selectivities under no-diluent conditions. The CH4 conversion
and C2 selectivity increased with increasing reaction temper-
ature up to 1073 K, whereas COx selectivity decreased. These
findings could be explained, at least partly, by a difference in the
activation energy between C2 and COx formation.5
The effect of catalyst components were examined using other
acid sources and other alkali metals. In the Li-doped acid-
promoted ZrO2 catalysts, the effect of acid precursors other than
(NH4)2SO4 was in the order: (NH4)2SO4 (25.8) > 0.5 m H2SO4
doped catalyst. Over catalysts promoted with 10 mass% of Li,
both CH4 conversion and C2 selectivity were slightly reduced
and, as a result, the C2 yield decreased to ca. 19%.
The effectiveness of the Li-doped sulfated ZrO2 catalysts on
the oxidative coupling of CH4 is still not clear. Since the effect
of Li doping on MgO support has been already reported,6 it
seems likely that the preparation of sulfated ZrO2 surface is a
key step in our catalyst system. In fact, the catalyst perform-
ances depend on the sulfate content (Fig. 2) and calcination
temperature: a maximum C2 yield is attained over the catalysts
which contain 6 mass% sulfate and are calcined at 923–973 K,
being closely related to the preparation conditions of sulfated
ZrO2 as solid superacids.7 If so, sulfated metal oxides other than
ZrO28 might be also effective as supports. We found that, for an
Li-doped sulfated SnO2 catalyst,§ 82.4% C2 selectivity at 1073
K is achieved at 30.5% CH4 conversion. Also, a ZrO2 catalyst,
impregnated with an aqueous solution containing both
(NH4)2SO4 and Li2CO3, followed by calcination in air at 873 K
for 3 h, showed much lower reactivity than that prepared by the
two-step method. By investigating all these findings, it is
concluded that Li doping over a super-acid surface is required
for the generation of high catalytic performances for the ZrO2-
based OCM reaction reported here.
(25.1)
> NH4Cl (22.9) > trimethyleneborate (22.3) >
(NH4)3PO4 (19.3) > NH4NO3 (16.5) > none (8.7). (The values
in parentheses represent the C2 yield at 1073 K.) For the Na- and
K-doped sulfated ZrO2 catalysts, the CH4 conversion was
almost the same as for the Li-doped catalysts, but C2 selectivity
decreased to ca. 50%, nearly 30% lower than that for the Li-
100
80
60
Further work is in progress not only to carry out reaction on
larger experimental scales, but also to elucidate the nature of the
Li-doped solid super-acid catalysts.
40
20
0
Footnotes
† In this run, after reaction for 5 h at 1073 K, methane conversion was
decreased (43.3 to 39%), whereas C2 selectivity was slightly increased (79.7
to 82.1%).
850
950
1050
‡ In this run, the amount of oxygen required to reach 30.7% CH4 conversion
could approximately correspond to that contained in the feedstock, if we
assume the following reaction steps including thermal cracking of ethane or
ethene to form ethene or acetylene, although further investigation will be
required for the reaction scheme. (1) 2CH4 + 1/2O2 ? C2H6 + H2O, (2)
C2H6 ? C2H4 + H2, (3) C2H4 ? C2H2 + H2, (4) CH4 + 3/2O2 ? CO +
2H2O, (5) CO + 1/2O2 ? CO2, (6) CH4 + H2O ? CO + 3H2. In fact, in this
run, the expected amount of hydrogen was observed by GC analysis. Also,
in a separate experiment using the same catalyst, we confirmed that efficient
C2H4 formation from C2H6 was observed even at 1023 K in the absence of
O2, consistent with a thermodynamical calculation.
T / K
Fig. 1 Effect of the reaction temperature on the oxidative coupling of
methane over 5% Li-doped 6 mass% SO422/ZrO2 catalysts without diluent.
Conditions: CH4 (90%)–O2 (10%), feed 50 ml min21, catalyst 0.5 g. CH4
conversion (2); selectivities: C2 (5), COx (8).
30
20
10
0
§ SnO2 impregnated with 6 mass% (NH4)2SO4 was calcined at 873 K for
3 h.
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10
SO
20
30
40
0
2–
Content (mass %)
4
Fig 2 Effect of sulfate content on the product yields. Conditions: CH4
(15%)–O2 (5%)–N2 (80%), feed 50 ml min21, 1073 K. The mass% of
sulfate content is defined as the ratio of SO4 to ZrO2. Selectivities: C2
22
(2), COx (5).
Received, 27th September 1996; Com. 6/06624K
222
Chem. Commun., 1997