Ind. Eng. Chem. Res. 1999, 38, 3225-3229
3225
Syntheses of Isobutane and Branched Higher Hydrocarbons from Carbon Dioxide and Hydrogen over Composite Catalysts Yisheng Tan,† Masahiro Fujiwara,* Hisanori Ando, Qiang Xu, and Yoshie Souma Osaka National Research Institute (AIST-MITI), 1 Midorigaoka, Ikeda, Osaka 563-8577, Japan
The hydrogenation of carbon dioxide was examined over various composite catalysts composed of Fe-Zn-M catalyst and zeolite. Fe-Zn-Zr/HY composite catalyst was confirmed to be efficient for the highly selective production of isobutane, which is regarded as a synthetic intermediate of MTBE (methyl tert-butyl ether). The composite catalyst of Fe-Zn-Zr catalyst with H-ZMS-5 formed C5+ hydrocarbons in good yield. The ratios of branched hydrocarbons in C5+ ones, which are suitable for high-octane gasoline, were over 90%. Introduction The emission of huge amounts of carbon dioxide is causing global warming. The fixation of carbon dioxide is an effective method to dissolve the problem. However, its fixation by chemical methods has not been accepted as a process to decrease carbon dioxide. The main reason the hydrogenation of carbon dioxide is not considered as a solution to global warming is that the hydrogen must be provided from procedures which are free from the discharge of carbon dioxide, such as solar cell-water electrolysis. Another problem of the hydrogenation of carbon dioxide is that its products are common and lowvalue materials such as methanol1-6 and methane.7-11 However, carbon dioxide is the most common and unlimited carbon resource. Therefore, the possibility to produce materials that are very valuable and used in large quantities in industry will promote the utilization of carbon dioxide. The composite catalyst systems comprised of methanol synthesis catalysts and zeolite catalysts are generally successful in the hydrogenation of carbon monoxide.12-17 This system is especially advantageous for production of C2+ hydrocarbons with the low productivity of methane, because the lowest hydrocarbon formed by the methanol-to-gasoline (MTG) reaction by zeolite is, in principle, ethylene.18 However, the productivity of C2+ hydrocarbons is poor in almost all composite catalysts reported, even when they are effective in the hydrogenation of carbon monoxide.19-21 We have revealed that common Cu-based methanol synthesis catalysts prepared by coprecipitation were disadvantageous to the composite catalysts.22-24 After various examinations, we made a breakthrough in the composite catalyst effective for the hydrogenation of carbon dioxide. Fe-Zn catalyst was a profitable component of the composite catalyst.25,26 This catalyst system produced C2+ hydrocarbons with good selectivity and a considerable ratio of olefins. We also revealed that the selectivity of the olefins is correlated with the formation of higher hydrocarbons.27 These achievements gave us the opportunity of producing two types of valuable hydrocarbons. * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-727-51-9629. † Permanent address: Institute for Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, P. R. China.
