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A study on methanol synthesis through CO2 hydrogenation over copper-based catalysts
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
505
A study on methanol synthesis through CO 2 hydrogenation over copper-based catalysts Son-Ki Ihm, Young-Kwon Park, Jong-Ki Jeon, Kwang-Cheon Park and Dong-Keun Lee* Dept. of Chem. Eng., Korea Advanced Institute of Science and Technology, 373-1 Kusungdong, Yusong-gu, Taejon 305-701, Korea
In CO2 hydrogenation over Cu/ZrO2 based catalysts, the methanol formation activity could be correlated with copper dispersion. The reaction intermediates of methanol synthesis were carbonate, formate, formaldehyde and/or methoxy, and the rate determining step for methanol synthesis seems to be the conversion of formate into formaldehyde or methoxy.
1. INTRODUCTION The synthesis of methanol over copper-based catalysts is an important industrial process and one of the most investigated catalytic reactions. As methanol can be very easily converted to other valuable materials, its add-value can compensate large energy costs needed. Although methanol synthesis from CO2 hydrogenation over supported copper catalysts has been widely investigated, there are still controversies concerning the methanol synthesis mechanism and the effect of copper on the catalytic activity[ 1-6]. In this work, the influence of the copper dispersion in Cu/ZrO2 catalyst on the catalytic activity in CO2 hydrogenation was investigated. In order to understand the reaction mechanism, FT-IR spectroscopy under reaction conditions and TPD of adsorbed methanol were performed.
2. EXPERIMENTAL Binary copper-based catalysts were prepared by coprecipitation method and some components were added as promoters into the binary catalysts. The methanol synthesis reaction was carried out in a continuous flow microreactor operated at 22 atm and at various temperatures. Reaction pathway of the methanol synthesis was investigated through FT-IR spectroscopy. For the catalyst with a copper content over 15wt%, the diffuse reflectance method (DRIFT) was applied, but for the catalyst with a copper content of 7wt%, the transmission technique was used. For more information about intermediates, TPD of adsorbed methanol was carried out and the products were analyzed using mass spectrometer.
*Present Address : Dept. of Chem. Eng., Res. Inst. Environ. Prot., Gyeongsang Nat. Univ., 900 Kajwa-dong, Chinju 660-701, Kyongnam, Korea Acknowldgement : This work was partially supported by Clean Energy Program by R&D Management Center for Energy and Resources of MTI (Korea).
506 3. R E S U L T S AND D I S C U S S I O N
Among copper based binary catalyst systems, CuO/ZrO2 was proved to be the most reactive toward methanol synthesis. The methanol synthesis activity of the CuO/ZrO2 catalyst was greatly affected by the copper dispersion (or copper crystallite size) ; the smaller the crystallite size, the higher the rate of methanol synthesis (Table 1). When some components of Ce, Cr, Pd, K, V and Zn were added as promoters into CuO/ZrO2, the crystallite size of copper particles changed significantly. CeO2 increased the copper crystallite size significantly, while ZnO made the copper crystallite size much smaller than those of the Cu/ZrO2 samples. Table 1. Ph~csical properties and methanol s)mthesis rate of Cu based catal~csts Catalyst SBET(m2/g) d(nm)* Methanol formation rate (104mol/g-cat 9min) Cu/ZrO2a(pH=7) 55 37 5.48 (pH=9) 78 41 4.99 (pH=l 1) 88 43 4.83 Cu/ZrOzb(pH=7) 60 53 4.75 Cu/ZrO2/CeO2 98 69 3.95 Cu/ZrO2/fr203 102 32 5.72 Cu/ZrO//PdO 69 40 5.39 Cu/ZrO2/K20 42 33 5.56 Cu/ZrO2/V205 90 70 3.38 Cu/ZrO~/ZnO 87 21 8.94 Cu/ZrO2:60/40 (wt%), Cu/ZrO2/MexOy:60/30/10 (wt%), precipitating agent ; aNaOH, bNa2CO3 * : Cu crystallite size by XRD, Temperature : 250~ Pressure : 22atm Fig. 1. shows the DRIFT spectra with time on stream in C O 2 hydrogenation. At 5min, bands at 1060, 1280, 1380, 1520 and 1580cm1 