Direct conversion of calcium carbonate to C1-C3 hydrocarbons

Article abstract of DOI:10.1039/c3ra40264a

With the objective of investigating the direct conversion of inorganic carbonates such as CaCO₃ to hydrocarbons, assisted by transition metal ions, we have carried out studies on CaCO₃ in an intimate admixture with iron oxides (FeCaCO) with a wide range of Fe/Ca mole ratios (x), prepared by co-precipitation. The hydrogen reduction of FeCaCO at 673 K gives up to 23% yield of the hydrocarbons CH₄, C₂H₄, C₂H₆ and C₃H₈, leaving solid iron residues in the form of iron metal, oxides and carbide particles. The yield of hydrocarbons increases with x and the conversion of hydrocarbons occurs through the formation of CO. While the total yield of hydrocarbons obtained by us is comparable to that in the Fischer-Tropsch synthesis, the selectivity for C ₂-C₃ hydrocarbons reported here is noteworthy. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra40264a

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COMMUNICATION
Direct conversion of calcium carbonate to C –C3
1
hydrocarbons3
Cite this: RSC Advances, 2013, 3, 7224
a
a
b
a
D. Jagadeesan, Y. Sundarayya, Giridhar Madras and C. N. R. Rao*
Received 17th January 2013,
Accepted 11th March 2013
DOI: 10.1039/c3ra40264a
www.rsc.org/advances
7
–13
With the objective of investigating the direct conversion of
inorganic carbonates such as CaCO to hydrocarbons, assisted by
transition metal ions, we have carried out studies on CaCO in an
chemical utilization of CO2 receives wider attention.
If an
3
efficient CO2 capture process and its subsequent chemical
transformation is developed, the cost of energy production can
3
intimate admixture with iron oxides (FeCaCO) with a wide range
of Fe/Ca mole ratios (x), prepared by co-precipitation. The
hydrogen reduction of FeCaCO at 673 K gives up to 23% yield
be significantly lowered. CO is sequestered in nature as solid
carbonate minerals, and these mineral rocks are abundant.
Inorganic carbonate minerals and the large volumes of industrial
2
of the hydrocarbons CH
4
, C
2
H
4
, C
2
H
6
and C
3
H
8
, leaving solid iron
CO which are being pumped into ocean beds as carbonates may
too be utilized profitably by converting them into hydrocarbons.
2
14
residues in the form of iron metal, oxides and carbide particles.
The yield of hydrocarbons increases with x and the conversion of
hydrocarbons occurs through the formation of CO. While the total
yield of hydrocarbons obtained by us is comparable to that in the
Inorganic mineral carbonates have the potential to become a
renewable resource for the production of fuels and chemicals as
they can be rapidly replenished by the carbonation process. Fossil
fuels such as coal and petrol are non-renewable for human use
because of the long lag time of their natural formation. On the
Fischer–Tropsch synthesis, the selectivity for C
reported here is noteworthy.
2 3
–C hydrocarbons
2
contrary, the rapid sequestration of CO to inorganic carbonates
will allow us to replenish it as a carbon resource quickly enough to
meet global energy demands, thereby leading to a steady state-like
Introduction
condition. The process of accelerating the reaction of CO
2
with
An important present-day global priority is to identify renewable
sources of energy that can meet the demands of economic growth.
It is important to remember that a serious emphasis is placed on
the environmental sustainability of any new process associated
with energy production. The combustion of fossil fuels, a major
alkaline oxides and silicates to form carbonates is moving towards
15
commercialization. The use of H
of CaCO to hydrocarbons is inevitable and can be considered a
non-green process. One hopes that it is not too optimistic to
assume the production of H will become an environmentally
benign commercial reality in the future, for instance, through the
photocatalytic splitting of H O. In this report, we discuss the
hydrogenation of CaCO3 to form C –C hydrocarbons, in the
2
during the catalytic reduction
3
2
source of energy in the present-day economy, releases a staggering
1
amount of CO into the atmosphere leading to adverse effects on
2
2
climate and the environment. A promising approach to addressing
the problems of energy availability, as well as environmental
1
3
presence of iron catalysts, a process which has the great potential
to be developed further.
