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Selective photocatalytic oxidation of methane into methanol on V-MCM-41 mesoporous molecular sieves
Recent Progress in Mesostructured Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
717 717
Selective photocatalytic oxidation of methane into methanol on V-MCM-41 mesoporous molecular sieves Yun Hu, Yasuhito Nagai, Masaya Matsuoka and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
The partial photocatalytic oxidation of methane with NO led to the selective methanol formation over the V-MCM-41 catalysts under UV irradiation at 295 K, while only a complete oxidation of methane proceeded in the presence of O2. The yield of methanol well corresponded to the yield of the photoluminescence due to the isolated tetrahedral V +-oxides species, indicating that the charge transfer excited triplet state of these species plays an importat role in the reaction. 1. Introduction The selective conversion of methane into higher hydrocarbons and/or more valuable oxygen-containing compounds such as methanol and formaldehyde has attracted much attention in the last decade since there are still difficulties in activating C-H bonds of CH4 and C2H6 [1, 2]. In most of these studies, high temperatures are required even for the low conversion levels. It has been reported that a silica supported vanadium oxide catalysts exhibit relatively high reactivity for the oxidation of methane into methanol and formaldehyde [3]. However, few studies on the selective photooxidation of methane have been successful [4, 5]. In this study, we report on the preparation of V-MCM-41 mesoporous molecular sieves which can accommodate the high concentrations of the isolated V5+-oxide species as active sites and their photocatalytic activities for the partial oxidation of methane with various oxidative gasses. This work was supported by the Japan Society for the Promotion of Science, Grant-inAid for JSPS Fellows.
718
2. Experimental Section
3. Results and Discussion
Yields of products / µmol •g-cat-1
V-MCM-41 gels were synthesized under acidic or basic conditions. Under both pathways, cetyltrimethylammonium bromide served as the template. For the acidic pathway, tetraethyl orthosilicate (TEOS) and NH4VO3 were used as the Si source and the V ion precursor, respectively. For the basic condition pathway, sodium silicate solution and SiO2 (Aerosil) or TEOS were used as Si source, while VOSO4-3H2O or NH4VO3 were used as the V ion precursor. After the as-synthesized products were recovered by filtration, washed with water and dried at 373 K for 12 h, calcination of the samples was carried out in air at 773 K for 8 h. Prior to the photocatalytic reactions and spectroscopic measurements, the catalysts were degassed at 773 K for 1 h, heated in O2 at 773 K for 2 h, and finally degassed at 473 K for 2 h. Photoluminescence spectra were measured at 295 K with a Spex Fluorog-3 spectrophotometer. The photocatalytic partial oxidations of methane with O2 and NO were carried out with the catalysts in a quartz cell under UV irradiation (A, > 270 nm) by a 100 W high pressure mercury lamp at 295 K. Reaction products were analyzed by gas chromatography. 15
CO CO22
The XRD patterns showed that all 10 10 CH 3 OH CHOH of the prepared V-MCM-41 catalysts have a siliceous MCM-41 meso5 porous structure. Figure 1 shows the results of the photocatalytic oxidation of methane with O2 and NO over the 0 V-MCM-41 (0.6 wt%) catalyst at 295 CH O 2= CH NO = CH 4//O =11 CH4//NO = 11 K. No products could be detected without the catalysts or UV light Fig. 1. Yields of CO2 and CH3OH in the irradiation. In the presence of O2, photocatalytic oxidation of CH4 with O2 or only the complete oxidation of NO on V-MCM-41(0.6 wt%) prepared by methane into CO2 and H2O proceeded acidic pathway. Amount of reactant: 4 under UV light irradiation. However, Torr. Reaction time: 3 h. the photocatalytic oxidation of methane with NO on V-MCM-41 resulted in the selective formation of methanol, accompanying the formation of the trace amounts of CO2 and acetaldehyde. The methane conversion and the methanol selectivity reached 6% and 88%, respectively, after the UV irradiation for 3 h. Figure 2 shows the effect of the V content of the V-MCM-41 catalysts on the reactivity for the photocatalytic oxidation of methane with NO. The yield of methanol was found to increase with the increase in the V content up to 0.6 wt%, and then decrease with a further increase in the V content. It was also found that V-MCM-41 exhibits photoluminescence at around 500 nm under excitation at 300 nm, as shown in Fig. 3, due to the radiative decay process 4
2
4
719
Yields of CH3OH / µmol• g- cat-1
10 8
6 4 2 0
0.15
0.6
3.6
V content //wt% wt%
Fig. 2. Relationship between the yields of CH3OH in the photocatalytic partial oxidation of CH4 with NO on V-MCM-41 prepared by acidic pathway and the intensities of the photoluminescence spectra.
