Cobalt catalysts for the conversion of CO2 to light hydrocarbons at atmospheric pressure

Article abstract of DOI:10.1039/c3cc46791k

A series of cobalt heterogeneous catalysts have been developed that are effective for the conversion of CO₂ to hydrocarbons. The effect of the promoter and loadings have been investigated.

Full text of DOI:10.1039/c3cc46791k

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Cobalt catalysts for the conversion of CO2 to light
hydrocarbons at atmospheric pressure†
Rhodri E. Owen,a Justin P. O’Byrne,b Davide Mattia,b Pawel Plucinski,b
Sofia I. Pascua and Matthew D. Jones*a
Cite this: Chem. Commun., 2013,
49, 11683
Received 5th September 2013,
Accepted 29th October 2013
DOI: 10.1039/c3cc46791k
www.rsc.org/chemcomm
A series of cobalt heterogeneous catalysts have been developed From XPS and pXRD the cobalt is present as Co3O4. Initial catalytic
that are effective for the conversion of CO2 to hydrocarbons. The results are shown in Table 1, the cobalt only system (entry 1) gave
effect of the promoter and loadings have been investigated.
good conversion but to predominately CH4, analogous to literature
precedent.4–6 It has been shown that the addition of a noble metals
One of the ‘‘holy grails’’ for the catalysis community for the 21st has a beneficial effect upon the catalysis.8 It has also previously been
century remains the development of robust and cheap systems for shown that adding alkali metals with cobalt increases selectivity to
the upgrading of CO2. With the dwindling supply of oil derived heavier HCs (for traditional FT reaction).9 Furthermore, in traditional
chemical feedstocks coupled with uncertainties of supply, now is FT catalysis the olefin–paraffin ratio increases with addition of alkali
unequivocally the time to act.1 The ‘‘shale-gas boom’’ in the US (and metals.10 Thus, we tested a range of Co:Pd:K systems Table 1 entries
Europe) remains, to date, a temporary life-line rather than a long- 2–9 with palladium present to aid CO production and potassium to
term solution. Therefore, it is of paramount importance to find a increase selectivity to heavier HCs. A cobalt loading of 20 wt% was
sustainable alternative. We and several other groups across the world found to be optimal in terms of conversion but with higher loadings
have already taken up this research challenge – mainly utilising iron producing lower quantities of CH4. Increasing potassium loading
promoted systems for the conversion of CO2 to hydrocarbons (HCs).2 was found to reduce selectivity towards CH4 and led to an increased
Far less attention has been applied to cobalt containing catalysts; yield of heavier HCs, this occurs at the cost of conversion with high
this is largely due to their preference to form CH4.3 One of the first potassium loadings leading to lower CO2 conversion. Comparing
examples of this catalysis was in 1950 by Miller with a series of entries 1 vs. 4 vs. 9 it is clear that for this system there is no
activated cobalt systems.4 In more recent years examples of active advantage, in-terms of selectivity, adding palladium to the catalyst.
catalysts include those of Somorjai who have prepared cobalt Comparing entries 1 and 9 there is a dramatic reduction in conver-
nanoparticles on MCF-17 mesoporous silica with the major products sion, with a concurrent increase in selectivity to heavier HCs. As
being CH4 and CO.5 It is observed that platinum can act as a expected the palladium containing systems produced lower level of
promoter for such processes. Willauer and co-workers have investi- olefins, entry 4 vs. 9, compared to the potassium promoted system.
gated a Co–Pt/Al2O3 system for the conversion of CO2 to HCs.6 The
The pore diameter of the SiO2 was varied (60, 250, 500 Å)
lowest selectivity to CH4 being 93.1% (T = 220 1C, P = 275 psi, Table 1 entry 4 (60 Å) vs. Table 2 entries 1 and 2. Generally as
H2 :CO2 1 : 1) higher selectivities towards CH4 where observed with the pore diameter is increased the HC yield decreases, however
H2 :CO2 ratios of 2 : 1 and 3 : 1. It was hypothesised that changing a noticeable increase in selectivity away from CH4 is observed.
this ratio lowered the methanation ability of the catalysts and However, the same trend is not observed for the 20 wt%Co/
conversely favoured chain growth.
