The inhibition effect of potassium addition on methane formation in steam reforming of acetic acid over alumina-supported cobalt catalysts

Article abstract of DOI:10.1246/cl.2008.614

The inhibition effect of potassium addition on methane formation in steam reforming of acetic acid over alumina-supported cobalt catalyst has been studied. Co-K/Al₂O3 catalyst showed much higher activity for hydrogen generation and much lower s

Full text of DOI:10.1246/cl.2008.614

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Chemistry Letters Vol.37, No.6 (2008)
The Inhibition Effect of Potassium Addition on Methane Formation in Steam Reforming
of Acetic Acid over Alumina-supported Cobalt Catalysts
1
;2
ꢀ1
Xun Hu and Gongxuan Lu
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, P. R. China
1
2
Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China
(Received February 4, 2008; CL-080131; E-mail: gxlu@lzb.ac.cn)
The inhibition effect of potassium addition on methane for-
mation in steam reforming of acetic acid over alumina-supported
cobalt catalyst has been studied. Co–K/Al2O3 catalyst showed
much higher activity for hydrogen generation and much lower
selectivity for methane than Co/Al2O3. Potassium addition
resulted in the inhibition of methanation process. The similar
effect was also observed in methanol and ethanol reforming
reactions.
2
Co–K/Al O3
Co/Al O3
2
50
150
250
350
450
550
650
750
Reduction temperature/°C
Currently renewable source to hydrogen is a very attractive
1
Figure 1. H2-TPR profiles for bare and modified Co catalysts.
topic owing to the fast depletion of fossil fuel. In general, biooil
is a complex mixture of organic compounds including many or-
formula: SCH (%) = 100 ꢃ (mole of CH4 generated)/(mole
4
2
ganic acids. Acetic acid is one of the main components in bio-
3
of acetic acid consumed ꢃ 2). The selectivities to others were
calculated in the similar way. Temperature-programmed reduc-
tion analysis (H2-TPR) was carried out by heating a sample
(30 mg) in a flow of 5 vol % H2/Ar mixture (40 mL/min).
Results of H2-TPR for Co/Al2O3 and Co–K/Al2O3 cata-
lysts were shown in Figure 1. The addition of potassium induced
a shift of the main reduction peak of cobalt oxide upwards to
higher temperature. However, the H2 uptake for Co–K/Al2O3
was much higher than that of Co/Al2O3. Higher H2 uptake im-
plied existence of more reducible Co species on catalyst surface,
oil. In addition, acetic acid is nonflammable; therefore, it is a
safe hydrogen carrier. Steam reforming of acetic acid always
gave significant amount of by-products such as methane, which
4
resulted in low hydrogen yield. Methane formation is highly de-
pendent on the amount and the density of acidic centers on cat-
alyst support,5 neutralization of acidic center by basic species
may lead to the significant variation of catalyst ability of inhib-
iting methane formation. In this paper, potassium was added to
the support to modify the properties of alumina support. Com-
paring studies on Co/Al2O3 and Co–K/Al2O3 catalysts, the re-
markable inhibition effect of potassium on methane formation
was found.
,6
7
which was active for steam-reforming reaction. It was reported
that potassium could lead to alumina carrier passivated and
8
became less reactive. Therefore, potassium addition resulted
in the significant enhancement of cobalt oxide reducibility in
Co–K/Al2O3.
Co/Al2O3 was prepared by impregnation method using
.
Co(NO3)2 6 H2O as a precursor. The cobalt loading was
3
(
0 wt % to Al2O3. Before impregnation, the support ꢀ-Al2O3
The catalytic properties of Co/Al2O3 and Co–K/Al2O3
2
ꢁ
ꢁ
129 m /g, 30–45 mesh) was stabilized in air at 600 C for 6 h.
After impregnation, the catalyst precursor was dried at room
were examined in the temperature region of 300–600 C. Com-
ꢁ
plete conversion of acetic acid was achieved above 450 C over
Co/Al2O3, the corresponding H2 selectivity was 80%. Below
ꢁ
temperature for 24 h and at 110 C for another 24 h. Finally,
ꢁ
ꢁ
the catalyst precursor was calcined at 500 C for 4 h. Co–K/
450 C, the activity of Co/Al2O3 was quite low. Pronounced
Al2O3 was prepared by coimpregnation method using a mixed
solution containing both Co(NO3)2 and KNO3. Potassium load-
ing amount was 8 wt %.