As part of our studies to convert inorganic carbonates to
3
hydrocarbons, we have demonstrated that CaCO containing
pollution, is to recycle CO
2
to produce commercially important
This approach is realistic and would help in
mitigating the increase in atmospheric CO levels, and reduce
our dependence on oil by producing carbon-neutral fuels and
1–4
chemicals.
2
5,6
transition metal ions such as Fe, Co and Ni can be transformed to
feedstock chemicals. Since many of the materials for CO2
sequestration may not be of practical use, it is important that the
16
CH in the presence of H . Under the experimental conditions
4
2
16
4
employed in this work, CH was the only major product and the
a
best catalyst contained Co, CoO and CaO. The earlier study
employed a reaction pressure of 2–5 atm and a hydrogen-rich
International Centre for Materials Science and CSIR Centre of Excellence in
Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O.,
Bangalore-560064, India. E-mail: cnrrao@jncasr.ac.in; Fax: 91 80 2365 3075;
Tel: 91 80 2208 2760
atmosphere which drove the reaction to form CH exclusively.
4
Interestingly, among the transition metal ions tested, iron
appeared to show the formation of hydrocarbons (HCs) beyond
b
Department of Chemical Engineering, Indian Institute of Science, Bangalore-
5
60012, India
4
CH . The catalytic role of iron-based catalysts in the production of
3
Electronic supplementary information (ESI) available: XRD, M ¨o ssbauer, XPS, time
on stream. See DOI: 10.1039/c3ra40264a
long chain HCs is known in the Fischer–Tropsch (FT) synthesis
7
224 | RSC Adv., 2013, 3, 7224–7229
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1
7,18
which utilizes CO + H
2
as the feed.
There is a renewed interest
Catalytic reaction
In a typical experiment, 0.04 g of Fe containing CaCO
was heated in a continuous flow, packed bed, stainless steel
reactor at a temperature ranging between 573 and 873 K in H gas
flowing at a rate of 3 mL min . Unlike the earlier work, the
reaction was carried out at atmospheric pressure. The gaseous
decomposition products of the carbonates were collected in a gas
bladder and were injected into the gas chromatograph after an
interval of 1 h. The reaction was carried out for 2 h, after which the
further decomposition of carbonates was not observed. No
external reactant feed containing CO or CO2 was used during
the reaction. The conversion of carbonates to products such as
CO, CO2 and hydrocarbons was calculated by subtracting the
amount of undecomposed carbonates remained after reaction
from the total amount of carbonates. For convenience, the solid
in Fe-based catalysts in the FT reaction due to their abundance,
3
(FeCaCO)
19
low-cost and unique ability to form oxygenates. Unlike cobalt,
the chemistry of iron catalysis involves the formation of oxide and
carbide phases in addition to the formation of metallic iron. The
exact role of each phase in the catalytic activity is a subject of
scientific debate. The motivation of the present work was to
explore the selective conversion of solid inorganic carbonates to
higher HC products using iron/iron oxides as catalysts. To our
knowledge, such studies have not been reported in the literature.
2
21
20
Experimental section
Synthesis of carbonates
Calcium carbonate compositions were prepared according to the
21
product remaining after the heat treatment in a H atmosphere
2
procedure reported earlier. In a typical synthesis, 4.2 g NaHCO3
was dissolved in 30 mL H O and maintained at 353 K. The
solution was continuously bubbled with CO from a pressurized
container at a rate of 50 mL min throughout the experiment. An
aqueous solution of 20 mL containing 0.59 g Ca(NO ?4H O and a
suitable quantity of FeCl was added according to the required Fe/
Ca mole ratio (x). The Fe salt solution was added dropwise to the
NaHCO solution under stirring. Precipitation was observed
will be referred to as FeCaD.
