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Catalytic reduction of carbon dioxide. The effects of catalysts and reductants
Energy Convers.
Pergamon
0196-8904(95)00070-4
Mgmt
Vol. 36, No. 69, pp. 573-576, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/95 $9.50 t 0.00
Catalytic Reduction of Carbon Dioxide - The Effects
of Catalysts
and Reductants
-
S.-E. Park, S.S. Nam, M.J. Choi and K.W. Lee* Korea Research Institute of Chemical Technology, P. 0. Box 107 Yusung, Taejon 305606, Korea
ABSTRACT Several trials were performed for the catalytic fixation of carbon dioxide by using hydrogen as well as methane as reductants in order to convert into useful chemicals, such as oxygenates and hydrocarbons and synthesis gas, respectively. As trials for the alleviation of chemical equilibrium limit in the CO hydrogenation into methanol, the hybridized catalysts such as H-zeolites and K-doped be/L catalysts were prepared by mixing with the methanol catalyst, Cu/ZnO/Al,O,. And the formation of oxygenated compounds and hydrocarbons, and the formation of methylformate were confirmed with the enhanced CO, conversions, respectively. Another trial was the FischerTropsch reaction approach to synthesize hydrocarbons directly with C02/H, over iron-based bimetallic catalysts. Fe-Co bimetallic catalysts showed over 60% CO, conversion. Finally, carbon dioxide reforming with methane was investigated over pentasil zeolite-supported nickel catalyst, which gave near equilibrium conversion of CO, and also near equilibrium yield on synthesis gas with high stability. And pentasil zeolite gave the superiority as a support, and alkaline promoters also attributed to have high dispersion and stability of nickel species.
INTRODUCTION Utilization of carbon dioxide as a raw material interests not only as carbon sources but also for the reduction of air pollution from the large amount of CO, in the flue gases. Many methods are proposed to maintain the concentration of atmospheric CO, or to reduce and recycle it. One of these is the chemical fixations, especially using catalysts.‘) Hydrogenation of carbon dioxide to methanol and its homologs over copper containing catalyst is one of the most widely studying subjects. The limitation in this reaction is the low conversion of CO, due to the chemical equilibrium and poor selectivity due to the reverse water gas shift reaction on the same Cu/ZnO catalysts. Therefore, many trials have been attempted not only to increase activity but also to facilitate the consecutive reactions of the primarily formed products for the diminution of CO selectivity. In our research lab,., we combined methanol synthesis catalysts with solid acid . These were proved to form drmethyl ether and hydrocarbons with improved conversion of CO, due to the consecutive conversions of methanol and CO. We also introduced K-Fe/L zeolite together with Cu/ZnO/Al,O, catalyst in order to reduce the CO selectivity as well as to improve CO, conversion that leads to methanol and methylformate. And hydrocarbon synthesis was attempted via the hydrogenation of CO, for the direct Fischer-Tropsch synthesis. That is another promising way to get chemicals and fuels. In this study, the synthesis of gasolines and fuels were intended by the hydrogenation of carbon dioxide. Based on the Fe/Al,O,-M-Ferrierite catalysts, bimetallic systems were introduced for hydrocarbon formation and possible product distribution. And we found our prepared catalysts produced higher conversion of CO,. and low molecular weight hydrocarbons. On the other hand, we carried out the reforming of CO, with methane. The limitation of methane reforming with CO, is that needs high temperature because of thermal equilibium. Recently, increasing the importance of the H&O ratio, the natural gas becomes more interest as the hydrogen source and olefin production. *) Group VIII metal catalysts have been known to give less carbon formation than nickel. However, nickel has been focused with special attention. Because, that is an active component of the catalysts, which have been used for steam reforming with economic reseasons. This study tried to figure out the behavior of pentasil zeolite-supported nickel catalysts and the effects of promoters in the CO, reforming of methane.
