CO2 Conversion and Utilization - American Chemical Society

Chapter 9

Effective Conversion of CO to Valuable Compounds by Using Multifunctional Catalysts 2

Tomoyuki Inui

Downloaded by MIT on May 22, 2013 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch009

Gas and Chemical Research Division, Air Water Inc., Sakai 592-8331, Japan (Fax: (+81)-722-44-8085; email: [email protected])

Indispensable conditions for C O mitigation by catalytic conversion are enumerated. These are very rapid conversion rate and high selectivity to valuable compounds. Since reduction of C O needs expensive hydrogen, the ways to get hydrogen inexpensively are described. One is the reduction of C O by methane or natural gas instead of hydrogen. In order to realize this reaction without coke formation, the four-component composite catalyst, the Rh -modified Ni-based catalyst with Ce O and Pt as additives has been developed by author et al. Another is the on-site heat supply by catalytic combustion to compensate the large endothermic heat of reforming of methane. Especially, the addition of more easily combustible hydrocarbons such as ethane, propane, and butane, which are contained in natural gas, made possible marked decrease in furnace temperature around as low as 570 - 600 K . Even such lower furnace temperatures, the catalyst temperature rises up to around 970 Κ and an equilibrium conversion of methane is observed even at a very short contact time such as 5 msec. Simultaneous reforming of methane with CO and H O also achieved. Ultra rapid methanation of C O on the Ni-based three -component composite catalyst, Ni-La O -Ru, highly effective 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.

131 synthesis of methanol by C O hydrogenation on the Pd-Ga-modified Cu-Zn-Cr-Al-Ox-Al-Ox catalyst, the effective synthesis of ethanol by C O hydrogenation on the multifunctional catalyst are then summarized. Finally, highly effective syntheses of light olefins and gasoline via methanol synthesis from C O hydrogenation using multi-step reactors connected in series are described. The sequential results of these catalytic conversions of C O shows a high potential to realize the processes for mitigation or recyclic use of C O . 2

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Hence carbon dioxide is the final combustion product of organic compounds, C 0 itself has little value, and in order to obtain new products by chemical reduction, the large amount of additional energy, in particular expensive hydrogen, is necessary. However, the catalytic hydrogénation of C 0 is superior to other chemical conversion methods for C 0 . Because, C 0 can be converted with an extremely higher rate on the well designed solid catalysts than other chemical conversion methods, and desired compounds can be synthesized very selectively. The largest problem in the catalytic hydrogénation would be concentrated in the effective and economic production method of the huge amount of H as the reducing reagent for C 0 . Development of the highly active Ni-based reforming catalyst which works at much lower temperatures than that of the conventional catalyst will be introduced first. Since the novel catalyst has high activity for not only steam-reforming but also C0 -reforming of methane, H for C 0 hydrogénation could be replaced by C H and H 0 . In this review paper, the focus will be concentrated to the C 0 reduction by H or C H to synthesize highly valuable major building blocks for petrochemical industries such as ethylene, propylene, methanol, ethanol, and high quality gaseous and liquid fuels such as substituted natural gas and high octane-number gasoline. These high value products have a potential to partly compensate the cost of reducing reagents, hydrogen or methane, and these C0 -conversion routes lead to a new paradigm for chemical industries and energy usage cycles in many fields. 2

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Rapid C0 -Reforming of Methane 2

The major conventional production method of H is the steam reforming of saturated hydrocarbons, in particular natural gas or methane, on the stabilized supported N i catalyst, and the reaction is operated at a high temperature around 1,170 K . In order to moderate the coke deposit, an excess concentration of steam than the stoichiometry of the reaction, Eq. [1] is added in the feed. A s the conventional catalyst has no ability for C 0 2

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

132

activation, C 0 once formed by the shift reaction, Eq. [2] via Eq. [3] cannot be converted to other molecules by the reaction between C 0 formed and methane unconverted as Eq. [4]. 2

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CH CO CH CH

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165 kJ/mol -41 kJ/mol ΔΗ° 206 kJ/mol Δ Η ° = 248 kJ/mol