were observed. As reaction time increased, the band at 1580cm-' grew apparently and reached a steady state after 30 min. The bands at 1580, 1380cm-1, the bands at 1520, 1280cm", and the bands at 1060-1080cm 1 could be assigned to bidentate formate, bidentate carbonate and methoxy, respectively[I-4]. It was found that the bands of bidentate carbonate, bidentate formate and methoxy continued to grow in transient state and that formate and methoxy reached steady state. A similar spectra were observed for Cu/ZrO2/ZnO. For ZnO catalyst, however, we found that the formate bands were observed but the methoxy band was hardly observed. Fig. 2 shows the DRIFT spectra with increasing temperature in CO2 hydrogenation over Cu/ZrO2. The Cu formate band at 1590cm1 decreased with temperature. The formaldehyde band at 1120-~1150cm1 increased with temperature upto 190~ but beyond that temperature the band decreased again. From the above results, copper formate, formaldehyde and methoxy were believed to be the intermediates of methanol synthesis on Cu/ZrO 2 catalysts. Temperature-programmed methanol decomposition was observed with DRIFT (Fig. 3). At 50~ monodentate formate, formaldehyde and methoxy were observed at 1600, 1130 and 1080cm~, respectively. As temperature increased, the bands due to monodentate formate and methoxy decreased slowly. The bands at 1580 and 1370cm" due to bidentate formate increased with temperature, but disappered over 270~ Gas phase products from the decomposition of adsorbed methanol were analyzed by mass spectrometer(Fig. 4). Methanol began to appear at around 70~ and disappeared about 200~ which supported the IR results
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Fig. 1. FT-IR spectra during CO 2 hydrogenation over Cu/ZrO 2 catalyst at 250~ and 22atm for (a)lmin (b)5min (c)lOmin (d)15min (e)aOmin (f)6Omin (g) 120min (h) 180min
~6oo
Fig. 2. FT-IR spectra during CO 2 hydrogenation over Cu/ZrO 2 catalyst under 22atm (a)100~ (b) 150~ (c)190~ (d)230~ (e)250~
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Wave number (cm"1) Fig. 3. FT-IR spectra during methanol TPD over Cu/ZrO2 catalyst at (a) 50~ (b) 70~ (c) 110~ (d)lS0~ (e)190~ (f) 230~ (g)270~ (h)300~
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Temperature (~ Fig. 4. Methanol TPD curves by GC-Mass over Cu/ZrO 2 catalyst
508 (Fig. 3) that methoxy peak disappeared above 200~ For C u / Z r O 2 / Z n O , similar results were also obtained. 2000 In addition to above results, a 1~o I transmission FT-IR spectra were obtained to confirm reaction intermediates over Cu/ZrO2 (7/93 in wt%). Fig. 5 shows the FT-IR spectra during CO2 hydrogenation. The band at 2130cm 1 was due to dissociation of CO2 into CO and O. Among reaction intermediates, carbonate and formate were 1410 confirmed through transmission IR spectra. The bands at 1410 and 1540cm -~ were due to carbonate species and the bands at 2930, ax26 I 1 I i I 2860, 2770, 1610 and 1360cm ~ were due to 2200 1800 1400 3000 2900 2800 2700 formate species. The intensities of small bands at about 1600cm-1 due to the water Wave number(cm "l) and the CO band at 2130cm 1 were higher than that of formate bands (2930, 2860cm-1). Fig. 5. IR spectra taken during CO2 hydrogenation This result showed that reverse water gas over Cu/ZrO2(7:93) catalyst under 22 atm at shit~ reaction was favored at lower (a)30~ (b)70~ (e)260~ temperature. This was supported by the reaction product analysis in which CO was the only product below 100~ The intensity of formate band increased with reaction temperature in good correlation with the results that methanol synthesis increased with temperature(not shown). However, formate bands were clearly shown at 70~ but methanol was not detected below 100~ It is believed that formate is difficult to be converted into formaldehyde or methoxy, and that the rate determining step is the conversion of formate into formaldehyde or methoxy.
REFERENCES
1. 2. 3. 4. 5. 6.
