Methanation of Alkali Carbonate and CO2 with Water in the Presence of Raney Fe-Ru/C Mixed Catalyst

石油学会誌 Sekiyu Gakkaishi, 42, (3), 157-164 (1999) 157

[Regular Paper] Methanation of Alkali Carbonate and CO2 with Water in the Presence of Raney Fe-Ru/C Mixed Catalyst

Kiyoshi KUDO* and Koichi KOMATSU

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011

(Received November 9, 1998)

The methanation of alkali metal carbonates and CO2 with water were investigated in the presence of Raney Fe- carbon supported ruthenium (Raney Fe-Ru/C) mixed catalyst in water. The methanation were not affected by Raney Fe alone, but the formation of alkali formate was found for the CO2 reaction. The combination of Ru/C with Raney Fe, however, created a high activity for the formation of methane at high temperatures (>300℃). The reactions were also controlled by the reaction temperatures, i.e. the selectivity and yield of methane increased with increasing temperature, suppressing the formation of metal formate. One notable characteristic in the present reactions is a rapid formation of a considerable amount of metal formate in the initial stage. It is assumed that the methanation occurs via the formation of metal formate and its subsequent hydrogenation by nascent hydrogen which is produced from water by action of Raney Fe. The apparent activation energies for the methanation of Na2CO3 and CO2 in aqueous NaOH solution on Raney Fe-Ru/C mixed catalyst were estimated to be 14 kcal mol-1 and 17 kcal mol-1, respectively.

1. Introduction convert some metal carbonates into hydrocarbons, but in spite of high temperature such as 800℃3) or 400℃4), Natural metal carbonates, in particular the abundant unfortunately the realized yields were exceedingly low. alkali metal and alkaline earth metal carbonates, are The authors previously reported that palladium chloride formed by various carbonization processes with atmos- is a very effective catalyst for the reduction of potassi- pheric CO2. They not only represent an important um carbonate to produce potassium formate in aqueous buffer system within the ecological carbon cycle, but solution in atmosphere of H25). The authors have now also play an important part in the natural fixation of developed the previous study, with particular attention CO2, which is one of the so-called "greenhouse gases" directed to search for the possibility to limit the reduc- and significantly contributes to the global warming, at tion of CO2 or alkali metal carbonates, with water as present. source of hydrogen and convert them into useful chem- Catalytic hydrogenation of CO2 to give valuable ical materials. chemicals and fuels such as methanol and methane has In this paper, the authors report details of the unique recently been recognized as one of important recycling process of methanation of alkali carbonates and CO2, technologies for recovered CO2. Although methana- using water-Raney alloy system as provider of molecu- tion1) or methanol synthesis2) were actively studied lar hydrogen in place of H2 (Eqs. (1)-(5), where Al recently, most of these previous ones were limited to cited in equations is aluminum metal derived from the use of hydrogen gas (H2) which is mainly prepared Raney alloy). Both methanations can not be carried by water gas shift reaction that produces CO2 as an out considering the catalysts with respect to Raney inevitable by-product. The direct utilization of water alloy. Methane, however, was obtained selectively in for the reduction of CO2 or metal carbonate is one ulti- acceptable yields. In addition, the results obtained for mate goal of chemists, since water is the vast natural both, the reactions of alkali carbonates and CO2, were resource for hydrogen, sufficient to reduce great quanti- compared with each other, and with previously reported ties of CO2 at a time. Furthermore, the reduction of results for H2-CO2 methanations. CO2 in aqueous solution is a particularly attractive 2/3Al(c)+2H2O(1)=H2(g)+2/3Al(OH)3(c) approach to the utilization of CO2, as water is the com- ΔH0=-68.1kcal mol-1,ΔG0=-94.7kcal mol-1 mon solvent for the recovery of CO2 from process fuel (1) gases. Na2CO3(aq)+4H2(g)=CH4(g)+2NaOH(aq)+H2O(1) Recently, some interesting attempts were made to ΔH0=-33.9kcal mol-1, ΔG0=-17.9kcal mol-1

(2) * To whom correspondence should be addressed.

