Novel Materials in Heterogeneous Catalysis - American Chemical

Chapter 22

Redox Cycle During Oxidative Coupling of Methane over PbO-MgO-Al O Catalyst 2

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Alvin H. Weiss, John Cook, Richard Holmes, Natka Davidova , Pavlina Kovacheva , and Maria Traikova 2

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Downloaded by STANFORD UNIV GREEN LIBR on April 26, 2013 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch022

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Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609 Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria 2

The partial oxidation of methane to ethane in a great oxygen deficiency was studied at 700, 770, and 820° C at methane to oxygen ratios of 10:1, 20:1, 50:1, 500:1, and infinity (no oxygen). W/F was 7.6, 20.3, and 41.3 ghr/mole in a 10 mm ID quartz tube containing 1 g catalyst mixed with quartz sand. The catalyst was mixed oxidePbO-MgO-Al O precipitated as crystals by a hydrotalcite preparation procedure and then oxidized at 750°C for one hour. BET surface area was 20.8 m /g before calcining, 8.7 after. DC Arc Plasma analysis confirmed equiatomic content of the metals. X-ray diffraction analysis showed that the calcined catalyst contained PbO in highly dispersed MgO - Al O matrix. XRD showed that PbO was reduced durine reaction to inactive Pb when oxygen was present in the gas feed in quantities insufficient to oxidize hydrogen produced. The oxygen in PbO was used for reaction, thereby deactivating the catalyst. Subsequent oxygen treatment converted the Pb back to PbO and restored catalytic activity, thereby completing the redox cycle. Maximum yield of ethylene plus ethane at 10:1CH /O was 13.7%. No acetylene was observed, and ethylene/ethane ratios were less than one percent of the value predicted by thermodynamic calculation. This suggests, but does not prove, that ethylene was produced thermally, not catalytically, on this catalyst. 2

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In r e c e n t years t h e shortage o f both ethylene and ethylene f e e d s t o c k s has r e s u l t e d i n a search f o r a l t e r n a t e sources o f these two commodities. Methane i s an obvious c h o i c e , s i n c e i t comprises up t o 85 mole 7. o f t h e hydrocarbons i n n a t u r a l gas.

0097-6156/90/0437-0243$06.00A) © 1990 American Chemical Society In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

244

NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS

A wide v a r i e t y o f metal oxides were screened by K e l l e r and Bhasin ( i ) i n t h e i r p i o n e e r i n g i n v e s t i g a t i o n into methane d i m e r i z a t i o n . Since then, many c a t a l y s t s have been developed and t e s t e d (2-6). However, due t o the thermal s t a b i l i t y of methane, even i n the presence of o x i d i z e r s , c o n v e r s i o n i s extremely low. To date, t o t a l y i e l d s o f C2 products o f 15 t o 207. (2, 4) a r e considered h i g h . Many d i f f e r e n t types of c a t a l y s t s have been employed, i n c l u d i n g both metal oxides, such as PbO ( 5 - 7 ) , which r e a c t i n a redox c y c l e t o Pb metal, and c a t a l y s t s c o n t a i n i n g metals with f i x e d valence, such as L i promoted MgO (2), which produce a c t i v e L i 0 " s i t e s f o r methane d i m e r i z a t i o n . Such c a t a l y s t s were d i s c u s s e d i n f o u r papers at the recent N i n t h I n t e r n a t i o n a l Congress on C a t a l y s i s (8). The most e f f e c t i v e c a t a l y s t s have s e v e r a l common t r a i t s . Low s u r f a c e a r e a has been found t o be very important (2, 9) i n the c o n v e r s i o n of methane. A l s o of importance i s the b a s i c i t y of the c a t a l y s t . (However, not every b a s i c m a t e r i a l causes C2 formation.) Iwamatsu, et a l (9), have shown t h a t , i n the absence of PbO, C2 y i e l d i n c r e a s e s from 4.4 t o 9.07. over MgO when s u r f a c e area i s c a l c i n e d from 70 t o 17 m /g. S i m i l a r e f f e c t s were shown on a l k a l i doped MgO, and the y i e l d s of C over 7 m /g 0.27. Na -MgO, 27. K - M § 0 , and 27. Cs -MgO were 9.8, 13.2, and 8.27., i n t h a t order. ( Y i e l d = (2x moles C hydrocarbons produced)/(moles CH4 i n f e e d ) ) . Bytyn and Baerns (5) showed t h a t the s e l e c t i v i t y obtained u s i n g supported PbO i n the absence of MgO depended on the a c i d i t y of the c a t a l y s t support. Y i e l d was best (157.) on S1O2 - g e l with pK^ = +6.8 and minimal (0.57.) on S1O2 - AI2O3 w i t h pK^ = -5.6. Hinsen, e t a l (6) showed an optimal e f f e c t o f PbO on C2 s e l e c t i v i t y , even though t h e r e was great l o s s of s u r f a c e area d u r i n g o p e r a t i o n at 1013K. Lee and Oyama (10) have p u b l i s h e d an e x t e n s i v e review on o x i d a t i v e c o u p l i n g of methane. In t h i s present work we show, by s t a r v i n g the r e a c t i o n f o r oxygen i n the gas phase, t h a t PbO c a t a l y s t then p r o v i d e s oxygen. I t i s reduced t o Pb°; and the c a t a l y s t d e a c t i v a t e s . The Pb° i s r e a c t i v a t e d t o c a t a l y t i c PbO by o x i d a t i o n , thereby demonstrating the redox c y c l e .