0 Torr (a)
Intensity / a.u.
Intensity of photoluminescence / a.u.
from the charge transfer excited triplet state of the isolated tetrahedral V4+-oxide species [5]. The addition of CH4 or NO molecules onto the catalyst leads to an efficient quenching of the photoluminescence, their extents depending on the amount of added gas, indicating that the CH4 or NO molecules interact with the photoexcited V5+-oxide species. The good correspondence between the yield of methanol and the intensity of the photoluminescence indicates that the isolated tetrahedral V5+-oxide species act as active sites for the photocatalytic oxidation of methane with NO into methanol.
0.007 Torr
0.04 Torr 0.1 Torr excess
400
500 600 Wavelength / nm
700
Fig. 3. Effect of the addition of CH, on the photoluminescence spectrum of VMCM-41 (0.6 wt%) prepared by acidic pathway. (a): after degassing at R.T. for 30 min.
The effects of the preparation method of the catalyst, such as pH value of the starting solution and kinds of V and Si source, on the reactivity for the photocatalytic oxidation of methane with NO were investigated. As shown in Table 1, the high conversion of methane and high selectivity for the formation of methanol were obtained for V-MCM-41 prepared in acidic solution using TEOS as the Si source. In the case of the V-MCM-41 prepared in basic solution using TEOS as the Si source, selective formation of methanol was also observed. On the contrary, quite low methanol selectivity was observed for VMCM-41 prepared in basic solution using sodium silicate as the Si source. It has been reported that pre-impregnation of the silica support with sodium strongly diminishes V=O concentration due to the formation of tetrahedral monomers with a high degree of symmetry, leading to the strong inhibition of the partial oxidation of methane on V/SiO2 [6]. The low methanol selectivity of V-MCM-41 prepared from sodium silicate can be attributed to the presence of sodium ion in the mesoporous framework structure. The low methane conversion on V-MCM-41 prepared in basic condition can be also ascribed to the efficient incorporation of V +-oxide species within the framework of MCM41 due to the high ordered framework of MCM-41 [7], which inhibits the efficient interaction of V5+-oxide species with gaseous reactants. Furthermore,
720
no significant difference in the methanol selectivity was observed between the catalysts prepared using NH4VO3 and VOSO4 as the V source. The activity on V-MCM-41 (acidic method) was found to be higher than that on V/SiO2, suggesting that the higher surface area and mesopore structure of MCM-41 could favor the dispersion of the V-oxide species, leading to a higher photocatalytic activity for methane oxidation. The reactivity on the V-MCM-41 catalysts remained at least after three circles. Table 1. The CH4 conversion and the selectivity of products in the photocatalytic oxidation of CH4 on V-MCM-41 (0.6 wt%) prepared by different methods. CH,
PH
V source
Si source
1
NH4VO3
11
VOSO,
11
sodium silicate, N H4V O 3 SiO2
0.7
11
sodium silicate, SiO2
3.7 3.6
VOSO,
Selectivity (% )
(%)
C H 3 OH
CO2
C2HX
TEOS
6.0
87 .6
4.2
TEOS
2.6
85 .5
7.6
i m p - V / S i O 2 (V : N H , V O , |
CjH,
CH3CHO
other
0.6
2.6
5.0
1.1
5.8
-
1 0.2
23.3
66.5