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Na2CO3(aq)+7H2O(1)+8/3Al(c) Table 1 Reduction of Metal Carbonates with Water in the =CH4(g)+2NaOH(aq)+8/3Al(OH)3(c) Presence of Raney Alloya)

ΔH0=-306.3kcal mor-1, ΔG0=-396.7kcal mol-1

(3)

2/3Al(c)+2H2O(1)=H2(g)+2/3Al(OH)3(c) (1) CO2(aq)+4H2(g)=CH4(g)+2H2O(1) ΔH0=-55.6kcal morl,ΔG0=-33.3kcal mol-1

(4)

CO2(aq)+6H2O(1)+8/3Al(c)=CH4(g)+8/3Al(OH)3(c) ΔH0-327.8kcal mol-1, ΔG0=-412kcal mol-1

(5)

2. Experimental

2.1. Materials Alkali and alkaline-earth metal carbonates were obtained commercially, and further dried by heating (at a) All the reactions were carried out with 50% Raney metal (0.2 g), 5% Ru/C (0.05g), carbonate (5mmol), and H2O (8ml) 180℃) in reduced pressure before use. Carbon diox- under Ar atmosphere at 380℃ for 2h. ide (99.9%), commercial grade, was supplied by Kyoto b) Yields were calculated on the basis of cabonate loaded. Teisan Co., and used without further purification. c) This reaction was carried out in hydrogen, pressure (7 atm), 50% Raney alloy, 5% carbon-supported ruthenium, and without Raney alloy. d) 10mmol. other metal catalysts were purchased from Nacalai Tesque Co. 2.2. Measurements The GC analysis was performed on a Hitachi the HPLC analysis of alkaline metal formate. GC-O23 using a column, packed with active carbon, In the CO2 reaction, the mixture of 50% Raney Fe for gaseous products. The peak areas were deter- (0.2g), Ru/C (0.05g), and aqueous 1.25mol/l-NaOH mined by using a Shimadzu chromatopac C-R6A inte- solution (8ml) was pressurized with CO2 (50atm at grator. The HPLC analysis for alkaline metal formate room temperature) in place of alkali carbonate. The was carried out on a Shimadzu LC-10A using a column yield of products cited in this paper is determined as a packed with SCR-101H (25cm×6mm) eluted with molar percent relative to employed carbonate or alkali aqueous perchloric acid (pH 2.1) solution. The con- base. tents of alkali and alkaline-earth metal carbonate and 2.4. Determined of Initial Rate hydrogencarbonate in the aqueous solution were deter- Kinetic measurement was carried out at reaction mined by classical titration method with 0.1M HCl, temperatures ranging from 240℃ to 380℃. First, a using phenolphthalein and methyl orange as an indica- mixture of 50% Raney Fe (0.2g), and 5%Ru/C (0.05 tor. g), and Na2CO3 or CO2 (50atm) was charged into the 2.3. General Procedure autoclave, as described above. After the autoclave The reaction was carried out in a batch reactor, using was preheated at a specified temperature (ca. 30min), abundance of water. The mixture of 50% Raney Fe water (8ml) was introduced into the autoclave by (0.2g), 5% carbon-supported ruthenium (Ru/C) (0.05 pump, and immediately the shaking was started and the g), and water (8ml) was charged into a shaking-type time was recorded as zero of the reaction time. After autoclave (made of stainless-steel; ca. 30ml) contain- a lapse of reaction time, the autoclave was allowed to ing several stainless-steel stir balls. The carbonate cool to room temperature, rapidly. The reaction and reaction was conducted, having alkali carbonate (5 analysis were carried out as above. The initial rate ν0 mmol) loaded into the mixture. After the atmospheric was determined from the experimental equation, y= air was carefully replaced with argon, the mixture heat- t/(at+b), where y=yield of methane (mmol), t=reac- ed and shaken constantly at 380℃ for 2h, the pressure tion time (min), and a and b=constants. Further, the increased to 220atm. After the autoclave was rapidly initial rate, v0=(dy/dt)0=1/b(mmolmin-1). cooled by blowing air, the reaction gas was collected in a gas-burette, and then determined by GC. The 3. Results and Discussion remaining reaction mixture was fed into water, after which the solid materials were filtered off. The fil- 3.1. Reaction of Alkali Carbonates trate was subjected to titration of alkali compounds and Table 1 presents the typical results showing the effi-