G.J. Milar, D.H. Rochester and K.C. Waugh, Catal. Lett., 14 (1992) 289 S. Fujita, M. Usui, E. Ohara and N. Takezuwa, Catal. Lett., 13 (1992) 349 S.G. Neophytides, A.J. Marchi and G.F. Froment, Appl. Catal. A:General, 86 (1992) 45 J.F. Edwards and G.L. Schrader, J. Catal., 94 (1985) 175 Y. Sun and P.A. Sermon, Catal. Lett., 29 (1994) 361 Y. Nitta, O. Suwata, Y. Ikeda, Y. Okamoto and T. Imanaka, Catal. Lett., 26 (1994) 345
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
505
A study on methanol synthesis through CO 2 hydrogenation over copper-based catalysts Son-Ki Ihm, Young-Kwon Park, Jong-Ki Jeon, Kwang-Cheon Park and Dong-Keun Lee* Dept. of Chem. Eng., Korea Advanced Institute of Science and Technology, 373-1 Kusungdong, Yusong-gu, Taejon 305-701, Korea
In CO2 hydrogenation over Cu/ZrO2 based catalysts, the methanol formation activity could be correlated with copper dispersion. The reaction intermediates of methanol synthesis were carbonate, formate, formaldehyde and/or methoxy, and the rate determining step for methanol synthesis seems to be the conversion of formate into formaldehyde or methoxy.
1. INTRODUCTION The synthesis of methanol over copper-based catalysts is an important industrial process and one of the most investigated catalytic reactions. As methanol can be very easily converted to other valuable materials, its add-value can compensate large energy costs needed. Although methanol synthesis from CO2 hydrogenation over supported copper catalysts has been widely investigated, there are still controversies concerning the methanol synthesis mechanism and the effect of copper on the catalytic activity[ 1-6]. In this work, the influence of the copper dispersion in Cu/ZrO2 catalyst on the catalytic activity in CO2 hydrogenation was investigated. In order to understand the reaction mechanism, FT-IR spectroscopy under reaction conditions and TPD of adsorbed methanol were performed.
2. EXPERIMENTAL Binary copper-based catalysts were prepared by coprecipitation method and some components were added as promoters into the binary catalysts. The methanol synthesis reaction was carried out in a continuous flow microreactor operated at 22 atm and at various temperatures. Reaction pathway of the methanol synthesis was investigated through FT-IR spectroscopy. For the catalyst with a copper content over 15wt%, the diffuse reflectance method (DRIFT) was applied, but for the catalyst with a copper content of 7wt%, the transmission technique was used. For more information about intermediates, TPD of adsorbed methanol was carried out and the products were analyzed using mass spectrometer.
*Present Address : Dept. of Chem. Eng., Res. Inst. Environ. Prot., Gyeongsang Nat. Univ., 900 Kajwa-dong, Chinju 660-701, Kyongnam, Korea Acknowldgement : This work was partially supported by Clean Energy Program by R&D Management Center for Energy and Resources of MTI (Korea).
506 3. R E S U L T S AND D I S C U S S I O N
Among copper based binary catalyst systems, CuO/ZrO2 was proved to be the most reactive toward methanol synthesis. The methanol synthesis activity of the CuO/ZrO2 catalyst was greatly affected by the copper dispersion (or copper crystallite size) ; the smaller the crystallite size, the higher the rate of methanol synthesis (Table 1). When some components of Ce, Cr, Pd, K, V and Zn were added as promoters into CuO/ZrO2, the crystallite size of copper particles changed significantly. CeO2 increased the copper crystallite size significantly, while ZnO made the copper crystallite size much smaller than those of the Cu/ZrO2 samples. Table 1. Ph~csical properties and methanol s)mthesis rate of Cu based catal~csts Catalyst SBET(m2/g) d(nm)* Methanol formation rate (104mol/g-cat 9min) Cu/ZrO2a(pH=7) 55 37 5.48 (pH=9) 78 41 4.99 (pH=l 1) 88 43 4.83 Cu/ZrOzb(pH=7) 60 53 4.75 Cu/ZrO2/CeO2 98 69 3.95 Cu/ZrO2/fr203 102 32 5.72 Cu/ZrO//PdO 69 40 5.39 Cu/ZrO2/K20 42 33 5.56 Cu/ZrO2/V205 90 70 3.38 Cu/ZrO~/ZnO 87 21 8.94 Cu/ZrO2:60/40 (wt%), Cu/ZrO2/MexOy:60/30/10 (wt%), precipitating agent ; aNaOH, bNa2CO3 * : Cu crystallite size by XRD, Temperature : 250~ Pressure : 22atm Fig. 1. shows the DRIFT spectra with time on stream in C O 2 hydrogenation. At 5min, bands at 1060, 1280, 1380, 1520 and 1580cm1 were observed. As reaction time increased, the band at 1580cm-' grew apparently and