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 pres- ent 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 metha- nation 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. 石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 42, No. 3, 1999 158 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- 石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 42, No. 3, 1999 159 ciency of combination of Raney alloys and an additive catalysts to promote the reaction of various alkali car- bonates 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 prod- uct molar percent relative to the principal carbonate. Individually, Raney Fe and Raney Ni exhibited differ- ent 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.