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Experimental Catalyst. One c a t a l y s t , prepared t o c o n t a i n equi-atomic amounts of l e a d , magnesium, and aluminum, was prepared f o r t h i s study. S y n t h e s i s was accomplished by the h y d r o t a l c i t e p r e p a r a t i o n procedure r e p o r t e d by R e i c h l e (12). I t i s a standard aqueous p r e c i p i t a t i o n and c r y s t a l l i z a t i o n procedure t h a t avoids f i l t e r i n g and washing problems a s s o c i a t e d with g e l p r e c i p i t a t e s . A s o l u t i o n of Pb, Mg, and A l n i t r a t e s was added under a g i t a t i o n t o a s o l u t i o n of NaOH and Na2CU3 and maintained a t 70°C f o r t h i r t y hours. T h i s caused p r e c i p i t a t i o n of c r y s t a l l i n e m a t e r i a l . Before being used as a c a t a l y s t , the m a t e r i a l was c a l c i n e d i n a i r at 750°C f o r 1.5 hours. A f t e r c a l c i n a t i o n , the c o l o r of the c a t a l y s t was b r i g h t yellow and a 257. weight l o s s was noted. BET s u r f a c e areas of u n c a l c i n e d and c a l c i n e d m a t e r i a l s were 20.7 and 8.7 nr/g, r e s p e c t i v e l y .

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

22. WEISS ET AL*

Redox Cycle During

Oxidative Coupling of Methane

245

Bulk phase analyses of samples (both c a l c i n e d and u n c a l c i n e d ) were made u s i n g a S p e c t r a m e t r i c s , Inc., SMI IV DC Arc Plasma A n a l y z e r and snowed nominally 1:1:1 atomic r a t i o s o f Pb:Mg:Al (1.00:1.05:0.92 and 1.00:1.15:0.69, r e s p e c t i v e l y ) . T h i s corresponds t o a PbO:MgOAl 0 weight r a t i o of 0.710:0.128:0.162. The c a l c i n e d c a t a l y s t has no h y d r o t a l c i t e s t r u c t u r e , but i s r e a l l y M§ and A l oxides on a support of PbO. T h i s i s shown by the x-ray d i f f r a c t i o n p a t t e r n of F i g u r e 1, obtained u s i n g a General E l e c t r i c XRD-5 X-ray D i f f r a c t i o n U n i t . The c a l c i n e d m a t e r i a l showed the e x i s t e n c e of two polymorphous m o d i f i c a t i o n s o f PbO (a-PbO and β-PbO) and low i n t e n s i t y c h a r a c t e r i s t i c s i g n a l s of α-ΑΙ^Οβ, γ - Α ^ Ο β , and MgO. Downloaded by STANFORD UNIV GREEN LIBR on April 26, 2013 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch022

