G. Lopes, A. Ferreira, A. P. Gonçalves and J. B. Branco
Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal
Unidade de Ciências Químicas e Radiofarmacêuticas
The applications of molten salts have been well recognized for more than a century. In spite of the use of high temperature corrosive liquids, molten salts offer unique opportunities. Low temperature multi
component molten salts, as well as room temperature ionic liquids have been developed for materials processing. Currently, molten salts are finding applications in fuel cell technology, in the field of
separation processes of minor actinides from the rest of the fission products that are contained in the irradiated nuclear fuel, for the direct catalytic conversion of methane under mild conditions and the
partial oxidation of methane to synthesis gas [1]. Here, we report the synthesis, characterization and behaviour for the partial oxidation of methane of potassium-cerium (K-Ce) molten carbonates.
All experiments were carried out using an K2CO3-Li2CO3 (50:50 wt.%) eutectic mixture (T=600 ºC) as solvent and the cerium molten carbonates prepared by the addition to this mixture of an appropriate
amounts of Ce2(CO3)3 (5 and 15 wt.%). The molten salts were characterized by Differential Scanning Calorimetry (PAC-ATD ADI prototype, DSC recorded under argon-50 mL/min, 20 oC (hold 15 min) until
1000 oC at a 10 oC/ min heating rate), X-Ray Powder Diffraction (XRD, reflection geometry with a PANalitycal X’Pert Pro diffractometer using Cu, ka monochromatic radiation l=1.5406 Å), Infrared
Spectroscopy (IR, recorded on a Bruker spectrometer with samples mounted as Nujol mulls) as well as Elemental Analysis (C,H,N, S and O, performed on a CE instrument EA1110 automatic analyzer).
DSC tests before catalytic test
XRD patterns for the molten salts
ΔT (ºC)
Intensity (a. u.)
0
300
350
400
450
500
550
600
650
Li2CO3-K2CO3-Ce2(CO3)3 (5%)
Li2CO3-K2CO3-Ce2(CO3)3 (15%)
600
700
T (ºC)
-0.5
 XRD tests were performed before and after
the catalytic tests.
 The melting of the salts leads to the CeO2
formation.
-1
-1.5
-2
-2.5
500
400
300
T = 485 ºC
-3
200
-3.5
-4
Phase diagram for the eutectic mixture
T = 504 ºC
100
Li2CO3
K2CO3
T = 506 ºC
-4.5
Li2CO3-K2CO3
-5
Li2CO3-K2CO3-Ce2(CO3)3 (5%)
CeO2
0
Li2CO3-K2CO3-Ce2(CO3)3 (15%)
20
30
40
50
60
70
80
2 Theta (degree)
 Melting temperatures between 484 ºC and 506 ºC;
 The addition and increase of cerium % decreases the
transition temperature
IR results for the molten salts
DSC tests after catalytic test
Transmittance (%)
ΔT (ºC)
0
100
300
350
400
450
500
550
600
650
700
90
T (ºC)
-0.5
80
-1
70
60
T = 496 ºC
-1.5
50
 New features are not seen before and after
the catalytic reaction.
 Slight contraction of the CeO2 lattice is
observed.
-2
-2.5
-3
40
30
20
10
T = 502 ºC
-3.5
ν(C=O) 878 cm-1
0
T = 506 ºC
-4
ν(C-O) 1056 cm-1
Li2CO3-K2CO3
Li2CO3-K2CO3-Ce2(CO3)3 (5%)
3700
3200
2700
2200
1700
1200
700
Wavenumber (cm-1)
Li2CO3-K2CO3-Ce2(CO3)3 (15%)
Li2CO3-K2CO3-Ce2(CO3)3 (5%)
Li2CO3-K2CO3-Ce2(CO3)3 (15%)
 Characteristic bands of CO3.
The study of the Gas Hourly Space Velocity (GHSV, mL of CH4 / g of catalyst.
h) and
CH4 /are
O2 molar
ratiobefore
was undertaken.
 New
features
not seen
and after The outlet gas composition was analyzed on-line by gas chromatography
(GC) with a thermal conductivity detector (TCD). Catalyst activity was defined
as the number
of mL of methane converted per g of catalyst and per hour (mLCH4/g.h), m≈25 g.
the catalytic
reaction.
Conv. CH4 (%)
Conv. CH4 (%)
14
 Slight contraction of the CeO2 lattice is
observed.
9
8
12
Selectivity (%)
46.7
50
45
7
10
42.6
40
6
Li2CO3-K2CO3-Ce2(CO3)3 (15%)
35
8
5
30
4
6
25
3
20
4
2
2
0
2500
1
10
0
3500
4500
5500
6500
7500
8500
0
1
2
GHSV (mL/g.h)
Li2CO3-K2CO3
Li2CO3-K2CO3-Ce2(CO3)3 (5%)
3
4
5
6
7
8
9
CH4/O2 molar ratio
Li2CO3-K2CO3-Ce2(CO3)3 (15%)
Effect of GHSV at CH4/O2 molar ratio = 2
Lower GHSV and CH4/O2 molar ratio influences the catalyst
activity. CO2 is the only product of the reaction.
10.2
15
Li2CO3-K2CO3
Li2CO3-K2CO3-Ce2(CO3)3 (5%)
0
Sel. CO
Li2CO3-K2CO3-Ce2(CO3)3 (15%)
Effect of CH4/O2 molar ratio at GHSV= 8520
Sel. CO2
Sel. C2
Sel. C3
Selectivity results for GHSV = 2863 and CH4/O2 molar ratio = 2
 Conversion of methane increases with the increase
of the cerium amount.
[1] a) B. Mishra et al., Journal of Physics and Chemistry of Solids, 2005, 66, 396; b) T.R. Griffiths et al., Journal of Alloys and Compounds, 2006,
418, 116; c) J.J. Peng, et al., Applied Catalysis A: General, 2000, 201, L55; d) Y.G. Wei, et al., Journal of Natural Gas Chemistry, 2007, 16, 6.
0.4
5
Molten carbonate with 15% Ce is the only that
shows selectivities to hydrocarbons
This work was supported by FCT, under contract number PTDC/QUI/72290/2006