larger-diameter
molecules on the catalyst surface in the co-feeding method.
Furthermore, the presence of polycyclic aromatic hydrocarbons
in coke species with methanol-only feeding would be condu-
cive to the occurrence of hydrogen transfer reactions, which
results in a decrease in the selectivity toward ethylene and
propylene.
Experimental Section
The designed Si/PLaHZSM-5 catalyst was prepared as follow: First,
the HZSM-5 zeolite with a SiO2/Al2O3 molar ratio of 150 obtained
from the Catalyst Plant of Nankai University was taken as the
parent material. Then, HZSM-5 was modified with H3PO4 and
La(NO3)3 solutions. Subsequently, the modified HZSM-5 catalyst
was silylated by using the chemical liquid deposition method with
the ethanol solution of tetraethoxysilane as a silicon source. Finally,
this precursor was calcined in air at 5508C for 4 h and steamed at
6508C for 4 h to obtain the Si/PLaHZSM-5 catalyst.
The reaction of toluene with methanol was further tested
for 120 h over the Si/PLaHZSM-5 catalyst, and the reaction re-
sults are illustrated in Figure 5. A toluene conversion of greater
than 15% and a methanol conversion of greater than 75%
were maintained for approximately 120 h. This long-term sta-
Keywords: ethylene · methanol · propylene · toluene · para-
xylene
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Received: December 14, 2013
Published online on February 12, 2014
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 713 – 718 718