Catalytic Methane Oxidation at Low Temperatures Using Ozone - ACS

Chapter 27

Catalytic Methane Oxidation at Low Temperatures Using Ozone 1

Downloaded by UNIV OF GUELPH LIBRARY on October 10, 2012 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch027

W. Li and S. Ted Oyama

Department of Chemical Engineering, Virginia Polytechnic Institute and State University, 133 Randolph Hall, Blacksburg, VA 24061

Methane oxidation using ozone at low temperatures (< 673 K) was studied both in the gas phase and with catalysts. Gas phase reactions were significant even at low temperatures, however, catalysts could improve the reactivity both under oxygen-rich and oxygen-deficient conditions. MgO was found to be active for the conversion of methane to CO andCO at oxygen-rich conditions, while Li/MgO catalyst promoted the production of formaldehyde at oxygen-deficient conditions. A selectivity to HCHO over 90% was achieved with a 4% methane conversion at 650 K on a Li/MgO catalyst, while no detectable conversion of methane was observed using oxygen as the oxidant. Methane oxidation to form formaldehyde was found to occur at the same temperature range when ozone decomposition happened, which suggested that the active oxygen species for methane oxidation were formed by ozone decomposition. Values of the ratio of converted CH /converted O above unity clearly indicated the involvement of a chain reaction mechanism. 2

4

3

The direct oxidation of methane to methanol or formaldehyde has been receiving considerable attention due to its fundamental as well as practical significance. Oxygen is the most commonly used oxidant. However, severe conditions (>400 °C) are usually necessary to achieve detectable conversion of methane, and high selectivities to the desired oxygenates occur only at small (), dried with a gas purifier (Alltech, Drierite and Molecular Sieve 5 A), through an ozone generator (OREC, Model V5-0), and its concentrations before and after the reaction were monitored by an ozone analyzer (H-l, IN-USA). In this study, the initial ozone concentration was always lower than 2 mol%. Methane (Air Products, 99.99%) and helium (Air Products, 99.999%) were also dried using the gas purifiers, and their flow rates were controlled by mass flow controllers (Brooks 5850E). The flow rate of the ozone and oxygen mixture was set by a needle valve (Nupro), and monitored by a mass flow meter (Aalborg, GFM17). The product compositions were determined by an on-line gas chromatograph (SRI, 8610) with a TCD and an FID detector. The gas phase reaction was evaluated with a reactor loaded with quartz chips (18-20 mesh), using oxygen-rich and oxygen-deficient conditions. The flow rates for these conditions are listed in Table I. Table I. Flow Rates for the Oxygen-Rich and Gas Oxygen-Rich He 6 0 CH 12 cm min 8.9 umol s" 0 (0 ) 800 cm^in" 595 umol s" (1.5 mol%0 ) (8.9 umol s' ) 3

1

1

4

1

2

1

3

1

3

Oxygen-Deficient Conditions Oxygen-Deficient 510cm min" 379 umol s" 60 cm^min" 45 umol s" 30 cnr'mm 22 umol s" (2 mol% Q ) (0.4 umol s' ) 3

1

1

1

1

1

1

1

3

Under all the reaction conditions studied, no methane conversion was observed when oxygen alone was used as the oxidizing agent. Methane conversion was

In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

HETEROGENEOUS HYDROCARBON OXIDATION

366

calculated by summing all of the products, and the converted CFLt/converted O3 ratio was calculated as (CH4 conversion x CH4 flow rate)/(03 Conversion x 0 flow rate). During the experiments, special caution was taken due to the high toxicity of ozone. The reactor system was carefully leak-tested, and the effluent gas was properly vented 3

Results and Discussions Gas Phase Reaction. The role of the gas phase reaction in methane oxidation by ozone was studied using a reactor filled with quartz chips (18-20 mesh). The quartz chips used had a very low specific surface area (< 0.2 m /g). The gas phase reaction was carried out under both oxygen-rich and oxygen-deficient conditions. As shown in Figure 1, significant gas phase reactions between ozone and methane were observed even at low temperatures, especially in an oxygen-rich atmosphere (Figure la). Under these conditions, the main product was C O 2 (Figure lb). A substantial amount of CO was produced only at temperatures higher than 550 K. Interestingly, the reaction rate was observed to decrease with increasing temperature. This can be understood as arising from the decreasing availability of ozone in the gas phase. With increasing temperature, ozone decomposes faster and is less available to react with methane. Thus, the observed kinetic behavior is a combination of the kinetics of ozone reaction with methane and of ozone decomposition to oxygen.