+ 2H 0 4H +C0 , + H 0 -* H + C0 , + H 0 - * 3 H + CO, +C0 2 H + 2CO, 2

2

ΔΗ° ΔΗ°

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[1] [2] [3] [4]

In recent years however, C0 -reforming of methane as expressed by E q . [4] has gathered a great deal of attention in the sequential international congresses on C 0 mitigation and/or utilization (1-10). Reflecting these situations, several elaborative reviews on this subject have been made by Fox III (11) in 1993, Rostrup-Nielsen (12) in 1994, Ross et al. (13) in 1996, Halmann and Steinberg (14) in 1999, and Bradford and Vannice (15) in 1999. Especially, the last one (15) summarized as many as 190 papers including classic papers. The most recent 16 papers including in this review were published in 1997. The focus of this review is concentrated mainly on the fundamental aspects such as activation of C H and C 0 , carbon deposition, kinetics, and reaction mechanisms. However, catalytic reaction-engineering aspects involving autothermal reforming and co-reforming such as C 0 + 0 and C 0 - H 0 - 0 are not mentioned. From the view points of the rapid reforming, which is applicable to industrialization, these reactionengineering aspects are indispensable, and indeed, in the most recent few years, the papers treated on these subjects are rapidly increasing (16-21). Including the most recent papers published until middle of 2000, the review article written by the author (22) on "Reforming of C H by C 0 Associated with 0 and /or H 0 " will be published from the Royal Society as Catalysis Vol. 16 in 2001. Therefore, here, only the essence of our recent research works on C0 -reforming is described below. It seems to be contradiction that in order to obtain H for C 0 hydrogénation, the additional fossil fuel is necessary to maintain the high reaction temperature, and moreover C 0 is produced as the by-product. However, if by the innovative improvement in catalyst structure, on which no coke deposit occurs and exhibits a very high reaction rate even at the low temperature range around 600 K , situation would become different. Because the heat to maintain such a medium-range temperature could be supplied by the waste heat of large scale facilities of industries and even by the accumulated heat of solar energy. Furthermore, the large endothermic heat of reaction could be supplied on-site by the catalytic combustion of more easily combustible hydrocarbons and/or a part of methane fed on the same catalyst surface, on which methane reforming reactions are advancing. In order to realize the rapid synthesis of hydrogen through methane reforming, the synergistic effect of composite catalysts and the combined reactions to compensate the large amount of endothermic heat were investigated. A Ni-based three-component catalyst such as Ni-Ce 0 -Pt 2


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133 supported on alumina-wash-coated ceramic fiber in a plate shape (23) was very suitable for the reforming of methane. The catalyst composition was set at 10 wt% N i , 5.6 wt% L a 0 , and 0.57 wt% Pt for example, or molar ratios of these components were 1: 0.2: 0.03. Even with such a low concentration, the precious metal enhanced the reaction rate markedly, and this synergistic effect was ascribed to the hydrogen spillover effect through the part of precious metal and it resulted in a more reduced surface of the main catalyst component. In particular, a marked enhancement in the reaction rate of C0 -reforming of methane was observed by the modification of a low concentration Rh to the Ni-Ce 0 -Pt catalyst (24). Fig. 1 shows that the four-component composite catalyst ( © in Fig. 1) exhibited a very high activity for C0 -reforming of methane, Eq. [4], with the stoichiometric ratio of the products approaching to the reaction equilibrium even such as a high space velocity (SV) 730,000 h" , or a very short contact time of the reaction gas of 5 msec, on the basis of the net volume of the catalyst and its support. The activity of the composite catalyst was much larger than the sum of the activities of the component catalysts, namely the Rh and the Ni-Ce 0 -Pt catalyst. This means that the composite catalyst involved a two-step spillover, i.e., hydrogen formed was adsorbed on the Rh part very rapidly, faster than on the Pt part, and then the spiltover hydrogen was abstracted by the Pt part, this being followed by its diffusion toward the major catalyst component, the N i part. Consequently, the N i part can be kept in a reduced surface state and the rapid reaction progress can occur on it. The role of C e 0 would be not only to promote dispersion of the N i component, but also to act as a transporting media for spiltover hydrogen. Hydrogen and C O were obtained in equivalent amounts by the reaction, and the space-time yield of H and C O obtained at 873 Κ was 7,690 mol/1 · h at 59% methane conversion. In a reaction of C H - C 0 - H 0 - 0 on the four components catalyst, an extraordinarily high space-time yield of hydrogen, 12,190 mol/l-h, could be realized under the conditions of very high space velocity (5 msec) (25). In order to overcome the restriction in conversion of the reaction equilibrium, the combustion of more easily combustible hydrocarbons such as ethane or propane, which are involved in the natural gas, was then combined by adding these hydrocarbons and oxygen. The aimed reactions to produce only syngas are expressed as the following Equations [5] and [6], including generation and consumption of heat by combustion and reforming, respectively. 2

