Chapter 5
Selective Conversion of Carbon Dioxide and Methanol to Dimethyl Carbonate Using Phosphoric Acid-Modified Zirconia Catalysts Downloaded by DICLE UNIV on November 14, 2014 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch005
Yoshiki Ikeda, Yutaka Furusawa, Keiichi Tomishige*, and Kaoru Fujimoto Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Direct synthesis of dimethyl carbonate from methanol and carbon dioxide was studied over heterogeneous catalysts. It is found that dimethyl carbonate can be synthesized selectively on zirconia catalysts. The additive effect of phosphates or sulfates to zirconia was investigated. Phosphoric acid was found to be very effective for the enhancement of the catalytic activity with high selectivity in this reaction. On zirconia and phosphoric acid-modified zirconia catalysts, the amount of by-products, dimethyl ether and carbon monoxide was below the detection limit. This reaction proceeded at much lower temperature on phosphoric acid-modified zirconia than on unmodified zirconia. It is suggested that DMC formation proceeds on the active sites derived from the interaction between phosphoric acid and zirconium hydoxide during the catalyst preparation.
© 2002 American Chemical Society In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Introduction The development of C0 -based methods for the synthesis of carbonic acid diesters is very attractive (i). Dimethyl carbonate (DMC), the lowest homologue of this family, is drawing attention as a safe, noncorrosive, and environmentally acceptable alternative to the carbonylating, carboxymethylating, and methylating agents C O C l , CH OC(0)CI, and dimethylsulfate or methyl halides, respectively. D M C has about 3 times the oxygen content as methyl terf-butyl ether and D M C has a good blending octane (2). D M C does not phase separate in a water stream like some alcohols do, and it is both low in toxicity and quickly biodegradable. D M C can also be used as an ocatne booster in gasoline. Furthermore the addition of D M C to the diesel fuel decreased the emission of the particulate matter. If D M C is used as the fuel additive, a large scale-up of current world D M C production would be necessary. Three kinds of large-scale production methods of D M C have been developed. First method is the stoichiometric reaction of methanol and phosgene in a concentrated sodium hydroxide solution (J): 2
Downloaded by DICLE UNIV on November 14, 2014 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch005
2
3
2CH OH + COCl 3
( C H 0 ) C O + 2HC1
2
3
2
Second method is the oxidative carbonylation of C H O H with carbon monoxide and oxygen catalyzed by cuprous chloride in a slurry reaction system (4-6), where the reaction proceeds in the redox cycle of copper ions as follows: 3
2CuCl + 2 C H O H + l / 2 0 2Cu(OCH )Cl + H 0 2Cu(OCH )Cl + CO ( C H 0 ) C O + 2CuCl 3
2
3
3
3
2
2
Third is an excellent D M C synthesis process based on the oxidative carbonylation using a palladium catalyst and methyl nitrite promoter (7): CO + 2 C H O N O 3
( C H 0 ) C O + 2NO 3
2
Methyl nitrite used in this process is synthesized by the following reaction, which proceeds at room temperature without any catalyst: 2 C H O H + 2NO + l / 2 0 -» 2 C H O N O + H 0 3
2
3
2
There are some routes of D M C synthesis from C 0 . The reaction of alcohols with urea is one potential route to carbonates (1): 2
2 C H O H + (NH ) CO 2NH + (CH 0) CO 2NH + C 0 (NH ) CO + H 0 3
2
3
2
2
3
2
2
3
2
2
In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
73 Ammonia is recycled to produce urea by reaction with C 0 . As a result, this process corresponds to the synthesis from C H O H and C 0 . There is another synthesis route of D M C via ethylene oxide route (8): 2
3
2
O
A
CO, Ο
H,C-CH,
ΛΟ CH
H,C-
2
Ο
Downloaded by DICLE UNIV on November 14, 2014 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch005
Ο
Λ
H C 2
H (
CH
\
Ρ»
H C-CH 2
\
0
+
V CH
2
2
V CH
3
3
The reaction of epoxide with C 0 is very rapid and high exothermic. This route produces ethylene glycol as a (1:1 molar) coproduct with D M C . This would limit the number of plants that could be built using this route. Recently the selective D M C synthesis via the reaction of supercritical C 0 and trimethyl orthoacetate using molecular catalyst Bu Sn(OCH ) has been reported (9): 2
2
2
CH C(OCH ) + C 0 3
3
3
3
(CH 0) CO + CH COOCH
2
3
2
3
2
3
It is known that D M C can be synthesized directly from C H O H and C 0 in the presence of dialkoxydibutyltin (10). The reactions were carried out under pressure of C 0 . It is assumed that C 0 is inserted into Sn-0 bond of Bu Sn(OCH ) followed by alcoholysis yielding carbonate and Bu Sn(OH) . This species is again esterified by alcohol, so that the tin catalyst can be reused: 3