5. 6
10.1
14.6
87 .8
5.8
1.0
39.4
22.9
7.4
5.4
-
4. Conclusion The photocatalytic partial oxidation of methane proceeds efficiently on the V-MCM-41 catalyst prepared in acidic condition, the reaction selectivity strongly depending on the kind of oxidative gasses. Selective formation of methanol was observed when NO was used as oxidative gas, while complete oxidation reaction proceeds in the presence of O2. The yield of methanol corresponded to the yield of the photoluminescence of the isolated tetrahedral V5+-oxide species, indicating that the photoexcited V5+-oxide species plays an important role in the photocatalytic oxidation of methane into methanol. The activity as well as the selectivity of V-MCM-41 greatly depends on the preparation methods of the catalysts. It has been elucidated that the methanol selectivity of V-MCM-41 can be enhanced dramatically by using TEOS as starting reagent for the Si source. 5. References [1] Z. Sojka, R. G. Herman and K. Klier, J. Chem. Soc. Chem. Commun. (1991) 185. [2] A. Parmaliana, F. Frusteri, A. Mezzapica, D. Miceli, M.S. Scurrell and N. Giordano, J. Catal. 143(1993)262. [3] M. M. Koranne, J. G. Goodwin Jr. and G. Marcelin, J. Phys. Chem. 97 (1993) 673. [4] K. Wada,K. Yoshida,Y.Watanabe and T. Suzuki,! Chem. Soc.,Chem. Commun.(1991)726. [5] Y. Hu, S. Higashimoto, S. Takahashi, Y. Nagai and M. Anpo, Catal. Lett. 100 (2005) 35. [6] S. Irusta, A.J. Marchi, E.A. Lombardo and E.E. Miro, Catal. Lett. 40 (1996) 9. [7] Y. Hu, N. Wada, M. Matsoka and M. Anpo, Catal. Lett. 97 (2004) 49.
717 717
Selective photocatalytic oxidation of methane into methanol on V-MCM-41 mesoporous molecular sieves Yun Hu, Yasuhito Nagai, Masaya Matsuoka and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
The partial photocatalytic oxidation of methane with NO led to the selective methanol formation over the V-MCM-41 catalysts under UV irradiation at 295 K, while only a complete oxidation of methane proceeded in the presence of O2. The yield of methanol well corresponded to the yield of the photoluminescence due to the isolated tetrahedral V +-oxides species, indicating that the charge transfer excited triplet state of these species plays an importat role in the reaction. 1. Introduction The selective conversion of methane into higher hydrocarbons and/or more valuable oxygen-containing compounds such as methanol and formaldehyde has attracted much attention in the last decade since there are still difficulties in activating C-H bonds of CH4 and C2H6 [1, 2]. In most of these studies, high temperatures are required even for the low conversion levels. It has been reported that a silica supported vanadium oxide catalysts exhibit relatively high reactivity for the oxidation of methane into methanol and formaldehyde [3]. However, few studies on the selective photooxidation of methane have been successful [4, 5]. In this study, we report on the preparation of V-MCM-41 mesoporous molecular sieves which can accommodate the high concentrations of the isolated V5+-oxide species as active sites and their photocatalytic activities for the partial oxidation of methane with various oxidative gasses. This work was supported by the Japan Society for the Promotion of Science, Grant-inAid for JSPS Fellows.
