Coupling of Methanol and Carbon Monoxide over H-ZSM-5 to Form Aromatics

Article abstract of DOI:10.1002/anie.201807814

The conversion of methanol into aromatics over unmodified H-ZSM-5 zeolite is generally not high because the hydrogen transfer reaction results in alkane formation. Now circa 80 % aromatics selectivity for the coupling reaction of methanol and carbon monoxide over H-ZSM-5 is reported. Carbonyl compounds and methyl-2-cyclopenten-1-ones (MCPOs), which were detected in the products and catalysts, respectively, are considered as intermediates. The latter species can be synthesized from the former species and olefins. ¹³C isotope tracing and ¹³C liquid-state NMR results confirmed that the carbon atoms of CO molecules were incorporated into MCPOs and aromatic rings. A new aromatization mechanism that involves the formation of the above intermediates and co-occurs with a dramatically decreased hydrogen transfer reaction is proposed. A portion of the carbons in CO molecules are incorporated into aromatic, which is of great significance for industrial applications.

Full text of DOI:10.1002/anie.201807814

A Journal of the Gesellschaft Deutscher Chemiker
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Chemie
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Accepted Article
Title: Coupling of Methanol and Carbon Monoxide over H-ZSM-5 to
Form Aromatics
Authors: Zhiyang Chen, Youming Ni, Yuchun Zhi, Fuli Wen, Ziqiao
Zhou, Yingxu Wei, Wenliang Zhu, and Zhongmin Liu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201807814
Angew. Chem. 10.1002/ange.201807814
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http://dx.doi.org/10.1002/ange.201807814

10.1002/anie.201807814
Angewandte Chemie International Edition
COMMUNICATION
Coupling of Methanol and Carbon Monoxide over H-ZSM-5 to Form
Aromatics
Zhiyang Chen, Youming Ni,Yuchun Zhi, Fuli Wen,Ziqiao Zhou,Yingxu Wei, Wenliang Zhu*,and
Zhongmin Liu*
enhanced by CO over the bifunctional ZnZrOx/H-ZSM-5
Abstract: The conversion of methanol to aromatics over
unmodified H-ZSM-5 zeolite is generally not high because the
hydrogen transfer reaction results in alkane formation. Here we
report an approximately 80% aromatics selectivity for the coupling
reaction of methanol and carbon monoxide over H-ZSM-5. Carbonyl
compounds and methyl-2-cyclopenten-1-ones (MCPOs) which were
detected in the products and catalysts, respectively, are considered
as intermediates. The latter species can be synthesized from the
former species and olefins. 13C isotope tracing and 13C liquid- state
NMR results proved that the carbon atoms of CO molecules were
incorporated into MCPOs and aromatic rings. A new aromatization
mechanism that involves the formation of the above intermediates
and co-occurs with a dramatically decreased hydrogen transfer
reaction is proposed. A portion of the carbons in CO molecules are
incorporated into aromatic, which is of great significance for
industrial applications.
catalysts in the conversion of syngas or intermediate methanol
because CO plays a role in the removal of H species and
subsequent formation of methanol. As a result, it can be
deduced that ZnZrOx is indispensable to the catalysis
mechanism.[6]
In the presence of CO, H-zeolites catalyse methanol
carbonylation to form methyl acetate (MAc) and acetic acid (HAc)
[7]
through the Koch reaction.
It has been proposed that the
formation of the first carbon-carbon bond occurs through
methanol carbonylation during the methanol to olefin reaction. [8]
Here we report an aromatics selectivity of 80% along with 65%
BTX for the coupling reaction of methanol and CO (CMTA) over
H-ZSM-5 zeolite. A new aromatization mechanism in which the
hydrogen transfer reaction is sharply decreased is proposed.
Aromatics, especially benzene, toluene and xylene (BTX), are
important bulk chemicals that are primarily produced from
petroleum via catalytic reforming or cracking.[1] Obtaining
aromatics from non-petroleum resources, such as coal, natural
gas or biomass, is very important because of the increase in
market demand for such products and the depletion of
petroleum resources. Methanol can be derived from these
alternative resources via syngas chemistry, and aromatics can
a 100
90
=
C -C
2 4
MeOH Conv
0
CH
4
80
70
60
50
40
30
20
10
0
be readily synthesized from methanol by
a methanol-to-
C
-C
BTX
Aromatics
2
4
[2]
aromatics (MTA) reaction.
