High throughput experiments on methane partial oxidation using molecular oxygen over silica doped with various elements

Applied Catalysis A: General 254 (2003) 45–58

High throughput experiments on methane partial oxidation using molecular oxygen over silica doped with various elements Yusuke Yamada1 , Atsushi Ueda, Hiroshi Shioyama, Tetsuhiko Kobayashi∗ Special Division for Green Life Technology, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received 7 October 2002; accepted 6 January 2003

Abstract Methane oxidation catalysis was measured for each of 43 elements doped in silica prepared by an impregnation method. The loading of each element was 0.1, 1 and 5 mol.% to drastically change the structure of the elements on the silica surface. Isomorphous substitution of an element in silicon was expected at 0.1 mol.% loading, if possible. A small cluster of metal oxide would be formed on the silica surface at 1% loading. Metal oxides can fully cover the silica surface at 5% loading. The catalytic performance was examined at around 773 K with GHSV = 4760 h−1 . The ratio of CH4 /O2 in the feed gas was varied from 90/10, 80/20 and 70/30 at atmospheric pressure. On the basis of the screening results on more than 120 catalysts, the elements suitable for methane selective oxidation were V, Fe, Sc, W, Mo and Os under these conditions. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Methane oxidation (partial); Silica; On-demand database; High throughput screening

1. Introduction Recently, combinatorial chemistry and high throughput experiments have attracted many catalyst developers and scientists [1–3]. Various tools for the high throughput experiments have been reported for catalyst preparation, catalysis evaluation and optimization. More than 100 catalysts have been prepared at a time with solution treating techniques such as the sol– gel technique [4,5]. Regarding catalysis evaluation tools, an IR thermograph [6–8], mass spectrometer [9–11], REMPI [12], photoaccoustic spectrometer [13], IR spectrometer with a multichannel detec∗ Corresponding author. Tel.: +81-72-751-9461; fax: +81-72-751-9630. E-mail address: [email protected] (T. Kobayashi). 1 Co-corresponding author.

tor [14], gas sensors [15], SEM [16], fluorescence imaging [17,18], dye utilization [19,20], etc. have been reported. Suitable multichannel reactors have also been developed [21–23]. As for optimization methodology, a genetic algorithm was adopted for the investigation of mixed oxide catalysts for oxidative dehydrogenation of ethane and propane [24,25] and propane selective oxidation [26]. Using these tools, we can obtain systematically designed catalysts and study their catalysis under the same reaction conditions within a short time. A great merit of the high throughput experiment is the achievement of an “on-demand” database. Catalysis depends not only on the catalyst itself but also on reaction conditions such as feed gas composition, pressure, catalyst shape and reaction temperature. Each researcher optimizes the reaction conditions for each catalyst to present its best catalytic activity; thus, the reaction

0926-860X/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(03)00262-X

46

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

conditions in the literature were varied if the treated reaction was the same. This fact makes it very difficult to build a systematic database from the literature and obtain “knowledge” from the literature. Catalyst design based on a combinatorial concept and testing a series of catalysts under the same conditions helps to obtain “knowledge” from the data. Selective oxidation of methane to formaldehyde or methanol using molecular oxygen or nitrous oxide is a challenging topic in catalyst research [27]. Several metal oxides such as SiO2 [28,29], Fe/SiO2 [30,31], V/SiO2 , Mo/SiO2 , [32–41], FePO4 [42] have been reported for the selective oxidation catalysis of methane.

The reaction conditions were also arbitrary and were determined by each researcher. It is not easy to recognize which catalyst is the best for the selective oxidation. Here, we show the methane oxidation catalysis with a series of silicas doped with 43 different elements at very small (0.1 mol.%), small (1%) or large (5%) loading. It is known that silica itself shows partial methane oxidation catalysis and that the addition of an appropriate chemical species on the silica surface enhances the selective oxidation catalysis [28,29]. The suitable structure for the reaction depends on each element. For example the suitable structure for vanadium

Scheme 1. Home-made five channel parallel reactor for the high throughput experiments.

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

is “vanadium oxide” because its lattice oxygen plays an important role in the reaction. On the other hand, an isolated form is effective with tungsten [43,44] and iron [30,31]. As for iron, a very small amount of iron (<0.1 mol.%) doped into silica also greatly enhances the partial oxidation catalysis; on the other hand, addition of a large amount of iron (>2%) decreases the formaldehyde selectivity [31]. In order to investigate the catalysis of differently structured metal oxides, the loading amounts were dramatically changed for each element. The metal ions or oxides with different structures should be treated as different chemical species. Three species can be considered for each element. The first is the isomorphous substitution into silica only for limited elements. Such substitution was expected for B, Cr, V, Al, Ge, Ga, Fe, Ti, Sn, Zn, Be, Mg, Zn, P, Mo, and Mn because their ionic radii is comparable to

47

that of Si and they can take tetrahedral coordination. The second is metal oxides partly covering the silica surface, and the third is a metal oxide fully covering the silica surface. These three chemical species can be formed at a molar ratio of metal ions to silicon for very small, small and large loadings. The methane oxidation catalysis was measured on silica doped with 43 elements at three loadings to build a database to determine which element is suitable for the reaction.