2
2
2
1
Results and discussion
3
)
2
2
We have carried out the hydrogen reduction of the FeCaCO
samples prepared by co-precipitation, wherein the carbonate was
in an intimate mixture with iron oxide. The carbonate itself
2
3
2
1
contained Fe in the form of Ca12yFe
in FeCaCO was varied over a wide range of Fe/Ca mole ratios (x =
.0–5.0). The starting carbonate was found to possess the calcite
y
CO
3
.
The proportion of Fe
immediately. The mixture was stirred at 353 K for 30 min, after
which it was allowed to cool to room temperature naturally. The
precipitate was filtered, washed in water and dried at 333 K for 2
days. The dried samples did not contain chloride ions, as found by
energy dispersive X-ray analysis.
0
20
structure, as expected, and excess Fe was present as Fe
x was small (x , 2), the calcite phase was the major component
while the Fe component increased with x. The X-ray diffraction
data was also supported by the M o¨ ssbauer data. The specific
surface area of xFeCaCO was found to be 95 m g for x = 5 and
3 m g for x = 0.5. The pores were in the range of 5–10 nm.
2 3
O . When
2 3
O
Characterization
2
21
3
The starting carbonate materials contained CaCO and Fe oxides
2
21
1
and are referred to as FeCaCO. These samples with different x
values, as well as the solid residues obtained after the reaction,
were examined by X-ray diffraction using a Bruker D8 instrument.
M o¨ ssbauer spectra were recorded in the transmission mode using
The temperature of the reaction was chosen to be 673 K, as the
conversion and yield were highest at this temperature (Table S1,
ESI
CO
FeCaCO compositions were heated in H at 673 K for 2 h. The
3
). In Fig. 1, we show the total mole conversion of carbonate
57
(
3
) to gaseous products for different values of x, when the
a
Co c-ray source in a rhodium matrix. The velocity and isomer
shift were calibrated using iron foil. The M o¨ ssbauer spectra were
analyzed using SITE and DIST options in the Win Normos-for-Igor
software package. The carbonate content of the samples was
estimated using a Thermo Scientific Flash 2000 CHNS analyzer,
using an SQS column at 350 K and a thermal conductivity
2
2 4 2 4
gaseous products that were detected include CO , CO, CH , C H ,
detector. The carbonates were decomposed to CO at 1100 K in He
2
gas. The gas samples were analyzed using a gas chromatograph
manufactured by Mayura Analyticals Ltd (India). The products
were separated using a Hayesep packed column using N as the
2
carrier gas. The column was connected to a Ru loaded methanator
at 623 K and a flame ionization detector. The specific surface area
was measured by the BET method by adsorbing N at 77 K using
2
Quantachrome autosorb IQ. The Fe/Ca ratio on the surface of the
samples were obtained by recording the X-ray photoelectron
spectroscopy (XPS) spectra using a EAC2000 SPHERA 547
spectrometer made by Omicron Nanotechnology ESCA, using
210
Mg-Ka X-ray radiation at 300 K under a 10
mbar pressure. The
area under the curve for the Fe2p and Ca2p spectra was calculated
after baseline and sensitivity corrections. TEM images were
recorded at 200 kV using a Technai instrument.
2
2
Fig. 1 Variation of the total mole conversion of CO3 to products (CO, CO and
2
22
HCs) with x, at 673 K (&). The ratio of the number of moles of CO
to the number of moles of Fe (nCCO3/nFe) (%) is shown as a function of x.
3
in the sample
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and C
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C
2
H
6
3
H
8
. The total mole conversion of CO
3
(%) was
a
Table 1 Gaseous products formed for different values of x at 673 K in 2 h
calculated as follows:
ꢀ
ꢁ
Yield (%)
nCCO {nCUD
x
3
Total mole conversion of CO3ð%Þ~
|100
CH
4
C
2
H
4
2
C H
6
C
3
H
8
CO
CO
2
nCCO3
0
0
0
2
3
5
0
1
2
2
4
5
0
2
3
3
5
7
0
2
5
2
3
4
0
0
0
1
1
4
0
1
2
8
7
6
5
7
12
17
34
21
Here, nCCO3 is the total number of moles of carbon present in
FeCaCO before the reaction and nCUD is the total number of moles
of carbon present in the sample after the reaction. The value of
nCUD was calculated by decomposing FeCaD at a higher
.1
.5
.5
3
temperature (900 K). The decomposition of CO did not go to
a
completion under the reaction conditions possibly because of the
inhomogeneous distribution of catalytically active Fe across the
CO matrix, and nC decreases as x increases. The total mole
Yield was obtained by multiplying the total mole conversion of CO
3
and the selectivity for the products.
