Oxidative coupling of methane in a catalytic membrane reactor: impact of the catalystmembrane interaction on the reactor performance.

Natural Gas Conversion VIII F.B. Noronha, M. Schmal, E.F. Sousa-Aguiar (Editors) © 2007 Published by Elsevier B.V.

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Oxidative coupling of methane in a catalytic membrane reactor: Impact of the catalystmembrane interaction on the reactor performance. Stephane Haag,a Magdalena Bosomoiu,a Andre C. van Veen,a Claude Mirodatosa a

Institut de Recherches sur la Catalyse – CNRS, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France

Results for the oxidative coupling of methane to higher hydrocarbons in dense membrane reactors with surface catalysts are reported. It is demonstrated that the concept is viable, but potential improvements are identified enhancing the activity of the oxidative coupling catalyst for highly permeable membranes. 1. Introduction Abundant resources of natural gas available at reasonable prices and the foreseeable shortage of petroleum reflected by a recent rise of prices to all-time highs stimulate renewed interest in the oxidative coupling of methane (OCM). Despite past extensive research efforts, suggesting mainly concepts based on fixed bed reactors, the need for sustainable solutions requires reviewing alternative reactor concepts with inherent need for adapted catalysts. One promising design solution could be a catalytic membrane reactor allowing potentially the use of air instead of oxygen. However, published works [1] used either porous membranes with fixed bed catalysts or dense membranes without catalysts having the obvious disadvantages to employ either the membrane only for oxygen distribution (feeding catalysts working in a conventional fashion) or comparably low productivity, respectively. Our previous work [2] indicated the interest to use membranes feeding oxygen directly the catalyst layer without passing via the gas-phase, thus allowing the dense perovskite membrane to provide activated oxygen to the catalyst for the methane oxidation. 2. Experimental Information on oxygen permeability, key property of dense mixed conducting membranes, was acquired in the temperature range of 800 to 1000°C.

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Reconstituted air was fed to the oxygen-rich membrane compartment, while helium was used as a sweep gas on the permeate side. The membrane disk (about 1 mm thick) was sealed with gold rings between two dense alumina tubes (outer diameter: 12 mm, inner diameter: 8 mm) heating to at least 900°C for 48h. The total pressure at the oxygen-rich side was adjusted to 1.1 bar passing a constant total flow rate of mixed O2 and N2 streams controlled by mass flow controllers. Gas chromatography (HP 5890 Series II, 13X packed column) allowed complete analysis of gases at both sides of the membrane. For the catalytic tests, an additional mass flow controller injected methane into the helium flow used as carrier gas on the reactant side. Maximum flow rates were 85 mL.min-1 for the CH4/He mixture and 100 mL.min-1 for reconstituted air, respectively. Various feed rates, partial pressures of O2 and CH4 and a suitable temperature range were explored. Experiments with a non-optimized but temperature stable Pt/MgO catalyst were conducted in the temperature range from 800 to 1000°C. A second gas chromatograph (TCD with HayeSep D packed column) was used to analyze carbon containing gases. 3. Results and Discussion 3.1. Comparison of the permeability for different membranes Performances of a Ba0.5Sr0.5Co0.8Fe0.2O3-į (BSCFO) membrane have been reported [2] using 100 mL.min-1 O2/N2 mixture as feed and 50 mL.min-1 of He as sweep gas on the permeate side. The oxygen fluxes for the activated permeation process and an estimation of the activation energy were presented and indicated a change in the rate determining step, i.e. for T < 725°C, the permeation of the oxygen was limited by surface steps and for T > 725°C, the diffusion through the bulk becomes the rate limiting step. In the case of a BaBi0.4Fe0.6Ox (BBFO) membrane [3], the reconstituted air feed was 100 mL.min-1, while the He flow rate was 100 mL.min-1. The oxygen permeation fluxes (Arrhenius plot) and an estimation of the activation energy are presented in Figure 1 (left side). Oxygen permeation fluxes are significantly lower than those obtained for the BSCFO membrane and are comparable to values reported in literature for a La0.6Sr0.4Co0.6Fe0.4O3-į (LSCFO) membrane [4]. The main drawback with this kind of membrane is its lack of thermal stability. In fact, the bulk BBFO membrane is not stable at temperatures above 960°C and cracking or partial melting of the disk is observed when increasing the temperature beyond that point. This is a substantial limitation for its utilization in a membrane reactor especially for the high temperature OCM reaction. The last membrane tested was a Ba0.5Sr0.5Mn0.8Fe0.2O3-į (BSMFO) disk. The reconstituted air feed was 100 mL.min-1 and the sweep flow was 100 mL.min-1 He. The oxygen permeation fluxes and an estimation of the activation energy