amount of by-products such as methane, CO, acetone, and
ketene were formed, which resulted in low H2 yield. The catalyt-
ic performance of Co/Al2O3 can be significantly improved
ꢁ
Catalytic tests were carried out in a fixed bed continuous
flow quartz reactor at atmospheric pressure. Typically, 0.5 g of
catalyst diluted with equal amount of quartz was reduced at
by the potassium addition at 350 C, as presented in Figure 2.
Acetic acid was converted completely even at temperature of
ꢁ
350 C over Co–K/Al2O3. Besides, much higher H2 selectivity
ꢁ
ꢁ
6
00 C for 3 h in situ with a 50 vol % H2/N2 mixture (flow rate:
(93.5%) was obtained at 350 C. The productions of the by-prod-
ucts were also remarkably suppressed compared to Co/Al2O3
catalyst. Very interestingly, only trace amount of methane was
60 mL/min) prior to experiment. The reaction mixture was fed
into a preheater by a syringe pump with a liquid hourly space
ꢂ1
ꢁ
velocity (LHSV) of 10.1 h under steam to carbon ratio (S/C)
found over Co–K/Al2O3 at 350 C. Moreover, in the whole
of 7.5:1. Product was analyzed by two on-line chromatographs
equipped with a thermal-conductivity detector (TCD) and a
flame ionization detector (FID). H2 selectivity was defined as
the fraction of H2 produced with respect to the H2 of theoretical
datum of full conversion of acetic acid (CH3COOH þ 2H2O !
range of tested temperature, much lower methane selectivity
was achieved over Co–K/Al2O3 (as shown in Figure 3). Since
production of 1 mol of methane will consume 1 mol of acetic
acid and result in loss of 4 mol of hydrogen in acetic acid reform-
ing reaction, the suppression of methane formation is very
important in enhancement of hydrogen yield.
4
H2 þ 2CO2). Selectivity to methane was defined by the
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.6 (2008)
615
1
00
Co/Al O
2 3
Xacetic acid
40
30
20
SH
2
Co–K/Al O3
2
8
6
4
2
0
0
0
0
0
SCO
2
SCH
4
S
CO
Sacetone
Sketene
10
0
300
350
400
450
500
550
600
Co/Al O3
Co–K/Al O3
Reaction temperature/°C
2
2
Figure 2. Acetic acid conversion and product selectivity at
ꢁ
50 C over Co/Al2O3 and Co–K/Al2O3.
Figure 4. Methanation reactions over Co/Al2O3 and Co–K/
Al2O3.
3
1
0
8
6
4
2
0
4
3
2
0
0
0
Co/Al O3
Co/Al
Co–K/Al O
2 3
2 3
O
2
Co–K/Al O3
2
10
0
300
350
400
450
500
550
600
Reaction temperature/°C
Methanol reforming
Ethanol reforming
Figure 3. CH4 selectivity vs. reaction temperature over Co/
Al2O3 and Co–K/Al2O3.
Figure 5. CH4 selectivity in methanol- or ethanol-reforming
reactions over Co/Al2O3 and Co–K/Al2O3.
In acetic acid reforming process, acetic acid decomposition
may occur according to the following way: CH3COOH !
CH4 þ CO2. In addition, methanation of carbon oxides may lead
methanol- or ethanol-reforming reactions was also studied
under the similar conditions. As shown in Figure 5, the selectiv-
ity for methane was significantly lower in both methanol- and
ethanol-reforming reactions over Co–K/Al2O3 than over Co/
9
to methane formation, while the steam reforming of methane
ꢁ
may decrease the detected amount of methane. However, our
blank tests indicated that both catalysts exhibited very low
activity for methane steam reforming at temperature below
Al2O3 at 400 C.
To the conclusion, potassium could promote the reduction
of cobalt oxide, resulting in the remarkable increase of the
low-temperature reforming activity of Co/Al2O3. The produc-
tion of methane in steam reforming of acetic acid, methanol,
and ethanol could be significantly reduced with potassium
addition to Co/Al2O3 because of the inhibition effects of potas-
sium on the methanation reactions.
ꢁ
600 C; therefore, steam reforming of methane was not consid-
ered in this study.
For proving decomposition reaction hypothesis, we carried
out an acetic acid decomposition experiment using pure acetic
ꢂ1
ꢁ
acid with a LHSV of 5.0 h at 400 C. Co–K/Al2O3 gave a
similar acetic acid conversion and methane selectivity to Co/
Al2O3 catalyst. Evidently, acetic acid decomposition was not
the main reason for the low CH4 selectivity over Co–K/Al2O3.
The methane formation was probably inhibited by potassium in
reforming reaction. Thus, the methanation reaction was subse-
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Published on the web (Advance View) May 3, 2008; doi:10.1246/cl.2008.614