Simultaneous conversion of methane and carbon dioxide to aromatics over silica supported chromium-based catalysts

Article abstract of DOI:10.1039/a904753k

Simultaneous activation of methane and carbon dioxide to aromatics (SAMCA) over sodium-modified silica supported chromium-based catalysts is reported.

Full text of DOI:10.1039/a904753k

Simultaneous conversion of methane and carbon dioxide to aromatics over
silica supported chromium-based catalysts
Hui Zhangab and Yuan Kou*a
a
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
E-mail: yuankou@pku.edu.cn
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b
Received (in Cambridge, UK) 14th June 1999, Accepted 29th June 1999
Simultaneous activation of methane and carbon dioxide to
aromatics (SAMCA) over sodium-modified silica supported
chromium-based catalysts is reported.
carrier) for CO, CH
columns (TCD, Argon as carrier) for H
4
, CO
2
and C
1
–C
3
, 13X and 5A sieve
, CH and CO, and a
2
4
quartz capillary column coated with SE-54 (FID) for benzene
and toluene. Both methane and carbon dioxide were 99.995%
pure.
Operating parameters significantly influence the activity of
the catalyst in the SAMCA reaction. Fig. 1(a) shows that both
the aromatics yield and the selectivity are increased by
increasing the reaction temperature. The yield and the selectiv-
ity of aromatics are also found to go through maxima at a
contact time of 0.79 h g l , indicating that deep oxidation may
occur when the contact time is > 0.79 h g l
Fig. 1(b) shows the effect of the CH /CO molar ratio on the
selectivities and the yields at 840 °C. Both the selectivity and
the yield of aromatics are enhanced with an increasing
In this decade, much attention has been paid to the aromatiza-
tion of methane under non-oxidative conditions following the
1
early work conducted by Bragin et al. Effective catalysts and
mechanisms have been recently reviewed.2 Molybdenum
oxide supported on HZSM-5 zeolite seems to be a uniquely
effective catalyst system for the aromatization of methane under
non-oxidative conditions.2 However, understanding the inter-
action of methane with the surfaces greatly depends upon
various observations. It appears, in general, that reduction and/
or carbidation of Mo species over Mo/ZSM-5 catalyst plays a
,3
–6
21
2
1
.
4
2
key role in the activation of methane to produce initial C
2
products, and the primary intermediates are then further
proportion of CH
maximum at a CH
selectivity to C2+ hydrocarbons essentially remains constant for
4
in the reactant mixture, and reach a
oligomerized to benzene on the B-acid sites within the channels
4
/CO molar ratio of 1.4. However, the
2
of ZSM-5 zeolite.2
,3,6
Recently, we have found that sodium-
modified silica supported chromium-based catalysts exhibit
considerable activity for the simultaneous activation of methane
and carbon dioxide to aromatics (SAMCA). This is the first
investigation, to our knowledge, to use non-zeolite supported
oxide catalysts for the non-free-oxygen activation of methane to
produce benzene and toluene in the presence of large amounts
of carbon dioxide.
CH
CO
4
/CO
2 4
molar ratios of > 1.1. When the molar ratio of CH /
2
is < 0.5 or > 2.5, both C2+ hydrocarbons and aromatics are
difficult to detect. It is of note that the yields of C2+
The catalysts were prepared as follows. CR18, as a typical
example, is described in detail. Given amounts of Cr
g) and/or (NH Cr (the molar ratio of Cr
NH Cr is 2+1) and NaNO (0.89 g) were slurried with
2
O
2 3
3
(0.35
O to
4
)
2
O
2 7
(
4
)
2
2
O
7
3
silica sol (42 wt% water, 45 g) for 3 h, and then dried overnight
at 120 °C. Several samples with different metal loadings and
different Cr(iii)/Cr(vi) ratios were prepared by controlling the
concentration of the sol. The resulting samples were calcined at
500 °C for 4 h and then at 800 °C for 4 h. All of the calcined
catalyst samples were crushed and sieved to 26–55 mesh. The
XPS spectrum in the Cr 2p region for the catalyst CR18 revealed
a strong peak at a binding energy of 579.0 eV and a shoulder of
this peak at a binding energy of 576.6 eV corresponding to
Cr(vi) 2p3/2 and Cr(iii) 2p3/2, respectively. The ratio of Cr(vi)/
Cr(iii) in CR18 on the basis of the XPS peak area is 5+1.
The catalytic runs were carried out at a pressure of 0.4 MPa
in a fixed-bed vertical-flow stainless steel reactor mounted
inside a tube furnace. The amount of the catalyst charged in the
reactor was 1.0 g. The catalyst was pretreated in highly pure
nitrogen (99.99%) at 840 °C for 30 min. Then, the reactant gas
mixture (CH
4
+CO
ml min . When the support (SiO
the reaction results also indicated small amounts of aromatics
yield < 0.07%, see Table 1). Isotopic trace experiments were
carried out under the same conditions but at a pressure of 0.3
2
= 7+5) was introduced at a flow rate of 22
2
1
2
) was used as a catalyst alone,
(
MPa. Pure 13CO
.3 MPa by high pressure CH
molar ratio of 7/5. The relative partial pressure of CO
CO mixture was calculated to be 1/3.
The products in the effluent stream were analyzed by an
online GC equipped with a Hayesep Q column (TCD,H as
obtained from Ba CO
and CO
13
was pressurized to
to give a CH /CO
in the
2
3
0
4
2
4
2
13
2
Fig. 1 Effect of operating parameters on the activity of the catalyst 5 wt%
2
Na–7.5 wt% Cr/SiO
in SAMCA. (a) Effect of reaction temperature (0.4
2
MPa, 0.79 h g l21, CH /CO = 7+5) and (b) effect of CH /CO molar ratio
4
2
4
2
(0.4 MPa, 0.79 h g l21, 840 °C).
2
Chem. Commun., 1999, 1729–1730
1729

