Langmuir 1999, 15, 6601-6604
Transformation of CO2 Alone and Combined with Ethanol Present in the Hydrogen-Accumulating Intermetallic System TiFe0.95Zr0.03Mo0.02, Pd/SiO2, and γ-Al2O3 M. V. Tsodikov,† V. Ya. Kugel,† E. V. Slivinskii,† V. G. Zaikin,† V. P. Mordovin,‡ G. Colo´n,§ M. C. Hidalgo,§ and J. A. Navı´o*,§ A. V. Topchiev Institute of Petrochemical Synthesis, RAS, 29 Leninsky prosp., 117912 Moscow, Russian Federation, A. A. Baykov Institute of Metalurgy and Materials Science, RAS, 49 Leninsky prosp., 117911 Moscow, Russian Federation, and Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, and Dpto. de Quı´mica Inorga´ nica/Universidad de Sevilla, Avda. Ame´ rico Vespucio, s/n, 41092-Sevilla, Spain Received February 16, 1999. In Final Form: May 17, 1999
1. Introduction Carbon dioxide (CO2) is one of the main gases responsible for the greenhouse effect. In the last few decades, general agreement between developing countries reflects the common idea to reduce anthropogenic greenhousegas emissions. At the same time, one important problem in the elaboration of energy- and resource-saving technologies is the development of catalytic processes for transformation of CO2 into valuable products of petrochemical synthesis. Carbon dioxide is of the greatest interest as a C1 feedstock because of the vast amounts of carbon which exist in this form and because of the low cost of bulk CO2. Although several experimental studies and reviews have been devoted to the search for active heterogeneous and homogeneous catalysts for these reactions, the problem still remains topical.1-4 Liquid carboxylation products, mono- and dicarboxylic acids, were produced in small amounts in the case of electrochemical activation of CO2 in the presence of alkenes and butadiene.5,6 When CO2 was hydrogenated in an autoclave using the heterogeneous catalyst Cu3Zr2Mn or palladium-containing homogeneous catalysts, solvents were found to have an effect on the formation of small amounts of esters and ethers.7,8 In this communication, we report the joint transformation of CO2 alone and combined with ethanol in the presence of the intermetallic compound TiFe0.95Zr0.03Mo0.02 * To whom correspondence should be addressed. † A. V. Topchiev Institute of Petrochemical Synthesis. ‡ A. A. Baykov Institute of Metalurgy and Materials Science. § Universidad de Sevilla. (1) Aresta, M. In Enzymatic and Model Carboxylation and Reduction reactions for Carbon Dioxide Utilization, NATO-ASI series C; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990; Vol. 314, p 1. (2) Brunstein, P.; Matt, D.; Nobel, D. Chem. Rev., 1988, 88, 747. (3) Jessop, P. G.; Kariya, T.; Noyori, R. Chem. Rev. 1995, 95 (2), 259. (4) Krylov, O. V.; Mamedov, A. Kh. Russ. Chem. Rev. 1995, 64 (9), 877. (5) Ford, P. C. In Electrochemical and Electrocatalytic Reactions of Carbon Dioxide, Sullivan, B. P., Krist, K., Guard, H. E., Eds.; Elsevier: Amsterdam, The Netherlands, 1993; Chapter 3, p 68. (6) Kushi, Y.; Nagao, H.; Nishioka, T.; Isobe, K.; Tanaka, K. J. Chem. Soc., Chem. Commun. 1995, 1223. (7) Evdokimova, E. V.; Lunin, V. V.; Afanasyev, P. V.; Moiseev, I. I. Mendeleev Commun. 1993, 1, 1. (8) Moiseev, I. I.; Evdokimova, E. V.; Lunin, V. V.; Afanasyev, P. V.; Gekhman, A. E.; Chromov, A. R. Dokl. Chem. Commun. 1993, 332, 195.
