Isotopic Labelling Studies of the Mechanism of the Catalytic Oxidative Coupling of Methane

A. Holmen et al. (Editors), Natural Gas Conversion 0 1991 Elsevier Science Publishers B.V., Amsterdam

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iSOTOPiC LABELLiNGSTUDiES OF THE MECHANiSMOF THE CATALYTIC OXiDATiVE COUPLING OF METHANE Peter F. Nelson', Eric M.Kennedy2 and Noel W. Cant2 ICSiRO Division of Coal and Energy Technology, P.O. Box 136, North Ryde, Australia 2113 2School of Chemistry, Macquarie Universfty, Australia 2109

SUMMARY The conversion and selectlvlties of methane-, ethane- and ethylene-oxygen mixtures have been determined for a LVMgO catalyst at temperatures of 600-750°C. Kinetic isotope effects and HID distributions have also been determinedfrom studies using D iabeiilng of ethane as C2D6 and CH3CD3. The importance of CH3O2 as a source of low temperature GO, from CH4 is demonstrated: by contrast the low thermal stability of C2H5O2, and the preferred reaction channel (to C2H4+H02) make conversion of C2H6 to C2H4 highly selective. iNTRODUCTlON The conversion of methane to ethylene and higher olefins by catalytic oxidative coupling provides an alternative to conventional synthesis gas based processes for producing petrochemical feedstocks and liquid transport fuels from naturai gas. Recently a number of catalytic systems which give methane converslon and C2 hydrocarbonselectivities of practical significance have been reported (ref. 1). The mechanism of oxidative coupling has been studied in greatest detail for U-promoted MgO (Li/MgO), a catalyst which exhibits moderate conversion rates and high selectivities to the desired Cp products. Details of the mechanism were flrst established by Lunsford and coworkers (ref. 2). They demonstratedthat the formation of C2 hydrocarbonsinvolves initially coupling of methyi radicals, producedfrom the methane, to form ethane. Secondary reactions of C2H6 are responsible for the formation of other higher hydrocarbons, principally ethylene. The participation of methyi radicals was also convincingly demonstrated by isotopic labelling studies (ref. 3). The ratecontroiiingstep in the reaction over Li/MgO was originally proposed (ref. 2.) to be reoxidation of the catalyst. However, the measurement of a kinetic isotope effect (KIE) of 1.5 at 750°C (ref. 3)shows C-H bond-breakingto be rate controlling at this temperature. An interestingobservation In this experiment was that selectivities were also affected, with CD4 producing more carbon oxides. it is clear that secondary reactions of the C2 hydrocarbonsare very important in the

conversion of ethane to ethylene, and in the oxidation of the Cp species to carbon oxides. We have previously demonstrated (ref. 4) that C2 oxidation can be responsible for 3040% of the

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carbon oxides at temperatures greater than 740°C over LVMgO. However lnsufflcient attention has been given to the study of the reactions of the C2 species under comparable conditions to those used for methane. Such studies under oxidative coupilng conditions are complicated since the large excess of methane usually present inhibits the homogeneousoxidation of the C2 species (ref. 5). In this study, the conversion and selectivities of methane, ethane and ethylene (each mixed in the same proportion with oxygen) are determined over a U/MgO catalyst. in addition the KIE is determined for C2H6 compared to c2D6 and CH3CD3; and hydrogen/deuterium distributions are determined for reactions of c2H&D&

and CH3CDgO2 mixtures. The results provide

further insights into the secondaly reactions of the C2 specles. EXPERIMENTAL Details of the procedures and apparatus have been given previously (refs. 3-4). Briefly, experiments were performed at atmospheric pressure using a flow system and a reactor constructed from fused alumina tubing. The catalyst (0.1Og) had a Li content of 0.77% w/w after firing at 850°C in air. High-purity N2 (99.99%) and high-purity 0 2 (99.9% mlnimum) were used without further purification. Hydrocarbon gases were: CH4 (99.9Y0 mlnimum), C2H6 (99% minimum), C2D6 (99.2 atom %D, MSD isotopes) and CH3CD3 (isotopic purlty: 99.2 atom %D, MSD Isotopes). Conversions and selectivities were determined by analysis of feed and product gases by gas chromatography. The reactor effluent was connected to the inlet system of a quadruple mass spectrometer (VG SX-200) operated in the multiple ion monitorlng mode. Samples of feed and product gas were also analyzed by FTlR spectroscopy using a lOcm path length gas cell and 0.25 cm-l resolution. RESULTS AND DISCUSSION Oxygen conversions and product selectivities were determined over the Li/MgO catalyst for a feed gas composition of 20% hydrocarbodlO%oxygen/He dliuent for temperatures of 600750°C. These experiments were performed with 100 mg catalyst and a total flow rate of 20 cm3/min. Results for oxygen conversion and hydrocarbon or carbon monoxide selectivities are given In Figs. 1 and 2 respectively. Oxygen conversion increased in the order CHq c C2H6 < C2H4 If the rates are compared in terms of hydrocarbon reacted then the reactivity order changes since the oxygen requirement for each reaction is different and somewhat temperature dependent. At 700°C hydrocarbon conversions are in the order CH4 G C ~ H ~ < in C the ~H~ approximate ratio 1:2.5:7. The observation that ethane reacts faster than ethylene is in agreement with the measurementsof Moraies and Lunsford (ref. 6) and in line with relative bond strengths. The higher reactivity of ethane relative to methane may reflect the possibilitles for chain reactions In the gas phase In the former case as noted later.