2

I

350

1

400

1

450

1

500

1

1

550

600

T

= = a

"'

650

I

I

700

750

Temperature / K Figure 1. Gas phase reactivity under both 0 rich and 0 deficient conditions 2

2

At the oxygen-deficient conditions, CO and C O 2 were the only products at low temperatures (Figure lc). However, the amount of formaldehyde produced increased sharply at about 523 K, and a very high selectivity to formaldehyde was


27. LI & OYAMA

367

Catalytic Methane Oxidation Using Ozone


observed at higher temperatures. At even higher temperatures (>750 K), the further oxidation of formaldehyde became important, and CO formation increased. Notice that the selectivities are different from those reported by Hutchings and coworkers (77,72). The reason is probably that the contact time used in this study (< 0.05 s) is much shorter than that used in their study (> 5 s). It is well accepted that selectivity depends strongly on the contact time in partial oxidation reactions. To better understand the relationship between ozone decomposition and its reaction with methane, we also carried out ozone decomposition with no methane under oxygen-deficient condition. Interestingly, the ozone decomposition was found to follow the same temperature profile as the reaction between ozone and methane (Figure 2). The reaction rates of both reactions were found to follow Arrenhius equation (Figure 3) within the temperature range of 473 to 523 K. Their activation energies were determined to be 74 klmol" for methane oxidation, and 80 kJmol" for ozone decomposition. This suggests that the active oxygen species for methane conversion are probably formed by ozone decomposition. 1

1

0.7

0.6-

-#— Ozone decomposition - • — Methane oxidation

-

0.5-

S

0.4

0.3" 0.2 0.1 0.0 350

400

450

500

550

600

650

Temperature / K Figure 2. Comparison of methane oxidation rate and ozone decomposition rate in the gas phase under 0 deficient condition 2

The mechanism of methane reaction with ozone in the gas phase was postulated to go through the following steps (18): (1) the decomposition of ozone to form an atomic oxygen species and an oxygen molecule, (2) the reaction between methane and the atomic species to form methyl radicals, (3) the further oxidation of methyl radicals to methanol and/or formaldehyde.



368


-4.01

•

1.8

1

•

1

1.9

•

1

2.0

^

2.1

1

•

2.2

1

2.3

1000/T/K-i Figure 3. Arrenhius plots for ozone decomposition and methane oxidation rates Our results are consistent with this mechanism, however, there exists a possible involvement of a chain reaction mechanism. The measurement of inlet and outlet ozone concentrations allowed calculation of converted CHVconverted O3 ratio (Figure 4). This ratio gives information on the stoichiometry of the reaction as well as insights

2.0

—1

300

1

1

•

400

1

500

1

1

600

•

1

1

—

700

Temperature / K Figure 4. Converted C H / Converted O 3 ratio in the gas phase reaction 4


27. LI & OYAMA


369


for the reaction mechanism. The ratio was found to be larger than 1 under certain conditions, which indicates that a chain reaction mechanism may be involved. The ratio for oxygen-rich conditions was found to be considerably higher than that for oxygen-deficient conditions, which suggests that molecular oxygen is involved in the chain propagation reaction. Reaction over Catalysts. MgO and 5 wt% Li/MgO were used to study the effect of catalysts on the reaction between ozone and methane. MgO. Under oxygen-rich conditions, MgO showed considerably higher reactivity than quartz for methane conversion, especially at lower temperatures (Figure 5a). However, the selectivities were comparable to those over quartz (Figure 5b). On the other hand, the gas phase reaction dominated under oxygen-deficient conditions (Figure 6a, b). Both the activity and the selectivities over the catalyst showed no difference from those over quartz. Therefore, under the latter conditions the reaction occurred mainly in the gas phase instead of on the MgO surface.

Temperature / K Figure 5. Reactivity of MgO under 0 rich condition 2

Notice that the selectivities under oxygen-deficient conditions were remarkably different with increasing temperature. At lower temperatures, the main products were CO and C 0 , while formaldehyde became the predominant product at higher temperatures. The production of CO and C 0 at lower temperatures suggests that they are not produced by the further oxidation of formaldehyde, even though the oxidation of formaldehyde to CO is observed at higher temperature (> 650 K). There 2

2



370

exist two reaction pathways: one to formaldehyde, which can be further oxidized to CO, and a separate one to CO and C 0 . The production of formaldehyde increased sharply around 523 K. This large temperature dependence indicated a high activation energy for the oxidation of methane to produce formaldehyde. 2

c

2.0


o

—i

«

1

300

«—*—i

400

>

500

1

1

600

700

Temperature / K Figure 6. Reactivity of MgO under O2 deficient condition Li/MgO. Under oxygen-rich conditions, Li/MgO showed similar reactivity to MgO. The methane conversion over Li/MgO was much lower than that over MgO, however, their areal rates were comparable because of the lower surface area of the Li/MgO catalyst (Figure 7a, b). C 0 was the main product. Only at temperatures higher than 550 K, was a substantial amount of CO formed. Under oxygen-deficient conditions Li/MgO showed higher activity and selectivity than MgO. A 4% methane conversion was achieved with the selectivity to formaldehyde exceeding 90% at 650 K (Figure 8a, b). Li/MgO is known as an effective catalyst for the oxidative coupling of methane, which involves the generation of methyl radicals and its coupling to C H6 (19). Surface O" species (Li 0") are proposed as the active centers. When ozone is used as the oxidizing agent, O" species and methyl radicals could be generated at much lower temperatures on the Li/MgO catalyst surface as well as in the gas phase. The reaction of methyl radicals with metal oxides has been studied by Lunsford and coworkers (20). C H O H and HCHO are formed by the reaction of methyl radicals with metal oxides at temperatures lower than 773 K. It was concluded that high selectivity and yield to oxygenates are possible to achieve if C H could be 2