1 wt%K/SiO2 system. Noteworthy, is that the large pore dia-
In this paper we report the utilisation of a new series of cobalt- meter silica appears to facilitate the formation of slightly larger
containing heterogeneous catalysts for the conversion of CO2 to HCs crystalline phases, which maybe related to the enhanced
at atmospheric pressure. The catalysts were prepared by the wet selectivity.†11 To assess the potential to scale-up this catalyst
impregnation of the appropriate metal salts on the silica support.† 7 system a larger particle size support was utilised, Table 2
The catalysts were characterised by TEM, SEM, XPS and pXRD.† entry 3. There was a reduction in conversion compared to the
35–70 mm material (Table 1, entry 4) potentially due to diffu-
a Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
sional effects; HC distribution was however observed to remain
similar. The removal of all promoters, giving a 20 wt%Co/
SiO2_500 catalyst (Table 2 entry 8) resulted in almost exclusively
E-mail: mj205@bath.ac.uk; Fax: +44 (0)1225 386231; Tel: +44 (0)1225 384908
b Department of Chemical Engineering, University of Bath, Claverton Down,
Bath BA2 7AY, UK
CH4 formation as observed with the use of 60 Å pore size silica
(Table 1, entry 1) showing the use of larger pore size silica alone
† Electronic supplementary information (ESI) available: Synthesis, characterisa-
tion details and tentative mechanism. See DOI: 10.1039/c3cc46791k
c
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Table 1 Initial catalyst screen
Entry
Catalysta
Con. (%)
CO yield (%)
HC yield (%)
C1
C2=
C2
0.3
0.3
2.7
5.4
6.4
C3=
C3
C4
C5+
1
2
3
4
5
6
7
8
9
20 wt%Co
20 wt%Co/1 wt%Pd
67.4
50.7
36.4
63.4
39.1
62.8
59.1
43.2
36.1
2.8
3.2
2.9
8.8
3.7
9.6
9.6
10.5
6.1
64.4
47.5
33.5
54.6
35.4
53.3
49.5
32.7
29.9
99.7
99.7
97.0
93.2
91.6
89.5
77.2
70.1
54.7
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
10.7
0
0
0.3
1.2
1.7
2.6
7.6
10.3
3.8
0
0
0
0
0.3
0.6
3.1
5.2
2.5
0
0
0
0
0.1
0
1.5
3.6
9.5
10 wt%Co/1 wt%Pd/1 wt%K
20 wt%Co/1 wt%Pd/1 wt%K
40 wt%Co/1 wt%Pd/1 wt%K
20 wt%Co/1 wt%Pd/0.5 wt%K
20 wt%Co/1 wt%Pd/1.5 wt%K
20 wt%Co/1 wt%Pd/3 wt%K
20 wt%Co/1 wt%K
7.3
10.6
10.9
11.5
2.0
a
Metal nanoparticles are supported on SiO2, pore diameter 60 Å. Test conditions: temperature, 643.15 K; pressure–atmospheric; H2/CO2 ratio 3 : 1
results were calculated after at least 5 hours on stream. Note: blank silica was tested but showed no observable catalytic activity.