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600

650

700

750

Temperature (OC) Flg. 1. Oxygen conversions as a function of temperature : (a) CH4, (b) C2ti6, (c) C2H4 over

LtlMgO (1OOmg).

Fig. 2. Selectivitiesas a function of temperature : (a) CH4, (b) C2H6, ( 4 C2H4

As observed by previous workers (ref. 2), the hydrocarbon selectivity with CH4 increases steadily with temperature from -20% at 600°C to -70% at 750°C. in the case of C2H6 selectivities to hydrocarbons, predominantly C2H4 remain relatively constant at -90% from 600700"C, and decline only slightly above 700°C. For C2H4 selectivities to CO are shown in Fig. 2.

The principal product is carbon dioxide (not shown) which accounts for 275% at 600" and 750°C and approximately 60% at the intermediate temperature of 650°C. Both C2 hydrocarbons also

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produce small amounts of CH4 which increase with temperature: for C2H6 the selectivity to CH4 is 0.3% at 650°C and 1.6% at 750°C; for C2H4 the corresponding figures are 1.6% and 4.4%. These results demonstrate that there is a fundamental difference in the mechanism of the conversion of methane and of ethane over this catalyst. With methane there is an important low temperature source of cafbon oxides which becomes relatively less important with increasing temperatures. This, however, is not the case with ethane which produces ethylene very selectively (>8O%) at all temperatures.

ld

5a,

-

E

C

0.05.

0 .-

2

2 O

O

O

100

200

300

E 400

Time on Stream (rnin) Fig. 3 Conversion rates (0 C2H6,o C2D6, ACH3CD3) as a function of tlme on stream. UlMgO catalyst at 680°C. The measurements of the KIE were performed at 680°C. The feed gas was 20% hydrocatbon/lO% oxygen and balance nitrogen at a total flow rate of 20 cm3/min. The experiment was set up so that C2H6, c2D6 or CH3CD3 could be mixed with O2 and Np at the same ethane and total flowrates. Under these conditions ethane conversion was small and differential reactor conditions could be assumed. The ethane feed was alternated from C2H6 to C2Dg and CH3CD3 to check for irreversibility or slow response of the catalyst to changes in the feed gas. Results for the overall ethane conversion rate in these experiments, calculated by summation of products, are given in Fig. 3. The catalyst was reasonably stable duilng the course of the experiment; the conversion rate with C2H6 decreasing by about 4%. Conversion rates with c2D6 were significantly less than those with C2H6, and results for CH3CD3, as might be predicted, fell in between. in Table 1 the KlEs (H/D) for these experiments with ethane are compared with our previous results (ref. 3) for methane, and in Fig. 4 selectivities to

93 TABLE 1 Comparisonof Klnetic Isotope Effect over Li1MgO at 680"Ca Methane ethane ethylene

Ethane

1.8

2.6

co

1.61

1.5 1.33 1.59

co2 overall

1.4 1.30 1.58

a With feed comprising 20% hydrocarbon, 10% oxygen, balance nitrogen at 20cm3/mln over 0.1009 of catalyst.

loo

r

CHqICDq

Fig. 4 Selectivities for CH4 compared to CD4 and

C2H6/C2D6 ethylene

compared to C2D6 CH4 and C2H6

hatched boxes. hydrocarbonsand carbon oxides are compared. The overall isotope effect is identical for both methane and ethane, and the only individual isotope effect which is signlflcantiy different is that of the ethylene. This difference is a reflection of the fact that, in the methane experiments,

ethylene is a secondary product arising from the further reactions of the ethane. Since the conversion of ethane to ethylene also exhibits an isotope effect, we would predict the effect wlth methane to be greater than that with ethane. The selectivities presented In Fig. 4 show that there is a significant effect when CD4 was used: more CO and C02 were produced mainly at the expense of ethane. However, the differences observed when C2D6 Is substituted for C2H6