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(x+5y) C H + χ C 0 + y C H + 3.5y 0 4

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ΔΗ (x+7z) C H + χ C 0 + ζ C H + 5z 0 4

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ΔΗ

-> (2x+7y) C O + (2x+13y) H

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7 7 3 Κ

= 259x - 240y kJ/mol

-> 7 7 3 Κ

[5]

(2x+10z) C O + (2x+18z) H

= 259x - 375y kJ/mol


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[6]

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Φ SV 73,000b- (CT 49.3ms) 1

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C H conversion

(%)

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Figure 1. Synergistic effect in Rh-modified Ni-Ce 0 -Pt catalyst caused by combination with each ingredient upon the C0 ~reforming of methane Catalyst supported on alumina washcoated ceramic fiber Φ 10.0wt% Ni - 6.0wt% Ce 0 Φ 1.0wt% Ft, Φ 0.2wt% Rh, φ lO.Owtfo Ni - 6.0wt% Ce 0 - 1.0wt% Pt, ® 10.0wt% Ni - 6.0 wt% Ce 0 - 0.2wt% Rh, @ lO.Owtfo Ni - 6.0 wt% Ce 0 - 1.0wt% Pt - 0.2wt% Rh Feed gas: 10 mol% CH -10 mol% C0 - 80mol% N 2

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SV:WBÊ 73,000 K ; È23 1

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730,000h

Dotted line: Equilibrium conversion level at 873 Κ


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φ

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(66.2%), 2,290 mol/l-h (36.2%), 16,800 mol/l-h

φ

Mffi

(82.2%), 3,400 mol/l-h (47.8%), 20,360 mol/l-h

mm

(80.8%), 3,160 mol/l-h


(57.3%), 24,870 mol/l-h 0

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J-

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25,000

Space-time yield of syngas (mol/l-h) Figure 2. Effect of catalytic oxidation of ethane or propane on the C0 reforming of methane at two kinds of space velocities 73,000 and 730,000 h Catalyst: The Rh-modified four-component catalyst Gas composition: φ 35% CH -10% CO - 55%N φ 35% CH -10% C0 - 5% Cflt -17.5% 0 - 32.5% N φ 35% CH -10% C0 - 3.3% CsH -16.5% 0 - 35.2%N Catalyst-bed temperature: 700 °C (Furnace temperature: 500 °C) Pressure: 1 atm. Numerals in parenthesis are methane conversion. 2

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In case of ethane or propane addition in C0 -reforming of methane, the catalyst temperature abruptly rose even at a very lower furnace temperature such as below 625 K, and it was maintained at much higher temperature than the furnace temperature, indicating that by the in-situ heat supply due to the catalytic combustion, the methane conversion was eventually induced at a very lower temperature range. As shown in Fig. 2, an extraordinarily high space-time yield of syngas, as high as 25,000 mol/l-h was obtained at a considerably low furnace temperature around 773 Κ (26). 2

