Catalytic Reduction of CO2 into Liquid Fuels: Simulating Reactions

Chapter 11

Catalytic Reduction of C O into Liquid Fuels: Simulating Reactions under Geologic Formation Conditions 2

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Downloaded by DICLE UNIV on November 7, 2014 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch011

D. Mahajan , C. Song , and A. W. Scaroni 1

Energy Sciences & Technology Department, Brookhaven National Laboratory, Building 815, Upton,NY11973-5000 Department of Energy and Geo-Environmental Engineering and the Energy Institute, Pennsylvania State University, University Park,PA16802

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In this paper, we describe two approaches that consider C O recycle via utilization as a viable option to sequester carbon in CO . First, we summarize the results of our study of the integration of C O capture with subsequent catalytic CO hydrogenation to methanol for application to stationary CO -emitting sources. We carried out room temperature CO solubility studies in amines and glycol solvents that are normally used to separate CO from flue gas in power plants. In polyethylene glycol (Peg-400), the solubility data obey Henry's Law up to 4.5 MPa whereas in triethanolamine (TEA), the solubility is dominated by facile formation of the TEA.CO adduct at CO partial pressure as low as 0.33 MPa. Preliminary results on catalyst design and evaluation to affect the C O / H reaction show that in these solvents, several transition metals are effective under mild conditions (T < 150°C and Ρ < 5 MPa) for methanol synthesis though rates and product selectivity need further improvement. Second, we address the H -cost issue that implicates geologic formations as natural slurry reactors for CO hydrogenation into liquid fuels wherein the needed H is produced from H O by naturally occurring transition metals in these formations. A catalytic reaction then reduces the buried CO into H -rich fuels. Successful development of the latter approach might close the natural carbon cycle in fossil fuels. 2

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© 2002 American Chemical Society In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Background Carbon management with respect to C 0 will define utilization of fossil fuels in the twenty-first century. A fundamental issue that will ultimately affect the implementation of any C 0 sequestration technology is its cost. Several options to sequester C 0 are described in the D O E technology roadmap entitled "Carbon Sequestration: State of the Science" (7). These options can be divided into two broad categories. These are: 1) C 0 burial and 2) C 0 recycle. The C 0 burial category includes ocean sequestration, depleted oil and gas reservoirs, abandoned coal mines and deep geological formations. O f these, recovery of stranded C H (by displacement with injected C 0 ) from coal mines has the benefit of offsetting some of the overall cost of carbon sequestration. For the subterranean C 0 sequestration options, the prohibitive cost remains an issue. Moreover, there is a growing concern about the long-term ecological impact of introducing large amounts of C 0 in these formations. The second category involves use of C 0 as a feedstock for making commodity products. This remediation option is attractive for its potential commercial value. One such preferred option involves recycling carbon in C 0 by converting it in to H rich synthetic fuels. Various aspects of thermal and photochemical activation of C 0 by metal complexes have been studied (2,3). A significant amount of work has been carried out with variations of Fe catalysts for synthesis of hydrocarbons (Equation 1) via the Fischer-Tropsch (F-T) route (4,5), modified Cu-ZnO catalysts for methanol synthesis (Equation 2) or further conversion of methanol to gasoline (MTG) (6) with heterogeneous metal catalysts. 2

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4

2

2

2

2

2

2

2

pC0 nC0

2

2

+ qH -> + (2n+l)H 2

(-CH -) + y H 0 + z C 0 -> C H OH + η H 0 2

2

n

2

n

2 n + 1

(1) (2)

2

2

These energy intensive transformations utilize heterogeneous catalysts that operate between 250°C to 400°C. It is to be noted that any C 0 transformation to H -rich fuels is highly endothermic ( C 0 is the thermodynamically stable product generated via combustion of fossil fuels). But it is the coproduction of H 0 that makes the overall reaction involving C 0 feedstock (for example reactions (1) and (2)) exothermic. Thus, the production of large-volume fuels, namely hydrocarbons and alcohols (specifically methanol), is attractive in the overall C 0 sequestration scheme but the cost of the required H remains a key issue. 2