The first study we wish to report here is the highly selective synthesis of isobutane using novel Fe-Znbased composite catalysts.28 Isobutane is a key intermediate for the synthesis of methyl tert-butyl ether (MTBE), which is regarded as a fuel substrate both for enhancing the octane value of gasoline and for purifying automobile exhaust.29 The second study in this paper is the synthesis of branched higher hydrocarbons from carbon dioxide over the composite catalysts with HZSM-5. Branched higher hydrocarbons are significant fuels as well as MTBE. The combination of methanol synthesis and MTG reaction by H-ZSM-5 seems to be a promising process to produce them. However, the yields of C5+ hydrocarbons have been poor in the case of the hydrogenation of carbon dioxide using these types of composite catalysts.20,21 Even our unique Cu-Zn-Cr catalysts effective for light alkane synthesis with HY zeolite produced ethane exclusively in the case of composite catalysts made of H-ZSM-5.22 Finally, we have achieved the effective synthesis of branched C5+ hydrocarbons by the hydrogenation of carbon dioxide over the composite catalysts. Experimental Section The Fe-Zn-M catalysts were prepared by coprecipitation using aqueous 1 M solutions of both the corresponding metal nitrates and NaOH. These two solutions were concurrently added to a 1 L beaker and continuously stirred at 65 °C. The pH value of the resulting solution was kept around 7. The precipitates were aged for 2 h and then filtered and washed 10 times with fresh distilled water. The gels were dried at 120 °C for 12 h and calcined at 400 °C for 4 h in air. The composite catalysts were obtained by the physical mixing of FeZn-M catalyst and zeolite. HY [JRC-Z-HY4.8(2), SiO2/ Al2O3 ) 4.9], NaY (JRC-Z-Y4.8, SiO2/Al2O3 ) 4.8), and H-ZSM-5 (JRC-Z5-25, SiO2/Al2O3 ) 24.6; JRC-Z5-70, SiO2/Al2O3 ) 80.1) were supplied from the Reference Catalyst of the Catalysis Society of Japan. Dealuminated Y-type zeolite (DAY, SiO2/Al2O3 ) 10.7) was obtained from Tosoh Co. H-Fe-silicate (SiO2/Fe2O3 ) 42.8) and H-Ga-silicate (SiO2/Ga2O3 ) 49.4) were provided from N. E. CHEMCAT Corporation. The hydrogenation of carbon dioxide was carried out using a pressurized flow-type fixed-bed reactor.26 The reactor was made of a stainless steel tube with an inner diameter of 9 mm. In a typical experiment, 1 g of
10.1021/ie980672m CCC: $18.00 © 1999 American Chemical Society Published on Web 08/18/1999
3226 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 Table 1. Hydrogenation of Carbon Dioxide over Fe-Zn-M Composite Catalystsa run
composite catalystb
1f 2 3 4 5 6 7 8 9 10 11
Fe-Zn/HY Fe-Zn-Zr Fe-Zn-Zrg Fe-Zn-Zr/HY Fe-Zn-Zrg/HY Fe-Zn-Zr/HYh Fe-Zn-Al/HY Fe-Zn-Cr/HY Fe-Zn-Mn/HY Fe-Zn-Ga/HY Cu-Zn-Al/HY
conv of conversion to (C-mol %) CO2 (%) HC oxyc CO 13.3 13.4 13.9 15.5 17.8 15.2 17.5 18.9 12.3 12.7 30.5
4.9 0.4 0.4 6.3 7.4 6.5 6.2 5.9 2.8 5.8 1.9
0.2 3.6 3.8 0.0 0.0 0.4 0 0 0 0.6 2.8
8.2 9.4 9.7 9.2 10.4 8.3 11.3 13.0 9.5 6.3 25.8
distribution of hydrocarbons (C-mol %) C1 C2 C3 C4 C5 C6 C7+ 8 100 100 2 1 6 3 3 4 5 10
15 0 0 10 9 16 9 10 14 15 29
14 0 0 17 18 17 19 18 16 14 41
27 0 0 47 48 38 46 46 42 37 16
18 0 0 18 18 14 18 18 15 18 4
16 0 0 5 5 8 5 5 9 9 0
2 0 0 1 1 1 0 0 0 2 0
ratio of olefin (%) yield of iso-C4 (C-mol %) C2dd C3de 80
30
73 77 74 68 72 70 80 0
7 0 24 3 6 0 29 0
1.1 0.0 0.0 2.5 3.0 2.1 2.4 2.3 1.0 1.8 0.0
a 360 °C, 5 MPa, SV ) 6000 mL/(g-cat h) to Fe-Zn-M catalyst, H /CO ) 3. Results after 6 h. b Fe/Zn/M ) 1:2:1 in atomic ratio. 2 2 Fe-Zn-M/HY ) 2:1 in weight ratio. c MeOH + MeOMe. d C2H4/(C2H4 + C2H6). e C3H6/(C3H6 + C3H8). f Reference 22; 350 °C, Fe/Zn ) g h 4:1, Fe-Zn/HY ) 1:1. Fe/Zn/Zr ) 1:1:1. Fe-Zn-Zr/HY ) 1:1.