2. Experimental section 2.1. Catalyst preparation Usually, the corresponding metal nitrate was used as the precursor of the loaded metal oxide. Vanadium citrate and [(diethylenetriamine)trioxotungsten(VI)]

Fig. 1. Production rate of formaldehyde and MeOH over alkali or alkali earth metal-doped silica. Alkali metal/silicon = 0.1/100, 1/100, 5/100; feed gas composition: CH4 /O2 = 70/30, 80/20, 90/10.

48

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

were used as the precursors of V/SiO2 and W/SiO2 , respectively. The silica support was obtained from Merck (extra pure, 50–270 mesh, 400 m2 /g) and was used without any other purification. The catalyst was prepared by the incipient wetness method. The precursor solutions were automatically prepared by an automated synthesizer, JEOL PTW-100, and each solution (1 ml) was also automatically delivered onto silica (1 g) in a glass tube. The swollen powder was dried at 343 K under atmospheric condition for several hours. The dried powder was calcined at 873 K for 5 h. Ge/SiO2 , Ti/SiO2 , Ta/SiO2 were manually prepared under Ar atmosphere using the Schlenck technique, because their precursors were unstable in air. Each cor-

responding ethanol solution of Ge(OEt)4 , Ti(O-i-Pr)4 or Ta(OEt)5 was impregnated into degassed silica and dried under reduced pressure at room temperature. The dried powder was calcined under air at 873 K for 5 h. 2.2. Catalysis measurement Each 0.63 ml (ca. 270 mg) of catalytic material was placed into a quartz glass tube (i.e. 12 mm) with silica wool. A thermocouple was placed in the center of the bed in order to measure the reaction temperature. Before the catalysis measurement, each catalyst was pretreated with flowing air (50 ml/min). A gas mixture of oxygen and methane (10/90, 20/80 or 30/70) was allowed to flow onto each catalytic material at 50 ml/min

Table 1 Methane reaction rate and selectivity for formaldehyde and MeOH over SiO2 doped with alkali or alkali earth metala CH4 /O2 = 90/10 RCH4 (nmol/s)

5% Li 1% Li 0.1% Li 5% Na 1% Na 0.1% Na 5% K 1% K 0.1% K 5% Rb 1% Rb 0.1% Rb 5% Cs 1% Cs 0.1% Cs 5% Mg 1% Mg 0.1% Mg 5% Ca 1% Ca 0.1% Ca 5% Sr 1% Sr 0.1% Sr 5% Ba 1% Ba 0.1% Ba

89 3.4 16 3.4 0.76 29 52 42 39 18 26 14 1.5 7.6 1.0 78 42 19 28 3.8 0.38 11 13 3.1 2.7 1.1 33

n.d.: not detected. a GHSV = 4760 h−1 , 773 K.

CH4 /O2 = 80/20 Selectivity (%) HCHO

MeOH

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. 67 60 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 57 n.d. 15 50 3.0 9.2 8.0 n.d. n.d. n.d. n.d. 5.9 50 n.d. n.d. 7.0

RCH4 (nmol/s)

0 0.76 0.38 0 0 2.3 0.38 0.77 10 0 16 2.7 0.38 0 3.9 22 15 4.2 20 7.7 2.7 23 14 1.1 16 7.3 5.7

CH4 /O2 = 70/30 Selectivity (%) HCHO

MeOH

50 100

50 n.d.

50 n.d. n.d. 59

17 n.d. n.d. 7.4

n.d. 29 n.d. 0.78 20 9.0 37 27 5.9 5 n.d. n.d. n.d. 33 n.d. n.d. 47

n.d. n.d. n.d. n.d. n.d. n.d. 2.6 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 6.7

RCH4 (nmol/s)

0 2.3 1.5 0 0 5.0 0.39 1.9 19 0 4.3 3.5 0 n.d. 7.8 22 22 6.6 17 6.2 5.0 23 12 4.2 21 15 14

Selectivity (%) HCHO

MeOH

17 n.d.

n.d. n.d.

31 n.d. 20 29

7.7 n.d. n.d. 4.1

n.d. 44

n.d. n.d.

5.0 8.6 22 24 13 6.3 n.d. n.d. n.d. 9.1 n.d. 5.0 21

5.0 n.d. 1.7 n.d. 2.2 n.d. n.d. n.d. n.d. n.d. n.d. 2.5 2.6

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

for a few minutes (GHSV = 4760 h−1 ). The effluents were analyzed by an Agilent technology M-200H micro gas chromatograph equipped with two thermal conductivity detectors and two capillary columns: a Poraplot Q column to separate CO2 , HCHO and water and a molecular sieve 5A column to separate CO, CH4 and O2 . Five catalysts were pretreated and heated at a time by a five channel parallel reactor built by Bel-Japan for high throughput experiments (Scheme 1). The catalysis of each sample was measured at 673, 723 and 773 K. Superior catalysis was confirmed by a conventional FID-GC equipped with a methanator. No significant difference was observed between the obtained results of the TCD-GC and the FID-GC. The silica used here shows no catalytic activity under these reaction conditions. No significant carbonaceous deposition was observed on each sample after reaction. The data reproducibility was confirmed by twice GC analysis. Sometimes, we obtained

49

an inconsistent data set, however, the outliers were removed by the re-measuremets. The superior catalysis was also confirmed by conventional FID-GC equipped with a methanator.