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Reaction. P u r i f i e d methane (99.07. min.), and t e n percent oxygen i n helium were obtained from Matheson Corp.. Both CP helium and a i r (>99.9X) were s u p p l i e d by A i r c o . A Hastings LF-50 mass flow meter was used t o monitor the flow of helium i n t o the system. A i r flow r a t e was measured by a Matheson ALL-50 mass flow meter. The methane f e e d was monitored by a Hastings LF-100 mass flow meter. A l l eases were passed over D r i e r i t e and A s c a r i t e t o remove moisture and CO2A t e n - i n c h l e n g t h of P o l y - F l o t u b i n g connected the a i r - c o o l e d r e a c t o r o u t l e t t o a water-cooled condenser - r e c e i v e r v e s s e l . The r e a c t o r was a two-foot-long p i e c e of quartz t u b i n g (OD = 12 mm ID = 10 mm). ( S t a i n l e s s s t e e l causes the complete o x i d a t i o n o f methane d u r i n g the methane a c t i v a t i o n process ( i ) . ) A 13.5 i n c h Lindberg heavy duty tube furnace was mounted v e r t i c a l l y around the r e a c t o r . Gas flow was downward. A d d i t i o n a l i n s u l a t i o n was used t o p r o t e c t the surrounding area from the h i g h temperatures generated and t o reduce heat l o s s from the top due t o f r e e c o n v e c t i o n . Experiments f l o w i n g helium through the empty r e a c t o r showed t h a t a 7 cm i s o t h e r m a l hot zone e x i s t e d s l i g h t l y above the c e n t e r of the r e a c t o r . The temperature i n s i d e the r e a c t o r was w i t h i n 5°C of t h a t o u t s i d e . I t was i n t h i s r e g i o n t h a t the c a t a l y s t bed was p l a c e d . No measurements were made d u r i n g r e a c t i o n t o e s t a b l i s h e i t h e r t r u e bed temperatures or the e f f e c t s of heat and mass t r a n s f e r . For a t y p i c a l experiment, 1 gram o f c a t a l y s t s i e v e d t o - 2 0 + 4 0 mesh was d i l u t e d i n 7 grams of -50 + 70 mesh white quartz sand ( A l d r i c h ) t o minimize exotherms. The c a t a l y s t bed was supported by a small p l u g of quartz wool. Two inches of quartz wool were a l s o p l a c e d above the c a t a l y s t bed. An 1/8 i n c h thermocouple was l o c a t e d at the c e n t e r of the hot zone but o u t s i d e of the reactor. The thermocouple was attached t o an Omega CN300 temperature c o n t r o l l e r , which both d i s p l a y e d and c o n t r o l l e d the r e a c t o r temperature. The main set of experiments c o n s i s t e d of p a s s i n g methane and oxygen over the atmospheric pressure c a t a l y s t bed at CH4/O0 r a t i o s of 10/1, 20/1, 500/1 and i n f i n i t y . The l a s t value corresponds t o no oxygen present i n the f e e d . Three temperatures, 700°C, t o 770°C, and 820°C were s t u d i e d at each r a t i o . W/F (grams of c a t a l y s t / m o l e of methane f e d per hour) was h e l d constant at 7.6 c-hr/mole. (This corresponded t o a methane flow r a t e of 55 cnç/min). In a l l t e s t s the t o t a l f e e d r a t e t o the r e a c t o r was 104 cnr/min. T h i s v a l u e was maintained through the use of helium as a make-up gas.