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and also in the presence of a catalytic composition mixture of the intermetallic compound, Pd/SiO2, and γ-Al2O3. 2. Experimental Section Catalytic experiments on the transformation of CO2 were carried out using a flow-circulation laboratory-type setup equipped with a dosing apparatus for the supply of the liquid reactant over a catalysts bed. To separate the gas and liquid phases, the reactor outlet was connected to a cooled separating vessel. The total volume of the system was 0.75 L, and the reaction area was 0.25 L. The intermetallic compound was charged in the reactor as the crushed 0.2-0.5 mm fraction (56 g). When compounds were used, the intermetallic one was mixed with the 0.1-0.2 mm fraction of 0.5% Pd/SiO2, which we had synthesized, and/or commercial γ-Al2O3 as beads weighing 3.0 g with a granular size of 1.5-2.0 mm. Hydrogenation was accomplished with hydrogen desorbed from the intermetallic compounds, with a mixture of desorbed hydrogen, or with dihydrogen alone. To identify the products of CO2 transformations, the experiments were also carried out under purified N2 and CO and with a small amount of ethylene added. Prior to each entry, a pressure test was carried out: the system was filled with dinitrogen to a pressure of 120130 atm that was maintained for 24 h. Then dinitrogen was replaced by dihydrogen, the surface being treated with dihydrogen for 8 h at atmospheric pressure and 100 °C, after which the system was cooled to 20 °C and the H2 pressure was increased to 120-130 atm. The amount of absorbed H2 was estimated using a standard manometer; then dihydrogen was replaced by CO2 at 15 atm of pressure. When the CO2 pressure became constant, a circulating pump was turned on, and the temperature of the reaction mixture was quickly increased to 300 °C. The turnover number for CO2 circulation through the heterogeneous system was 5 L h-1. When a specified temperature had been attained, ethanol was supplied to the catalyst bed at a rate of 10-12 cm3 h-1. Each experiment lasted 3 h; after the experiment, the setup was cooled, and the volume of gas in the system was determined with the standard manometer at =20 °C. The initial gas and that formed during hydrogenation were analyzed by GC using a “Biokhrom” chromatograph (TC detector, a 150 cm × 0.4 cm column, SKT 0.25-0.50 mm, using Ar as a carrier gas). The composition of gaseous hydrocarbons was determined using a LKhMM-8 MD chromatograph (flameionization detector, a 150 cm × 0.4 cm column, 0.25-0.50 mm R-Al2O3 modified with 5% squalane, using He as a carrier gas). The content of the gas components was calculated by comparison with standard mixtures. The degree of CO2 conversion was determined by the manometric method using the following procedure: after an experiment was conducted and the system was cooled to 20 °C, the total gas volume in the system was determined from the manometer reading; the concentration of CO2 converted during the reaction (∆CO2) was calculated. The degree of conversion was found as follows: R (% v/v) ) [∆CO2(L)/CO2init(L)] × 100%. After that, concentrations and volumes (in liters) of the gas components formed during the reaction were found. Since ethane and ethylene were also formed from ethanol, the experiments were carried out under identical conditions; CO2 was replaced by dinitrogen. On the basis of the quantities of converted CO2 and the gaseous products formed, the amount of CO2 spent for gas formation was determined. The rest of the converted CO2 (% v/v) was characterized as the selectivity with respect to the formation of liquid products by the following expression:
S ) [∑∆CO2(L) - ∆CO2(into gases)(L)/∑∆CO2] × 100% The data of the GC analysis were also used to determine the amount of H2 desorbed from the intermetallic compound during the experiment. The composition of liquid products was studied by GC/MS using a Kratos MS 25TF mass spectrometer (a 200 cm × 0.4 cm
10.1021/la990156g CCC: $18.00 © 1999 American Chemical Society Published on Web 07/27/1999
6602 Langmuir, Vol. 15, No. 19, 1999
Notes
Figure 1. Dependence of the proportion of CO (1) and C1-C4 hydrocarbons (2) in the gaseous products formed during the cycles with a bath of intermetallics: first (A), second (B), and third (C) cycles (T ) 623 K, P ) 8 atm). Table 1. Degree of CO2 Conversion and Composition of the Reaction Products (During a Circulation Time of 40 h) T/K
degree of CO2 conv
293 373 593 623
1.2 2.0 11.5 28.4
composition of reaction products (% vol) CO CH4 C2-C4 1.0 1.5 7.8 20.0
0.2 0.2 1.8 6.0
0.3 1.9 2.4
packed column filled with poly(ethylene glycol)). At the same time, the composition of liquid products was also determined by GLC on a “Khrom-5” chromatograph (flame-ionization detector, with a 200 cm × 0.4 cm packed column, Celite 545 modified with 5% polypropylene glycol sebacinate). The palladium-containing catalyst was prepared using the alkoxo method. The precursors PdCl2 and tetraethoxysilane (TES), from Aldrich, were used as initial reactants. Methanol (10 mL) and several drops of glacial acetic acid were added to TES (10 g) with continuous stirring. Palladium chloride (0.17 g) was dissolved in water (2.4 g) containing several drops of concentrated hydrochloride acid (d ) 1.18 g/mL) and mixed with ethanol (3 mL). A solution of palladium chloride in aqueous ethanol was slowly added with intense stirring to the mixture of TES with ethanol; then the resulting sol was thoroughly stirred for an additional 30 min and allowed to stand in the dark. The gel was formed over a period of 2 days. The freshly prepared gel was evacuated at 180-200 °C, and the resulting catalyst was determined by atomic absorption spectroscopy using a PerkinElmer 400 spectrometer. According to this analysis, the concentration of palladium in the Pd/SiO2 samples was 0.45-0.53%.
