The Oxidation of Ethane to Ethylene over Strontium Containing Methane Coupling Catalysts

H.E. Curry-Hyde and R.F. Howe (Editors), Natural Gas Conversion I1 0 1994 Elsevier Science B.V. All rights reserved.

THE OXIDATION OF ETHANE TO ETHYLENE CONTAINING METHANE COUPLING CATALYSTS

199

OVER

STRONTIUM

Jeffrey S. Church and Noel W. Cant School of Chemistry, Macquarie University, NSW 2109, Australia 1. ABSTRACT The oxidative dehydrogenation of ethane to ethylene over three types of strontiumcontaining catalysts has been investigated. The catalysts are much more active and selective than for the corresponding methane coupling reaction and ethylene yields of up to 40% are possible with a 20%ethane/lO%oxygen feed. Lanthana and magnesia based catalysts are the most active but strontium carbonate/aluminosilicate catalysts are more selective. Although DRIFTS measurements of the latter show the presence of strontium silicates and aluminates in the bulk XPS indicates that the surface layers are largely SrCO, and that the high selectivity is related to the presence of some sodium. Carbonates also dominate at the surface of the other catalysts but the magnesia-based ones show oxides as well in conformity with DRIFTS measurements of the bulk. 2. INTRODUCTION The oxidative coupling of methane is unusual for a catalytic reaction in that selectivity to the desired products usually increases with temperature [l]. This may arise because the desired reaction to ethane and ethylene is second order in surface-generated methyl radicals while formation of undesired carbon oxides is of lower order [2]. As a result highly active catalysts capable of operation at very high temperature may outperform lower activity catalysts with intrinsically higher selectivity at lower temperature. Thus although Li/MgO is particularly selective over the temperature range 700-750°C [ 11 most studies with industrial applications in mind have concentrated on very active catalysts operated at 800°C to 900°C [3,4] where Li/MgO is unstable. Strontium is often included in such high temperature catalysts, either as a promoter for rare earth oxides such as L%O, [ 5 ] , or as a major ingredient in combination with other materials [6]. Little work exists on the oxidation of ethane over such catalysts although the reaction has been investigated in some detail for both Li/MgO [7] and rare earth oxides [8]. In both cases the oxidation of ethane is faster than that of methane by a factor of three to five as expected from the difference in C-H bond energies. The selectivity is also higher and remains so at hydrocarbon conversions much above those feasible for methane so that ethylene yields above 50% are possible [7]. In the present work we have investigated ethane oxidation over several types of strontium-containing catalysts which perform well for methane coupling. Complementary surface and bulk characterisation of the catalysts has been carried out by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared spectroscopy (DRIFTS).

200

3. EXPERIMENTAL

The catalytic oxidation of methane and ethane was carried out in a single pass flow system similar to that described earlier [8] using 100 mg samples of catalysts. The standard feed swam comprised 20% hydrocarbon, 10% oxygen, balance helium to a total pressure of one atmosphere with a combined flow rate of 100 cm3/minute. In the case of methane two additional tests were carried out with methane/oxygen mixtures of 35%/5% and 95%/5% respectively. Product analysis was done by gas chromatography. The seven catalysts listed in Table 1 were investigated. The lanthana was of 99.99% purity (Aldrich Inc.). The strontium in catalysts B and C was introduced by incipient wemess using a nitrate solution followed by calcination for 24 hours in air at 400°C. The strontium/magnesium carbonate catalysts (D and E) and the sodium-promoted strontium carbonate/aluminosilicate ones (F and G) were development catalysts provided by CSIRO and had been calcined at 1000°C after preparation. XPS measurements were carried out with a Kratos Axis 800 cpi system. Binding energies are referenced to the 1s line of adventitious carbon (284.6 eV) with quantitation using standard sensitivity factors. DRIFTS measurements were carried out on samples diluted with KBr with 128 scans at 4 cm-' resolution using a Mattson Cygnus 100 FTIR instrument with an MCT detector. 4. RESULTS AND DISCUSSION Figures 1 and 2 show oxygen conversions and selectivity to ethylene for the oxidation of ethane over strontium/lanthana catalysts. All three catalysts are very active with oxygen conversions in excess of 90% at 650°C compared to ~ 1 0 %at 800°C in the blank tests. The rise in conversion with temperature is steeper for the two strontium-containing samples. The 1% Sr/L%O, catalyst (B) is more active than L%O, at all temperatures while the 5% Sr/La,O, one (C) is less active than La203 below 600°C but more active Table 1 Catalyst Composition and Substances Detected by DRIFTS Catalyst A B C D E F

G

'