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ciency of combination of Raney alloys and an additive catalysts to promote the reaction of various alkali carbonates with water at 380℃ using the batch system operation. This reaction temperature, 380℃, was determined by preliminary experiments as the optimum temperature for methanation. When a mixture of Na2CO3 (5mmol),50% Raney Fe (0.2g), and 5% Ru/C

(0.05g) in water (8ml) was heated at 380℃ for 2h, the total pressure increased to 220 atm, and methane Scheme 1 was obtained as a main product along with an extreme- ly small amount of sodium formate and a significant amount of H2 as by product (Run 1 in Table 1). The yield of products listed in Table 1 is defined as product molar percent relative to the principal carbonate. Individually, Raney Fe and Raney Ni exhibited different activities for the reaction, i.e. Raney Fe alone did not exhibit any activity for the formation of methane and sodium formate from Na2CO3, while Raney Ni alone exhibited activity for the reaction as shown in Runs 2 and 4 in Table 1. Furthermore, when a catalytic amount of Ru/C was added to the Raney Fe system, methane was obtained more effectively than added to Raney Ni system (Runs 1 vs. 4 in Table 1). Although the additive effect of Ru/C catalyst on the Raney Ni system for the reaction of alkali metal car- Conditions: Na2CO3 (5mmol), Raney Fe(0.2g), 0.5% Ru/C bonate was much smaller than that on the Raney Fe (0.05g), H2O(8ml), Ar (5atm), 2h. ●: HCO2Na, ■: CH4, system (Run 5 in Table 1), the greater effects were ○: Conversion. observed, especially for the reactions of alkaline-earth Fig. 1 Effect of Reaction Temperature on the Reduction of metal carbonates (Runs 9 vs. 10, 12 vs. 13 in Table 1). Na2CO3 with Water in the Presence of Ru/C-Raney Fe Other additive catalysts such as PdCl2, Pd/C. Rh/C, and Catalyst Ru/alumina were found inferior to that of Ru/C in yield and selectivity of the methanation. Such efficient methanation over Ru catalyst was reported in previous carbonates, such as magnesium and calcium carbonate, studies on the reduction of CO2 with H26)-11). It is of showed somewhat lower yields than that of alkali metal interest that the formation of CO6),11)and C2-C4 hydro- carbonates (Runs 9-14 in Table 1). Barium carbon- carbons12),13), reported by others on the CO2-H2 metha- ate did not show appreciable yield of the products, pre- nations, were not observed at all in the present reac- sumably because of its high stability under given conditions. It should be noted that when only Ru/C catalyst tion, since the starting material was recovered was used for the reaction of Na2CO3 with H2 (10 atm), unchanged. It is noteworthy that ammonium carbon- yields of both methane and sodium formate were ate, (NH4)2CO3, also gave methane in almost compara- almost negligible (Run 3 in Table 1). This unexpect- ble yield with that of alkaline-earth metal carbonates in ed result, which is an important feature in the study, the same conditions (Run 15 in Table 1). The higher suggests both effective participation of a nascent reactivity of Cs2CO3 has been observed in the authors' hydrogen (H*) formed by the reaction of Raney alloy previous study: a reductive capture of CO2 with CO to and water, and the synergistic effect between Raney Fe give oxalate14). The difference in reactivity among the and Ru catalyst in the methanations. A control experi- alkali metal carbonates is attributed to the differing ment conducted in the absence of these metal carbon- sizes of the alkali metal ions. The cesium ion, which ates, gave neither methane nor metal formate except for has the greatest ionic radius, is considered to be only H2 liberated. This result implies that the carbon of weakly paired with the counter-anion of the carbonate. methane and metal formate originates from the carbon- This would presumably facilitate the activation of the ate carbon employed. carbonate by interaction with the surface of the catalyst The authors also compared the reactivities of several as will be described later (Scheme 1). alkali and alkaline-earth metal carbonates. As is The difference in the catalytic features of Raney Ni apparent from Table 1, alkali metal carbonates, espe- and Raney Fe for the reactions (Runs 2 and 4 in Table cially Cs2CO3 (Runs 7 and 8 in Table 1), gave higher 1) is essentially attributed to the differing oxidation- yields in the methanation. Other alkaline-earth metal reduction potential of the metals. It is well known