718
2. Experimental Section
3. Results and Discussion
Yields of products / µmol •g-cat-1
V-MCM-41 gels were synthesized under acidic or basic conditions. Under both pathways, cetyltrimethylammonium bromide served as the template. For the acidic pathway, tetraethyl orthosilicate (TEOS) and NH4VO3 were used as the Si source and the V ion precursor, respectively. For the basic condition pathway, sodium silicate solution and SiO2 (Aerosil) or TEOS were used as Si source, while VOSO4-3H2O or NH4VO3 were used as the V ion precursor. After the as-synthesized products were recovered by filtration, washed with water and dried at 373 K for 12 h, calcination of the samples was carried out in air at 773 K for 8 h. Prior to the photocatalytic reactions and spectroscopic measurements, the catalysts were degassed at 773 K for 1 h, heated in O2 at 773 K for 2 h, and finally degassed at 473 K for 2 h. Photoluminescence spectra were measured at 295 K with a Spex Fluorog-3 spectrophotometer. The photocatalytic partial oxidations of methane with O2 and NO were carried out with the catalysts in a quartz cell under UV irradiation (A, > 270 nm) by a 100 W high pressure mercury lamp at 295 K. Reaction products were analyzed by gas chromatography. 15
CO CO22
The XRD patterns showed that all 10 10 CH 3 OH CHOH of the prepared V-MCM-41 catalysts have a siliceous MCM-41 meso5 porous structure. Figure 1 shows the results of the photocatalytic oxidation of methane with O2 and NO over the 0 V-MCM-41 (0.6 wt%) catalyst at 295 CH O 2= CH NO = CH 4//O =11 CH4//NO = 11 K. No products could be detected without the catalysts or UV light Fig. 1. Yields of CO2 and CH3OH in the irradiation. In the presence of O2, photocatalytic oxidation of CH4 with O2 or only the complete oxidation of NO on V-MCM-41(0.6 wt%) prepared by methane into CO2 and H2O proceeded acidic pathway. Amount of reactant: 4 under UV light irradiation. However, Torr. Reaction time: 3 h. the photocatalytic oxidation of methane with NO on V-MCM-41 resulted in the selective formation of methanol, accompanying the formation of the trace amounts of CO2 and acetaldehyde. The methane conversion and the methanol selectivity reached 6% and 88%, respectively, after the UV irradiation for 3 h. Figure 2 shows the effect of the V content of the V-MCM-41 catalysts on the reactivity for the photocatalytic oxidation of methane with NO. The yield of methanol was found to increase with the increase in the V content up to 0.6 wt%, and then decrease with a further increase in the V content. It was also found that V-MCM-41 exhibits photoluminescence at around 500 nm under excitation at 300 nm, as shown in Fig. 3, due to the radiative decay process 4
2
4
719
Yields of CH3OH / µmol• g- cat-1
10 8
6 4 2 0
0.15
0.6
3.6
V content //wt% wt%
Fig. 2. Relationship between the yields of CH3OH in the photocatalytic partial oxidation of CH4 with NO on V-MCM-41 prepared by acidic pathway and the intensities of the photoluminescence spectra.
0 Torr (a)
Intensity / a.u.
Intensity of photoluminescence / a.u.
from the charge transfer excited triplet state of the isolated tetrahedral V4+-oxide species [5]. The addition of CH4 or NO molecules onto the catalyst leads to an efficient quenching of the photoluminescence, their extents depending on the amount of added gas, indicating that the CH4 or NO molecules interact with the photoexcited V5+-oxide species. The good correspondence between the yield of methanol and the intensity of the photoluminescence indicates that the isolated tetrahedral V5+-oxide species act as active sites for the photocatalytic oxidation of methane with NO into methanol.
0.007 Torr
0.04 Torr 0.1 Torr excess
400
500 600 Wavelength / nm
700
Fig. 3. Effect of the addition of CH, on the photoluminescence spectrum of VMCM-41 (0.6 wt%) prepared by acidic pathway. (a): after degassing at R.T. for 30 min.