Generally, acidic ZSM-5 zeolites
are selected as MTA catalysts due to their unique topology and
three-dimensional micropore systems. However, acidic H-form
ZSM-5 zeolite (H-ZSM-5) without metal modifications usually
has low aromatics selectivity because of the corresponding
formation of alkanes via hydrogen transfer.[3] Although
[2a, 2c, 2e]
modifications with metals such as Zn,
Ga,[2d] and Ag[2b]
increase aromatics formation by enhancing the dehydrogenation
of alkanes via catalysis by these metal species,[4] some
methanol decomposition and the formation of unrecoverable
catalyst structure due to metal evaporation,segregation and
aggregation are inevitable. [2a,5]
1
2
3
4
5
6
7
8
9
Time on stream（ hr）
b 100
90
80
70
60
50
40
30
20
10
0
Recently, Cheng et al. reported that aromatization could be
=
MeOH Conv
o
CH4
C2-C4
C2-C4
BTX
Aromatics
MAc+HAc
[*]
Z.Chen,Dr. Y.Ni, Dr.Y.Zhi, F.Wen,Z.Zhou, Prof. Dr. Y.Wei, Prof. Dr.
W. Zhu, Z. Liu
National Engineering Laboratory for Methanol to Olefins
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023 (P.R. China)
E-mail: liuzm@dicp.ac.cn
1
2
3
4
5
6
7
8
9
wlzhu@dicp.ac.cn
Time on stream（ hr）
Z. Chen,F.Wen,Z.Zhou
University of Chinese Academy of Sciences,Beijing 100049
Figure 1. MTA (a) and CMTA (b) performance over Z-25 .
Reaction conditions: T=673 K;WHSV =0.15 g g-1h-1 ;N2 or CO/CH3OH=113;
P=4 Mpa.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201807814
Angewandte Chemie International Edition
COMMUNICATION
The CMTA and MTA reactions were performed at 673 K under
4.0 MPa over Z-25 (H-ZSM-5 with the SiO2/Al2O3 ratio = 25).
The results are shown in Fig.1a, an approximate 40% initial
aromatics selectivity with a 53% C2-C4 paraffin selectivity was
obtained under N2, which agree with the typical hydrogen
transfer mechanism.[3] Interestingly, the initial aromatics
selectivity was dramatically increased to 80% along with 65%
BTX when methanol was coupled with CO, while the CH4 and
MAc and HAc(Fig.S6a). The other products were C2H4 and C4H8.
When the temperature rose to 523 K, the selectivity for MAc and
HAc reached 92.4% with ca. 40% methanol conversion in the
beginning, and then rapidly dropped to a very low value with the
generation of hydrocarbons and complete methanol conversion.
Upon catalyst deactivation, the aromatics selectivity decreased
and the carbonyl compounds appeared again (Fig.S6b). It is
widely accepted that the MTH reaction is an autocatalytic
C2-C4 paraffin selectivity was only 3% and 20%, respectively (Fig. reaction, and critical levels of species such as
1b). The detailed selectivity of aromatics is shown in Fig.S1.As
presented in Fig.S2, increasing the reaction temperatures of the
CMTA reaction was advantageous to aromatics generation
because C2-C4 paraffin formation was suppressed, however a
much higher temperature (723 K) led to lower aromatics
selectivity due to sharp increases in CH4.
polymethylbenzenes in the hydrocarbon pool are necessary to
initiate autocatalytic reactions.[12]Thus we propose that carbonyl
compounds could be transformed into species in the
hydrocarbon pool to initiate the autocatalytic MTH reaction,which
can explain the decrease in carbonyl compounds with the
increase in methanol conversion. Overall, we suppose that
carbonyl compounds take part in the formation of aromatics.
H-ZSM-5 zeolites with SiO2/Al2O3 ratios ranging from 25 to 175
were employed to study the effect of acidity on aromatization in
the CMTA reaction. The XRD, N2 absorption-desorption and
NH3-TPD results are depicted in Fig.S3, Table.S1 and
Fig.S4,respectively. A lower SiO2/Al2O3 ratio results in more
acidic sites.[9] As seen in Fig.2, the aromatics selectivity
increased as the SiO2/Al2O3 ratio decreased (or the number of
acidic sites increased). Notably,1.1～1.2% CO was converted,
whereas approximately 65% CO2 selectivity from CO was
obtained. As shown in Table S2,almost equivalent CO2 amount
were detected when co-fed CO-MeOH over H-ZSM-5 and CO-
H2O over quartz sand,thus we consider that CO2 was primarily
To clarify the effect of the carbonyl compounds, methanol was
co-fed with MAc over Z-25 under N2 at 0.1 Mpa. As listed in
Table.S3, increasing the MAc concentration enhanced the
selectivity for aromatics. The selectivity for aromatics increased
with increasing CO partial pressure(shown in Fig.S7)，which
agrees with previous works showing that the carbonylation rates
increase linearly with the CO partial pressure.[7b,7c] This
relationship suggests that carbonylation could have a close
relationship with aromatization. We also investigated the
intermediates formed at reaction temperature as high as 673 K
by shortening the contact time. It is obvious from Fig. S8 that the
derived from the water-gas-shift reaction（CO + H2O CO2 +H2, carbonyl compounds were detected when the contact time was
H2O was formed through the MTH reaction）caused by the
materials(such as Fe、Ni and Cr) of the stainless steel tubular
reactor. Therefore, approximately 35% of the carbon atoms in
the consumed CO are incorporated into the hydrocarbon
products. At a CO/MeOH ratio =20, the hydrocarbons yield will
increase ca. 8% compared to the yield from methanol
conversion, which is of great significance to industrial
applications. Considering that the nanostructure of H-ZSM-5 is
beneficial to prevent coke formation,[2c,2f] we chose a nanosized
H-ZSM-5 with a SiO2/Al2O3 ratio =41 to study the catalytic
stability of the CMTA reaction. As shown in Fig.S5, the
aromatics selectivity remained at approximately 70% as
methanol was completely converted to products in 100 h on
stream.