PARK et ul.: CATALYTIC
574
REDUCTION
OF CARBON
DIOXIDE
EXPERIMENTAL The methanol synthesis catalyst, Cu/ZnO/AI,O,(CuO:ZnO:AI,O =24:38:38),prepared by coprecipitation method were mixed with H-zeolites and K-Fe/L zeo?ite, which was prepared by treating H-L with 7% solution of K$[Fe(CN),] 3H,O. Hydrocarbons synthesis catalysts(F-T catalyst) were prepared by typical Impregnation method. The metal content was 10% for single and 5% for each as a second metal. Zeolite-supported nickel catalysts were prepared by solid-state reaction method and impregnation using typical incipient wetness method. The loading of nickel as a NiO was 6.6 wt.% and the molar ratio of KNiCa oxides was K:Ni:Ca = 0.08:1.0:2.2. The total metallic oxides content was 17.9 wt.%. Hydrogenation of carbon dioxide reactions(HJC0, = 3) were carried out in a fixed-bed reactor which is filled with reduced catalyst under 20-30 Kg/cm’, at 200-300°C and 3,080 ml/ cat.h space velocity and analyzed by on-lined GC. 8 arbon dioxide reforming reaction was carried out over supported Ni catalysts which were reduced in the reactor with hydrogen stream at 700% for 1 h prior to each catalytic measurement. With the help of several spectroscopic tools, In-situ IR measurements were performed. The adsorption of methanol, methyl formate, H,O, and CO was performed under 8, 6, 10, and 700 Torr, respectively. The intrusion temperatures for adsorption and for surface reaction and after adsorption of CO and methanol were r.t. and 15O”C, respectively.
RESULTS AND DISCUSSION Dimethyl ether, synthesized from the dehydration of methanol a key intermediate for MTG(methanol-to-gasoline) process. Table 1. Activity and selectivity Solid
SiOJAI,O,
acid silica silicalite H-ZSM-5 H-ZSM-5 H-ZSM-11 E$er. H-Y Reaction conditions
over solid acid catalyst, is
change of CO, over Cu/ZnO/Al,O.&24:38:38) Conv.(%)
Selectivity(%)
Yield(%)
ratio
CC*
co
CH,OH
infinite 1800 50
20.30 20.70 21.70 22.60 22.80 23.02 23.10 24.00
58.80 56.30 50.30 46.00 45.00 42.00 41.20 37.40
41.20 43.70
0.00 0.00
::*:: 19:70 17.98 17.20 12.50
10.90 34.20 35.30 40.10 41.60 50.10
:: !.: 1:5 : T=250°C,
P=2.03MPa,
+ Zeolites(l:l)
F/W=0.67ml/gcat.sec.,
DME
DME+CH,OH 8.40 9.10 10.70 12.20 12.54 13.32 13.60 15.40
R=HJCO,=3
As it is shown in Table 1, H-Zeolites were effective in increasing the activity and the selectivity for CH,OH and DME. And the formation of dimethyl ether was greately dependent upon the acid strength of zeolite in this hybrid system. The mixed catalysts systems were investigated as a trial giving multifunction to the catalysts in order to consecutively react CO and merhanol, which were supposed to be yield as primary products via Cu/ZnO-methanol catalyst. Not only the diminution of CO selectivity but also the consecutive conversion of methanol would be considered as a prominent trial for the alleviation of chemical limit in the hydrogenation of CO,. We introduced K-Fe/L zeolite together with Cu/ZnO/Al,O, catalyst in order to reduce the CO selectivity with the improved CO conversion. Cu-based methanol synthesis catabsts produce mainly CO and methanol with H,O in the methanol synthesis with CO, and H,. Cu/ZnO/AbO, catalyst only gave high CO selectivity and low methanol selectivity with poor CO, conversion. However, when a K-Fe/L zeolite catalyst was mixed with methanol synthesis catalyst, this gave higher CO, conversions with the reversed selectivities on CO and methanol. And moreover, small quantities of methyl formate were confirmed, which were yield more at lower reaction temperature. The i.r. spectrum of K-Fe/L zeolite after treating adsorbed CO and methanol at 150°C followed by flash evacuation gave evident characteristics of formed methylformate on KFe/L zeolite surface(Fig. la). Shoulder peaks in the C-H streching vibration bands due to HCOO- in methylformate. And also C=O of formate at 1683 cm-’ and C-H bending vibration
PARK e/ ol.:
CATALYTIC
REDUCTION
OF CARBON
DIOXIDE
575
bands at about 1450 and 1368 cm-’ could be confirmed as the characteristic bands of adsorbed methylformate. Similar experiment was performed by using CD,OD instead of CH OH in order to get rid of OH deformation band at near 1640 cm-‘. Fig. lb shows several peais of various C-O species, which indicated also the strong evidence on the formation of methylformate via methanol carbonylation on the K species in zeolite L catalyst. This had C=O streching vibration as well as carbonate anion peaks(1596 cm”) with the combination of formate peaks adsorbed on surface(1644 cm-‘) and potassium site(1665 cm*‘). And their C-O stretching and symmetric band were seemed to appear at 1448 cm” and 1325 cm-‘, respectively. This observation was tried to be figured out and confirmed via in-situ vaporphase formation of methylformate in IR cell with the adsorption of methanol and CO. This is because the carbonylation of methanol would be guessed as a plausible route for the formation of methylformate among several possibilities in the reaction environment. The reversed higher methanol selectivities than CO selectivities could be explained as a result of consecutive hydrogenolysis of methylformate into two molecules of methanol under high pressure of hydrogen.