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Table 2 Catalytic Activity for the Reduction of CO2 with Watera)

Conditions: Na2CO3 (5mmol), Raney Fe (0.2g), 0.5% Ru/C

(0.05g), H2O (8ml), Ar (5atm), 2h. ●: HCO2Na, ■: CH4, a) The reaction were carried out in a stainless-steel autoclave (30 ○: Conversion. ml). Conditions: Raney alloy (0.2g), additive catalyst (0.05 g), CsOH (10mmol), H2O (8ml), CO2 (50atm at room tem- Fig. 2 Time-course of Reduction of Na2CO3 with Water at perature), 380℃, and 2h. 280℃ b) Determined by HPLC and GLC, and based on CsOH employed. c) 50% Raney alloy powder. d) 5% carbon-supported Ru powder. that, at high temperature, iron metal has a higher tend- e) NaOH (10mmol) used in place of CsOH. ency to be oxidized by water compared to that of nickel f) 5% carbon-supported Rh powder. metal (H2O+Fe=FeO+H2)15). In addition, Aresta et g) 5% carbon-supported Pd powder. h) 5% alumina-supported Rh. al.16) isolated the Ni-CO2 complex in which the CO2 ligand is coordinated through the carbon atom and one of the oxygen atoms. In the case of Raney Ni, there- fore, the carbonate or CO2 would be fixed as a ligand reaction, there was little change in product yields, giving an active surface species which would be hydro- though the amount of H2 liberated (5.7mmol) is suffi- genated stepwise to formate and methane. This cient to meet the need of the reaction. As will be dis- methanation, however, is not catalyzed by Raney Ni. cussed below, the presence of a maximum point in This seems, likely, to cause carbon deposition17). As a yield of the formate is of interest relative to the activa- result, the Raney Fe-Ru/C mixed catalyst was subjected tion step of carbonate. This discovery suggests that to further investigation. this overall reaction consists of three steps, i.e. rapid Shown in Fig. 1 are the effects of the reaction tem- liberation of nascent hydrogen (H*) from water by perature on the yields and products distribution in action of Raney alloy as the essential first step (Eq. Na2CO3 methanation. The formation of methane (6)), and first reduction of Na2CO3 or CO2 to sodium increased in concert with the formation of sodium for- formate (Eq. (7)) followed by the slower Ru-catalyzed mate as the temperature was increased; the highest hydrogenation to give methane (Eq. (8)). This is also yield and selectivity of methane were achieved at about supported by observing that the reaction of sodium for- 380℃. Decreasing the temperature to 250℃ greatly mate (10mmol) in place of a metal carbonate in condi- retarded the methanation, and the yield of sodium for- tion similar to that in Run 1 shown in Table 1, but mate increased. Since the critical temperature of lower temperature of 320℃ instead of 380℃, gave water is 374.2℃, the change in the physical property of 18% yield of methane. the medium could cause such variation in reactivity. 2/3Al+2H2O→2H*+2/3Al(OH)3 (6) At the temperatures below a critical point of water, the Na2CO3(or CO2/NaOH)+2H*→ reaction proceeds mostly in liquid phase, while at HCO2Na+NaOH(or H2O) (7) supercritical region, the water is expected to be mostly HCO2Na+6H*→CH4+NaOH+H2O. (8) in gas-like phase. H*: nascent hydrogen Shown in Fig. 2 is a typical time-yield profile of the 3.2. Reaction of CO2 in Aqueous Alkaline Solu- products in the reaction of Na2CO3 at 280℃ and 70 tion atm. At an early stage (a reaction time of ca. 0.5h), Table 2 presents the results of preliminary investi- the yield of sodium formate reached its maximum gations of catalysts for promoting the methanation of value, and then the yield of methane gradually CO2 with water in the presence of CsOH at 380℃. increased in concert with the formation of formate as When a mixture of CsOH (10mmol) and 50% Raney the reaction proceeded. The rate of reaction gradually Fe (0.2g), and 5% Ru/C (0.05g) in water (8ml) was decreased at a later stage. Even after a prolonged heated under the pressure of CO2 (50atm at room tem-