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In this paper, we describe two research themes that relate to C 0 conversion into liquid fuels. The first theme emphasizes integration of C 0 capture with subsequent catalytic hydrogénation of C 0 to methanol, a molecule that can be used as a building block for fuels and chemicals. Here, the key challenge was to develop a highly efficient catalyst that operates in an amine or glycol solvent 2

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In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

168 that is normally used to separate C 0 from the C0 -emitting source. We, therefore, carried out C 0 solubility studies in amines and glycol solvents. We also present here preliminary results on catalyst design and evaluation to affect the C 0 / H reaction in these solvents under mild conditions of temperature and pressure. The second theme then outlines a concept to address the H -cost issue that implicates C 0 reduction in geologic formations. The approach is to consider geologic formations as natural slurry reactors for C 0 hydrogénation into liquid fuels wherein the needed H is produced from H 0 by a natural phenomenon in the formations. A concomitant decrease in C 0 concentration is achieved by catalytic reduction of the buried C 0 into H -rich fuels. 2

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Experimental Batch Unit A commercially available A E Zipperclave stirred batch unit was modified at BNL and used in these studies. The unit consisted of a 0.55 L pressure vessel and was fitted with a dispersimax six-blade impeller, a removable metal ring, inserted into the vessel to break up any vortices, that might form during stirring. The unit had the following provisions: 1) heating/cooling through a Pantemperature controller and 2) several inlet and outlet ports for sampling of gases and liquids. The maximum working pressure and temperature were 20 MPa at 350°C. A dual channel Omega chart recorder was attached to the unit to monitor any change in temperature and pressure during a reaction.

Solubility Studies In a typical run, a solvent of desired composition was purged with argon, loaded into the reactor and the reactor was sealed. C 0 was slowly introduced in the reactor at room temperature over the liquid through one of the ports until the desired pressure was attained. The gas inlet valve was closed and the vessel was allowed to stabilize for one minute. The solution was stirred and the pressure drop was measured. Typically, the pressure drop was constant within ten seconds. The solubility data were then computed. 2

Catalyst Evaluation Studies In a typical run, the selected metal catalyst, any additive, and solvent were loaded into the pressure vessel. The vessel was pressurized with feed gas containing C 0 and H , heated to a desired temperature, and the pressure drop 2

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169 was followed as a function of time on the chart recorder. Gas and liquid samples were taken at the start, during and after the run. A l l samples were analyzed on gas chromatographs. For colored solutions, U V / V I S spectra were recorded on Perkin-Elmer Lambda 4B spectrophotometer

A n Integrated Approach to Catalytic CO2 Hydrogénation Figure 1 outlines a general scheme for aquifer/ocean sequestration of C 0 . The steps involved are: 1) C 0 is captured by using amines (typically mono or diethanolamines) or glycols in an exothermic reaction, 2) the captured C 0 is released in a stripper by heating up to 150°C (an endothermic step) and the solvent recycled, and 3) free C 0 is delivered via pipeline to a chosen site for deep burial. In the overall C 0 sequestration scheme, the C0 -capture step is the most energy-intensive step and accounts for up to 70% of the total sequestration cost (/). We propose to address the cost issue by integrating the C0 -capture step with subsequent catalytic hydrogénation of C 0 to methanol. The integrated scheme is shown in Figure 2. In this scheme, the stripper is replaced with a catalytic reactor wherein the heat is utilized to catalyze C 0 hydrogénation to methanol. Note that H needed for the reaction is assumed to be generated by direct C H decomposition (C and 2H ) or from biomass (nonC 0 sources). Our approach to develop an integrated system includes the following essential elements: 2

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2

2

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2

4

2

2

• •

Utilize amine, sodium or potassium carbonate or glycol solvents. H 0 is invariably produced as a byproduct during C 0 transformations (Equations 1 and 2). Therefore, include H 0 as a cosolvent. Limit reaction temperature for catalytic C 0 hydrogénation to H 0 . 3