246


A second study examined t h e e f f e c t o f v a r y i n g methane W/F. Again, a s t h e W/F f o r methane was .varied t h e t o t a l flow t o t h e c a t a l y s t bed was maintained at 104 cm /min. The W/F values examined were 7.6, 20.3 and 41.3 g-hr/mole. Methane t o O2 r a t i o was h e l d constant a t 10/1. When the r e a c t o r was a t temperature, methane was d i v e r t e d t o the r e a c t o r and t h e f i r s t sample was taken a f t e r two minutes. Samples were taken every f i v e minutes on l i n e a f t e r t h a t . Once the c a t a l y s t had d e a c t i v a t e d , methane flow was h a l t e d and the system was purged with helium f o r one hour. The r e a c t o r was found t o be etched a f t e r use. A i r a t 26 cnr/min was f e d t o t h e r e a c t o r t o regenerate t h e catalyst. Again helium was used t o keep t h e t o t a l flow a t 104 cnr/min. Regeneration was allowed t o proceed f o r one hour, a f t e r wnich the r e a c t o r was e i t h e r taken o f f - l i n e i n order t o x-ray the c a t a l y s t or was again purged with helium t o prepare f o r a second methane r e a c t i o n . I n the l a t t e r case, the r e a c t o r was purged with helium f o r one hour t o remove oxygen from the l i n e s . A f t e r t h e helium purge was completed, methane was again f e d t o the r e a c t o r . Blank runs, i n which p o r c e l a i n c h i p s were used t o f i l l t h e c a t a l y s t zone i n the quartz r e a c t o r , were made a t 820°C. Methane at 10/1 CH4/O2 r a t i o (55 cc/min CH4 + 55 cc/min 107, O2 i n He) was 1.57· converted a t 1007· s e l e c t i v i t y t o ethane. No c o n v e r s i o n o f methane was measured i n the absence o f Oo. Ethane c o n v e r s i o n a t 5/1 CoHg/02 a t a t o t a l gas r a t e o f 82.5 cc/min over the p o r c e l a i n was 85.07·; and s e l e c t i v i t i e s were 3.07· C0 , 867· C9H4, and 117· CH . I n the absence o f O2, C^^Q g r e a t l y d i f f e r e n t , 787·; and s e l e c t i v i t i e s were 987· t o C2H4 and 2% t o CH4. T h i s suggests t h a t most o f t h e ethylene i n t h e c a t a l y t i c methane runs t h a t a r e t h e subject o f t h i s paper was produced t h e r m a l l y , not c a t a l y t i c a l l y , from ethane formed by the d i m e r i z a t i o n . A check was made a t 820° C o f t h e behavior o f ethane a t 5/1 C2Hg/U2 over one gram o f t h e c a t a l y s t a t 82.5 cc/min t o t a l . Conversion ranged from 65.5 t o 81.87. at p r a c t i c a l l y 1007. s e l e c t i v i t y t o methane. The reason f o r no o b s e r v a t i o n o f C2H4 o r CO2 i n t h e product i s not known. Reactor e f f l u e n t was c o n t i n u o u s l y f e d t o a 0.25 ml sampling loop l o c a t e d i n s i d e a Hewlett-Packard Model 2520 Gas A n a l y z e r . S e p a r a t i o n o f the products was over two s e r i a l columns, 6' χ 1/8" Poropak Q 80/100 mesh, f o l l o w e d by 10 χ 1/8" molecular s i e v e 5A 60/80 mesh. Thermal c o n d u c t i v i t y d e t e c t i o n was used with helium c a r r i e r gas. Columns were isothermal a t 60°C.