Table 1 Catalytic performances of a series of chromium-based catalysts in the SAMCA reactiona
Conversion Selectivity
Cr
Na
(wt%)
S
m g
BET
2
/
Aromatics
yield (%)
21
3
21
+
Arb
7.6
23.7
38.4
48.7
41.2
1.8
Catalyst (wt%)
V/cm g
CH
4
CO
2
C
2
CO
SiO
2
0
0
0
2
5
10
2
2
10
2
204.48
202.44
10.05
3.35
46.8
46.17
2.29
0.76
0.43
2.11
2.19
0.51
2.13
0.9
2.7
2.4
3.8
4.0
1.4
2.3
1.9
2.1
5.1
21.4
10.6
16.0
7.1
11.9
0.0
4.1
4.8
4.1
3.6
4.0
30.5
23.1
80.4
76.3
57.5
46.5
54.7
94.6
65.3
64.2
74.7
0.07
0.6
0.9
1.9
1.7
0.03
0.7
0.1
CR16
CR17
CR18
CR19
CR7
CR8
7.5
7.5
7.5
7.5
11.4
8.9
7.5
7.5
1.87
9.80
10.12
2.11
9.51
c
13.5
16.6
d
30.7
5.2
2.2
e
e
CR15
CR10
0.05
a
Reaction conditions: 0.4 MPa, 840 °C, 20–23 ml min21, CH = 7/5. b Benzene + toluene. c Na–Cr(iii)/SiO . d Na–Cr(vi)/SiO . e CH
4 2 2 2 4
/CO feed only.
hydrocarbons and aromatics are very low when only methane is
fed into the reaction mixture as demonstrated by the results for
CR10 and CR15 in Table 1.
3+
6+
Variation of the Cr /Cr ratio in the preparation of the
catalyst precursors has a considerable effect on the activity of
the catalyst in SAMCA. The reaction results obtained from a
series of chromium-based catalysts are presented in Table 1.
2 3 2
Over the Cr O /SiO catalyst (CR7 in Table 1), the dominant
product is carbon monoxide (94.6%) although C2+ and
aromatics are detectable. This situation is very similar to the
7
results over a Cr
O
2 3
catalyst reported by Asami et al. except
7
VI
that they observed no aromatics. However, addition of Cr to
the Cr /SiO catalyst led to significantly increased SAMCA
O
2 3
2
reactivity. In particular, over the catalyst containing a 2+1 molar
6+
ratio of Cr3+ to Cr and modified with 5 wt% sodium promoter
(
CR18), CH
4
conversion reaches 3.8%, while selectivity to
Fig. 2 MS spectra of the products formed in the SAMCA reaction with
CO -labelled reactant.
2
13
VI
aromatics increases to 48.7%. By comparison, when only Cr
species are present on the fresh catalyst surface (CR8), the CH
4
conversion decreases to 2.6%, and the selectivity to aromatics
also decreased to 13.9%. Interestingly, when the highest
selectivity to aromatics is obtained over CR18 (5 wt% Na and
necessary for the formation of benzene, however, the definate
requirement of zeolite channels as proposed based on the Mo/
ZSM-5 system is questioned. This research may therefore lead
to some new insights on the mechanism for the aromatization of
methane.
Further work is in progress, not only to carry out the reaction
on a larger experimental scale, but also to elucidate the nature of
the mixed site-symmetry/oxidation state catalysts by XAFS
analysis.
We greatly appreciate Professor Shikong Shen at Beijing
Petroleum University for his helpful suggestions and discus-
sions. We thank Drs Changchun Yu and Chunyi Li for
experimental assistance.
3+
6+
2
+1 molar ratio of Cr to Cr ), the selectivity to CO is at a