2

+

3

4


27. LI & OYAMA


371

-•—MgO -•—Li/MgO


•—MgO ••—Li/MgO

Temperature / K Figure 7. Comparison of activities of MgO and Li/MgO under 0 rich condition 2

U — i

300

•

*

i

400

1

i

1

500

600

•

1

700

Temperature / K Figure 8. Reactivity of Li/MgO under 0 deficient condition 2


372


converted to CH « radicals at moderate temperatures. When ozone is used as the oxidant, active oxygen species can be produced through ozone decomposition. The formed oxygen species can react with methane to generate methyl radicals. In the experiments here the formation of HCHO was observed to increase sharply at about 523 K, which is the thermal decomposition temperature of ozone. Therefore the results are consistent with a radical reaction mechanism shown earlier, with the calculation of converted CH^converted 0 ratio confirming the occurrence of the chain reaction mechanism (Figure 9). 3


3

7

O Deficient

6

2

t; CD

5H

o U

4

X u

3

€

2

CD

O Rich 2

H

O

U

0 400

500

700

Temperature / K Figure 9. Converted C H / Converted 0 ratio over Li/MgO 4

3

Conclusions The direct oxidation of methane using ozone at low temperatures (< 673 K) was studied both in the gas phase and over MgO, Li/MgO catalysts. Although significant reaction in the gas phase was observed, catalysts could improve the reactivity both under oxygen-rich and oxygen-deficient conditions. MgO was found to be active for the conversion of methane to CO and C 0 at oxygen-rich conditions, while Li/MgO catalyst promoted the production of formaldehyde at oxygen-deficient conditions. A 4% methane conversion with a selectivity to HCHO over 90% was achieved at 650 K on a Li/MgO catalyst using ozone as the oxidant. Our calculations of converted CHVconverted O 3 ratio clearly indicate the involvement of a chain reaction mechanism. 2


27.

LI & OYAMA


373

Acknowledgments We gratefully acknowledge the financial support for this work by the Director, Division of Chemical and Thermal System of the National Science Foundation, under Grant CTS-9311876.


Literature Cited

(1) Gesser, H. D.; Hunter, N. R. In Methane Conversion by Oxidative Processes; Wolf, E.E., Ed.; Van Nostrand Reinhold: New York, 1992, 403. (2) Spencer, N. D. J.Catal.1988, 109, 187. (3) Barbaux, Y.; Elamrani, A. R.; Payne, E.; Gengembre, L.; Bonnelle, J. P.; Grzybowska, B. Appl. Catal. 1988, 44, 17. (4) Spencer, N. D.; Pereira, C. J. J.Catal.1989, 116, 399. (5) Parmaliana, A.; Frusteri, F.; Mezzapica, A.; Miceli, D.; Scurrell, M. S.; Giordano, N. J.Catal.1993, 143, 262. (6) Smith, M. R.; Ozkan, U. S. J.Catal.1993, 141, 124. (7) Smith, M. R.; Zhang, L.; Driscoll, S. A.; Ozkan, U. S.Catal.Lett. 1993, 19, 1. (8) Liu, H.-F.; Liu, R.-S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J. Am. Chem. Soc. 1984, 106, 4117. (9) Khan, M. M.; Somorjai, G. A. J. Catal.,1985, 91, 263. (10) Zhan, K. J.; Khan, M. M.; Mak, C. H.; Lewis, K. B.; Somorjai, G. A. J.Catal.1985,94,501. (11) Hutchings, G. J.; Scurrell, M. S.; Woodhouse, J. R. Appl. Catal. 1988, 38, 157. (12) Hutchings, G. J.; Scurrell, M. S.; Woodhouse, J. R. Methane Conversion, Bibby, D. M.; Chang, C. D.; Howe, R. F.; Yurchak, S., Eds.; Elsevier: Amsterdam, 1988. (13) Gesser, H. D.; Hunter, N. R.; Das, P. A.Catal.Lett. 1992, 16, 217. (14) Otsuka, K.; Wang, Y. Chem. Lett. 1994, 1893. (15) Otsuka, K.; Wang, Y.Catal.Lett. 1994, 24, 85. (16) Ullmann's Encyclopedia of Industrial Chemistry, Vol. A18, 350, VCH, 1991 (17) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J. Am. Chem. Soc., 1985, 107, 5062. (18) Toby, S.; Toby, F. S. J. Phys. Chem., 1989, 93, 2453. (19) Lunsford, J. H. Angew. Chem. Int. Ed. Engl.,1995, 34, 970. (20) Pak, S.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. 1994, 98, 11786.


Catalytic Methane Oxidation at Low Temperatures Using Ozone - ACS

Recommend Documents