reached a steady state after 30 min. The bands at 1580, 1380cm-1, the bands at 1520, 1280cm", and the bands at 1060-1080cm 1 could be assigned to bidentate formate, bidentate carbonate and methoxy, respectively[I-4]. It was found that the bands of bidentate carbonate, bidentate formate and methoxy continued to grow in transient state and that formate and methoxy reached steady state. A similar spectra were observed for Cu/ZrO2/ZnO. For ZnO catalyst, however, we found that the formate bands were observed but the methoxy band was hardly observed. Fig. 2 shows the DRIFT spectra with increasing temperature in CO2 hydrogenation over Cu/ZrO2. The Cu formate band at 1590cm1 decreased with temperature. The formaldehyde band at 1120-~1150cm1 increased with temperature upto 190~ but beyond that temperature the band decreased again. From the above results, copper formate, formaldehyde and methoxy were believed to be the intermediates of methanol synthesis on Cu/ZrO 2 catalysts. Temperature-programmed methanol decomposition was observed with DRIFT (Fig. 3). At 50~ monodentate formate, formaldehyde and methoxy were observed at 1600, 1130 and 1080cm~, respectively. As temperature increased, the bands due to monodentate formate and methoxy decreased slowly. The bands at 1580 and 1370cm" due to bidentate formate increased with temperature, but disappered over 270~ Gas phase products from the decomposition of adsorbed methanol were analyzed by mass spectrometer(Fig. 4). Methanol began to appear at around 70~ and disappeared about 200~ which supported the IR results
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Wave Number (cm"1)
Wave number(cm "1)
Fig. 1. FT-IR spectra during CO 2 hydrogenation over Cu/ZrO 2 catalyst at 250~ and 22atm for (a)lmin (b)5min (c)lOmin (d)15min (e)aOmin (f)6Omin (g) 120min (h) 180min
~6oo
Fig. 2. FT-IR spectra during CO 2 hydrogenation over Cu/ZrO 2 catalyst under 22atm (a)100~ (b) 150~ (c)190~ (d)230~ (e)250~
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Wave number (cm"1) Fig. 3. FT-IR spectra during methanol TPD over Cu/ZrO2 catalyst at (a) 50~ (b) 70~ (c) 110~ (d)lS0~ (e)190~ (f) 230~ (g)270~ (h)300~
50
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Temperature (~ Fig. 4. Methanol TPD curves by GC-Mass over Cu/ZrO 2 catalyst
508 (Fig. 3) that methoxy peak disappeared above 200~ For C u / Z r O 2 / Z n O , similar results were also obtained. 2000 In addition to above results, a 1~o I transmission FT-IR spectra were obtained to confirm reaction intermediates over Cu/ZrO2 (7/93 in wt%). Fig. 5 shows the FT-IR spectra during CO2 hydrogenation. The band at 2130cm 1 was due to dissociation of CO2 into CO and O. Among reaction intermediates, carbonate and formate were 1410 confirmed through transmission IR spectra. The bands at 1410 and 1540cm -~ were due to carbonate species and the bands at 2930, ax26 I 1 I i I 2860, 2770, 1610 and 1360cm ~ were due to 2200 1800 1400 3000 2900 2800 2700 formate species. The intensities of small bands at about 1600cm-1 due to the water Wave number(cm "l) and the CO band at 2130cm 1 were higher than that of formate bands (2930, 2860cm-1). Fig. 5. IR spectra taken during CO2 hydrogenation This result showed that reverse water gas over Cu/ZrO2(7:93) catalyst under 22 atm at shit~ reaction was favored at lower (a)30~ (b)70~ (e)260~ temperature. This was supported by the reaction product analysis in which CO was the only product below 100~ The intensity of formate band increased with reaction temperature in good correlation with the results that methanol synthesis increased with temperature(not shown). However, formate bands were clearly shown at 70~ but methanol was not detected below 100~ It is believed that formate is difficult to be converted into formaldehyde or methoxy, and that the rate determining step is the conversion of formate into formaldehyde or methoxy.
REFERENCES
1. 2. 3. 4. 5. 6.
G.J. Milar, D.H. Rochester and K.C. Waugh, Catal. Lett., 14 (1992) 289 S. Fujita, M. Usui, E. Ohara and N. Takezuwa, Catal. Lett., 13 (1992) 349 S.G. Neophytides, A.J. Marchi and G.F. Froment, Appl. Catal. A:General, 86 (1992) 45 J.F. Edwards and G.L. Schrader, J. Catal., 94 (1985) 175 Y. Sun and P.A. Sermon, Catal. Lett., 29 (1994) 361 Y. Nitta, O. Suwata, Y. Ikeda, Y. Okamoto and T. Imanaka, Catal. Lett., 26 (1994) 345