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perature) at 380℃ for 2h, the total pressure increased uted to those different properties of CO2 and the metal to 320 atm, and methane was obtained as the main carbonate for producing transition metal complexes. product (45.3% based on the CsOH employed or 90.8 It is well known that introduction of CO2 into a transi- mol per Ru atom) with a significantly small amount of tion metal-hydrogen bond produces a formate complex by product cesium formate (1.2%), and liberation of H2 which reacts with water to yield formic acid18). (30%) (Run 5 in Table 2). First, the authors also Further, the addition of Ru/C catalyst to the Raney Fe compared the efficiency of some Raney alloys, such as system realized methanation more efficiently than in Raney Fe, Raney Ni, Raney Co, and Raney Cu, for CO2 other catalytic system, just as that for metal carbonate methanation in the conditions described above. As (Runs 5 and 7-10 in Table 2). shown in Table 2, Raney Ni gave methane with high In the same way as that occuring in carbonate reac- selectivity, as high as that of the carbonate reaction, tions, the authors have also found significant effect of while Raney Fe, Co, and Cu gave cesium formate as a alkali metal ion added, especially cesium ion, i.e. the main product, even in the absence of Ru/C catalyst yield of methane in an aqueous CsOH solution was (Runs 1-4 in Table 2). These results differ from that about double that in an aqueous NaOH solution (com- found in the reaction of alkali carbonates, by Raney Fe pare Runs 5 and 6 in Table 2). alone (Run 2 in Table 1) in which alkali formate was Further, the yield of sodium formate increased con- not formed. This difference could possibly be attrib- tinuously with increasing concentration of NaOH, whereas the yield of methane increased with increase in the concentration of NaOH up to 1.25mol l-1, then levelling off in the same way as in the liberation of H2. The effect of the partial pressure of CO2 on the yields of methane and sodium formate was significant at low pressure (<2atm), but approached zero-order as pressure was raised (>5atm). Shown in Fig. 3 is the effect of the reaction temperature on the yields and distribution of products in CO2 reaction, using the Raney Fe-Ru/C mixed catalyst in an aqueous NaOH solution. The reaction temperature has a marked effect on product distribution. At low temperatures (<240℃), the sum of the yield of sodium formate and that of methane was less than 14 mol%, and the formate was obtained as main product. The Conditions: Raney Fe (0.2g), 0.5% Ru on carbon (0.05g), formation of methane increased in concert with the for- NaOH (10mmol), H2O (8ml), CO2 (50atm at room tempera- mation of sodium formate as the temperature is ture), 2h. ●: HCO2Na, ■: CH4, ○: H2. increased, the highest yield (26%) and selectivity

Fig. 3 Effect of Reaction Temperature on the Reaction of CO2 (95%) of methane being achieved at about 380℃. in the Presence of Raney Fe-Ru on Carbon Mixed Figures 4a and 4b show the time-yield profile of the Catalyst products in the CO2 reaction in varying amounts of

Conditions: CO2 (50atm at room temperature), NaOH (10mmol), Raney Fe (0.2g), H2O (8ml), 280℃. a) mol/mol-NaOH. ○: Ru/C=0, □: Ru/C=0.01g, ●: Ru/C=0.03g, ■: Ru/C=0.05g.