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/2

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2

The solubility data in Peg-400 as a function of pressure ( 0 . 4 - 4 . 5 MPa) were also collected at room temperature. Henry's Law holds in this solvent because unlike T E A , there is no chemical interaction (Figure 3). From the data in Figure 3, Henry's constant (H) was calculated to be 9.4 MPa. For our proposed catalytic studies (vide infra), C 0 solubility data as a function of temperature (25-150°C) and pressure (0.1-0.5 MPa) are needed. The measured data in Table 1 and Figure 3 can be used to extrapolate solubility values at higher temperatures in various solvents by literature methods (8). 2

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Figure 3. Equilibrium pressure-gas mol fraction isotherm for C0 solubility in Peg-400 at room temperature. 2

Specifically in water, the correlation for solubility as a function of temperature is based on the following equation (P): log xwt = A + B/T + C log Τ + D T

(6)

where xwt is weight fraction solubility of gases in water at 1 atm, Τ is absolute temperature, and A , B , C, and D are regression coefficients for C 0 . The corresponding correlation for Henry's Law constant (H) can be calculated: 2


173 log H = A + Β/Τ + C log Τ + D T

(7)

Methanol Synthesis Catalyst Design Considerations It is known that catalytic hydrogénation of C 0 produces several products (10,11). The products include formic acid, formaldehyde, C O , methanol, methane, higher alcohols and hydrocarbons that are thermodynamically related: 2


C0

2

->

CO

->

HCOOH

->

ROH

RH

(8)

We considered the following catalyst systems as the basis for our catalyst design. •

The homogeneous hydrogénation of C 0 to formic acid can be catalyzed by [(dppp)Rh(hfacac)] (dppp is a bisphosphine ligand and hfacac is hexafluoroacetylacetonate) in dimethylsulfoxide solvent in the presence of triethylamine at 23°C and 40 atm pressure of C 0 / H (1/1). The ratedetermining step in the catalytic cycle appears to be the liberation of formic acid (12). One of the most active homogeneous catalyst systems for formic acid synthesis comprises of Ru-phosphine complexes in supercritical C 0 solvent. A t 50°C and 205 atm, turnover frequency (TOF) of 1400 h" has been reported (13). For comparison with the same catalyst in THF, the TOF is 80 h" . The Ru (CO)i /KI homogeneous catalyst system in N-methyl-2-pyrrolidone (NMP) solvent yields a mixture of CO, C H O H , and C H at 240°C and under 8 M P a of C 0 / H (1/3) (14). Electrochemical reduction of C 0 to C O catalyzed by Pd complexes has been reported (15). Photochemical reduction of C 0 to formate catalyzed by metal complexes has been reported (16). The process uses a R u complex to capture sunlight and affect this transformation via electron relay, but the drawback is the consumption of expensive electron donor additives that are added to make the overall process catalytic. 2

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2+

The representative catalyst systems and the corresponding product selectivity are shown in Table II. It is clear that poor selectivity, low productivity, severe reaction conditions or other system limitations make these catalysts commercially unattractive. We specifically focused on developing catalysts that allow homogeneous liquid phase synthesis of methanol from C 0 : 2



3

Slurry

Ru(acac)

Homogeneous

Highly dispersed slurry

Fe/KOR 9 25

34

60

66

66

40

40

50

60

50

-

MeOH/ Glyme E-164/ Peg-400

THF

3

DMSO/ Et N THF

Oil

Solvent

3 0

250

120

270

270

23

260

Τ °C

3

2

20

130

4

5

Ρ MPa

r

3

3

3

4

CH OH >99% HCOOH > 99% CH OH > 80% HC > 90% CH OH > 99% C C -OH > 90%

Product

1

25

17

2

2

13

6

Reference

*0.5L A E Zipplerclave Reactor dppp = bis(diphenylphosphino)propane; hfacac = hexafluoroacetylacetonate; H C = hydrocarbons; L = Ligand; K O R = potassium alkoxide; E-flo-164 = C hydrocarbon solvent; Peg: polyethyleneglycol.