3. Results and discussion Each of 43 elements was added to SiO2 to improve the catalysis of SiO2 . The structure of each element on the SiO2 surface was controlled by its loading amount. Metal ions would be occluded into the silica framework at 0.1 mol.% concentration if the ion can substitute at the silicon position. Sometimes a coordinatively unsaturated ion shows unique catalysis as has been reported for Fe/SiO2 . For example a methane oxidation catalyst of Fe/SiO2 highly depends on the Fe loading amount [31]. On the other hand, a kinetic study with a molecular oxygen isotope on V/SiO2 , which is the

Fig. 2. Production rate of formaldehyde and MeOH over IIIB elements, Ge-, or P-doped silica. Metal/silicon = 0.1/100, 1/100, 5/100; feed gas composition: CH4 /O2 = 70/30, 80/20, 90/10.

50

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

most famous catalyst for methane partial oxidation, indicates that the lattice oxygen of vanadium oxide or peripheral oxygen plays an important role. If the peripheral bridged oxygen plays an important role, a higher loading would diminish its superior catalysis. A loading of 1% is sufficient to form metal oxide particles but not to fully cover the surface of our used silica. A loading amount of 5% seems to be sufficient to cover the entire surface of the silica. Three loadings of 0.1, 1 and 5% were selected to form different metal oxide species on the silica. 3.1. Effect of alkali and alkali earth metals Fig. 1 shows the production rate of MeOH and formaldehyde on alkali (Li, Na, K, Rb, Cs) and alkali earth metal (Mg, Ca, Sr, Ba)-doped SiO2 . The feed gas composition was changed for CH4 /O2 = 70/30, 80/20 and 90/10. Their production rate of HCHO and MeOH was relatively small when compared with other classes of elements. As for the alkali metal concentration, 0.1% loading was better than a higher loading in all cases; however, no clear correlation was found between the feed gas composition and the production rate of HCHO and MeOH. In alkali earth metal-loaded silica, no apparent tendency was found among the

loading amount, feed gas composition and production rate of formaldehyde and MeOH. For Mg/SiO2 , 1% loading was optimal at all feed gas compositions. For Ca/SiO2 , 5% Ca/SiO2 showed a higher production ratio than 1 and 0.1% Ca/SiO2 at feed gas compositions of 70 and 80% CH4 . No production of formaldehyde and MeOH occurred over all Ca/SiO2 ratios with a feed gas of 90% CH4 . 0.1% Sr/SiO2 showed better catalysis than 5 and 1% Sr/SiO2 . Its catalysis became better at high CH4 concentration. A higher production rate was observed at 0.1% Ba/SiO2 than at 1 and 5% Ba/SiO2 . The reason why higher production rate of silica doped with magnesium could be due to its smaller ionic radii than the other alkali earth metals. Magnesium can easily form mixed oxide with silica which shows higher ability for C–H activation than the other alkali earth metal ions. Table 1 summarizes the methane reaction rate (RCH4 ) and the selectivity sum for formaldehyde and MeOH over alkali and alkali earth metal-doped silica. When compared with RCH4 of other classes, the RCH4 of alkali and alkali earth metals-loaded silica was small. At feed gas compositions of CH4 /O2 = 80/20 and 70/30, alkali earth metal-loaded silica showed a larger RCH4 than did the alkali metal-loaded samples. On the other hand, alkali metal-loaded silica showed

Table 2 Methane reaction rate and selectivity for formaldehyde and MeOH over SiO2 doped with Al, Ga, In, Ge, Pa CH4 /O2 = 90/10 RCH4 (nmol/s)

5% Al 1% Al 0.1% Al 5% P 1% P 0.1% P 5% Ga 1% Ga 0.1% Ga 5% Ge 1% Ge 0.1% Ge 5% In 1% In 0.1% In

1.4 2.3 36 0 0 0 2.3 60 4.9 6.1 46 8.7 3.9 84 34

CH4 /O2 = 80/20 Selectivity (%) HCHO

MeOH

× × 102

0.027 1.5 5.2

0.0057 0.33 n.d.

× 102

1.7 6.9 38 69 9.1 70 n.d. n.d. 10

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

102

× 102

n.d.: not detected. a GHSV = 4760 h−1 , 773 K.