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Oxidative coupling of methane in a catalytic membrane reactor

are presented in Figure 1 (right side). The oxygen permeation flux increases with increasing temperature but to a much smaller extend compared to the former described membranes. The comparably high activation energy at high temperatures indicates that the oxygen diffusion through the bulk of the BSMFO membrane is much more difficult compared to the other perovskite samples. This behavior could relate to a low oxygen mobility in the volume of the membrane caused by the relatively small amount of vacancies present in the structure. However, further studies, e.g. the determination of the nonstoichiometry as a function of temperature, are required to confirm.

-2

ln (J(O2) / mL.min .cm )

-2

ln (J(O2) / mL.min .cm )

-0,6

-1

-1

-0,8 -1

Ea = 102 kJ.mol

-1,0 -1,2 -1,4 -1,6 0,80

0,82

0,84 1000 K / T

0,86

0,88

-3,2 -1

-3,4

Ea = 196 kJ.mol

-3,6 -1

-3,8

Ea = 77 kJ.mol

-4,0 -4,2

0,80

0,82 0,84 1000 K / T

0,86

Figure 1: Arrhenius plot of the oxygen permeation for a BBFO (left) and BSMFO (right)

membrane using a reconstitued air: 100 mL.min-1 / He: 100 mL.min-1 gradient

3.2. Comparison of the sol-gel and wash-coat catalyst performance A catalytic surface modification of the membrane was performed to improve efficiency of the membrane reactor, i.e. to enhance selectively CH4 conversion to C2 products. In this work, MgO was chosen as support and platinum was selected as highly stable active species. Two routes were tested for the deposition of the oxide, a sol-gel and a wash-coating method, followed by calcination at 800°C. Scanning electron microscopy (Fig. 2a, left) revealed that about half of the membrane surface is coated by MgO applying the sol-gel method. At one magnitude lower magnification it can be observed that, the solgel method yields non-uniform deposition of the catalyst whereas a wash coat layer covers well the whole surface (Fig. 2b, right). Platinum was added by impregnation, using tetraammine platinum(II) nitrate as metal precursor (Sigma-Aldrich). Permeation tests were performed with both modified membranes and compared to results obtained to a bare membrane. In fact, oxygen fluxes are similar for the bare and the sol-gel modified membranes while the wash-coat modified membrane shows an about 2.5 times higher oxygen permeability (Fig. 3).

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b

a

10 μm

100 μm

Figure 2: SEM of a surface modified BSCFO membrane by a sol gel method (a, left) and by a wash-coat method (b, right)

BSCFO

-1

J(O2) / mL.min .cm

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3,5 3,0

(Pt/MgO wash-coat)

2,5 2,0 1,5

(bare)

1,0 0,5 950

(Pt/MgO gel) 1000 1050 1100 1150 1200 T/K

Figure 3: Oxygen permeation through BSCFO-based modified membranes (reconstitued air: 50 mL.min-1 / He: 100 mL.min-1)

3.3. Comparison of the performance of BSCFO and BSMFO membranes The catalytic tests for the OCM used both kinds of modified BSCFO membranes. The amount of platinum on the MgO support was estimated to be around 2%. Surprisingly, a non-negligible amount of gaseous oxygen is found on the reactant side for the sol-gel prepared sample while it was not the case for the wash-coat modified one. Given the higher oxygen permeability of the washcoated sample, the disadvantage of the sol-gel catalyst is less related to an insufficient methane activation but to a shortcoming in contacting oxygen and methane. Obviously, this is caused by the imperfect membrane coverage in the sol-gel case where oxygen desorbs from the membrane material without intimate contact to the catalyst. Conversion of methane was higher with the