less than 0.15 s, and that prolonging the contact time decreased
the selectivity for HAc,MAc,and olefins and increased that for
aromatics. These results indicate that the carbonyl compounds
act as intermediates to form aromatics.
100
90
80
70
60
50
40
30
20
10
0
1.5
1.4
1.3
1.2
1.1
1.0
CH4
C2=-C4
C20-C4
=
CO Conv
CO2
0
C5+
Aromatics
The enhanced aromatization in the CMTA reaction could not be
explained by the dehydrogenation of alkanes[4] or the synthesis
of methanol[11] due to the absence of metal species. We also
confirmed that propane was slightly converted to products over
Z-25 at 673 K and 4 MPa in the presence of CO.CO was
inactive when fed alone or syngas (H2: CO=2:1) over Z-25 at
673 K and 4 MPa.
157
38
65
25
SiO2/Al2O3 ratio
Figure 2. Effect of SiO2/Al2O3 ratio of H-ZSM-5 on CMTA performance
Reaction conditions:T=673 K; WHSV=0.5 g g-1h-1;CO/CH3OH=20;P=5 MPa
time on stream=4 h;PS:C5+ excludes aromatics.
To illustrate the evolution from carbonyl intermediates to
aromatics, the reactions at 523 K were quenched by liquid
nitrogen and active species were extracted and analyzed by GC-
MS following Guisnet’s method.[13]The results are shown in Fig.
3, and the catalytic performance is shown in Figs. S9 and S6b.
The primary organics retained in the catalyst after the MTA
In the presence of CO, carbonyl compounds including MAc and
HAc were also detected at the beginning of deactivation at 673
K (Fig.1b).When the reaction temperature dropped to 493K,
methanol conversion remained at 11% with a 91% selectivity for
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10.1002/anie.201807814
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COMMUNICATION
reaction under N2 were polymethylbenzenes,and we observed a
few methyl-2-cyclopenten-1-ones (MCPOs),which have been
detected in the conventional MTH reaction.[14] Interestingly, the
introduction of CO in the reaction strikingly enhanced the
amount of MCPOs. Thus we propose that MCPOs act as
intermediates during the conversion of carbonyl compounds to
aromatics. From the catalysis results in Figs.1 and S8, it can
also be found that the selectivity of aromatics increased with
decreasing selectivity for olefins and carbonyl compounds.
Therefore, MCPOs may be generated by the reaction of low
olefins and carbonyl compounds.
and
paraffin
could
have
derived
from
the
13C
polymethylbenzenes, as it is widely accepted that hydrocarbon
pool species could produce light olefins.[15] Because benzene
containing one 13C atom was predominant, we consider that one
CO molecule took part in forming an aromatic ring and that
olefins containing a 13C atom from aromatics led to the inclusion
of more than one 13C atom in aromatics. We also analyzed the
13C incorporation into MCPOs intermediates. As shown in Figs.
4b and S13, the mass of the base peak of MCPOs increased by
1 when 12CO switched to 13CO, indicating that only one CO
molecule was involved in the formation of one MCPO
intermediate. Considering that only one CO molecule takes part
in the formation of aromatics, we believe that MCPOs act as the
intermediates to form aromatics.
To identify the product from the reaction of olefins with carbonyl
compounds, excess acetic anhydride was absorbed onto the
ZSM-5 catalyst, which was then sealed with C3H6 (5% C3H6,
95% N2) or N2. The products formed at 523 K were analyzed by
Guisnet’s method. As shown in Fig.S10, MCPOs were hardly
detected when acetic anhydride alone was converted over the
ZSM-5 catalyst. However, a sharp increase in MCPOs occurred
after the addition of propene, indicating that MCPOs formed
from olefins and carbonyl compounds.