3
UD
conversion of CO
in FeCaCO that decomposed under the reaction conditions to
form various products. The total mole conversion of CO for
3 3
represents the percentage of the moles of CO
3
In a trend similar to the total mole conversion of CO , the
3
specific mole conversion of CO to HCs also increases with x, as
3
FeCaCO is around 5% when x = 0.0 (i.e. pure CaCO ) and increases
3
shown in Fig. 2. In the absence of Fe (x = 0.0), the specific molar
as x increases, suggesting that the presence of Fe favors the
conversion to HC is zero. At x = 0.0, CO is the main product
2
decomposition of CO
for the same weight of FeCaCO, the number of moles of CO
decreases as the value of x increases. As a result, the number of
moles of Fe present for every mole of CO (nCCO3/nFe) increases
3
. From elemental analysis, it was found that
formed, with only traces of CO. The presence of Fe is clearly
3
essential for the conversion of CO
to CO and HCs. In other words,
3
Fe is not only essential for enhancing the decomposition of CO to
3
3
2
CO , but also for the subsequent hydrogenation to yield the HCs.
with x as shown in Fig. 1 (broken line) which explains the increase
in the total mole conversion of CO3 with x. The slope of the
increase in the mole conversion is large for smaller values of x,
which suggests a higher activity for Fe.
Treatment of the starting carbonate material FeCaCO with
2
hydrogen was found to yield CO and CO as well as hydrocarbons.
Increasing x or the concentration of Fe in FeCaCO does not result
in a linear increase in the conversion to HCs. In order to estimate
the catalytic activity of Fe, the specific mole conversion to HCs was
normalized with respect to nFe in the sample. In Fig. 2, the
turnover number for HCs (HC TON) is plotted against x (broken
line). The plot represents the number of moles of carbon in CO
3
In Fig. 2, the specific mole conversion of CO (%) is plotted against
3
converted to HCs by one mole of Fe (present in FeCaCO) per
second. The HC TON is high when x = 0.1, and then decreases
gradually and proceeds asymptotically as x increases. Although the
the mole ratio x. The specific mole conversion of CO3 (%) is
calculated as follows.
3
specific mole conversion of CO to HCs is higher at high Fe
Specific mole conversion of CO3 to HCð%Þ~
concentrations, the activity of Fe (conversion per n ) does not
Fe
ꢀ
ꢁ
nCCO {ðnCUDznCCOznCCO Þ
3
2
follow the trend. This observation is consistent with our inference,
|100
nCCO
3
3
based on the total mole conversion of CO in Fig. 1, that the
activity of Fe is high in the lower x regime.
Here, nCCO and nCCO2 represent the total number of moles of
CO and CO respectively.
2
Fig. 2 Plot of turnover number of the HCs (HC TON) on the Fe catalysts (#) and the
Fig. 3 Variation of the yields (%) of different HCs such as CH4 (&), C H ($), C H
2
4
2 6
22
specific mole conversion of CO
3
to HC ($) against different mole ratios x, at 673 K.
(#) and C H8 (%) with x, at 673 K.