Fi . 1. IR spectra of K-! e/L zeolite after surface reactions at 150 “C with adsorbed CO and CH,OH(a) and adsorbed CO and CD,OD(b).
~ & X 2 .$ 2 E
u
I
5%
a
1
Table 2 showed the conversion and product distribution from the hydrogenation of CO, over iron based catalysts. Table 2. Catalytic activity and hydrocarbon distribution in the hydrogenation iron based catalysts. Conv.(%)
of CO, over
Distribution of H.C.s (%)
Selectivity (%)
Catalysts
ZX? 0 FeCo/Al 203 FeNi/Al b 3 Fe/Al,C!~+i&rFer. Fe/AI,O,+CoFer. Fe/AI,O,+NiFer.
CO,
CO
HCs
C,
C,
C,
C,
C,
60.0 57.9 62.9 !!?z:z
19.7 14.8 28.9 17.0 16.6 17.4 20.9
80.3 85.2 71.1 83.0 83.4 82.6 79.1
40.4 30.6 44.9 48.4 39.9 43.9 59.3
16.0 14.9 15.3 17.9 16.1 16.0 12.1
28.3 19.1 18.0 8.2 19.5 18.3 9.1
17.1 17.9 14.9 17.0 16.8 14.9 10.2
8.3 7.4 6.9 8.0 7.7 6.9 9.3
61.8 58.2
Reaction conditions : P = 2.03 MPa , T = 300 “C The pure Fe is fastly deactivated in this system. The mixed metal catalysts showed a great influence on the catalytic activity and selectivity. The hydrocarbons were formed from The main products were paraffins and olefins, and CO, conversion the range of C, to C could be as high as 86%. The combination of Fe and Co showed maximum conversion up to 63% with moderate hydrocarbon distribution. This result suggests the combination of Fe and Co enhance catalytic activity because hydrogen and CO, are easily chemisorbed and activated on these surface. The combination of Fe and Mn increased the C,, hydrocarbon selectivity and the combination of Fe and Ni increased the CH, formation. Although the addition of mangnese component to iron catalyst showed a lower activity due to lower extent
576
PARK e/ ul.: CATALYTIC
REDUCTION
OF CARBON
DIOXIDE
of reduction, it gave a considerably higher selectivity towards C, hydrocarbons as well as lower olefins. Even though ion exchanged (Co, Mn, Ni) zeolite was physically mixed with iron catal st, that showed similar results. Therefore, it is an excellent application to introduce g ischer-Tropsch catalysts in the the hydrogenation of CO, and the product distribution can be controlled by these combinations. Supported Ni catalysts were investigated in the carbon dioxide reforming with methane and displa ed in Table 3. And catalytic activies were compared with the modified catalyst via K and 8 a as promoters. The addition of K and Ca oxides on pentasil zeolite supported Ni catalyst by the use of solid-state reaction method exhibited superior coke resistance in this reaction. It was reported that the addition of alkali promoters is effective in preventing the coke formation from methane during steam reforming. Fujimoto et al. pointed out that the CaO-supported Ni catalyst exhibited stable activity without coke formation due to high basicity of CaO. Thus, the low coke forming ability of KNiCa/ZSI(I) should be attributed to the favorable interaction between basic metals and support with nickel and the basicity induced by added Ca and K promoters. Table 3. Catalytic activities of the CO, reforming of methane at 7OO’C Conversion(%) Catalyst NiO/o-Al,O, NiO/SiO, NiO/ZSI K-Ni-Ca-0 /cc-A&O, K-Ni-Ca-OfSiO K-Ni-Ca-O$ZSP
S,,dm2/g)
CH,
Yield(%) CO,
320::
;:
47
287
78
58’ 17
2;: 137
;; 79
Reaction Conditions : P(CHS = 24 kPa, CO&H,
?A
CO
H,
46 76 77
46
:; 79
:; 78
5;
= 1, WHSV = 60,00Oh-’