Rapid C0 -Methanation 2

Catalytic hydrogénation of carbon oxides was found by Sabatier and Senderens (27) at the beginning of twentieth century. Since then, a numerous studies have been done (28, 29). Industrially, this reaction has been applied to the purification of H -rich gas and production of substituted natural gas (30). Two decades ago, methanation reactions, i.e., inverse reactions of Eqs. [3] and [1] were applied to energy saving devices and processes. A typical one known as EVA-ADAM process (30) was achieved in Germany. The 2


136 process consists of two reaction systems. The one (EVA) is endothermic steam reforming of methane, i.e., Eqs. [1-3]. this requires a large amount of energy at a temperature level of more than 1,200K, which is supplied form the high temperature gas furnaces. Another one (ADAM) is exothermic methanation of CO and C 0 , i.e., inverse reaction of Eqs. [3] and [1], where heat is set free over a wide range of temperature. The energy taken up in EVA is transformed into a cold H -rich gas that is transported to ADAM, where heat is released according to the requirements of the heat market. Recently, C 0 methanation has had a new significant object concerning with the global warming problem. Since methanation of C 0 needs four times molecules of H , it seems to be disadvantageous. However, as the rate of C 0 methanation is far beyond all other methods for chemical conversion of C 0 to hydrocarbons and oxygenates. The largest consumption of hydrogen per unit C 0 mole equivalents to that the largest energy taking into methane molecules. Therefore, the methane synthesized by hydrogénation of C 0 is expected as the transportation media of the energy injected to the reaction. When the rapid C 0 methanation progress at a low temperature range around 470 - 570 K, the large exothermic reaction heat itself is available, and furthermore, when the methane formed is used as the fuel, a high temperature even 1,300 Κ can be obtained. Therefore, it is thought that the rapid C0 -methanation is significant as one kind of the chemical heat pump, by which a low temperature, i.e., a low-value energy, can be transformed into a high temperature, i.e., a high-value energy, through chemical reactions. Most recently, Halmann and Steinberg reviewed C 0 methanation (32) in their monograph. As shown in this review, the most of the studies on C 0 methanation used supported single component catalysts, except small number of studies, such as Co/Cu/K catalyst (33), Rh/Ce0 /Si0 (34), FeMn doped with Rh and La (35), Fe-Zr-Ru (36), and Ni-La 0 -Ru supported on spherical silica or alumina reported by the author et al. (37). The last one had been developed more than two decades ago, however, this catalyst is regarded as one of the most active methanation catalysts among the whole catalysts, as recognized by Halmann and Steinberg in their review. The author has described in detail the sequential results on this catalytic performance and the reason of exerting the ultra-rapid reaction rate in the review article of "Methanation" in "Encyclopedia of Catalysis" (38), which is now under preparation with editing by Professor Horvâth et al., and will be published soon. Therefore, here, only the essence of these works is described below. The author et al. have studied on the C 0 methanation since 1970's, and developed the Ni-based three components composite catalyst supported on the spherical silica support having meso-macro bimodal pore structure (37). The catalyst contained La 0 by 1/5 mol of Ni, and Ru by 1/30 mol of Ni. As shown in Fig. 3, combination of these three kinds of catalyst components exerted an evident synergistic effect on the methanation rates of C 0 , and showed the high conversion rate to synthesize methane exclusively (24, 37). 2

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137 The catalyst made possible the C O - C 0 co-methanation (39), and therefore, the process of C 0 elimination in the conventional S N G process could be omitted. The cause of the high performance was elucidated as follows; the adsorption capacity of catalyst for C 0 was increased by the weak basicity of L a 0 , and hydrogen adsorption was markedly enhanced by combining Ru, which worked as the porthole of hydrogen spillover. The meso-macro bimodal pore structure has the roles of the supported bed for catalytic substances and the pass for quick diffusion of the reactants, respectively (37). 2

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5.0wt%Ni |


0.7wt%Ru

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a

Ratio ofSTY >

11.4

5.5wt%-0.8wt%Ru 4.6wt%-2.6wt%La 0 2

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4.3wt%Ni-2.5wt%La O -0.7wt%Ru 2

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Relative methanation activity Figure 3.

Activity of C0 methanation for the Ni-La

CO2 Conversion and Utilization - American Chemical Society

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