Fig. 4 Effect of Amount of Ru/C on Yields of Methane (a) and Sodium Formate (b)

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Conditions: Na2CO3 (5mmol), Raney Fe (0.2g), 5% Ru/C (0.05g), H2O (8ml), Ar (5atm), 2h. a) mol/mol-Na2CO3. □: 280℃, ●: 320℃, ○: 350℃, ■: 380℃.

Fig. 5 Effect of Reaction Temperature on Na2CO3 Methanation (a) and Arrhenius Plots (b)

Reaction conditions: CO2 (50atm at room temperature), Raney Fe (0.2g), 5% Ru/C (0.05g), NaOH (10 mmol), H2O (8ml). a) mol/mol-NaOH. ■: 240℃, △: 260℃, □: 280℃, ●: 310℃, ○: 340℃.

Fig. 6 Effect of Reaction Temperature on CO2 Methanation (a) and Arrhenius Plots (b)

Ru/C catalyst. When only Raney Fe was used as cata- the change of the reaction rate with temperature was lyst, sodium formate (marked ○ in Fig. 4b) is a main examined in the range from 240℃ to 380℃ and appar- product over the reaction times. As shown in Fig. 4a, ent activation energy (Ea') for Na2CO3 and CO2 metha- an addition of Ru/C catalyst to the Raney Fe system nation were estimated. The results are shown in Figs. caused the formation of methane, and the rate of 5 and 6. The initial rates (v0) were determined by dif- methane formation increased with increasing amount of ferentiation of the experimental expressions of the Ru/C added. On the other hand, the formation of time-yield curves (Figs. 5a and 6a) at early period sodium formate was observed in an early stage (Fig. (-2h) to avoid possible complications attributed to the 4b) of sodium carbonate reaction, i.e. the yield of sodi- factors such as a change in catalytic activity. Summa- um formate reached its maximum value in the early rized in Table 3 are initial rates (v0) interpolated to stages (ca. 0.5h), and the maximum value decreased 253℃ and values of Ea', together with data in those with increasing amount of Ru/C accompanying an references where a continuous flow system was used. enhancement of the rate of methanation. These find- The initial rate (v0) of 4.6×10-6mol min-1 g-cat-1 for ings indicate that the Ru catalyst plays an essential role the Na2CO3 methanation is much smaller than 71.1× in conversion of the formate into methane, and that the 10-6 mol min-1 g-cat-1 for the CO2 methanation, in formate is an intermediate in the methanation. similar conditions. The initial rates obtained for Na2CO3 and CO2 methanations are plotted in Figs. 5b 4. Kinetic Studies and 6b in Arrhenius fashion. A trend of decreasing activation energy (i.e., departure from the initial linear For the purpose of comparison among obtained data, slope) with increasing temperature was observed in

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Table 3 Comparison of Kinetic Data with Previous Studies for Methanation

a) Apparent activation energy. b) This study. c) This value was estimated by interpolation from higher temperature. d) Data from Ref. 6. e) Data from Ref. 20. f) Data from Ref. 21.