The morphologies of ZSM-5 supported nickel catalysts were observed(data not shown). The KNiCa catalyst prepared by impregnation method as well as Ni only supported catalysts via both methods formed much filamentous coke after the reaction even still they showed considerable activity. However, the morphology of KNiCa catalyst via solid-state reaction had no difference from that of ZSM-5 support itself . This told the importance of promoters as well as the preparation method for the coke resistance. CONCLUSION 1. Hydrogenation of carbon dioxide (i) In the case of the Cu/ZnO/Al 0 -H-zeolite hybrid catalyst, the H-zeolite catalysts promoted the formation of CH$eH, by the dehydration of methanol. Thus, the CO, conversions were slightly enhanced due to the acidic function of H-zeolite through the consecutive conversion of methanol into DME. (ii) Methylformate was formed in the reaction of hydrogen and carbon dioxide by mixing with K-Fe/L catalyst. This was resulted from the secondary reaction of primary products such as methanol and CO. The route for the formation of methylformate would be plausible via carbonylation of methanol. (iii) Fischer-Tropsch catalysts, Fe-Co/Al,O, and Fe/Al,O,-Co-Ferrierite showed over 62% of CO, conversion. 2. Pentasil-type zeolite could be used as a support for nickel catalysts in the CO, reforming of methane. It exhibited high activity and stability due to not only the basicity of alkaline promoters but also the incorporation with zeolite support. These contributions of promoters could be enabled by the solid-state reaction method. By these catalysts, the conversion of CO, with methane gave near equilibrium as well as yielded on the ratio of CO and H, is one.
References 1. G. A. Sormorjai, Catal. Today, 18, 113 (1993). 2. S.-E. Park, J.S. Chang and K. W. Lee, Chem., Ind & Tech. 12 (1) 17-29(1994). 3. D. Liang et al., Proceeding of 1st lnt4 CO workshop, KRICT, Taejon, 155-163 (1993). 4. K. Fujimoto and K. Yokota, Chem. Sot. &n. Catal. Lett., 559 (1991). 5. T. Ishihara, K. Eguchi and H. Arai, J. Catal., 30, 225-238 (1987).
Pergamon
0196-8904(95)00070-4
Mgmt
Vol. 36, No. 69, pp. 573-576, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/95 $9.50 t 0.00
Catalytic Reduction of Carbon Dioxide - The Effects
of Catalysts
and Reductants
-
S.-E. Park, S.S. Nam, M.J. Choi and K.W. Lee* Korea Research Institute of Chemical Technology, P. 0. Box 107 Yusung, Taejon 305606, Korea
ABSTRACT Several trials were performed for the catalytic fixation of carbon dioxide by using hydrogen as well as methane as reductants in order to convert into useful chemicals, such as oxygenates and hydrocarbons and synthesis gas, respectively. As trials for the alleviation of chemical equilibrium limit in the CO hydrogenation into methanol, the hybridized catalysts such as H-zeolites and K-doped be/L catalysts were prepared by mixing with the methanol catalyst, Cu/ZnO/Al,O,. And the formation of oxygenated compounds and hydrocarbons, and the formation of methylformate were confirmed with the enhanced CO, conversions, respectively. Another trial was the FischerTropsch reaction approach to synthesize hydrocarbons directly with C02/H, over iron-based bimetallic catalysts. Fe-Co bimetallic catalysts showed over 60% CO, conversion. Finally, carbon dioxide reforming with methane was investigated over pentasil zeolite-supported nickel catalyst, which gave near equilibrium conversion of CO, and also near equilibrium yield on synthesis gas with high stability. And pentasil zeolite gave the superiority as a support, and alkaline promoters also attributed to have high dispersion and stability of nickel species.