Initial Formulation

DRIFTS Analysis (after use)

1% Sr(NO,),/L%O, 5% Sr(NO,),/L%O, Sr,,M&., carbonate a Sro.2Mg,,8Li-q,02 carbonate a 80 wt % SrCO, plus 20 wt % bentonite 80 wt % SrCO, plus 20 wt % SiO, and A1,0,

L%O,, L%O,CO,, LaOH(s) L%O,, LaOH(s), SrCO, L%03, LaOH(s), SrCO, MgO, SrCO, MgO, SrCO,, LaOH(s) SrCO,, Sr silicates, aluminates SrCO,, Sr silicates, aluminates

-

D was prepared by coprecipitation and E by subsequent impregnation with La(NO,),. Both were calcined at 1000°C. Preparation as per reference [61. Catalyst G is a synthetic analogue of catalyst F with similar overall elemental composition (including some added sodium).

20 1

0 400

I

600

1

400

800

Temperature, "C

Temperature, "C

Fig. 1 Oxygen conversions for ethane oxidation over Fig. 2 Ethylene selectivity for ethane oxidation over catalysts A, B and C and in blank experiments (W). catalysts A, B and C and in blank experiments (m).

than it above that temperature. The relationship between selectivity and temperature is very similar for all three catalysts with the 1% Sr/La,O, sample slightly more selective than the unpromoted material throughout. Figures 3 and 4 show the corresponding results for ethane oxidation over the other four catalysts, D through G. As shown in Figure 3 the strontidmagnesia catalysts are much more active than the SrCO&duminosilicate ones. The strontidmagnesia catalyst promoted by lanthanum (catalyst E) is the most active with a conversion versus temperature curve similar to the 5% Sr/La,O, one. The selectivities for the magnesia-based catalysts are similar to those of the lanthana-based ones with maximum selectivities near 70% above 750°C. Both aluminosilicate containing catalysts exhibit higher selectivities with maximum values of approximately 80% at 750°C and slightly lower values above that. Table 2 provides a comparison of catalyst activity (as measured by the temperature for 80% oxygen conversion) and performance at 800°C. At this temperature some methane is produced with all catalysts, probably through decomposition of ethane in the gas phase and reaction of the methyl radicals so formed with ethane. Catalysts A, B, D and E produce

Temperature, "c

Fig. 3 Oxygen conversion for ethane oxidation over catalysts D, E, F and G.

-

O400

Temperature, "C

Fig. 4 Ethylene selectivity for ethane oxidation over catalysts D, E, F and G.

202

Table 2 Reaction Characteristics for Ethane Oxidation at 800OC under Standard Conditions ~~

~

Catalyst

Activity" T I 3 0

OC

A

B C D E F G

650 565 625 720 645 790 >8ood

Exit pressures, am x

Id

Sel. to

Conv.

%

%

CZH4

CO

C02

CH4

H2

71' 71 66 73 72 77 86

56 52 41 56 55 52 27

7.9 7.4 5.4 8.2 7.9 8.0 4.7

2.4 1.5 0.7 1.9 1.6

3.5 3.7 4.4 3.3 3.5 3.5 1.0

0.7 0.7 0.4 0.9 1.0 0.4 0.2

4.5 5.1 1.7 3.8 4.1 2.1 0.8

C2H4

c2H6

' Temperature for 80% oxygen conversion. 819°C rather than 800°C.

0.8

0.4

GH,

yield. %

39' 37 27 41 40 40 23

~ r gas y basis.

38% oxygen conversion at 800°C.

somewhat more hydrogen and carbon monoxide than the others and this enables slightly higher ethane conversions through better oxygen utilisation. Catalysts B (1% Sr/L%O,), E (2% WSrO-MgO) and F (SrCOfientonite) are the most active in each series and together with catalysts A and D can achieve ethylene yields (i.e. selectivity times conversion) in excess of 35%. Catalysts G and C could also do so if the flow rate was reduced. Since the reaction is oxygen limited somewhat higher yields would be possible with higher oxygen partial pressures. Yields exceeding 50% have been reported for Li/MgO catalysts when using an equimolar feed mixture [7] but those catalysts are much less active. The general pattern of activity and selectivity for methane coupling paralleled that evident in Figures 1 to 4 for ethane oxidation. Figures 5 and 6 show results for methane coupling over catalysts B, E and F using the standard feed. The lanthana- and magnesiabased ones (B and E) are highly active with conversions over 90% at 700°C compared to -27% at 800°C for catalyst F. The reverse behaviour is m e in respect of the selectivity to ethylene plus ethane. Catalyst F exhibits a selectivity of -65%, which approaches that of Li/MgO under the same conditions [9], while the more active catalysts B and E show selectivities of approximately 45%.

