A highly selective and coking-resistant catalyst for methane dehydrocondensation

Article abstract of DOI:10.1246/cl.2002.418

Acid reflux-dealuminated HMCM-22 supported Mo catalyst exhibits highly selective and coking-resistant catalytic performance for methane dehydrocondensation reaction, compared with conventional catalysts. The excellent catalytic stability of the dealuminated catalyst is based on the remarkable suppression of coke formation owing to the effective decrease of Broensted acid sites, which are demonstrated by TPO, ¹H NMR and NH₃-TPD methods.

Full text of DOI:10.1246/cl.2002.418

418
Chemistry Letters 2002
A Highly Selective and Coking-Resistant Catalyst for Methane Dehydrocondensation
Yuying Shu, Ryuichiro Ohnishi, and Masaru Ichikawaꢀ
Catalysis Research Center, Hokkaido University, Kita-ku, N-11, W-10, Sapporo 060-0811
(Received December 5, 2001;CL-011229)
Acid reflux-dealuminated HMCM-22 supported Mo catalyst
DRX-400 spectrometer fitted with a MAS probe using 4-mm
ZrO2 rotors at spinning rate of 8 kHz.12;13 A sodium 4,4-dimethyl-
4-silapentane sulfonate was taken as references for 1H chemical
shifts.
XRD patterns of synthesized MCM-22 were consistent with
that reported previously.7 After dealumination treatment, the
intensities of peaks were comparable to those of the original
MCM-22. In addition, the surface area and micropore volume of
dealuminated HMCM-22 did not change as compared to those of
the parent zeolite. Accordingly, it is suggested that the
microporous structure of HMCM-22 zeolite was unchanged
before and after dealumination treatment.
It is worthy to note that the dealumination of HMCM-22 and
HZSM-5 exert a significant effect on the catalytic performance in
the methane aromatization as shown in Table 1. Compared with
6% Mo/HZSM-5 and 6% Mo/HMCM-22 catalysts, the acid
reflux-dealuminated HMCM-22 and HZSM-5 supported Mo
catalysts give much higher benzene formation selectivity owing
to the lower coke formation. Figure 1 shows the formation rates of
benzene, naphthalene, hydrogen, C2 hydrocarbons and methane
exhibits highly selective and coking-resistant catalytic perfor-
mance for methane dehydrocondensation reaction, compared
with conventional catalysts. The excellent catalytic stability of
the dealuminated catalyst is based on the remarkable suppression
of coke formation owing to the effective decrease of Bronsted
¨
acid sites, which are demonstrated by TPO, 1H NMR and NH3-
TPD methods.
The catalytic dehydrocondensation of methane towards
benzene and naphthalene for the petrochemical feed stocks and
bulky hydrogen for the fuel cell has attracted significant attention.
It has been reported that Mo or Re supported on selected
ꢀ
microporous zeolites with pore size of 5–6 A and protonic acid
sites exhibit the effective performance for methane aromatization
reaction.1{8 Nevertheless, the obvious decrease in catalytic
activity with time-on-stream due to the coke accumulation on
catalyst surface is still the main obstacle for the potential
industrialization of this process. To solve this problem, we have
previously reported that the addition of a few percent CO and CO2
to methane feed made a dramatic enhancement of the catalyst
stability on Mo/HZSM-5 and Re/HZSM-5.3;9;10 Here we report an
alternative but simple method to suppress the coke formation by
pre-dealumination of HMCM-22 zeolite using an acid reflux
treatment. Significant improvement in catalytic stability and
selectivity towards benzene has been realized on this deal-
uminated HMCM-22 supported Mo catalyst owing to the efficient
suppression of coke formation.
The parent MCM-22 zeolite was synthesized using hexam-
ethyleneimine as a template.7 The dealumination treatment of
HMCM-22 zeolite was performed with aqueous 6 M HNO3
solution at reflux temperature.11 As a result;the Si/Al atomic ratio
increased from 15 to 19. Six weight percent loading of Mo
supported with parent and dealuminated HMCM-22 zeolite were
prepared by wet impregnation method,7 and denoted as 6% Mo/
HMCM-22 and 6% Mo/HMCM-22-(D), respectively. The
methane dehydrocondensation reaction was carried out in a
continuous flow system at 973–1023 K, 0.3 MPa and 15 ml minꢁ1
of methane flow rate. All the products were on-line analyzed by
two GCs with FID and TCD as described in previous report.3;9;10
1H MAS NMR spectra of samples were measured on a Bruker
Figure 1. Catalytic performance of 6% Mo/HMCM-22-D
catalyst at 1023 K, 0.3 MPa and 2700 ml gꢁ1 hꢁ1 versus time
on stream; þ, ꢃ, Ã, 4, and } represent methane conversion,
formation rates of benzene, naphthalene, C2 hydrocarbons, and
hydrogen (ꢄ1=2), separately. Solid circle (l) represents
formation rate of benzene on 6% Mo/HMCM-22 catalyst under
same reaction conditions.