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Oxidative coupling of methane in a catalytic membrane reactor

wash-coat catalyst but selectivity was lower than that obtained for the sol-gel catalyst (Fig. 4). Higher conversions could relate to a better spread of the catalyst on the membrane while the decreased selectivity relates probably to a higher degree of oxidation of the catalyst as demonstrated by the enhanced oxygen permeation. 7 BSCFO-Pt/MgO

X (CH4) / %

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6

15 10 5 0 1050

BSCFO-Pt/MgO

5

(wash-coat)

S (C2) / %

25

(gel)

4

(gel)

3 2 (wash-coat)

1 0

1100

1150 1200 T/K

1250

1100

1150

1200

1250

T/K

Figure 4: Comparison of CH4 conversion (left) and C2 selectivity (right) between for a sol-gel deposited and wash-coated catalysts at low CH4 conc. in the reactant feed (CH4 concentration: 10% / reactant flow rate: 85 mL.min-1)

Although the BBFO membrane covers an interesting permeability window, its brittleness did not allow obtaining performance data for a sufficient time span. This membrane may be reconsidered when more active catalyst allow operation at lower temperature where the poor thermal stability does not present a significant issue. Finally, the BSMFO membrane with sol-gel catalyst showed reasonable results (Fig 5) with a C2 selectivity of 46 % and a CH4 conversion of 6.6 % (yield: 3 %) at 950°C. The C2 selectivity is much higher than that observed for the equivalent BSCFO membrane exhibiting comparable methane conversions. Unfortunately, the membrane was not stable under the severe OCM conditions and after some cycles, it disintegrated – most probably because of deep reduction of the perovskite, being even that severe for a washcoated membrane that no clear results may be reported. The improved selectivity using a BSMFO instead of a BSCFO membrane remains the most significant finding. The obvious difference in oxygen permeability between those membranes (BSCFO, BSMFO) allows comparing the catalytic performance of the catalyst at different oxygen supply rates. On the other hand, the partial surface coverage (sol-gel deposition) might also give rise to side reactions on the bare membrane surface. However, the membrane surface contribution does not seem dominant as methane activation clearly relates to the catalyst demonstrated by increased conversions when improving the catalyst deposition (wash-coat instead of sol-gel).

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24 60 C2 selectivity

S (C2) or X (CH4) / %

50 40 30 20

CH4 conversion

10 0

1120

1160

1200

1240

1280

T/K

Figure 5: Catalytic activity of the BSMFO membrane (CH4 concentration: 10% / reactant flow rate: 85 mL.min-1)

4. Conclusions It may be concluded that the BSCFO membrane reactor suffers of decreased selectivity as the oxygen supply rate is too high compared to the conversion rate for the given catalyst loading and conditions. Opting to limit the oxygen supply as demonstrated with the BSMFO membrane might help to increase selectivity, but poor performance and stability present major inconveniences. Thus, aiming on stable reactors with high performance, enhancements in the catalytic activity are obviously required to balance the oxygen permeation when using well performing membrane material like BSCFO, which will give then access to better performing membrane reactors. Acknowledgements The research stay of M.B. was supported by the EC Marie Curie program (contract HPMT-2000-00160) and the work of S.H is supported by the European research project “TOPCOMBI” (contract NMP2-CT2005-515792). References 1. S. Liu, X. Tan, K. Li, R. Hughes; Catal. Rev. 43 (2001) 147. 2. M. Rebeilleau-Dassonneville, S. Rosini, A.C. van Veen, D. Farrusseng, C. Mirodatos; Catal. Today 104 (2005) 131. 3. A.C. van Veen, D. Farrusseng, M. Rebeilleau, T. Decamp, A. Holzwarth, Y. Schuurman, C. Mirodatos; J. Catal. 216 (2003) 135. 4. Y. Zeng, Y.S. Lin, S.L. Swartz; J. Mem. Sci. 150 (1998) 87.