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are not nearly so marked. These observations are related to the absence of a low temperature source of carbon oxides when C2H6/O2 mixtures are fed to the catalyst (see Flg. 2). This is in contrast to CH4/O2 mixtures which exhibit high selectivities for COX at low temperatures. Lunsfordand co-workers (ref. 2)postulated that the formation of and subsequent homogeneous reactions of the methyl peroxy radical, CH3O2, were responsible for this low temperature COX Recent measurements (ref. 7) of the equilibrium constant for (1):

show that significant amounts of CH3O2 would be present under the experimental conditions of temperature, CH4/O2 ratios and pressure used for oxldative coupling reactions. However from measurementsof the KIE at different flowrates Nelson and co-workers (ref. 3) concluded that the surface was probably Involved in the conversion of CH3O2 to CO,. The thermal stability of the ethylperoxy radical, C2H5O2, Is considerably less than that of CH3O2 (ref. 8) and at temperatures greater than 600K reaction (2):

is the dominant reaction pathway. in this system C2H4 could also be produced homogeneously by decomposltion of C2H5 or heterogeneously. in addition the H02 may induce further conversion of C2H6 by a chain process. Thus, conversion of C2H6 to C2H4 is facile even at the lower temperatures of this study, and CH4 exhibits a low temperature route to COXdue to the significant thermal stability of CH3O2 under these conditions. Lower CH4/O2

ratios and higher pressures will favour the formation of CH3O2 and thus lead to lower hydrocarbon selectivities. This places constraints on the operating conditions for a practical reactor. Conversion of ethane to ethylene was further investigated by studying H/D distributions in experiments with C2H6/C2D6/02 and CH3CD3/02 mixtures at 740°C. Hydrocarbon: oxygen ratios and flowrates were the same as those for the KIE measurements. For both mixtures a KIE was observed in making H2 versus D2 and the H2 : HD : Dp was at equilibrium as one

expects. in the case of the ethylenes, the H/D distribution was determined by FTlR spectroscopy as ~ a KIE was observed with C2H4 described Pr~ViOuSly(ref. 3). For the C ~ H G / C $mixture

made preferentially to the C2D4 in the ratio of 2.51. The C2H4 and C2D4 made up at least 83% of the ethylenes; small amounts of C2H3D and other ethylenes were also observed. For

the experiment with CH3CD3, CH2CD2 made up at least 86% of the total ethylenes; small amounts of C2H3D and C2HD3 were also observed. Thus, in both cases, the reaction

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proceeds without significant exchange processes. These observations are consistent with an essentially homogeneousprocess for ethane to ethylene conversion for this catalyst. However a significant contrlbutlon from the surface for other catalysts Is not precluded, and, indeed, appears to be necessary to account for very high ethylene to ethane ratios at relatively low temperatures (ref. 9).

REFERENCES 1 J.S. Lee and S.T. Oyama, Oxidative coupling of methane to higher hydrocarbons, Catal.

Rev. - Scl. Eng., 30 (1988) 249-280 and referencestherein. 2 T. Ito, J.-X. Wang, C.H. Un and J.H. Lunsford, Oxldatlve dlmerisatlon of methane over lithium-promoted magnesium oxide catalyst, J. Amer. Chem. Soc., 107 (1985) 5062-5068. 3 P.F. Nelson, C.A. Lukey and N.W. Cant, Measurementsof kinetic Isotope effects and hydrogeddeuteriumdistributions over methane oxidative coupling catalysts, J. Catal., 120 (1989) 216-230. 4 P.F. Nelson and N.W. Cant, Oxidation of C2 hydrocalibon products during the oxidative coupling of methane over a LMMgO catalyst, J. Phys. Chem., 94 (1990), 3756-3761. 5 J.C. Mackie, J.G. Smith, P.F. Nelson and R.J. Tyler, Inhibitionof C2 oxidation by methane under oxldatlve coupling conditions, Energy and Fuels, 4 (1990), 277-285. 6 E. Morales and J.H. Lunsford, Oxidative dehydrogenatlonof ethane over a lithium-promoted magnesiumoxide catalyst, J. Catal., 118 (1989), 255-265. 7 I.R. Slagle and D. Gutman, Kinetics of polyatomicfree radicals produced by laser photoiysis. 5. Study of the equilibrium CH3+O2 + CH3O2 between 421 and 538°C. J. Am. Chem. SOC.,107 (1985), 5342-5347. 8 I.R. Slagle, Q. Feng and D. Gutman, Kinetics of the reaction of ethyl radicals with molecular oxygen from 294 to 1002K, J. Phys. Chem., 88 (1984), 3648-3653. 9 J. Williams, R.H. Jones, J.M. Thomas and J. Kent, A comparison of the catalytic performance of the layered oxychlorides of bismuth, lanthanum and samarium in the conversion of methane to ethylene, Catal. Lett., 3 (1989), 247-256.