3. Results and Discussion From the data presented in Table 1, one can see that CO and CH4 are formed at room temperature. Nonetheless, noticeable conversion of CO2 in the gaseous phase was observed at 593-623 K. In this temperature range, decomposition of the hydride phase of the intermetallic compound and the desorption of hydrogen occur.9 In the first cycle, CO2 is mostly converted into CO (20%) (Figure 1), and in the subsequent cycles, the proportion of C1-C4 hydrocarbons increases. This variation of the composition products may be associated with a catalytic effect of the iron incorporated into the intermetallic compound upon synthesis of hydrocarbons. Transformation of CO2 to liquid products was observed in a combined conversion mixture of CO2 and ethanol in (9) Kalachev, B. A.; Shalin, R. E.; Llyin, A. A. Allows-Storage of Hydrogen; Dictionary/Metallurgy: Moscow, 1995; p 384.
the presence of these intermetallic compounds. As can be seen in Table 2, the reaction with the hydrogen-contained intermetallics allows the formation of acetaldehyde [AcH], ethyl acetate [EtOAc], and a slight amount of n-butanol. In an atmosphere of dinitrogen, AcH and EtOAc are not produced; however, traces of n-butanol are detected. It is difficult to draw definite conclusions concerning participation of CO2 in the formation of this compound. The amount of AcH and EtOAc substantially increased when a catalytic composition consisting of the intermetallics Pd/SiO2 and γ-Al2O3 was used (see Table 2). As has been noted previously,5,6 a slight amount of oxygen-containing carboxylic products can be formed upon CO2 interaction with alkenes. We found that addition of a slight amount of ethylene (up to 0.075%) to the reaction mixture results in a sharp decrease in the degree of CO2 conversion during the subsequent appearance of ethane in the gaseous reaction products; ethane is likely to be formed upon hydrogenation of ethylene on the surface of the activated intermetallic compound (see Table 2, entry 13). It is very well-known that alcohol can be transformed into ethylene10 in the presence of Al2O3 at temperatures > 523 K. Therefore, alumina was added to the catalyst, so the ethylene could be formed in situ on its surface. In view of the known ability of some palladiumcontaining systems to accomplish catalytic activation of CO2, we used a Pd/SiO2 system.1-3,7,8 The composition of liquid reaction products obtained with a binary system comprising the intermetallics and Pd/SiO2 is close to that obtained in entries 1 and 2 (see Table 2). However, the concentration of oxygen-containing compounds is higher than that for entry 1, in which only intermetallics were used as the heterogeneous catalyst, and lower than that in entry 2, in which the ternary composition consisting of the intermetallics, Pd/SiO2, and γ-Al2O3 was used. The composition of the products changes considerably in the presence of the binary intermetallics and γ-Al2O3 system (see Table 2, entry 4). With the overall degree of CO2 conversion R ) 34% and the selectivity S ) 77%, the liquid products of the synthesis contain large proportions of alkanes and alkenes (up to 40%), the content of branched isomers being as high as ∼40%. The transformation of ethanol under N2 in the presence of catalytic compositions containing Pd/SiO2 and γ-Al2O3 yields substantial amounts of diethyl ether (DEE), ethylene, ethane, and traces of methane, whereas, in the presence of only intermetallic compounds possessing no acidic properties, no DEE was detected (see Table 2, entries 9-12). The introduction of gaseous H2 as a reactant into the system markedly decreases the selectivity of the transformation of CO2 into liquid oxygen-containing products and enhances gas formation. In the presence of only H2, CO2, or EtOH, the selectivity in the formation of oxygencontaining liquid products drops almost to zero (see Table 2, entries 5-7). The use of dihydrogen in the reaction catalyzed by the intermetallics and γ-Al2O3 binary composition decreases the selectivity from 75% to ∼5%; however, the composition of the hydrocarbon fraction remains virtually the same as that observed with hydrogen desorbed from the inter(10) (a) Kung, H. H. In Transition Metal Oxides: Surface Chemistry and Catalysis; Studies in Surface Science and Catalysis Vol. 45; Elsevier: Amsterdam, 1989. (b) Sokolovskii, V. D. Principles of Oxidative Catalysis on Solid Oxides. Catal. Rev.-Sci. Eng. 1990, 32 (1 & 2).