The effects of the preparation method of the catalyst, such as pH value of the starting solution and kinds of V and Si source, on the reactivity for the photocatalytic oxidation of methane with NO were investigated. As shown in Table 1, the high conversion of methane and high selectivity for the formation of methanol were obtained for V-MCM-41 prepared in acidic solution using TEOS as the Si source. In the case of the V-MCM-41 prepared in basic solution using TEOS as the Si source, selective formation of methanol was also observed. On the contrary, quite low methanol selectivity was observed for VMCM-41 prepared in basic solution using sodium silicate as the Si source. It has been reported that pre-impregnation of the silica support with sodium strongly diminishes V=O concentration due to the formation of tetrahedral monomers with a high degree of symmetry, leading to the strong inhibition of the partial oxidation of methane on V/SiO2 [6]. The low methanol selectivity of V-MCM-41 prepared from sodium silicate can be attributed to the presence of sodium ion in the mesoporous framework structure. The low methane conversion on V-MCM-41 prepared in basic condition can be also ascribed to the efficient incorporation of V +-oxide species within the framework of MCM41 due to the high ordered framework of MCM-41 [7], which inhibits the efficient interaction of V5+-oxide species with gaseous reactants. Furthermore,
720
no significant difference in the methanol selectivity was observed between the catalysts prepared using NH4VO3 and VOSO4 as the V source. The activity on V-MCM-41 (acidic method) was found to be higher than that on V/SiO2, suggesting that the higher surface area and mesopore structure of MCM-41 could favor the dispersion of the V-oxide species, leading to a higher photocatalytic activity for methane oxidation. The reactivity on the V-MCM-41 catalysts remained at least after three circles. Table 1. The CH4 conversion and the selectivity of products in the photocatalytic oxidation of CH4 on V-MCM-41 (0.6 wt%) prepared by different methods. CH,
PH
V source
Si source
1
NH4VO3
11
VOSO,
11
sodium silicate, N H4V O 3 SiO2
0.7
11
sodium silicate, SiO2
3.7 3.6
VOSO,
Selectivity (% )
(%)
C H 3 OH
CO2
C2HX
TEOS
6.0
87 .6
4.2
TEOS
2.6
85 .5
7.6
i m p - V / S i O 2 (V : N H , V O , |
CjH,
CH3CHO
other
0.6
2.6
5.0
1.1
5.8
-
1 0.2
23.3
66.5
5. 6
10.1
14.6
87 .8
5.8
1.0
39.4
22.9
7.4
5.4
-
4. Conclusion The photocatalytic partial oxidation of methane proceeds efficiently on the V-MCM-41 catalyst prepared in acidic condition, the reaction selectivity strongly depending on the kind of oxidative gasses. Selective formation of methanol was observed when NO was used as oxidative gas, while complete oxidation reaction proceeds in the presence of O2. The yield of methanol corresponded to the yield of the photoluminescence of the isolated tetrahedral V5+-oxide species, indicating that the photoexcited V5+-oxide species plays an important role in the photocatalytic oxidation of methane into methanol. The activity as well as the selectivity of V-MCM-41 greatly depends on the preparation methods of the catalysts. It has been elucidated that the methanol selectivity of V-MCM-41 can be enhanced dramatically by using TEOS as starting reagent for the Si source. 5. References [1] Z. Sojka, R. G. Herman and K. Klier, J. Chem. Soc. Chem. Commun. (1991) 185. [2] A. Parmaliana, F. Frusteri, A. Mezzapica, D. Miceli, M.S. Scurrell and N. Giordano, J. Catal. 143(1993)262. [3] M. M. Koranne, J. G. Goodwin Jr. and G. Marcelin, J. Phys. Chem. 97 (1993) 673. [4] K. Wada,K. Yoshida,Y.Watanabe and T. Suzuki,! Chem. Soc.,Chem. Commun.(1991)726. [5] Y. Hu, S. Higashimoto, S. Takahashi, Y. Nagai and M. Anpo, Catal. Lett. 100 (2005) 35. [6] S. Irusta, A.J. Marchi, E.A. Lombardo and E.E. Miro, Catal. Lett. 40 (1996) 9. [7] Y. Hu, N. Wada, M. Matsoka and M. Anpo, Catal. Lett. 97 (2004) 49.