x

50

50

2

2

Feedgas . %CO %H

%co

NiL /KOR

3

Homogeneous

Homogeneous

Slurry

Lurgi

(dppp)Rh(hfacac) Ru(acac)

Mode

Catalyst

Table II. Liquid Phase Catalytic Hydrogénation of CO2/CO


175 C0

2

+ 3H

CH3OH

->

2

+

1

H 0

A H ° 2 9 8 = - 52.8 kJ.mor

2

(9)

Our approach to C 0 hydrogénation is based on the liquid phase low temperature (LPLT) concept that was specifically developed at BNL to design enhanced carbon utilization catalyst for syngas conversion into methanol (77). 2

CO

+

2H

2

->

CH3OH

ΔΗ°

2 9 8

= - 128.6 kJ.mol

1

(10)

With C 0 as the reactant, the challenge was to initiate the water-gas-shift (WGS) reaction that can be combined with Reaction (10) to yield methanol. Since carbonyls of Mo, Co, Cu, Pt, Ru are known WGS catalysts, we conducted a preliminary screening study with these complexes to assess their ability to reduce CO to methanol. Peg-400/base or TEA itself provided the basic medium in these runs. The data are shown in Table III. Runs conducted in the batch unit with initial syngas (CO/H ) charge of 5 MPa showed that except for Co, all other metal carbonyls evaluated consumed syngas. The gas consumption rate was 2 mmol/min at 130°C with Cu and 0.7 mmol/min at 150°C with Mo. The gas chromatographic analysis indicated that methanol was indeed formed in all these runs. Further work is underway to select the best metal that will yield high methanol selectivity under mild conditions of temperature (< 150°C) and pressure (< 5 MPa).


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Table III. Catalytic hydrogénation screening runs Catalyst

T,

Cu(OMe) K PtCl Co (CO) Ru (CO) Mo(CO) 2

2

6

2

8

3

12

6

°c

130 150 130 160 150

1

Rate, mmol/min

Gas Consumed ,mmol

1.2 2.0 0 1.2 0.7

230 190 —

185 201

0.5 L A E Zipperclave batch unit; catalyst: 1 mmol; Base: Potassium methoxide = 100 mmol; solvent: TEA or Glycol = 120 mL; Syngas: H /CO ~ 2; Pi = 5 MPa at room temperature. 2

Geologic Formations: The Ultimate Slurry Reactor for Catalytic CO2 Reduction In the previous section, we described the results of our laboratory study to develop a highly efficient catalyst for methanol synthesis via C 0 hydrogénation 2


176 that operates in an amine or a glycol solvent. Successful development of this novel system has the potential of lowering the cost of C 0 sequestration by integrating the C 0 capture and utilization aspects but the cost of H remains an issue. Our aim is to design a highly effective catalyst for C 0 hydrogénation in the laboratory and apply these results to ultimately conduct C 0 reduction in geologic formations. Therefore, we envision geologic formations as slurry reactors to produce H from H 0 that is subsequently utilized to hydrogenate C 0 in these formations. We have based this approach on what is known about temperature and pressure conditions that exist in oil and gas reservoirs. We discuss below the basis of such an approach and the consequence of utilizing this pathway to recycle C 0 by its conversion into fuels. 2