RCH4 (nmol/s)

51 17 16 1.2 0 1.2 54 14 9.1 0.77 0 0 3.2 × 102 17 33

CH4 /O2 = 70/30 Selectivity (%) HCHO

MeOH

4.6 11 n.d. 33 7.0 33 0.0 2.7 47 50

1.5 4.7 n.d. 33 22 33 0.0 2.7 8.7 50

0.24 16 35

0.12 2.3 3.7

RCH4 (nmol/s)

37 18 15 18 5.6 11 65 14 41 0.39 0.77 0 2.2 × 102 27 83

Selectivity (%) HCHO

MeOH

1.0 11 n.d. 13

1.1 4.3 n.d. 4.3

41 0.0 2.8 22 100 50

6.9 0.60 0.0 1.9 n.d. n.d.

1.3 23 19

0.54 2.9 1.9

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

better selectivity for formaldehyde and MeOH than the alkali earth metal-loaded samples. A correlation between a large RCH4 and low selectivity for useful oxygenate has often been observed with various selective oxidation catalysts. The lower selectivity of alkali earth metal-doped silica seems to be due to its higher RCH4 . 3.2. Effect of typical elements (Al, P, Ga, Ge and In) Fig. 2 shows the formation rate of MeOH and formaldehyde for methane oxidation over Al-, P-, Ga-, Ge- or In-loaded SiO2 with three feed gas compositions at 773 K. Al, Ga, In are trivalent (IIIB metals), Ge is tetravalent (IVB) and P (VB) is pentavalent. Among IIIB metals, In was the best additive for formaldehyde and MeOH formation. As for the loading of In, 0.1% loading was the best at all feed

51

gas compositions. A higher oxygen concentration yielded a higher production rate. As for Ga/SiO2 and Al/SiO2 , the optimal concentration was 0.1% for Ga and 1% for Al. The formation rate of formaldehyde and MeOH over Ga/SiO2 was independent of the loading amount of Ga. The best feed gas composition was the highest methane concentration of 90%. The production rate over pentavalent P had no correlation with the loading amount but rather with feed gas composition. On the contrary, in the case of Ge, a lower methane concentration was favorable. The methane reaction rate and selectivity sum for formaldehyde and methanol are summarized in Table 2. Among the IIIB metal-loaded silicas, their methane reaction rate strongly depends on the feed gas composition. As a whole, a larger RCH4 was obtained at higher methane partial pressure. As for selectivity, each 5% metal oxide-doped silica showed lower selectivity, although their RCH4 values were

Fig. 3. Production rate of formaldehyde and MeOH over first transition-metals-doped silica. Metal/silicon = 0.1/100, 1/100, 5/100; feed gas composition: CH4 /O2 = 70/30, 80/20, 90/10.

52

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

larger than those of 1 and 0.1% metal oxide-doped silica. Ge-doped silica shows a similar tendency with a larger RCH4 obtained at CH4 /O2 = 90/10 than at 80/20 and 70/30. On the contrary, P-doped silica showed a larger RCH4 at CH4 /O2 = 70/30 than at 80/20 and 90/10. These results might be related to their difference in valence. 3.3. Effect of first row transition elements The group of first transition element-doped silicas includes the most favorable species for methane partial oxidation. Fig. 3 shows the formation rate of formaldehyde and MeOH over first transition element-loaded

silica. V/SiO2 shows an extremely high activity for the formation of formaldehyde and MeOH at 1 and 5% loadings. The formation rate became faster at higher vanadium loadings. This result matched other previous reports that the lattice oxygen of the vanadium oxide is closely related to the formaldehyde or MeOH formation. The reason for a sudden decrease in the production rate over 5% V/SiO2 at a feed gas composition of CH4 /O2 = 90/10 is due to an extremely large RCH4 . Vanadium oxide formation seems to be suitable for the methane selective oxidation. Table 3 summarizes the methane conversion rate and the selectivity sum for formaldehyde and MeOH over silicas doped with first transition element. The

Table 3 Methane reaction rate and selectivity for formaldehyde and MeOH over SiO2 doped with first row transition elementa CH4 /O2 = 90/10 RCH4 (nmol/s)

5% Sc 1% Sc 0.1% Sc 5% Ti 1% Ti 0.1% Ti 5% V 1% V 0.1% V 5% Cr 1% Cr 0.1% Cr 5% Mn 1% Mn 0.1% Mn 5% Fe 1% Fe 0.1% Fe 5% Co 1% Co 0.1% Co 5% Ni 1% Ni 0.1% Ni 5% Cu 1% Cu 0.1% Cu 5% Zn 1% Zn 0.1% Zn a

57 4.6 49 69 4.5 1.5 1.6 2.1 4.2 2.1 1.5 6.6 5.6 1.6 3.4 2.6 91 45 2.0 2.4 81 3.9 2.7 2.7 1.9 3.8 57 5.0 49 69

× 103 × 102 × × × × × × ×

103 103 102 102 102 102 102

× 103 × 102 × × × × ×

102 102 102 103 102

GHSV = 4760 h−1 , 773 K.