Table 1. The typical reaction results of Mo supported zeolite catalysts before and after the dealumination treatmenta
Sample
CH4 conv./%
Rate of product formation/103 nmol g-catꢁ1ꢂsꢁ1
Selectivity of product/%
C6H6
C10H8
Coke
C6H6
C10H8
Coke
6%Mo/HMCM-22-(D)
6%Mo/HMCM-22
6%Mo/HZSM-5-(D)
6%Mo/HZSM-5
6.5
7.5
6.7
6.9
1.1
1.2
1.1
1.0
0.2
0.1
0.4
0.3
0.5
1.1
0.4
0.8
52.0
46.2
47.3
41.8
9.0
3.7
18.2
13.1
22.7
43.4
18.1
35.1
a973 K, 0.3 MPa, 2700 ml gꢁ1 hꢁ1, and the data were taken at the stable period of 240 min time on stream. (D): dealuminated zeolite.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
419
conversion on the 6% Mo/HMCM-22-(D) catalyst at 1023 K,
0.3 MPa and 2700 ml gꢁ1hꢁ1, in comparison with those of
benzene on the 6% Mo/HMCM-22. As seen, the initial formation
rate of benzene on both catalysts is around 1500 nmol g-catꢁ1 sꢁ1
.
However, the formation rates of benzene on the 6% Mo/HMCM-
22 declined drastically after 10 h of time on stream and eventually
reached to 10% of its initial value. In contrast, the 6% Mo/
HMCM-22-(D) showed quite stable performance by keeping the
benzene formation rate at 85% level for the prolonged 24 hours.
This high stability of the methane aromatization reaction at high
temperature as 1023 K under pure methane feed without any
additives such as CO and CO2 has never been reported before.
TPO experiments of coked 6% Mo/HMCM-22-(D) and 6%
Mo/HMCM-22 were conducted and the results are displayed in
Figure 2. There are two peaks existed on coked 6% Mo/HMCM-
22, the low temperature oxidation peak was attributed to carbon
associated with molybdenum while the high temperature peak to
Figure 3. 1H MAS NMR spectra of parent HMCM-22 (bold
line) and dealuminated HMCM-22 (narrow line), respectively.
the carbonaceous deposits on the zeolite Bronsted acid sites.3;9
¨
After dealumination treatment, the amount of coke deposited on
the Bronsted acid sites decreased greatly and less pronounced.
¨
for catalyst modification efficiently suppresses the coke forma-
tion in methane dehydrocondensation reaction owing to the
distinct decrease of the Bronsted acid sites. The higher catalytic
¨
activity and longer stability are achieved as compared with
conventional catalysts.
This work is partially supported by the New Energy and
Industrial Technology Development Organization (NEDO) of
Japan. The authors thank Professors of Xinhe Bao, Xiuwen Han
and Yide Xu, Dalian Institute of Chemical Physics for their kind
help in NMR experiment.
References
1
2
3
4
5
6
7
8
9
L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, and Y. Xu, Catal.
Lett., 30, 135 (1995).
D. Wang, J. H. Lunsford, and M. P. Rosynek, Topics Catal., 3,
289 (1996).
S. Liu, Q. Dong, R. Ohnishi, and M. Ichikawa, Chem.
Commun., 1997, 1455.
R. W. Borry, E. C. Lu, Y. H. Kim, and E. Iglesia, Stud.Surf.
Sci.Catal. , 119, 403 (1998).
C. Zhang, S. Li, Y. Yuan, W. Zhang, T. Wu, and L. Lin, Catal.
Lett., 56, 207 (1998).
L. Wang, R. Ohnishi, and M. Ichikawa, Catal.Lett. , 62, 29
(1999).
Y. Shu, D. Ma, L. Xu, Y. Xu, and X. Bao, Catal.Lett. , 70, 67
(2000).
R. Ohnishi, L. Xu, K. Issoh, and M. Ichikawa, Stud.Surf.Sci.
Catal., 136, 393 (2001).
Figure 2. TPO profiles of coked 6% Mo/HMCM-22 (a) and
6% Mo/HMCM-22-D (b) after reaction at 1023 K for 24 h.
(TPO experiments were carried out using 30 mg of catalyst at a
10% O2/He flow rate of 15 ml minꢁ1 and temperature raising
rate of 10 K minꢁ1.)
1H MAS NMR experiments were carried out to study the
variation of zeolite acidity before and after dealumination
treatment, and the corresponding spectra are shown in Figure 3.
As presented, HMCM-22 gave peaks at 1.6, 3.7 and 6.0 ppm
which were reasonably attributed to silanol group, framework
Al(Si)-bridging OH groups (Bronsted acid sites) and water
¨
R. Ohnishi, S. Liu, Q. Dong, L. Wang, and M. Ichikawa, J.
Catal., 182, 92 (1999).
adsorbed on Lewis sites (another kind of Bronsted acid sites),
¨
respectively.12;13 It is demonstrated that the Bronsted acid sites of
¨
10 L. Wang, R. Ohnishi, and M. Ichikawa, J.Catal. , 190, 276
(2000).
11 P. Wu, T. Komatsu, and T. Yashima, Micropor.Mesopor.
Mater., 22, 343 (1998).
12 Y. Shu, D. Ma, X. Liu, X. Han, Y. Xu, and X. Bao, J.Phys.
Chem.B , 104, 8245 (2000).
13 D. Ma, Y. Shu, X. Han, X. Liu, Y. Xu, and X. Bao, J.Phys.
Chem.B , 105, 1786 (2001).
HMCM-22 are suppressed drastically after dealumination treat-
ment. The following NH3-TPD experiments also confirmed the
effective decrease of strong acid sites after such treatment. Basing
on those findings, we can propose that the remarkable suppression
of coke formation on the dealuminated HMCM-22 supported Mo
catalyst is related to an effective decrease of the Bronsted acid
¨
sites.
In conclusion, the acid reflux pre-dealumination treatment