catalyst was packed in the reactor and was activated in a stream of diluted hydrogen (5% H2 in N2) at 340 °C for 14 h at atmospheric pressure. After this prereduction, a reaction gas (H2/CO2 ) 3) was introduced into the reactor under 5 MPa. All parts of tubing from the catalyst bed to the gas chromatographs were heated at 100-150 °C to prevent any condensation of the products. After reaction under the prescribed conditions for 6 h, all effluent gases were analyzed for catalytic activity by on-line gas chromatographs using Porapak Q for carbon dioxide, MS-13X for methane and carbon monoxide, and Porapak Q and Porapak R in series for methanol, dimethyl ether, and C2+ hydrocarbons. Isomers of liquid hydrocarbons (C5 and C6) were detected with a capillary column (CHROMPACK PLOT Fused Silica, column length 50 m, inside diameter 0.32 mm). C-mol % means the amount of the products based on carbon mole: [(carbon number of the product) × (yield or selectivity of the product)]. For example, the yield of isobutane was 4(yield of isobutane) C-mol %. Powder X-ray diffraction was carried out using Cu KR radiation at 40 kV and 30 mA on a Rigaku X-ray diffraction meter. The Na contents in catalysts were determined by atomic absorption spectroscopy (Hitachi Z-8200). Results and Discussion Selective Isobutane Synthesis. Table 1 summarized the results of the hydrogenation of carbon dioxide over various Fe-Zn-based catalysts with HY zeolite. After a great number of experiments for the modifications of reaction conditions, we considered that the conditions described in Table 1 were optimum. We have already reported25,26 that the Fe-Zn/HY catalyst was not effective for the selective synthesis of isobutane, because of its low yield (1.1 C-mol %; run 1). However, the productivity of isobutane was the best of those of all hydrocarbons. These results encouraged us to improve the Fe-Zn catalyst for the selective synthesis of isobutane. Next we prepared the Fe-Zn-M catalysts where M was not reducible. Reducible metal such as copper promotes the reduction of Fe species to form F-T catalyst sites,30,31 which prevents the combination system of methanol synthesis and MTG reaction. Zr, Al, Cr, Mn, and Ga were tested as the third metal component of Fe-Zn-M catalysts. As we have already discussed regarding the effect of Na in the composite catalysts on the catalytic activity,32,33 the remaining Na from NaOH during the coprecipitation was reduced to
C3d > C4d, as obviously observed in the F-T catalyst.38 On the other hand, the ratio of isobutane to C5+ hydrocarbons was high (iso-C4/C5+ ) 1.7) from carbon dioxide over Fe-Zn-Zr/HY (run 5 in Table 1), while the ratio was reported to be 1.1 in the reaction of methanol over HY (iso-C4, 31%; C5+, 28%).15 It is thought that the reactivity of HY zeolite for the hydrogentransfer reaction of isobutylene to isobutane, which terminates the carbon homologation into C5+ hydrocarbons, was enhanced by the mixing of Fe-Zn-Zr with HY. Methanol synthesis catalysts employed in the composite catalysts such as Cu-Zn-Cr and Pd/SiO212-17,19-23 were generally active hydrogenation catalysts of olefins. It is well discussed that the hydrogenation of olefins by these catalysts prevents the carbon homologation into higher hydrocarbons with MTG reaction.14-17,23,27 Moreover, we have reported that methanol synthesis catalyst bearing low activity for the hydrogenation of olefins was advantageous to higher hydrocarbon formation.27 The activity of olefin hydrogenation of Fe-Zn-Zr is thought to be inferior to those of common catalysts such as Cu-Zn-Cr. It is a factor that Fe-Zn-Zr/H-ZSM-5 produced C5+ hydrocarbons in good yield. However, this factor cannot explain all features of the composite catalysts with H-ZSM-5. The characters of the two composite catalysts of H-ZSM-5 with Cu-Zn-Cr22 and Fe-Zn-Zr (runs 4 and 7 in Table 2) were completely different. It seems that the influences of mixed metal oxides on H-ZSM-5 are more serious than those on HY. The mechanism of these effects is now under investigation. Conclusions The hydrogenation of carbon dioxide produced isobutane over Fe-Zn-Zr/HY composite catalysts with high selectivities. The mechanism of isobutane formation was the combination of the methanol synthesis and MTG reaction. The light olefins were confirmed to be important intermediates for isobutane formation. FeZn-Zr catalyst was also effective as a component of the composite catalyst with H-ZSM-5 to afford branched C5+ hydrocarbons in good selectivity. These results indicated that the proper use of zeolite with Fe-Zn-Zr catalyst enabled us to produce specified hydrocarbons from carbon dioxide.
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3229
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Received for review October 19, 1998 Revised manuscript received March 22, 1999 Accepted July 7, 1999 IE980672M