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Basis for the Proposed Approach Oil and gas reservoirs have broad geographic distribution in the U.S. The porous and permeable reservoir rocks in oil and gas reservoirs include limestone, dolomite, and sandstone. The cap rock and basement rock which have a far lower permeability than the reservoir rock act as a seal to prevent the escape of oil and gas from the reservoir rock. Typical cap rock and basement rocks are clays and shale, that is, strata in which pores are much finer than those of reservoir rocks. The impermeable cap rock usually consists of shales that are clay minerals. The type of oil and gas reservoir depends on the boundary structure between the reservoir rocks and the cap rock. Many oil and gas accumulations (reservoirs) are trapped in either anticlines or salt domes. The major metal elements found in reservoir rocks are Na, Ca and M g . There are many other metal elements that occur in low concentrations in the rocks, including transition metals such as Fe, N i , M o , Pd. In essence, any element that has been identified in seawater can be found in the reservoir rocks. Oil and gas reservoirs typically contain H 0 . The water may occur as wetting films around the sand grains as well as in some completely filled pores. The water in most oil and gas reservoirs is heavily salted and different in terms of detailed composition, but is generally neutral with a pH close to 7, as in most eastern reservoirs in the U.S. (18). 2

The temperature and pressure gradients of the earth vary regionally and vertically. The global average geothermal gradient is about 22°C/km, but ranges from as low as 10°C in old shield areas to as much as 50°C/km in active zones of sea floor spreading (19). Geothermal gradients in sedimentary basins


177 generally range from 15-50°C/km; the average may be taken to be 30°C/km (20). The pressure gradients range from 0.434 psi for fresh water, 0.465 psi per foot for "standard" water containing 100 parts per thousand of dissolved solids, and 0.50-0.55 psi per foot for strong brines. The geostatic pressure depends on the bulk wet densities of the rocks including their fluids. The higher end of the overburden pressure gradient is 1 psi per foot (27). The estimated geologic conditions of many oil and gas reservoirs in Pennsylvania range from 1.1 to 5.7 miles in depth, 16°C to 150°C in temperature, and 17 to 86 M P a pressure.


The role of the sedimentary sulfur cycle in the diagenesis of organic matter is of wide interest. The sulfur content in crude oils varies from < 0.05 to 14 wt%. The subject has been reviewed in a recent book (22). The foregoing discussion sets the stage for the proposed concept. The essential elements of a catalytic system that can convert the sequestered C 0 into liquid fuels in geological formations are as follows: 2

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•

•

The cap rock and basement rock, which have a far lower permeability than the reservoir rock act as a seal to prevent the escape of fluids from the reservoir rock and can be viewed as a large natural reactor vessel. The reservoir rock must possess fluid-holding capacity (porosity) and fluid transmitting capacity (permeability). Transition metals such as Fe, N i , M o , Pd, that are normally found, though at ppm or ppb concentrations, in the reservoir rocks can be viewed as highly dispersed supported heterogeneous catalysts. The presence of oil and H 0 can be viewed as the available reaction solvents. H 0 will also serve as a natural source of H . The decreased concentration of 0 as a function of depth provides a natural reducing atmosphere for the proposed consecutive reduction reactions, i.e. reduction of H 0 to H followed by C 0 / H conversion to H -rich fuels. A t the reservoir depth of interest, the temperature and pressure range from: T: 16-150°C and P: 17-86 MPa. Sulfur could be implicated as a catalyst in the formation of hydrocarbons. 2

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2

Some direct supporting evidence for the catalytic effects of certain metals in naturally occurring rocks and the feasibility of H production from water by the action of transition metal species can be found in several recent papers (23,24). In these studies, it was shown during a one-year experiment that carbonaceous rocks (Monterey Formation, California, Eocene) that were low maturity and organic- rich (composition: 14 wt% total organic C, 7.5 wt% S, 350 ppm N i and 560 ppm V ) produced C H from n-octadecene-1 and H at 190°C. The reaction is believed to proceed via the hydrogenolysis of the Ο unsaturated hydrocarbon to C H at a rate of ~10" g CH /g. rock/d. The addition of H 0 , up to ~ 2.5 wt%, 2

4

2

ΐ 8

7

4

4

2


178

appeared to increase both catalytic activity and product selectivity. It remains to be established the nature of the catalytically active species for H production from H 0 , which is a key issue in the proposed C 0 reduction scheme. 2