both cases. A previous kinetic study in CO2-H2 the hydrogenation step of a surface carbon to methane methanation demonstrated that the rate-determining has been proposed as the rate determining step23). step and, hence, the activation energy change with tem- There has been considerable controversy in the liter- perature, i. e. the activation energy decreases with ature regarding the mechanism of CO2 hydrogenation, increasing temperature11),19) This behavior is mainly i.e., whether it proceeds by transformation of CO2 to attributed to following two reasons in the present reac- CO followed by CO hydrogenation or a unique set of tions. One is the difference of reaction phase of elementary steps. It is known7),that CO2-H2 methana- water, i. e. non-critical and supercritical conditions. In tion mechanism over Ru catalyst involves, firstly, the the supercritical conditions, the reaction most likely CO2 dissociation to CO and subsequent hydrogenation follows a gas-solid heterogeneous reaction, wherein the to methane, according to the CO-H2 mechanism. density of water is significantly low20) and a solution In the present methanations of Na2CO3 and CO2, a and dissociation of the carbonate become unfavorable. rapid formation of considerable amounts of formate This assumption was also inferred from the complete was seen in early stage, the production of CO was not heterogeneous gas-solid reaction, i.e. the reaction of observed even if without base, and both the values of Run 1 in Table 1; however, the use of H2 (10 atm) in Ea' were roughly in agreement. These suggest that place of water showed quite low reactivity to give both of the CO2 and the alkali carbonate processes methane (2.1%). Another reason is that the deposition involve a common intermediate, or a formate species. of carbonaceous materials on the surface of ruthenium Although it is difficult to rationalize the exact mecha- catalyst is occurring to deactivate the catalyst, especial- nism for the present methanations, it is possible to pre- ly at high temperatures. In the case of the CO2 metha- sume that the present methanation would proceed via nation, the departure from the initial linear slope was the formation of formate-like intermediate (the "surface observed at lower temperature (about 280℃), lower formate") 2, as shown in Scheme 1; thus, the carbon- than that of the carbonate methanation. This is mainly ate and hydrolyzed CO2 (CO2+NaOH=NaHCO3) are caused by the latter reason, suggesting that CO2 is more first activated as adsorbed carbonate 1. The carbonate liable to cause deposition of carbonaceous materials on 1 is then subjected to reduction by ruthenium hydride the ruthenium catalyst than that by carbonate. Such (RuH) resulting from the nascent hydrogen and rutheni- tendencies were also reported for other systems11,21). um species to form 2, which would be in equilibrium The apparent activation energy values (Ea') were with alkali metal formate. The formate 2 is decom- obtained from the initial linear slope of the Arrhenius posed through the reductive dissociation into an active plots (Figs. 5b and 6b). The value of Ea' for the surface carbon, which could be then hydrogenated to Na2CO3 methanation was around 14kcal mol-1 (in the methane. The presence of such intermediate formate range of 280-350℃), close to the value of 17kcal mol-1 species has been proposed for CO2-H2 on rhodium and (in the range of 240-280℃) for the CO2 methanation. ruthenium catalyst7)-9)23)-25) These values are also close to the values previously In conclusion, this work clearly shows that when the reported for CO2-H2 methanation on Ru/C (19.7kcal aqueous alkaline carbonate or CO2 solution was heated mol-1)6), Ru/SiO2 (17.2kcal mol-1)19), and Ru/Al2O3 its the presence of Ru/C-Raney Fe mixed catalyst at (16.1kcal mol-1)22). Further, the methanation of CO temperatures over 300℃, methane was exclusively with H2 reported in recent work6) proceeds with a obtained by the reduction with water instead of additive greater activation energy of 26.2kcal mol-1, in which molecular hydrogen. This system might be worthy of

