Selective synthesis of C2-C4 olefins and C5+ hydrocarbons over unpromoted and cerium-promoted iron catalysts supported on ion exchanged (H, K) zeolite-Y

Article abstract of DOI:10.1039/a900457b

Hydrogenation of CO₂ to hydrocarbons is carried out on unpromoted and Ce-promoted iron catalysts supported on ion exchanged (H, K) zeolite-Y; the results suggest that the Fe-Ce/KY catalyst has significant advantages being highly selective for C₂-C₄ olefins and C₅₊ hydrocarbons.

Full text of DOI:10.1039/a900457b

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344 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 344^345y
Selective Synthesis of C2 C4 Olefins and C5‡
Hydrocarbons over Unpromoted and
Cerium-promoted Iron Catalysts Supported on
Ion Exchanged (H, K) Zeolite-Yy
Sang-Sung Nam,* Gurram Kishan, Myung-Woo Lee,
Myoung-Jae Choi and Kyu-Wan Lee
Chemical Technology Division-I, Korea Research Institute of Chemical Technology, P.O. Box 107,
Yusong, Taejon 305-600, Korea
Hydrogenation of CO2 to hydrocarbons is carried out on unpromoted and Ce-promoted iron catalysts supported on
ion exchanged (H, K) zeolite-Y; the results suggest that the Fe^Ce/KY catalyst has significant advantages being
highly selective for C2 C4 olefins and C5‡ hydrocarbons.
The environmental impact of high concentrations of CO2 in
the atmosphere has been of acute concern to the global
community.1 One of the ways to mitigate this problem is
to convert CO2, at the generation point, into valuable indus-
trial feed stocks such as lower ole¢ns and commercially
important high molecular weight hydrocarbons. Lower
alkenes, such as ethene and propene, are primary feedstocks
for the manufacture of polymers and petrochemicals. Pro-
duction of higher hydrocarbons, in particular the long chain
ones, has immense signi¢cance, considering that many
industrial R&D teams are working towards this goal. Long
chain hydrocarbons, particularly their branched counterparts,
can replace aromatics as octane boosters. Iron based catalysts
double isotherm method, after reducing the catalyst at 723 K
for 16 h in hydrogen. Hydrogenation of CO2 was carried
out in a ¢xed bed £ow reactor made of stainless steel. Prior
to the initiation of the reaction the catalyst was reduced
in situ in hydrogen at 723 K for 24 h. The reaction was car-
ried out at 573 K under 10 atm, using a reactant gas mixture
in the ratio H2/ : CO2 ˆ 3 : 1 at
a
£ow rate of
1900 ml g 1 h 1. Products were analysed by on-line GC using
both FID and TCD with the help of GS-Q capillary and
Porapack Q columns respectively.
Reaction results are compared in Table 1. Methane was
found to be the major product on Fe/HY catalyst. The
CO2 conversion was also not that high, the major part being
converted into CO and saturated hydrocarbons. The presence
of potassium not only increased the conversion of CO2,
tremendous improvements also being observed in selectivities
to desired products such as ole¢ns and higher hydrocarbons.
This clearly shows that the potassium exchanged catalyst
has a bifunctional character as it not only increases the con-
version but also alters the hydrocarbon distribution. On
the other hand, the highly acidic Fe/HY catalyst produced
mainly methane. The high selectivity for methane can be
attributed to the low concentration of active carbon on
the surface, which is due to low dissociation of CO2 for
acidic catalysts.10 The presence of potassium in Fe/KY is
expected to enrich the local electron density of the active iron
metal.11 There are several reports in the literature of
improved selectivities upon potassium addition to iron
catalysts.2;3 It is speculated that potassium strengthens the
have shown great promise in the hydrogenation of CO and
4
CO2.2
Potassium has been reported to be an e¡ective
promoter, as it facilitates elementary steps, reverse water-gas
4
shift (RWGS) and Fischer^Tropsch (FT) reactions.2 Lee
et al.2;3 have reported higher selectivities for the formation
of lower ole¢ns on unsupported Fe^K catalysts in the
CO2 hydrogenation reaction. This selectivity enhancement
was attributed to the formation of iron carbides on these cat-
8
alysts. There were several reports5 that the presence of rare
earth metal oxides like La and Ce exert an in£uence on
CO hydrogenation over supported metal catalysts. For
instance, Barrault et al.5 reported that lanthanum or cerium
oxide promoters improved the total activity and increased
the selectivity to alkenes and higher hydrocarbons on car-
bon-supported cobalt and ruthenium catalysts. However,
there were no reports pertaining to CO2 hydrogenation
(unlike CO hydrogenation), in which selective synthesis of
both the lower ole¢ns and long chain hydrocarbons has been
achieved. The present study reports selective synthesis of
C2 C4 ole¢ns and C5‡ hydrocarbons on Ce-promoted iron
catalysts supported on potassium exchanged zeolite-Y. The
results have been compared with the results obtained on
unpromoted catalysts.
Ta b l e 1 CO2 hydrogenationa over unpromoted and Ce-promoted
iron catalysts supported on HY and KY zeolites
Catalyst (17 wt%)
CO2 conversion
Fe/HY
10.14
Fe/KY
17.95
Fe-Ce/HY Fe-Ce/KY
11.58
20.08
The ion exchanged (H, K) zeolite-Y was prepared as
described elsewhere9 and the crystallinity of the thus
obtained material was veri¢ed by XRD. To prepare
unpromoted and Ce-promoted catalysts, the H, K-Y zeolites
were impregnated with corresponding aqueous solutions of
iron nitrate and iron nitrate ‡ cerium nitrate (17 wt% Fe,
atomic ratio of Fe:Ce was 9:1) through the incipient wetness
method. Thus obtained catalysts were dried at 383 K for
16 h and calcined in air at 773 K for 24 h. Chemisorption
measurements of CO2 were carried out at 298 K by the
Selectivity (C mol%)
CO
HC
39.14
60.86
31.35
66.49
31.33
68.67
34.61
65.39
Hydrocarbon distribution (C mol%)
C1
72.56
0.02
15.23
0.07
7.64
ö
12.54
8.94
3.18
13.02
3.24
10.40
3.83
70.48
0.02
10.23
4.96
ö
2.73
0.74
10.83
8.86
8.73
2.21
15.79
ö
12.19
2.62
49.59
ˆ
C2
C2
ˆ
C3
C3
ˆ
C4
C4
C5‡
2.98
1.49
44.50
* To receive any correspondence.
Selectivity for olefins (C2 C4) (C mol%)
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
Ol./(Ol. ‡ Para.)
0.36
75.93
41.29
88.37
1
aCO2 hydrogenation at 1900 ml g
h
1, 573 K, and 10 atm.