2
2
3
2
2
2
2
2CH OH + C 0 3
2
2
(CH 0) CO + H 0 3
2
2
It has also been reported that D M C was synthesized from C H O H and C 0 in the presence of Sn(IV) and Ti(IV) alkoxides and the metal acetates (11). These alkoxide catalysts react with the water produced with D M C and deactivate. Recently we have reported that D M C was selectively synthesized from C H O H and C 0 using zirconia catalysts (12). On some other catalysts, dimethyl ether (DME) was formed and D M C was not detected at all. It is characteristic that D M E formation on Z r 0 was below the detection limit. The amount of D M C formation showed the volcano-type dependence on the calcination temperature of zirconium hydroxide (12, 13). It was found that the D M C formation rate was strongly dependent on the structure of Z r 0 . We calculated the equilibrium level of C H O H conversion under our reaction conditions to be 3
3
2
2
2
2
3
In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
74 around 1% ( D M C = 0.96 mmol). This value is higher than the experimental results, but this seems to be because our calculation did not consider H 0 as an impurity. The reactor, reactants, and catalyst surface should contain H 0 as an impurity, the amount of which is difficult to estimate. H 0 as an impurity may decrease the equilibrium level of D M C formation (12). In this article, the modification effect of Z r 0 with phosphates and sulfates in D M C synthesis from C H O H and C 0 was investigated. Especially, the catalytic properties of phosphoric acid-modified Z r 0 catalysts were focused. 2
2
2
2
3
2
2
Downloaded by DICLE UNIV on November 14, 2014 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch005
Experimental Z r 0 was prepared by calcining a commercially available zirconium hydroxide (Zr0 -JcH 0) at 673 Κ for 3 h under air atmosphere. The calcination temperature was optimized by our privious study (12, 13). Modified Z r 0 catalysts were prepared by impregnating Z r 0 ' x H 0 with the aqueous solution of phosphates or sulfates. The solvent was removed by heating and the sample was dried at 393 Κ for 10 h, followed by calcining at different temperatures (573-923 K ) for 3 h under air atmosphere. These catalysts are represented by M ^ P 0 y Z r 0 or M ^ ( S 0 y Z r 0 (M: H , K , Cs, Mg, Ce, Zn, Fe, x,y = 1, 2, 3). The loading of modification reagents is denoted as the molar ratio in parentheses; e.g. H P 0 / Z r 0 (P/Zr = 0.05). Another preparation method was attempted. Phosphoric acid was loaded directly on Z r 0 which had prepared by calcining Z r 0 ' x H 0 at 673 Κ for 3 h. After impregnation, the catalyst was prepared by the same procedure as described above. This catalyst is represented by H P 0 / Z r 0 ( D ) . 2
2
2
2
2
4
2
4
3
4
2
2
2
2
2
3
4
2
2
The reaction was carried out in a stainless steel autoclave reactor with an inner volume of 70 ml. The standard procedure is as follows: 6.1 g C H O H (192 mmol) and 0.5 g catalyst were put into an autoclave, then the reactor was purged with C 0 . 8.8 g C 0 (200 mmol) was introduced and the initial pressure was about 4 MPa at room temperature. The reactor was heated and magnetically stirred constantly during the reaction. The reaction was carried out at different temperatures (383-443 K ) for 2 h. Products in both gas phase and liquid phase were analyzed by gas chromatograph (GC) equipped with FID and T C D . In this experiment the reproducibility is in the range of ± 0 . 0 1 mmol. In the gas phase, no products were observed. C O was below the detection limit of FID-GC equipped with methanator. On Z r 0 and H P 0 / Z r 0 catalysts under all reaction conditions shown in this paper, D M C was the only product and D M E was below the detection limit of FID-GC. B E T surface area, X R D spectra, and L R S spectra of the catalysts were measured with Gemini 2360 (Micromeritics, N adsorption), RINT-2400 (Rigaku, Cu K„), and L A B R A M I B (JOBIN-YBON, He-Ne laser), respectively. 3
2
2
2
3
4
2
2
In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Results and Discussion The results of C H O H + C 0 reaction over heterogeneous catalysts are listed in Table I. On some solid acid catalysts like A 1 0 , T i 0 , H-ZSM-5, H - U S Y , H Mordenite and Η-β, D M E was formed and D M C formation was not detected at all. D M C was formed selectively on Z r 0 catalysts and S n 0 . On other catalysts, neither D M C nor D M E was observed. The amount of D M C formation on Z r 0 (calcined at 673 K ) at 443 Κ for 40 h was 0.35 mmol, and D M E formation was not detected at all. D M E formation rate was extremely low (