I0

Temperature, "C

Fig. 5 Oxygen conversions for methane oxidation over catalysts B, E and F.

Temperature, "C

Fig. 6 C2 selectivity for methane oxidation oxidation over catalysts B. E and F.

203

The 20% CH&O% 0, feedstream used in these standard tests is not conducive to good selectivity and as is shown in Table 3 higher values could be obtained with 35% CHd5% 0, and 95% CHd5% 0, mixtures. With the latter mixture catalyst F could achieve 85% C, selectivity at nearly 80% oxygen conversion. The best magnesia-based catalyst (E) is slightly better than the best lanthana-based one (B) under these conditions with selectivities of 69 and 65% respectively, both at near total oxygen consumption. Catalyst G is remarkably selective (90%) with the same mixture but the conversion is the lowest of all the catalysts tested. The results of X P S analyses on used catalysts are summarised in Table 4 in comparison with results for two reference materials. Perhaps the most significant finding is the detection of sodium in catalysts F and G. This is the likely reason for their relatively low activity and high selectivity. In conformity with this interpretation catalyst G, with the 2.0% sodium. is less active and more selective than catalyst F with 1.5% sodium. Although the amounts of sodium are low, similar quantities have been shown to drastically alter the properties of other catalysts for methane coupling [lo] and to a lesser extent for ethane oxidation [ 111. The surface layers of catalysts F and G appear to be largely SrCO, since neither silicon nor aluminium could be detected by XPS. The D R F I 3 spectra (Table 1) of these samples showed that SrCO, is also dominant in the bulk but bands characteristic of strontium silicates and aluminates were readily detectable. The similarity in spectra for these samples indicates that the bentonite starting material in catalyst F is largely converted to the same structures formed from the separate silica and alumina phases from which G was derived. XPS analyses for the magnesia based catalysts (D and E) show that more oxygen is present as oxide than as carbonate. It seems likely that the magnesium is largely present as MgO while the strontium is present as SrCO, since SrCO, is likely to be stable under the carbon dioxide pressures prevailing during ethane oxidation [12]. The same conclusions are reached from the bulk analyses by DRIFTS (Table 1). DRIFTS results for the pure lanthana catalyst (A) showed that some L%O,CO, was present after use although the major components appeared to be L%O, and LaOH groups (probably at the surface). The XPS results for the surface layers are consistent with this since they show that more oxygen exists as oxide than as carbonate. The DRIFTS Table 3 Characteristics of Methane Coupling with Various Feed Compositions * ~~~

20% CHdlO% 02/8000C

B C D E F G a

35% CHd5% O,R5O0C

Conv.O,, %

Sel. C,, %

Conv.02, %

Sel. C,, %

98 27 91 99 26 11

39 34 44 43 64 75

94 73 87 94 65 28

50 39 47 56 67 83

With flowrate of 20 cm3/min for the 35% CH&% notmeasured.

~

95% CH&% 0,/800°C Conv.02, % Sel. q,%

93

65

b

b

82 88' 77 45

69 72' 85 90

feedstream and 100 cm3/min for the other two.

750°C.

204

Table 4 XPS Analyses of Used Catalysts a La3' 835eV

133eV 88eV ~~~

A B C D E F G L+0,C03 SrCO,

8 8 8

0.5 10

Mg2+

S?'

Na' 1071 eV

02-

co,2-(ols)

CO,"(Cls)

-529eV

-531 eV

289 eV

56

20 78 74 39 29 68 73 55 61

16 11 16 5 5 15 8 14 22

~

2 3 2.3 1.8 15 17

b

b

35 32

17

Atom % excluding adventitious carbon.

1.5 2.0

19 32 21

Detection limit -5%.

measurements of used 1% Sr/La203 and 5% Sr/L%03 catalysts (B and C) again showed strong bands due to L%03and surface LaOH species together with some SrCO, in rough proportion to the loading but no residual L%02C03. However the XPS results indicated that the oxygen was present as carbonate with no oxide detectable. This might suggest that the surface layers are largely SrC03 but the amount of S?' is too low, and the amount of La3' too large, for this interpretation to be tenable. The tentative conclusion is that although the bulk of catalysts B and C is largely oxide the surface layers are a mixed strontium-lanthanum carbonate. The amount of S?' is somewhat greater for catalyst C than B and this may correlate with the lower activity and selectivity of the latter sample. Acknowledgments: This work was supported by a grant from the Australian Research Council. The authors are grateful to A.M. Maitra and R.J. Tyler for the provision of catalyst samples D through G and for helpful discussions.

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