3
7
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In Fig. 5, we present TEM images of two FeCaD samples (x =
0
.5 and 5.0). The TEM images of both of the samples showed
regions of darker contrast (white arrow marks) and regions of
lighter contrast (black arrow marks). The darker regions may be
Fe-rich regions because of the higher bulk density of the Fe phases
observed in this reaction. The electron diffraction patterns and the
elemental ratio of the samples at the different regions are
presented in Fig. S4, ESI.
nm in diameter for x = 0.5 (Fig. 5a) and 30–50 nm for x = 5
Fig. 5b). In contrast to the x = 0.5 sample, the regions of darker
3
The size of these dark spots was 2–10
(
contrast are more densely crowded in the x = 5 sample. The
change in the size of the Fe-rich particles with x explains the trend
3
observed in the total mole conversion of CO and the specific mole
conversion of CO to HCs (see Fig. 1 and 2), where an apparent
3
decrease in the activity of the Fe catalysts (conversion per n ) is
Fig. 4 M o¨ ssbauer spectra of FeCaD (x = 5) showing the various phases of iron. The
Fe
black open circles show the observed data and the red solid line is the fit to the
observed with an increase in x. XPS spectra were recorded for the
FeCaCO and FeCaD samples (x = 1 and 5) in order to determine
the Fe/Ca ratio on the surface of the carbonates as the reaction
observed data. Each of the sub-spectra from left to right are identified as c-Fe
489 kOe), Fe (A-site)–c-Fe (467 kOe), Fe (B-site) (437 kOe), a-FeOOH (390
kOe), a-Fe (335 kOe), h-Fe C (302 kOe), h-Fe C (230 kOe), x-Fe (152 kOe), x-Fe
90 kOe) and superparamagnetic Fe (narrow doublet), respectively. The numbers
in the brackets represent the corresponding hyperfine field values.
2 3
O
(
3
O
4
2
O
3
3 4
O
3
3
5
C
2
5 2
C
3+
3
proceeded (Fig. S5, ESI ). The Fe/Ca ratio is comparable for both
(
the x = 1 and 5 FeCaD samples. The surface content of Fe varies as
the reaction proceeds, an aspect which requires further investiga-
tion.
The decomposition of CaCO
occurs at a lower temperature in a H
reaction, the carbonate decomposes to CO
reduction to form CO and HCs. The formation of CO is crucial for
the formation of the HCs as CO and H which adsorb on the
surface of the catalyst can give rise to RCH species (R = H, CH or
). The chain growth, followed by their desorption from the
surface of the catalyst, results in HCs of varying molecular weights.
A dynamic equilibrium between CO and CO involving H and
3
containing transition metal ions
The yields of various products formed during the reaction are
presented in Table 1. The yields of HCs are shown graphically in
Fig. 3. The total yield of HCs increases as the value of x increases,
suggesting a higher selectivity for HCs. Comparing the trends
across the HCs, the relative yield of C H was higher than the
22,23
2
atmosphere.
During the
2
, which undergoes
2
2
4
other HCs for all values of x, while the yield of C
x. This trend suggests that higher concentration of Fe favors C–C
coupling over dehydrogenation. HCs higher than C were not
3 8
H increased with
2
2
24
CH
3
3 8
H
detected. It is noteworthy that the total yield of HCs obtained here
is comparable to that in the FT synthesis.
2
2
2
H O(g) is possible under the reaction conditions in the form of the
25
water–gas shift reaction. In a separate experiment, when we
passed a mixture of CO and H (g) over FeCaD (x = 2) at 673 K, CO
was converted to CO . Similarly, formation of CO was observed
when a mixture of CO and H was passed over the catalyst under
After 2 h of heat treatment, the decomposition products,
FeCaD, were examined by X-ray diffraction and M o¨ ssbauer
spectroscopy. The X-ray diffraction pattern of FeCaD (x = 0.5)
showed the presence of Fe O which may have formed by the
2
O
2
2
2
3
4
the same conditions. This observation is to be expected as Fe-
based catalysts in FT reactions are known to catalyze the water–gas
shift reaction. Interestingly, trace amounts of HCs were also
observed at 673 K within 15 min of reaction. The equilibrium
reduction of Fe
2 3
O . The X-ray diffraction pattern of FeCaD (x = 5.0)
showed the presence of a-FeOOH, c-Fe O , Fe O , a-Fe, h-Fe C,
2
3
3
4
3
25
x-Fe
supported by the analysis of the M o¨ ssbauer spectrum in Fig. 4 (See
Tables S2 and S3, ESI ).