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Effort at BNL AND PSU The proposed concept integrates C 0 sequestration with subsequent conversion in geologic formations. Catalytic activation of small gas molecules, namely C H , CO, C 0 and H that are major constituents of coal or natural gasderived synthesis gas is a subject of ongoing efforts both at B N L and P S U (25,26). The new concept in the proposed research seeks to constitute a new and catalytic path for C 0 conversion into reduced and useful products such as hydrocarbon fuels that only uses naturally available geothermal energy in oil and gas reservoirs following C 0 sequestration in such geologic formations. 2


4

2

2

2

2

H 0 2

-»

H

+

2

(0)

surroun

dings

(11)

In Reaction 1, the Ο from H 0 is stabilized by naturally occurring materials in the surroundings. The produced H then catalytically reacts with C 0 to produce reduced products (according to Reactions (1) and (2)) that can be recovered after prolonged storage in geologic formations. The initial challenge is to demonstrate the reaction sequence for C 0 to liquid fuels in micro pressure vessels under simulated geologic formation conditions in the laboratory. For this study, two Ni-rich natural rock samples, Niccolite (Ni As ) and Pentlandite (Fe, N i ) S , have been procured and their characterization is underway at the B N L National Synchrotron Light Source (BNL/NSLS). Runs are planned with these natural rocks in a micro-reactor to demonstrate: 1) catalytic production of H from H 0 and 2) catalytic hydrogénation of C 0 under the pressure and temperature conditions that are relevant to oil and gas reservoirs. The focus is on the feasibility of the production of liquid fuels from buried C 0 under natural reservoir conditions. We then plan to utilize these laboratory data and design a system to selectively enhance the desirable reactions by adding trace amounts, in a vapor or a liquid form, to the "natural reactor" i.e., a selected geologic formation site. The proposed concept of seeding the geologic formation site with a highly active catalyst to enhance the H 0 to H production and the subsequent C 0 / H reaction to produce energy liquids closes the natural carbon cycle in fossil fuels. Interestingly, the concept is an alternative to "seeding the ocean with Fe to increase the growth of phytoplankton to enhance the C 0 uptake. 2

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Finally, it should be mentioned that there may be other undesirable reactions or more desirable reactions in geologic formations in addition to what


179 we have considered; a number of issues need to be clarified, and many fundamental questions remain to be answered by further study.

Concluding Remarks The production of H -rich synthetic fuels by catalytically recycling carbon in C 0 is an attractive C 0 sequestration option. In this paper, the C0 -capture step that typically utilizes an amine or glycol solvent is integrated with subsequent catalytic hydrogénation of C 0 to methanol. The challenge is to design a highly active catalyst that can produce methanol from C 0 / H in basic medium under mild operating conditions. In the C 0 / H system, C 0 is first reduced to CO via the reverse water-gas-shift reaction. Preliminary data are presented for several metal catalysts that show activity for CO reduction. Further work is underway to select the best metal catalyst that will yield high methanol selectivity under mild conditions of temperature (< 150°C) and pressure (< 5 MPa). 2

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Future work will extend these laboratory studies to address a key issue of H cost in the overall scheme of C 0 sequestration via C 0 hydrogénation. Here, we envision geologic formations, specifically abandoned oil and gas reservoirs, as heated slurry reactors under pressure to catalytically produce H from H 0 that is subsequently utilized to catalytically hydrogenate C 0 in these formations. Knowledge gainedfromthe laboratory studies will allow selection of active catalyst(s) that could be added to the formations to hasten reaction kinetics. This approach, if successful, holds an enormous potential for carbon sequestration. 2

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Acknowledgments We thank Dr. Arun C. Bose of the US Department of Energy (US DOE) National Energy Technology Laboratory (NETL) for his valuable input. This work was partially funded by US DOE under Contract No. DE-AC0298CH10886.

References 1.

Carbon Sequestration: State of the Science. Office of Science and Office of Fossil Energy, U.S. Department of Energy: Washington, DC, February 1999.


180 2. 3. 4. 5. 6. 7. 8.


9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

23. 24. 25. 26.

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Catalytic Reduction of CO2 into Liquid Fuels: Simulating Reactions

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