CH4 /O2 = 80/20 Selectivity (%) HCHO

MeOH

n.d. n.d. n.d. n.d. 25 n.d. 0.97 19 27 n.d. n.d. n.d. n.d. 0.48 1.9 n.d. 11 24 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.014 n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. n.d. 0.16 1.64 n.d. n.d. n.d. 0.40 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

RCH4 (nmol/s)

31 19 4.3 13 1.5 0.75 1.2 × 22 1.9 1.3 × 2.6 × 8.1 × 3.7 × 1.3 × 2.2 × 2.6 × 45 25 5.0 × 2.7 × 72 3.4 × 46 5.7 × 2.4 × 8.1 × 2.2 × 46 12 14

103

103 103 102 102 102 102 102

102 102 102 102 103 102 102

CH4 /O2 = 70/30 Selectivity (%) HCHO

MeOH

8.6 29 18. 5.9 25 50 6.2 29 40 n.d. n.d. n.d. n.d. n.d. 3.3 n.d. 12 31 n.d. n.d. n.d. n.d. n.d. n.d. 0.070 0.78 8.9 8.4 6.3 2.7

1.2 4.2 9.1 n.d. n.d. n.d. 0.16 1.8 20 0.029 0.025 n.d. 0.10 n.d. n.d. n.d. n.d. n.d. n.d. 0.14 0.52 0.11 0.82 n.d. 0.042 0.091 0.35 1.7 3.1 2.7

RCH4 (nmol/s)

52 38 14 22 6.2 1.1 9.5 45 22 8.7 1.1 1.7 2.8 97 1.6 2.4 46 22 6.0 80 41 2.0 31 9.7 2.5 6.0 1.9 38 14 22

× 102 × × × ×

102 103 102 102

× 102 × 102 × 102 × 102 × × × ×

102 103 102 102

Selectivity (%) HCHO

MeOH

11 23 19 3.6 19 67 7.3 14 14 n.d. n.d. n.d. n.d. n.d. 3.8 n.d. 14 25 n.d. 3.8 11 1.3 n.d. n.d. n.d. 0.32 1.9 20 19 2.5

1.5 2.1 2.7 1.8 n.d. n.d. 0.20 1.7 3.6 n.d. 0.032 0.22 0.14 n.d. n.d. n.d. n.d. n.d. 0.12 0.95 1.9 0.57 1.2 0 0.028 0.064 n.d. 4.7 3.5 2.5.

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

reaction rates of the silicas loaded with Ti to Ni decreased in proportion to the CH4 partial pressure decrease. This tendency indicates that these catalysts are suitable for methane activation which is rate-determining step of the reaction. The selectivity sum for methanol and formaldehyde highly depends on the RCH4 value. The selectivity sum for formaldehyde and MeOH as a function of the methane reaction rate of first transition element-doped silicas is depicted in Fig. 4. The remarkable additive elements are indicated in the figure. Selective catalysis was observed on 0.1% V, 1% V, 1% Sc, 0.1% Fe and 5% V doped silica measured at CH4 /O2 = 80/20. Silica with 0.1% Ti shows high selectivity at a feed gas composition of 70% CH4 and 30% O2 , although its reaction rate was very low. Silica with 1 and 5% V showed a certain selectivity at a high methane reaction rate. Based on Fig. 4, vanadium is the most suitable element for methane partial oxidation. The addition of vanadium is effective at a wide range of

53

its concentration (0.1–5%) and also independent of feed gas composition from 70 to 90% CH4 . On the other hand, silicas doped with Fe, Sc and Ti show the catalysis at limited condition on loading and feed gas composition. This result indicates that not only compositional optimization of the catalyst but also reaction condition optimization is required in order to obtain new catalytic system. 3.4. Effect of second row transition elements Formaldehyde and/or MeOH formation was found on Y, Zr, Nb, Mo, Ag and Cd/SiO2 . Noble metals of Ru, Rh, Pd added to silica were tested under a high methane concentration due for safety reasons based on their high combustion ability. Fig. 5 summarizes the production rate of HCHO and formaldehyde over the above catalysts. The optimal concentration of these elements seems to be 1% except for Ag. These results indicate that the metal oxide

Fig. 4. Selectivity sum for formaldehyde and MeOH as a function of methane reaction rate over first transition-metal-doped silica. The elements in parenthesis are the results of second or third transition-metal-doped silicas.

54

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

Fig. 5. Production rate of formaldehyde and MeOH over second transition-metal-doped silica. Metal/silicon = 0.1/100, 1/100, 5/100; feed gas composition: CH4 /O2 = 70/30, 80/20, 90/10.

partially covering the silica is meaningful for the reaction. The methane reaction rate and the selectivity sum for formaldehyde and MeOH are listed in Table 4. Except for Mo/SiO2 , no remarkable selective oxidation catalysis was found. A series of Mo/SiO2 compositions showed high selectivity at a feed gas composition of 80% CH4 and 20% O2 . Their data are plotted in Fig. 4. Especially, 0.1% Mo/SiO2 showed highly selective catalysis, although its conversion was small. 3.5. Effect of third row transition elements Third transition elements do not seem to be suitable for the partial oxidation of methane. Their catalytic activities were extremely low compared with those of other groups. Fig. 6 shows the production rate

of formaldehyde and MeOH over silica doped with third row transition elements. Among the lanthanide elements, 0.1% La/SiO2 produced a small amount of formaldehyde and MeOH at 90% methane feeding. As for Ta/SiO2 , a higher loading was favorable. The production amount of formaldehyde and MeOH increased in proportion to the loading of Ta. On the other hand, the production amount decreased in proportion to the loading for W/SiO2 . The tendency was clearly observed at a feed gas concentration of 70% methane. The partial oxidation catalysis of W isolated on the silica surface has been reported previously [43,44]. Our observed results support the isolation effect of the W ion. No clear trend was found for Os/SiO2 , although a certain amount of oxygenate was obtained. Ir- and Pt-doped SiO2 showed methane combustion activity even at a high methane concentration. The activity