INTRODUCTION Utilization of carbon dioxide as a raw material interests not only as carbon sources but also for the reduction of air pollution from the large amount of CO, in the flue gases. Many methods are proposed to maintain the concentration of atmospheric CO, or to reduce and recycle it. One of these is the chemical fixations, especially using catalysts.‘) Hydrogenation of carbon dioxide to methanol and its homologs over copper containing catalyst is one of the most widely studying subjects. The limitation in this reaction is the low conversion of CO, due to the chemical equilibrium and poor selectivity due to the reverse water gas shift reaction on the same Cu/ZnO catalysts. Therefore, many trials have been attempted not only to increase activity but also to facilitate the consecutive reactions of the primarily formed products for the diminution of CO selectivity. In our research lab,., we combined methanol synthesis catalysts with solid acid . These were proved to form drmethyl ether and hydrocarbons with improved conversion of CO, due to the consecutive conversions of methanol and CO. We also introduced K-Fe/L zeolite together with Cu/ZnO/Al,O, catalyst in order to reduce the CO selectivity as well as to improve CO, conversion that leads to methanol and methylformate. And hydrocarbon synthesis was attempted via the hydrogenation of CO, for the direct Fischer-Tropsch synthesis. That is another promising way to get chemicals and fuels. In this study, the synthesis of gasolines and fuels were intended by the hydrogenation of carbon dioxide. Based on the Fe/Al,O,-M-Ferrierite catalysts, bimetallic systems were introduced for hydrocarbon formation and possible product distribution. And we found our prepared catalysts produced higher conversion of CO,. and low molecular weight hydrocarbons. On the other hand, we carried out the reforming of CO, with methane. The limitation of methane reforming with CO, is that needs high temperature because of thermal equilibium. Recently, increasing the importance of the H&O ratio, the natural gas becomes more interest as the hydrogen source and olefin production. *) Group VIII metal catalysts have been known to give less carbon formation than nickel. However, nickel has been focused with special attention. Because, that is an active component of the catalysts, which have been used for steam reforming with economic reseasons. This study tried to figure out the behavior of pentasil zeolite-supported nickel catalysts and the effects of promoters in the CO, reforming of methane.
PARK et ul.: CATALYTIC
574
REDUCTION
OF CARBON
DIOXIDE
EXPERIMENTAL The methanol synthesis catalyst, Cu/ZnO/AI,O,(CuO:ZnO:AI,O =24:38:38),prepared by coprecipitation method were mixed with H-zeolites and K-Fe/L zeo?ite, which was prepared by treating H-L with 7% solution of K$[Fe(CN),] 3H,O. Hydrocarbons synthesis catalysts(F-T catalyst) were prepared by typical Impregnation method. The metal content was 10% for single and 5% for each as a second metal. Zeolite-supported nickel catalysts were prepared by solid-state reaction method and impregnation using typical incipient wetness method. The loading of nickel as a NiO was 6.6 wt.% and the molar ratio of KNiCa oxides was K:Ni:Ca = 0.08:1.0:2.2. The total metallic oxides content was 17.9 wt.%. Hydrogenation of carbon dioxide reactions(HJC0, = 3) were carried out in a fixed-bed reactor which is filled with reduced catalyst under 20-30 Kg/cm’, at 200-300°C and 3,080 ml/ cat.h space velocity and analyzed by on-lined GC. 8 arbon dioxide reforming reaction was carried out over supported Ni catalysts which were reduced in the reactor with hydrogen stream at 700% for 1 h prior to each catalytic measurement. With the help of several spectroscopic tools, In-situ IR measurements were performed. The adsorption of methanol, methyl formate, H,O, and CO was performed under 8, 6, 10, and 700 Torr, respectively. The intrusion temperatures for adsorption and for surface reaction and after adsorption of CO and methanol were r.t. and 15O”C, respectively.