5 2 2 4
C and CaFe O . The presence of these phases was also
between CO and CO plays an important role in adjusting the CO/
2
3
H ratio, which in turn affects the formation of HCs. Under the
2
present experimental conditions, excess H
ture (673 K) may not favour the reverse water–gas shift reaction.
The theoretical conversion of the reverse water–gas shift reaction
2
and the low tempera-
26
27
calculated at this temperature is less than 5%.
In the present study, the decomposition of CO
at around 400 K, and the formation of CO occurred at around 500
K for all values of x. In Fig. S7, ESI, we compare the temperature-
dependent product formation during the decomposition of CaCO
and xFeCaCO (x = 5). For x = 0.0, CO was the only product
3
2
to CO started
3
3
2
observed (see Fig. 1), suggesting that Fe was necessary for the
hydrogenation of CO to CO. Note that the reduction of Fe ions in
2
FeCaCO to form different phases of iron occurs simultaneously
with the decomposition of CO
could be affected by the concentration of H
3
. The final products based on Fe
Fig. 5 TEM images of FeCaD showing the regions of dark contrast (white arrow)
and the regions of lighter contrast (black arrow) for (a) x = 0.5 and (b) x = 5.
28
2 2
, CO and CO. As the
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concentration of CO
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2
and CO changes during the reaction, it is
important starting materials in the chemical industry. Possible
difficult to control the composition of the active catalyst. Since the
proportion of the active metal phase formed increases with x (as
reasons for this selectivity could be the following: (a) a H
atmosphere is conducive for the hydrocracking of longer HCs, and
(b) reaction mixtures which are rich in H do not attain the
necessary CO : H ratios to form longer HCs.
Although lower hydrocarbons have uses as feed stock
chemicals in industry, it is useful to calculate the energy obtained
during their combustion to calculate the energy efficiency of the
2
-rich
evidenced by the formation of Fe
mole conversion of CO to HCs also increases with x. Fe
Fe formed in the reaction can actively reduce CO to CO, as
Fe O is an active water–gas shift catalyst. The formation of CH4
3
O
4
, Fe and Fe
5
C
2
), the specific
2
3
2
O
3
and
2
3
O
4
2
25
3
4
and other higher HCs was observed above 500 K, around which
temperature the reduction of oxides to metallic Fe occurs (523–623
process. The energy balance (E
the energy gain or loss that occurs in synthesizing (E
hydrogenating (E ) one mole of CaCO and the energy obtained
during the combustion (E
reaction.
b
) of the reaction can be defined as
20
K). Metallic a-Fe can therefore be actively involved in the
formation of CH and HCs. Thus, the mixtures of Fe metal and
iron oxides supported on Ca-rich oxides catalyze the conversion of
CO to CO and HCs. We observe the presence of crystalline
CaFe O at higher concentrations of Fe.
) and
p
4
h
3
c
) of the hydrocarbons produced in the
2
2
4
E = E + E 2 E
c
b
p
h
Since the amounts of CO and CO produced increases with x,
2
and the proportion of CO
of CO and the HCs for a particular value of x, it is likely that the
rate of decomposition of CO to CO is generally higher than that
of the reduction of CO to CO or HCs. This explains the high yield
2
is always significantly greater than that
The reaction involving the synthesis of CaCO3 from the
precursors can be represented by the following equation.
3
2
2
1
0NaHCO + 2Ca(NO ) + 3FeCl A 3FeCO + CaCO +
3
3 2
2
3
3
2
of CO (Table 1). The results of the HC yields presented in Table 1
4
NaNO
3
+ 4NaCl + 5H
2
O + Na
2
CO
3
2
+ 5CO + CaCl
2
can be explained based on the value of x and the types of phases
2
9
that are formed. From Fig. S2 and S3, ESI,
that there is an increase in the tendency for the formation of
carbide phases of Fe as x increases. Since Fe is associated with
C–C coupling, we speculate that the particle size plays an
3
it can be observed
2
1
Ep = 142 kJ mol
5 2
C
Assuming 100% hydrogenation, and the same ratio of products
as obtained in the experiments for FeCaD with x = 5, the
20,30
important role in the selectivity of the reaction.