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

55

Table 4 Methane reaction rate and selectivity for formaldehyde and MeOH over SiO2 doped with second row transition elementa CH4 /O2 = 90/10 RCH4 (nmol/s) 5% Y 1% Y 0.1% Y 5% Zr 1% Zr 0.1% Zr 5% Nb 1% Nb 0.1% Nb 5% Mo 1% Mo 0.1% Mo 5% Ru 1% Ru 0.1% Ru 5% Rh 1% Rh 0.1% Rh 5% Pd 1% Pd 0.1% Pd 5% Ag 1% Ag 0.1% Ag 5% Cd 1% Cd 0.1% Cd

1.7 × 62 21 5.3 × 80 73 42 57 0 0 50 2.7 6.0 × 6.2 × 8.8 × 6.5 × 6.9 × 1.5 × 4.1 × 3.3 × 2.6 × 6.9 × 4.2 × 14 57 0.76 0.76

102

102

103 103 102 103 103 103 103 103 103 102 102

CH4 /O2 = 80/20 Selectivity (%) HCHO

MeOH

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.40 5.8 23 57 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. 1.9 3.7 n.d. 2.8 n.d. n.d. n.d. 0 47 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

RCH4 (nmol/s) 88 54 3.0 8.9 × 1.1 × 4.4 × 32 0.39 100 6.7 15 4.7 – – – – – – – – – 1.0 × 8.8 × 26 31 6.2 3.9

102 102 102

103 102

CH4 /O2 = 70/30 Selectivity (%) HCHO

MeOH

0.43 9.1 25 n.d. 2 n.d. 13 0 6.7 5.0 44 58

0.43 0.70 0 0.041 0.33 0.086 1.2 100 12 23 2.6 8.3

n.d. n.d. 9.0 n.d. 6.3 20

n.d. 0.087 1.5 n.d. 6.3 n.d.

RCH4 (nmol/s) 88 50 8.4 1.6 × 1.1 × 3.5 × 32 6.9 5.9 n.d. 16 32 – – – – – – – – – 5.2 × 2.9 × 39 31 8.5 4.3

103 102 102

102 102

Selectivity (%) HCHO

MeOH

2.6 13 14 n.d. n.d. n.d. 9.6 40

0.43 0.76 4.5 0.022 0.70 n.d. 2.4 3.8

29 11

2.4 1.2

n.d. 0.14 10 n.d. n.d. 9.1

n.d. 0.27 2.0 n.d. 4.5 9.1

–: not measured, n.d.: not detected. a GHSV = 4760 h−1 , 773 K.

Table 5 Methane reaction rate and selectivity for formaldehyde and MeOH over SiO2 doped with third row transition elementa CH4 /O2 = 90/10 RCH4 (nmol/s) 5% La 1% La 0.1% La 5% Ce 1% Ce 0.1% Ce 5% Sm 1% Sm 0.1% Sm 5% Eu 1% Eu 0.1% Eu 5% Ta 1% Ta

1.0 40 30 5.0 91 1.4 1.6 52 32 1.6 53 1.9 63 5.7

× 102 × 102 × 102 × 102 × 102

CH4 /O2 = 80/20 Selectivity (%) HCHO

MeOH

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 5.4 33

n.d. n.d. 33 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

RCH4 (nmol/s) 86 20 7.6 72 12 0.78 24 45 0 3.4 × 102 24 1.5 23 1.9

CH4 /O2 = 70/30 Selectivity (%) HCHO

MeOH

n.d. n.d. 15 n.d. n.d. n.d. n.d. n.d. 0 0.34 1.7 50 11 40

n.d. n.d. 5 0.54 n.d. n.d. n.d. n.d. 0.34 3.3 n.d. n.d. n.d.

RCH4 (nmol/s)

Selectivity (%) HCHO

MeOH

73 20 8.0 78 12 0.78 29 34

n.d. n.d. 9.5 1.0 n.d. n.d. n.d. n.d.

n.d. n.d. n.d. 0.50 n.d. n.d. n.d. n.d.

3.0 × 102 31 9.0 27 2.7

0.13 7.5 8.7 14 43

0.26 2.5 4.3 n.d. n.d.