RESULTS AND DISCUSSION Dimethyl ether, synthesized from the dehydration of methanol a key intermediate for MTG(methanol-to-gasoline) process. Table 1. Activity and selectivity Solid
SiOJAI,O,
acid silica silicalite H-ZSM-5 H-ZSM-5 H-ZSM-11 E$er. H-Y Reaction conditions
over solid acid catalyst, is
change of CO, over Cu/ZnO/Al,O.&24:38:38) Conv.(%)
Selectivity(%)
Yield(%)
ratio
CC*
co
CH,OH
infinite 1800 50
20.30 20.70 21.70 22.60 22.80 23.02 23.10 24.00
58.80 56.30 50.30 46.00 45.00 42.00 41.20 37.40
41.20 43.70
0.00 0.00
::*:: 19:70 17.98 17.20 12.50
10.90 34.20 35.30 40.10 41.60 50.10
:: !.: 1:5 : T=250°C,
P=2.03MPa,
+ Zeolites(l:l)
F/W=0.67ml/gcat.sec.,
DME
DME+CH,OH 8.40 9.10 10.70 12.20 12.54 13.32 13.60 15.40
R=HJCO,=3
As it is shown in Table 1, H-Zeolites were effective in increasing the activity and the selectivity for CH,OH and DME. And the formation of dimethyl ether was greately dependent upon the acid strength of zeolite in this hybrid system. The mixed catalysts systems were investigated as a trial giving multifunction to the catalysts in order to consecutively react CO and merhanol, which were supposed to be yield as primary products via Cu/ZnO-methanol catalyst. Not only the diminution of CO selectivity but also the consecutive conversion of methanol would be considered as a prominent trial for the alleviation of chemical limit in the hydrogenation of CO,. We introduced K-Fe/L zeolite together with Cu/ZnO/Al,O, catalyst in order to reduce the CO selectivity with the improved CO conversion. Cu-based methanol synthesis catabsts produce mainly CO and methanol with H,O in the methanol synthesis with CO, and H,. Cu/ZnO/AbO, catalyst only gave high CO selectivity and low methanol selectivity with poor CO, conversion. However, when a K-Fe/L zeolite catalyst was mixed with methanol synthesis catalyst, this gave higher CO, conversions with the reversed selectivities on CO and methanol. And moreover, small quantities of methyl formate were confirmed, which were yield more at lower reaction temperature. The i.r. spectrum of K-Fe/L zeolite after treating adsorbed CO and methanol at 150°C followed by flash evacuation gave evident characteristics of formed methylformate on KFe/L zeolite surface(Fig. la). Shoulder peaks in the C-H streching vibration bands due to HCOO- in methylformate. And also C=O of formate at 1683 cm-’ and C-H bending vibration
PARK e/ ol.:
CATALYTIC
REDUCTION
OF CARBON
DIOXIDE
575
bands at about 1450 and 1368 cm-’ could be confirmed as the characteristic bands of adsorbed methylformate. Similar experiment was performed by using CD,OD instead of CH OH in order to get rid of OH deformation band at near 1640 cm-‘. Fig. lb shows several peais of various C-O species, which indicated also the strong evidence on the formation of methylformate via methanol carbonylation on the K species in zeolite L catalyst. This had C=O streching vibration as well as carbonate anion peaks(1596 cm”) with the combination of formate peaks adsorbed on surface(1644 cm-‘) and potassium site(1665 cm*‘). And their C-O stretching and symmetric band were seemed to appear at 1448 cm” and 1325 cm-‘, respectively. This observation was tried to be figured out and confirmed via in-situ vaporphase formation of methylformate in IR cell with the adsorption of methanol and CO. This is because the carbonylation of methanol would be guessed as a plausible route for the formation of methylformate among several possibilities in the reaction environment. The reversed higher methanol selectivities than CO selectivities could be explained as a result of consecutive hydrogenolysis of methylformate into two molecules of methanol under high pressure of hydrogen.
Fi . 1. IR spectra of K-! e/L zeolite after surface reactions at 150 “C with adsorbed CO and CH,OH(a) and adsorbed CO and CD,OD(b).
~ & X 2 .$ 2 E
u
I
5%
a
1
Table 2 showed the conversion and product distribution from the hydrogenation of CO, over iron based catalysts. Table 2. Catalytic activity and hydrocarbon distribution in the hydrogenation iron based catalysts. Conv.(%)
of CO, over
Distribution of H.C.s (%)
Selectivity (%)
Catalysts
ZX? 0 FeCo/Al 203 FeNi/Al b 3 Fe/Al,C!~+i&rFer. Fe/AI,O,+CoFer. Fe/AI,O,+NiFer.