however result from the deactivation of x-Fe
h-Fe C can
We do not
3
31
5
C
2
.
hydrogenation of CaCO may be represented by the following
3
observe the carbide phases at low values of x (y0.5). The lower
equation.
chemisorption potential of H on the smaller-sized catalysts favors
2
CaCO + 2H A CaO + 0.29CO + 0.12CO + 0.06CH +
the reduction of the oxides of carbon, compared to the larger-sized
3
2
H
2
4
32
0.12C
2
4
+ 0.06C
2
H
6
3 8 2
+ 0.06C H + 1.3H O
particles. From this result, one should expect higher HC yields at
low x values. However, from Table 1, we observe that the HC yield
is higher as x is increased. The observation in our work differs
from the earlier work because an increase in the concentration of
21
h
E = +72 kJ mol
Fe also increases the rate of decomposition of CO
3
, as observed
It can be found that an energy, E
during the combustion of 0.06 moles of CH
.12 moles of C H . Thus, an energy, E = 231 kJ, can be gained by
c
= 449 kJ, can be obtained
from the nCUD values. The formation of C H is explained on the
2
4
4
, C and C and
2
H
6
3 8
H
basis of the rearrangement or b-elimination of C intermediates
2
0
2
4
b
33
adsorbed on the iron. A higher selectivity for C
2 4
H at low values
3 2
producing and utilizing one mole of CaCO . Furthermore, CO
of x found in this study are in accordance with the results on the
produced during the combustion of the hydrocarbons can be
directly used to produce hydrocarbons, or used in the carbonation
of CaO. We note that the energy efficiency of the process can be
further improved if the selectivity is shifted towards higher
hydrocarbons. The calculation, however, does not take into
account the energy that may be consumed in the production of
29
size dependent selectivity of Fe-based FT catalysts. Smaller
particles have a lower H chemisorption potential which favors the
2
32
formation of olefins, rather than C–C coupling. Although the
results present only the trends on the product formation, we shall
attempt to explain the selectivity of the process in relation to
particle size and chemisorption. The explanation is based on the
general conclusions drawn by earlier workers. The formation of
2
H (which can be reduced with new emerging technologies
regarding the production of H ) or the extraction of mineral
2
C H and C H is probably preceded by the formation of C H .
2
6
3
8
2
4
carbonates, which is beyond the scope of the present work.
Hydrogenation of C
higher hydrocarbons, is favored depending on the chemisorption
of H and CO, which in turn is a function of particle size and
crystalline phase of the catalyst. The chemisorption potentials of
and CO are closely related as the M–CO bond is electron-
2 4 2 6
H to yield C H , or chain elongation to
Conclusions
2
From the present study, we conclude that Fe-containing CaCO is
3
H
2
a useful material for the conversion of CO to HCs with selectivity
3
3
2
accepting while the M–H bond is electron-donating. Thus, as the
for C –C products. Such carbonate materials can be prepared
1
3
value of x increases, both C H and C H are formed in relatively
readily even if they do not exist in nature. Iron, present in the form
of metal, metal oxides and metal carbide particles supported on
2
6
3 8
higher yields. It is noteworthy that the decomposition of CO to
3
HCs is highly selective for the formation of C
1
–C
3
HCs, which are
3 2
Ca-rich oxides, is involved in the decomposition of CO to CO and
7
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its subsequent reduction to CO and HCs. Since many of the iron-
based phases are present in active Fischer–Tropsch catalysts, it is
possible that the conversion of CO3 to HCs follows a similar
mechanism. However, more detailed studies are required to
establish the mechanism of the processes described here. The
process reported by us will be economically attractive and
environmentally benign if the selectivity for higher hydrocarbons
is improved. We are carrying out research along this pathway. By
suitably controlling the decomposition of carbonates and the
formation of iron-based catalytic phases, it may be possible to
achieve better selectivity and higher levels of conversion.
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