56

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

Table 5 (Continued ) CH4 /O2 = 90/10 RCH4 (nmol/s)

CH4 /O2 = 80/20 Selectivity (%) HCHO

0.1% Ta 5% W 1% W 0.1% W 5% Os 1% Os 0.1% Os 5% Ir 1% Ir 0.1% Ir 5% Pt 1% Pt 0.1% Pt 5% Au 1% Au 0.1% Au

0.0 0.0 0.0 2.6 4.2 13 5.7 4.3 4.2 4.0 2.6 2.6 2.9 2.3 58 52

× × × × × × ×

103 103 103 103 103 103 102

n.d. 36 46 40 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

RCH4 (nmol/s)

MeOH

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

CH4 /O2 = 70/30 Selectivity (%) HCHO

0.0 0.79 0.78 2.0 3.9 1.6 8.4 – – – – – – 2.1 × 102 3.9 2.3

RCH4 (nmol/s)

MeOH

n.d. n.d. 40 10 25 33

50 50 20 10 n.d. 9.5

n.d. n.d. 17

0.19 n.d. 17

0 4.7 8.1 13 13 5.8 13 – – – – – – 2.4 × 102 6.6 8.2

Selectivity (%) HCHO

MeOH

8.3 19 15 12 47 39

8.3 4.7 2.9 2.9 6.7 3.0

n.d. 5.9 9.5

0.16 n.d. 4.8

–: not measured, n.d.: not detected. a GHSV = 4760 h−1 , 773 K.

Fig. 6. Production rate of formaldehyde and MeOH over third transition-metal-doped silica. Metal/silicon = 0.1/100, 1/100, 5/100; feed gas composition: CH4 /O2 = 70/30, 80/20, 90/10.

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

of Au/SiO2 was negligible. During the reaction, 5% Au/SiO2 became heterogeneous due to the coagulation of gold particles from the silica. The methane reaction rates and selectivity for formaldehyde and MeOH are summarized in Table 5. Remarkable selectivity was observed on 0.1% Eu and 0.1% W at a feed gas composition of 80% CH4 and 20% O2 and on 1% Os at a feed gas of 70% CH4 and 30% O2 ; however, their reaction rates were very low. The observed trend in the results is that not only redox active metals but also redox inactive metals is effective at limited reaction condition. The redox active metals help the dissociative adsorption of oxygen which directly reacts with methane [45]. The reason why the redox inactive metal are effective is speculated that the metals encourage the formation of strained siloxane bridge formed by the dehydration of adjacent silanol groups or that the acidity of silica was enhanced by the metals. The effective concentration of these additive metals was low (1%) or very low (0.1%). This fact indicates that their metal oxides itself does not suitable for the partial methane oxidation.

4. Conclusion Various typical and transition element-doped silicas at different loadings were each tested for partial methane oxidation at three feed gas compositions. The elements suitable for partial methane oxidation were V, Fe, Sc, W, Mo and Os. The isolated species by isomorphous substitution was effective only for Fe/SiO2 . V/SiO2 , Fe/SiO2 , W/SiO2 and Mo/SiO2 have been already reported, but Sc/SiO2 is a new catalyst for the partial methane oxidation. The catalyst is active at low CH4 concentration although the catalysis is not prominent at high CH4 concentration. The combinatorial survey of not only catalyst composition but also reaction conditions should be explored. References [1] B. Jandeleit, D.J. Schaefer, T.S. Powers, H.W. Turner, W.H. Weinberg, Angew. Chem. Int. Ed. 38 (1999) 2494. [2] J. Scheidtmann, P.A. Weiss, W.F. Maier, Appl. Catal. A: Gen. 222 (2001) 79. [3] S. Senkan, Angew. Chem. Int. Ed. 40 (2001) 312.