Table 2 Effect of noble metal, pore size and additional promoter on catalytic efficiency
Entry
Catalysta
Con. (%)
CO yield (%)
HC yield (%)
C1
C2=
C2
C3=
C3
7.0
C4
C5+
1.1
1
2
3
4
5
6
7
8
20 wt%Co/1 wt%Pd/1 wt%K_250
20 wt%Co/1 wt%Pd/1 wt%K_500
20 wt%Co/1 wt%Pd/1 wt%K_60b
20 wt%Co/1 wt%Pt/1 wt%K_500
20 wt%Co/1 wt%Ru/1 wt%K_500
20 wt%Co/1 wt%Pd/1 wt%Li_500
20 wt%Co/1 wt%Pd/1 wt%Na_500
20 wt%Co_500
20 wt%Co/1 wt%Li_500
20 wt%Co/1 wt%Na_500
20 wt%Co/1 wt%K_500
20 wt%Co/1 wt%Mo_500
44.9
41.6
45.7
36.5
45.1
39.5
41.9
64.3
39.3
51.2
47.6
64.8
60.9
62.0
42.7
43.9
3.0
5.7
3.6
7.6
5.7
7.6
8.5
4.3
8.4
11.1
8.1
4.2
13.9
4.3
8.4
6.9
41.9
36.8
42.2
28.9
39.4
31.9
33.4
60.0
30.9
40.1
39.5
60.7
47.1
57.7
34.3
37.0
76.9
62.8
91.6
52.4
60.2
69.5
60.7
98.2
74.3
53.7
60.4
94.7
98.2
97.9
72.4
45.5
0
0
0
0.3
0.5
0
0
0
0.6
2.1
1.4
0
0
0
13.0
17.0
6.3
15.5
13.3
18.9
14.9
1.8
14.8
13.4
13.0
5.0
0
0
0
2.0
4.7
0.4
6.9
7.0
2.2
4.7
0
2.5
8.2
6.6
0
11.6
1.6
9.9
5.9
6.9
8.5
0.1
4.7
5.1
5.2
0.3
0.1
0.1
4.9
6.4
3.7
0.1
12.1
6.5
2.4
9.2
0
0.6
6.4
4.4
0
2.9
6.7
0.2
2.0
0
2.6
11.1
9.0
0
0.1
0
4.4
11.1
9
10
11
12
13
14
15
16
20 wt%Co/1 wt%Cr_500
20 wt%Co/1 wt%Mn_500
20 wt%Co/1 wt%Na/1 wt%Mn_500
20 wt%Co/1 wt%Na/1 wt%Mo_500
1.6
2.0
13.1
15.8
0.1
0
3.5
9.1
0
0
1.0
10.4
0.8
1.7
a
Metal nanoparticles are supported on SiO2, _X where X is the pore diameter of the SiO2. b SiO2 particle size 1–2 mm (N.B. all other SiO2 are in the range
35–70 mm). Test conditions: temperature, 643.15 K; pressure–atmospheric; H2/CO2 ratio 3 : 1 results were calculated after at least 5 hours on stream.
is not sufficient to significantly influence product selectivity. SiO2_60 Å systems (Table 1, entries 4 and 9). CH4 selectivity is also
The 500 Å pore size SiO2 material was taken forward and the noble observed to be lower for these two catalysts with the Co–Na catalyst
and group 1 metal were systematically varied, Table 2 entries 4–7. performing best. In direct contrast to the noble metal promoted
Typically, the HC distribution obeyed the Anderson–Schulz–Flory catalysts (Table 2 entries 2, 6 and 7) the systems promoted with
distribution.†
only alkali metals show a high degree of selectivity towards olefins.
The nature of the noble metal was varied (Table 2 entries 2 vs. The improved HC selectivity observed upon alkali addition has
4 and 5). An increase in conversion is observed with the presence of been attributed to an enhancement in surface to molecule charge
ruthenium and CH4 selectivity is also seen to decrease with transfer,12 increasing CO binding strength and reducing the H2
selectivity to C5+ HCs almost doubling. Replacement of palladium binding strength, thus, resulting in an increased chain growth
with platinum led to a large decrease in selectivity towards CH4 probability and higher alkene selectivity.13
with C5+ selectivity increasing to ca. 12%. This improvement in
With results showing that the addition of expensive noble
selectivity occurs with only a slight decrease in conversion. The metals are not needed to obtain good product distributions
variation of alkali metal was investigated (entries 2 vs. 6 and 7). The some cheaper more abundant transition metal alternatives
lithium system performed worse both in terms of conversion and were also investigated. The addition of manganese to cobalt
selectivity relative to the potassium promoted system. The sodium catalysts used for the CO-fed FT process has shown an increase
catalyst on the other hand gives greater selectivity to C5+ HCs with in water–gas shift activity.14 Molybdenum has been shown to be
no decrease in conversion.