Notes
Langmuir, Vol. 15, No. 19, 1999 6603 Table 2. Products of the Hydrogenation of CO2 with Ethanol in the Presence of an Intermetallic Compound (TiFe0.95Zr0.03Mo0.02), Pd/SiO2, and γ-Al2O3 (P ) 15 atm, 573 K) product composition
entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
catalytic composition
reactant
RCO
2
S (% v/v)
liquide (% w/w)
gaseous (% v/v)
89
CO 1.7, CH4 0.85, C2-C4
36.0
80
CO 1.0, CH4 2.5, C2-C4
CO2, H2 ethanol
26.0
67
CH4 3.0, C2-C4 0.9c
CO2, H2,a ethanol
34.0
77
CH4 0.35, C2-C4 0.92c
CO2, H2,a H2,b ethanol
35.0
50
CH4 6.0, C2-C4 4.4c
CO2, H2,b ethanol
20.0
0
CH4 10.0, C2-C4 5.0c
,a
intermetallic compound
CO2, H2 ethanol
intermetallic compound, Pd/SiO2, γ-Al2O3 intermetallic compound, Pd/SiO2 intermetallic compound, γ-Al2O3 intermetallic compound, Pd/SiO2, γ-Al2O3 intermetallic compound, Pd/SiO2, γ-Al2O3 intermetallic compound intermetallic compound, γ-Al2O3 intermetallic compound intermetallic compound, Pd/SiO2, γ-Al2O3 intermetallic compound, Pd/SiO2 intermetallic compound, γ-Al2O3 intermetallic compound intermetallic compound, γ-Al2O3 intermetallic compound intermetallic compound, Pd/SiO2, γ-Al2O3 intermetallic compound, Pd/SiO2, γ-Al2O3
,a
CO2, H2 ethanol ,a
,b
CO2, H2 ethanol CO2, H2,b ethanol
24.0
15.0 10.0
0 ∼5
CO 2.5, CH4 7.5, C2-C4 CH4 0.5, C2-C4 4.0c
0.44c
2.5c
AcH 1.5, EtOAc 6.7, MeOH 1.4, BuOH 4.0 AcH 7.9, EtOAc 14.0, BuOH 1.8, DEE 7.0 AcH 3.2, EtOAc 9.6, BuOH 1.7, DEE 2.0 alkanes + alkenes (normal) 23.8, (iso) 17.0, BuOH 1.7, DEE 8.2 EtOAc 2.0, BuOH 2.0, DEE 2.0 BuOH 2.0, DEE 1.5
2.7c
N2, H2,a ethanol N2, H2,a ethanol
CH4 0.05, C2H4 2.0, C2H6 5.0d CH4 0.1, C2H4 traces, C2H6 6.6d
BuOH traces alkanes + alkenes (normal + iso) 2.0, DEE 7.0 DEE traces DEE 10.0
N2, H2,a ethanol
CH4 0.12, C2H4 traces, C2H6 5.5d
BuOH traces, DEE 2.3
N2, H2,aethanol
CH4 0.01, C2H4 7.2, C2H6 5.7d
BuOH traces, DEE 15.0
CO2, H2,a EtOH CO, H2,a EtOH