57

[4] D.E. Akporiaye, I.M. Dahl, A. Karlsson, R. Wendelbo, Angew. Chem. Int. Ed. 37 (1998) 609. [5] J. Klein, C.W. Lehmann, H.W. Schmidt, W.F. Maier, Angew. Chem. Int. Ed. 37 (1998) 3369. [6] W.F. Maier, G. Kirsten, M. Orschel, P.A. Weiss, Chim. Oggi Chem. Today 18 (2000) 15. [7] A. Holzwarth, P.W. Schmidt, W.F. Maier, Angew. Chem. Int. Ed. 37 (1998) 2644. [8] F.C. Moates, M. Somani, J. Annamalai, J.T. Richardson, D. Luss, R.C. Wilson, Ind. Eng. Chem. Res. 35 (1996) 4801. [9] P. Cong, R.D. Doolen, Q. Fan, D.M. Giaquinta, S. Guan, E.W. McFarland, D.M. Poojary, K. Self, H.W. Turner, W.H. Weinberg, Angew. Chem. Int. Ed. 38 (1999) 483. [10] P. Cong, A. Dehestani, R. Doolen, D.M. Giaquinta, S. Guan, V. Marcov, D. Poojary, K. Self, H. Turner, W.H. Weinberg, Proc. Natl. Acad. Sci. U.S.A. 96 (1999) 11077. [11] Y.M. Liu, P.J. Cong, R.D. Doolen, H.W. Turner, W.H. Weinberg, Catal. Today 61 (2000) 87. [12] S.M. Senkan, Nature 394 (1998) 350. [13] A. Ueda, Y. Yamada, T. Kobayashi, Appl. Cat. A. 209 (2001) 391. [14] C.M. Snively, G. Oskarsdottir, J. Lauterbach, Catal. Today 67 (2001) 357. [15] Y. Yamada, A. Ueda, Z. Zhao, T. Maekawa, K. Suzuki, T. Takada, T. Kobayashi, Catal. Today 67 (2001) 379. [16] A.M. Cassell, S. Verma, L. Delzeit, M. Meyyappan, J. Han, Langmuir 17 (2001) 260. [17] H. Su, E.S. Yeung, J. Am. Chem. Soc. 122 (2000) 7422. [18] H. Su, Y.J. Hou, R.S. Houk, G.L. Schrader, E.S. Yeung, Anal. Chem. 73 (2001) 4434. [19] E. Reddington, A. Sapienza, B. Gurau, R. Viswanathan, S. Sarangapani, E.S. Smotkin, T.E. Mallouk, Science 280 (1998) 1735. [20] A. Hagemeyer, B. Jandeleit, Y.M. Liu, D.M. Poojary, H.W. Turner, A.F. Volpe, W.H. Weinberg, Appl. Catal. A: Gen. 221 (2001) 23. [21] C. Hoffmann, A. Wolf, F. Schuth, Angew. Chem. Int. Ed. 38 (1999) 2800. [22] C. Hoffmann, H.W. Schmidt, F. Schuth, J. Catal. 198 (2001) 348. [23] S. Thomson, C. Hoffmann, S. Ruthe, H.W. Schmidt, F. Schuth, Appl. Catal. A: Gen. 220 (2001) 253. [24] O.V. Buyevskaya, D. Wolf, M. Baerns, Catal. Today 62 (2000) 91. [25] O.V. Buyevskaya, A. Bruckner, E.V. Kondratenko, D. Wolf, M. Baerns, Catal. Today 67 (2001) 369. [26] Y. Yamada, A. Ueda, K. Nakagawa, T. Kobayashi, Res. Chem. Int. Med. 28 (2002) 389. [27] H.D. Gesser, N.R. Hunter, Catal. Today 42 (1998) 183. [28] S. Kasztelan, J.B. Moffat, J.C.S. Chem. Commun. (1987) 1663. [29] A. Parmaliana, F. Frusteri, D. Miceli, A. Mezzapica, M.S. Scurrell, N. Giordano, Appl. Catal. 78 (1991) L7. [30] T. Kobayashi, K. Nakagawa, K. Tabata, M. Haruta, J. Chem. Soc. Chem. Commun. (1994) 1609. [31] T. Kobayashi, N. Guilhaume, J. Miki, N. Kitamura, M. Haruta, Catal. Today 32 (1996) 171.

58

Y. Yamada et al. / Applied Catalysis A: General 254 (2003) 45–58

[32] M. Kennedy, A. Sexton, B. Kartheuser, E.M. Coda, J.B. McMonagle, B.K. Hodnett, Catal. Today 13 (1992) 447. [33] D. Miceli, F. Arena, A. Parmaliana, M.S. Scurrell, V. Sokolovskii, Catal. Lett. 18 (1993) 283. [34] B. Kartheuser, B.K. Hodnett, H. Zanthoff, M. Baerns, Catal. Lett. 21 (1993) 209. [35] A. Parmaliana, F. Frusteri, A. Mezzapica, M.S. Scurrell, N. Giordano, J. Chem. Soc. Chem. Commun. (1993) 751. [36] A. Parmaliana, V. Sokolovskii, D. Miceli, F. Arena, N. Giordano, ACS Symp. Ser. 523 (1993) 43. [37] S.Y. Chen, D. Willcox, Indus. Eng. Chem. Res. 33 (1994) 832. [38] A. Parmaliana, V. Sokolovskii, D. Miceli, F. Arena, N. Giordano, J. Catal. 148 (1994) 514.

[39] F. Martinjimenez, J.M. Blasco, L.J. Alemany, M.A. Banares, M. Faraldos, M.A. Pena, J.L.G. Fierro, Catal. Lett. 33 (1995) 279. [40] M. Faraldos, M.A. Banares, J.A. Anderson, H.C. Hu, I.E. Wachs, J.L.G. Fierro, J. Catal. 160 (1996) 214. [41] S. Irusta, A.J. Marchi, E.A. Lombardo, E.E. Miro, Catal. Lett. 40 (1996) 9. [42] R.L. McCormick, G.O. Alptekin, D.L. Williamson, T.R. Ohno, Top. Catal. 10 (2000) 115. [43] A. de Lucas, J.L. Valverde, P. Canizares, L. Rodriguez, Appl. Catal. A: Gen. 184 (1999) 143. [44] A. de Lucas, J.L. Valverde, L. Rodriguez, P. Sanchez, M.T. Garcia, J. Mol. Catal. A: Chem. 171 (2001) 195. [45] K. Otsuka, Y. Wang, Appl. Catal. A 222 (2001) 145.