active for the FT process itself and addition to cobalt based FT
As with the analogues SiO2_60 Å supported systems the catalyst has resulted in an improved catalyst performance.15
removal of the noble metal to give alkali metal only promoted The addition of small loadings of these metals along with
systems (Table 2 entries 9–11) still significantly outperformed chromium, whose addition has been shown to increase selectivity
the cobalt only system in terms of product selectivity. Conver- towards longer chained HCs with some cobalt based FT catalysts,
sion values recorded for the lithium catalyst with and without were investigated (Table 2 entries 12–14).
palladium remain very similar, however, both sodium and
A slight drop in conversion was observed upon the addition
potassium systems show an increase in conversion in the of chromium to the system combined with a higher selectivity to
absence of palladium, the opposite to that observed for the CO, this resulted in a reduced HC yield. Little effect on conversion
c
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yielding the best HC distribution was chosen, 20 wt%Co/
1 wt%Na/1 wt%Mo_500, Fig. 1. Conversion remained stable for
the entirety of the time on stream with an average conversion of
48.6%, a 5% increase over that observed for the system after a
2 h reduction. There was no evident difference in the time taken
for product selectivity to stabilise with the catalyst requiring
4–6 h to stabilise after both the 2 and 15 h pre-treatments. HC
distribution showed very little difference with C5+ selectivity
B10% for the system with both pre-treatment times.
In conclusion a range of cobalt based catalysts were pre-
pared and tested for their ability to hydrogenate CO2 at atmo-
spheric pressure. Conversion values although reduced upon the
introduction of some promoters still compares favourably with
other catalysts, even iron-based systems currently being studied
for the process. Product selectivities significantly superior to
other cobalt based catalyst reported from CO2 hydrogenation
are observed. Further studies are on-going to gain an under-
standing of silica pore size effects coupled with computational
studies to gain further insight into the electronic effects of the
promoters, specifically molybdenum and sodium, on the cobalt
component.
Fig. 1 Top: conversion vs. time plot for 20 wt%Co/1 wt%Na/1 wt%Mo_500.
Bottom: selectivity vs. time plot for 20 wt%Co/1 wt%Na/1 wt%Mo_500.
and HC yield was observed with molybdenum and manganese
addition. The chromium system showed little difference in CH4
selectivity relative to the non-promoted cobalt system (Table 2
entry 8). A more significant decrease in CH4 selectivity was obtained
upon the addition of molybdenum and manganese. Molybdenum
addition was found to be the most effective of the three transition
metals tested although its promotional abilities pale in comparison
to those obtained with the addition of alkali metals (Table 2
entries 9–11). The improvement observed upon the addition of
molybdenum can likely be attributed to the suppression of CH4
formation as observed with cobalt catalysts for FT catalysis.15
In order to ascertain if the promotional abilities of the alkali
metal systems could be combined with the slight improvement
observed with the manganese and molybdenum systems two
catalyst systems were prepared containing a combination of
1 wt% sodium (the best performing alkali metal) with 1 wt%
of each of the best performing transition metals (Table 2
entries 15 and 16). The combination of manganese and sodium
as promoters shows a lower conversion than that observed for
both the non-promoted system and the individually promoted
systems. Product selectivity was observed to be intermediate
between the sodium only and manganese only promoted
system. The dual promotion with sodium and molybdenum
proved far more successful. The catalyst gives a greater selectivity
to heavier C5+ HCs similar to that observed for the Co–K–Pt
system (Table 2 entry 4) however conversion was found to be
higher. CH4 selectivity was found to be 45.5%, the lowest
observed for any of the catalyst systems tested. No change in
cobalt binding energy was observed by XPS with sodium and
molybdenum addition. However, the addition of molybdenum
may favour the formation of alkylidene species, which are
important in the chain growth mechanism.†
The Royal Society (SIP), the EPSRC (EP/H046305) and UoB
are thanked for funding.
Notes and references
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 11683--11685 11685