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J. CHEM. RESEARCH (S), 1999 345
arrangement results in the weakening of the C^O bond,
and so in enhanced dissociation of CO2. As discussed above,
the potassium will also enhance the dissociation of CO2. The
combined e¡ect of potassium ion exchanged Y zeolite and
cerium oxide has enhanced the selectivities of C2 C4 ole¢ns
and C5‡ hydrocarbons. Another feature of these catalysts is
reduction of methane, which is also a greenhouse gas. As
can be seen from Fig. 1, there is some trend of CO2 uptake
and selectivity of methane, C5‡ hydrocarbons and C2 C4
ole¢ns. Formation of large amounts of C5‡ hydrocarbons
on cerium-promoted basic Y zeolites may be explained
through polymerization of (CHx†
species followed by
ad
hydrogenation of the surface hydrocarbon precursors.15 Here,
both an increase of the rate of C-O bond dissociation and/ or
a decrease of the hydrogenation rate of surface carbon species
can contribute to the enhanced formation of long chain
hydrocarbons. Moreover, a decreased hydrogenation rate,
which could be related with a lower population and mobility
of hydrogen atoms over the oxide promoted catalysts, may
facilitate the formation of ole¢nic hydrocarbons. Lower
hydrogen uptake over these catalysts further supports this
argument. Thus our results clearly show that though cerium
alone is not a good promoter, its coexistence with potassium
indeed improves the performance in selective synthesis of
C5‡ hydrocarbons and C2 C4 ole¢ns in CO2 hydrogenation.
Fig. 1 Correlation between CO2 chemisorption and hydrocarbon
distribution over unpromoted and Ce-promoted catalysts supported
on (H, K) zeolite-Y
bond between iron and carbon, while simultaneously weak-
ening the carbon^oxygen bond, facilitating easier formation
of a C-H bond. Some reports suggest that the ole¢n forma-
tion is initiated through an iron carbide mechanism.2;3
Our temperature programmed decarburization results concur
with those observations, as increased quantities of carbide
species has been observed on Fe/KY catalysts. Higher uptake
of CO2 (Fig. 1) is a further proof of the enhanced basicity of
the potassium promoted catalyst.
Received, 18th January 1999; Accepted, 10th February 1999
Paper E/9/00457B
The CO2 conversion, selectivity to CO and total
hydrocarbons on Fe-Ce/HY catalyst was not signi¢cantly dif-
ferent to that with Fe/HY catalyst. Moreover, the CO2 con-
version increased marginally and the selectivity of ole¢ns
increased markedly from 0.36 to 41.29. This enhancement
might be due to formation of new active sites. Barrault et
al.5 also observed the enhancement of selectivity for
C2 C4 ole¢ns with cerium promoted CO catalysts supported
on carbon. Cerium promoted iron supported on a KY cata-
lyst showed improved selectivity for C2 C4 ole¢ns and high
molecular weight hydrocarbons compared with that of Fe/KY
catalysts. Some sort of synergy appears to be responsible for
this enhanced selectivity of both C2 C4 ole¢ns and C5‡
hydrocarbons, as cerium promotion alone (in Fe^Ce/HY)
did not result in this enhancement. The promotion e¡ect
of cerium oxide might be due to partially reduced cerium
oxide, which provides binding sites for the oxygen ends of
the carbon dioxide molecule facilitating its dissociation.
These results agree with those on ruthenium/alumina
catalysts promoted by cerium oxide.12;13 The promotion e¡ect
has been explained in terms of newly formed active sites
(metal^promoter ensembles) at which the carbon monoxide
is bonded with its carbon atom to a metal atom and with
its oxygen atom to a partially reduced promoter ion.14 This
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