CO,
CO
HCs
C,
C,
C,
C,
C,
60.0 57.9 62.9 !!?z:z
19.7 14.8 28.9 17.0 16.6 17.4 20.9
80.3 85.2 71.1 83.0 83.4 82.6 79.1
40.4 30.6 44.9 48.4 39.9 43.9 59.3
16.0 14.9 15.3 17.9 16.1 16.0 12.1
28.3 19.1 18.0 8.2 19.5 18.3 9.1
17.1 17.9 14.9 17.0 16.8 14.9 10.2
8.3 7.4 6.9 8.0 7.7 6.9 9.3
61.8 58.2
Reaction conditions : P = 2.03 MPa , T = 300 “C The pure Fe is fastly deactivated in this system. The mixed metal catalysts showed a great influence on the catalytic activity and selectivity. The hydrocarbons were formed from The main products were paraffins and olefins, and CO, conversion the range of C, to C could be as high as 86%. The combination of Fe and Co showed maximum conversion up to 63% with moderate hydrocarbon distribution. This result suggests the combination of Fe and Co enhance catalytic activity because hydrogen and CO, are easily chemisorbed and activated on these surface. The combination of Fe and Mn increased the C,, hydrocarbon selectivity and the combination of Fe and Ni increased the CH, formation. Although the addition of mangnese component to iron catalyst showed a lower activity due to lower extent
576
PARK e/ ul.: CATALYTIC
REDUCTION
OF CARBON
DIOXIDE
of reduction, it gave a considerably higher selectivity towards C, hydrocarbons as well as lower olefins. Even though ion exchanged (Co, Mn, Ni) zeolite was physically mixed with iron catal st, that showed similar results. Therefore, it is an excellent application to introduce g ischer-Tropsch catalysts in the the hydrogenation of CO, and the product distribution can be controlled by these combinations. Supported Ni catalysts were investigated in the carbon dioxide reforming with methane and displa ed in Table 3. And catalytic activies were compared with the modified catalyst via K and 8 a as promoters. The addition of K and Ca oxides on pentasil zeolite supported Ni catalyst by the use of solid-state reaction method exhibited superior coke resistance in this reaction. It was reported that the addition of alkali promoters is effective in preventing the coke formation from methane during steam reforming. Fujimoto et al. pointed out that the CaO-supported Ni catalyst exhibited stable activity without coke formation due to high basicity of CaO. Thus, the low coke forming ability of KNiCa/ZSI(I) should be attributed to the favorable interaction between basic metals and support with nickel and the basicity induced by added Ca and K promoters. Table 3. Catalytic activities of the CO, reforming of methane at 7OO’C Conversion(%) Catalyst NiO/o-Al,O, NiO/SiO, NiO/ZSI K-Ni-Ca-0 /cc-A&O, K-Ni-Ca-OfSiO K-Ni-Ca-O$ZSP
S,,dm2/g)
CH,
Yield(%) CO,
320::
;:
47
287
78
58’ 17
2;: 137
;; 79
Reaction Conditions : P(CHS = 24 kPa, CO&H,
?A
CO
H,
46 76 77
46
:; 79
:; 78
5;
= 1, WHSV = 60,00Oh-’
The morphologies of ZSM-5 supported nickel catalysts were observed(data not shown). The KNiCa catalyst prepared by impregnation method as well as Ni only supported catalysts via both methods formed much filamentous coke after the reaction even still they showed considerable activity. However, the morphology of KNiCa catalyst via solid-state reaction had no difference from that of ZSM-5 support itself . This told the importance of promoters as well as the preparation method for the coke resistance. CONCLUSION 1. Hydrogenation of carbon dioxide (i) In the case of the Cu/ZnO/Al 0 -H-zeolite hybrid catalyst, the H-zeolite catalysts promoted the formation of CH$eH, by the dehydration of methanol. Thus, the CO, conversions were slightly enhanced due to the acidic function of H-zeolite through the consecutive conversion of methanol into DME. (ii) Methylformate was formed in the reaction of hydrogen and carbon dioxide by mixing with K-Fe/L catalyst. This was resulted from the secondary reaction of primary products such as methanol and CO. The route for the formation of methylformate would be plausible via carbonylation of methanol. (iii) Fischer-Tropsch catalysts, Fe-Co/Al,O, and Fe/Al,O,-Co-Ferrierite showed over 62% of CO, conversion. 2. Pentasil-type zeolite could be used as a support for nickel catalysts in the CO, reforming of methane. It exhibited high activity and stability due to not only the basicity of alkaline promoters but also the incorporation with zeolite support. These contributions of promoters could be enabled by the solid-state reaction method. By these catalysts, the conversion of CO, with methane gave near equilibrium as well as yielded on the ratio of CO and H, is one.
References 1. G. A. Sormorjai, Catal. Today, 18, 113 (1993). 2. S.-E. Park, J.S. Chang and K. W. Lee, Chem., Ind & Tech. 12 (1) 17-29(1994). 3. D. Liang et al., Proceeding of 1st lnt4 CO workshop, KRICT, Taejon, 155-163 (1993). 4. K. Fujimoto and K. Yokota, Chem. Sot. &n. Catal. Lett., 559 (1991). 5. T. Ishihara, K. Eguchi and H. Arai, J. Catal., 30, 225-238 (1987).