90
013
113
213
C
C
C
a
80
70
60
50
40
30
20
10
0
benzene
toluene
propene
propane
Figure 4. The results of 13CO isotope tracing method.
Figure 3. GC-MS chromatograms of organic materials retained in catalysts.
Reaction Condition:T=523 K;WSHV=0.15 g g-1h-1; CO/CH3OH=113; P=4 Mpa.
The catalysts were removed at 120 min during the methanol conversion in (N2
(bottom) and CO (top)).The enlarged region of retention time from 6.2 to 7.4
min are shown in the inset, where dimethyl-2-cyclopenten-1-one species are
labeled 1*, trimethyl-2-cyclopenten-1-one is labeled 2* while 3-Methyl-2-
cyclohexen-1-one is labeled 3*.
(a)13C distribution in products of coupling reaction of methanol and 13CO
Reaction conditions:T=673 K;WHSV=0.15 g*g-1h-1; CO/CH3OH=28 ;P=1 Mpa.
(b) MS spectra of 3-methyl-2-cyclopenten-1-one in 12CO (bottom) and 13CO
(top) during the coupling reaction of methanol and CO
Reaction conditions: T=523 K; WHSV=0.15 g g-1h-1; CO/CH3OH=28 ;P=1 Mpa.
The catalysts were removed at 120 min during the reaction.
The products of 13CO/CH3OH feeds and used catalysts were
analysed by 13C liquid-state NMR spectroscopy. The 13C NMR
spectra of the liquid products collected from the 13CO/CH3OH
feed are shown in Fig.S14.The signal at 128 ppm was assigned
to the C atom of the aromatics ring. [16] Signals of methyl groups
were not observed in the 13C NMR spectra.These results prove
that the 13C atoms of 13CO are indeed introduced into the
aromatic rings. The used ZSM-5 catalyst was extracted by
CH2Cl2 following Guisnet’s method, and the resultant liquid
solution was analysed through 13C liquid-state NMR
spectroscopy (Fig.S15). The signal at 209 ppm was assigned to
keto C while the signals corresponding to the other C atoms in
To further confirm that CO participates in forming aromatics, 13
C
isotope tracing was utilized to examine the reactions, and the
catalytic performance is shown in Fig.S11. The MS spectra of
products in the presence of 12CO or 13CO are displayed in Fig.
S12. The 13C distribution in the products can be calculated on
the basis of the 12C distribution. As determined from Fig.4a, the
amount of 13C incorporated into benzene and toluene was
significantly higher than that incorporated into propene and
propane — crucial evidence supporting the involvement of CO in
the formation of aromatic rings. Moreover, this result indicates
that the principal mechanism of aromatization in the coupling
reaction of methanol with CO is not conventional olefin
dehydrogenation because the amount of 13C incorporated into
aromatics, olefins and paraffin differed greatly. The 13C in olefins
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10.1002/anie.201807814
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COMMUNICATION
MCPOs(2-cyclopenten-1-one was used as stand reference)
were not observed, which indicates that the keto C in MCPOs
comes from CO.
over the H-ZSM-5 catalyst. A new aromatization mechanism that
includes the formation of carbonyl compounds and MCPO
intermediates and that co-occurs with a drastically decreased
hydrogen transfer reaction is proposed. A portion of the carbons
in CO molecules are incorporated into aromatics, which is of
great significance for industrial applications.
Because CO participates in the formation of aromatics via
carbonyl compounds and MCPO intermediates, we propose a
new mechanism of aromatization, as shown in Fig.5 and the
reaction formula is shown in Scheme S1. First, carbonyl
compounds such as HAc and MAc are produced by a Koch-type
carbonylation reaction, which has been repeatedly proven by our
results and past studies.[7] Our work demonstrate that carbonyl
compounds reacts with olefins to generate MCPOs. As MCPOs
have been reported to easily transform into aromatic
compounds,[13] we propose MCPOs with 5-MR were transformed
into aromatics with 6-MR by dehydrolysis. Since the isotope
tracing results indicate that a portion of aromatics do not contain
carbon from CO, the conventional aromatization mechanism of
olefin hydrogen transfer co-occurs with the new mechanism.
Experimental details, catalyst characterization, enlarged part of
GC-MS chromatograms are provided in the Supporting
Information
Acknowledgements
The work was supported by the National Natural Science
Foundation of China (Grant No. 21606224) .
Keywords：heterogenous catalysis• aromatics•methanol •CO
•H-ZSM-5
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