minimum, and the molar ratio of benzene to CO is ca. 1+1. In
this case, small amounts of H were observed at a CO/H molar
ratio of 3. However, after being on-stream for 6 h, the catalyst
CR18 shows behavior more closely resembling that of Cr
CR7). Correspondingly, XPS analysis of the quenched catalyst
indicates the Cr(iii) 2p3/2 peak only. As proposed pre-
2
2
2 3
O
(
viously,8
–10
it is highly probable that mixing of octahedral and
tetrahedral site-symmetries on the surface of the supported
oxide catalyst presents catalytically active sites for the SAMCA
reaction.
Asami et al. have proposed the reaction given by eqn. (1) to
explain the formation of ethene:7
2CH
4
+ 2CO
2
? C
H
2 4
+ 2CO + 2H
2
O
(1)
Notes and references
We tentatively suggest a subsequent additional step here to
explain the formation of benzene for SAMCA in which the CO
molecules can be obtained from either reaction (1) or the partial
1 A. V. Bragin, T. V. Vasina, A. V. Preobrazhenskii and K. M. Minachev,
IZV. Akad. Nauk. SSSR Ser. Khim., 1989, 3, 750.
2
3
4
D. Wang, M. Rosynek and J. H. Lunsford, J. Catal., 1997, 169, 347.
J. Zhang, M. Long and R. Howe, Catal. Today, 1998, 44, 293.
Y. Xu, S. Liu, L. Wang, M. Xie and X. Guo, Catal. Lett., 1995, 30,
135.
reduction of CO
+ 3CO ? C
DG = 2183.2 kJ mol at 840 °C)
2
in the hydrogen atmosphere.
2C
2
H
4
H
6 6
+ CO + H
2
2
O
2
1
(
(2)
5 L. Chen, L. Lin, Z. Xu, X. Li and T. Zhang, J. Catal., 1995, 157,
90.
Y. Shu, Y. Xu, S. Wong, L. Wang and X. Guo. J. Catal., 1997, 170,
1.
1
In order to confirm whether CO
2
participates in the formation
6
of benzene, isotopic trace experiments using 13CO
in CH
co-feed have been carried out under a pressure of 0.3 MPa.
The 13CC
species, which was not observed by using the
normal CH –CO mixture as the feed gas, is identified by a peak
at m/z 79.17 in the online mass spectrum (Fig. 2), indicating that
the carbon in CO demonstrably takes part in the construction of
the benzene ring.
2
4
and
1
CO
2
7 K. Asami, T. Fujita, K. I. Kusakabe, Y. Nishiyama and Y. Ohtsaka,
Appl. Catal. A, 1995, 126, 245.
8 H. Zhang, J. Niu and Y. Kou, T. Tanaka and S. Yoshida, J. Solid State
Chem., 1998, 137, 325.
5 6
H
4
2
9
Y. Kou and H. Wang, 5th China–Japan Bilateral Symposium on
Effective Utilization of Carbon Resources (EUCR-V) Proceedings,
Chengdu, China, 1997, p. 5.
2
Despite their low reactivity, this work has demonstrated that
methane and carbon dioxide can be simultaneously activated.
The reaction producing aromatics over silica supported chro-
mium-based catalysts is accompanied by an induction period of
ca. 20 minutes. Ethene, as the primary intermediate, is
1
0 H. Zhang and Y. Kou, 5th China–Japan Bilateral Symposium on
Effective Utilization of Carbon Resources (EUCR-V) Proceedings,
Chengdu, China, 1997, p. 137.
Communication 9/04753K
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Chem. Commun., 1999, 1729–1730