石油学会誌 Sekiyu Gakkaishi, Vol. 42, No. 3, 1999 164 attention from the viewpoint of the increasing the con- 10) Lunde, P. J., Kester, F. L., Ind. Eng. Chem., Process Des. certration of atmospheric CO2. Develop., 13, 27 (1974). 11) Weatherbee, G. D., Bartholomew, C. H., J. Catal., 77, 460 (1982). Acknowledgments 12) Ross Hastings, W., Cameron, C. J., Thomas, M. J., Baird, M. C., The authors are grateful to Mr. M. Yasumoto for his Inorg. Chem., 27, 3024 (1988). valuable contribution in manufacturing and assembling 13) Inui, T., Funabiki, M., Takegami, Y., J. Chem. Soc., Faraday the high-pressure experimental equipment. Trans. 1, 76, 2237 (1980). 14) Kudo, K., Ikoma, F., Mori, S., Komatsu, K., Sugita, N., J. Chem. Soc., Perkin Trans. 2, 679 (1997). References 15) Grenoble, D. C., Estadt, M. M., J. Catal., 67, 90 (1981). 16) Aresta, M., Nobile, C. F., Albano, V. G., Forni, E., Manassero, 1) For example: a) Inui, T., Funabiki, M., Takegami, Y., Ind. Eng. M., J. C. S. Chem. Comm., 1975, 636. Chem., Prod. Res. Dev., 19, 385 (1980); b) Weatherbee, G. D., 17) Lunde, P. J., Kester, F. L., Ind. Eng. Chem., Process Des. Bartholomew, C. H., J. Catal., 68, 67 (1981). Develop., 13, 27 (1974). 2) For example: a) Ramaroson, E., Kieffer, R., Kiennemann, A., J. 18) Inoue, Y., Izumida, H., Sasaki, Y., Hashimoto, H., Chem. Lett., Chem. Soc., Chem. Commun., 1982, 645; b) Denise, B., Sneeden, 1976, (8), 863. R. P. A., Appl. Catal., 28, 235 (1980); c) Amenomiya, Y., Appl. 19) Weatherbee, G. D., Bartholomew, C. H., J. Catal., 87, 352 Catal., 30, 57 (1987); d) Inui, T., Takeguchi, T., Catal. Today, (1984). 10, 95 (1991). 20) Antal, M. J., Mok, W. S. L., Raissi, T. A., J. Anal. Appl. Pyrol., 3) Reller, A., Padest, C., Hug, P., Nature, 329, 527 (1987). 8, 291 (1985). 4) Akiyama, F., Bull. Chem. Soc. Jpn., 69, 1129 (1996). 21) van Meerten, R. Z. C., Vollenbroek, J. G., de Croon, M. H. J. 5) Kudo, K., Sugita, N., Takezaki, Y., Nippon Kagaku Kaishi, M., van Nisselrooy, P. F. M. T., Coenen, J. W. E., Appl. Catal., 1977, 302. 3, 29 (1982). 6) Moreno-Castilla, C., Salas-Peregrin, M. A., Lopez-Garzon, F. J., 22) Lunde, P. J., Kester, F. L., J. Catal., 30, 289 (1973). J. Mol. Cat. A: Chemical, 95, 223 (1995). 23) Jackson, S. D., Moyes, R. B., Whyman, R., J. Chem. Soc., 7) Solymosi, F., Erdohelyi, A., Kocsis, M., J. Chem. Soc., Faraday Faraday Trans. 1, 83, 905 (1987). Trans. 1, 77, 1003 (1981). 24) Amenomiya, Y., Pleizier, G., J. Catal., 76, 345 (1982). 8) Solymosi, F., Erdohelyi, A., Bansagi, T., J. Chem. Soc., Faraday 25) Deluzarche, A., Hindermann, J. P., Kieffer, R., J. Chem. Res., Trans. 1, 77, 2645 (1981). 1981, (S) 72; (M) 934. 9) Solymosi, F., Erdohelyi, A., Bansagi, T., J. Catal., 68, 371 (1981).

要旨

Raney Fe-Ru/C混合触媒存在下, アルカリ炭酸塩および炭酸ガスの水によるメタン化反応

工藤清, 小松紘一

京都大学化学研究所, 611-0011京都府宇治市五ケ庄

Raney Fe-Ru/C混合触媒の存在下, アルカリ金属炭酸塩およタン化反応が進行した。また, いずれの反応においても反応初

びCO2の水によるメタン化反応を検討した。Raney Fe単独系期にギ酸塩の生成が見られ, このメタン化反応はギ酸塩を経由ではメタン化反応は起こらないが, CO2との反応ではギ酸塩がしていることが示唆された。なお, Raney Fe-Ru/C混合触媒系生成した。しかしながら, 触媒量のRu/Cを加えた Raney Fe- での炭酸ナトリウムおよびCO2のメタン化反応に対する見か Ru/C混合触媒系では, 低温 (<250℃) ではギ酸塩が主生成物けの活性化エネルギーは, それぞれ14kcal/molおよび17 であるが, 反応温度の上昇とともにメタンの選択率および収率 kcal/molの値が得られた。が増加し, 380℃付近でいずれの反応においても高選択的にメ

Keywords Carbon dioxide, Hydrogenation, Alkali carbonates, Methanation, Water, Raney alloy

石油学会誌 Sekiyu Gakkaishi, Vol. 42, No. 3, 1999