3

2

4

w a s n o t

1

Results and Discussions F i g u r e 2 shows t h a t lower CH4/O2 values produce much g r e a t e r conversions a t the t h r e e temperatures s t u d i e d . When the r a t i o was kept low, 10/1, 20/1, 50/1, t h e r e was no s i g n i f i c a n t d e a c t i v a t i o n i n the course o f one thousand minutes. However when t h e r a t i o was i n c r e a s e d t o 500/1 and i n f i n i t y ( i . e . , a great d e f i c i e n c y o f O2), d e a c t i v a t i o n o f the c a t a l y s t was observed i n an hour. X-ray d i f f r a c t i o n p a t t e r n s on F i g u r e 3 show t h e presence o f m e t a l l i c l e a d i n c a t a l y s t d e a c t i v a t e d by o p e r a t i o n i n the absence o f


22. WEISS ET AL

Redox Cycle During Oxidative Coupling ofMethane

3000C A T A L Y S T C A L C I N E D FOR 1.5 H O U R S AT 7 5 0 ° C

2000 :

(75


1000-

15

ι ι ι ι ι ι ι ι 25 35 45

THETA

55

65

75

DEGREES

85

Figure 1. XRD spectra of -the unused catalyst showing PbO and no Pb°. φ = MgO; Ο = ex-PbO; · = β -PbO; A = 7 -AI O . 2

3

820 C

0.16 A

0.08 ^X Ο

0.00 LL.

770 C

OQ.16 Ο œ .08 0

Lu >

-ζ. Ο o.oo ο 0.16

, « -

CH4/O, CH4/O, -

10/1 20/1

700 C

0 - NO 0 , PRESENT W/F - 7Tôg-hr/meée

0.08

0.00

1 I 1 1 uni

i

1

1 ι ΓΓΠιιΓ

10 TIME,

M I N1 U00 TES

Figure 2. GH conversion vs. time on stream at 820°C, 770°C and 700°C. Parameters of CH /0 r a t i o at W/F =7.6 g-hr/mole 4

#

4

2

American Chemical Society Library 1155 ISth St., N.W. In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; Washington, D.C. Society: 20Q36 Washington, DC, 1990. ACS Symposium Series; American Chemical

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248


oxygen. When c a t a l y s t was operated i n an excess o f oxygen, P b ° was not v i s i b l e i n t h e XRD p a t t e r n (Figure 4 ) . When deactivated c a t a l y s t was r e o x i d i z e d by an a i r - h e l i u m mixture, a c t i v i t y was r e s t o r e d , a l b e i t a t a somewhat lower l e v e l than t h a t o f t h e f i r s t r e a c t i o n c y c l e . F i g u r e 5 shows t h a t a f t e r t h e r e g e n e r a t i o n o f t h e d e a c t i v a t e d c a t a l y s t , t h e metal P b ° was transformed t o β-PbO. Analogous r e s u l t s were obtained with c a t a l y s t operated a t 500 CH4/O2 ( a l s o a d e f i c i e n c y o f O2). The r e s u l t s a r e i n accord with t h e observations o f Asami, e t a l ( £ ) , which were t h a t t h e PbO c o u l d be c y c l e d i n a redox c y c l e i f O2 and CH4 were n o t simultaneously f e d t o t h e catalyst. The y i e l d s o f products C2H4 and 0 2 % were h i g h e s t a t 820° C. F i g u r e 6 shows t h e e f f e c t o f CH4/O9 r a t i o i n t h e f e e d a t W/F = 7.6 g-hr/mole f o r C9H4, 0 2 % , and CÙ2. I t was r a r e t o observe a t r a c e o f CO i n t h e product; and no acetylene was observed i n any experiment. Consequently, t h e r e a c t i o n s t h a t satisfy the s t o i c h i o m e t r y f o r O2 consumption are: 2CH

4

+ l/20 — > C H

6

+ H 0

2CH

4

+

4

+ 2H 0

CH

4

2

+

2

0 —>C H 2

2

2

2

2 0 — > C 0 + 2H 0 2

2

2

(These r e a c t i o n s should not be regarded as mechanistic, s i n c e oxygen comes from PbO produced i n t h e redox c y c l e ) . Table I compares f o r each temperature and f o r CH4/O2 r a t i o s i n the f e e d o f 10/1, 20/1, 50/1, and 500/1, t h e r a t i o o f oxygen needed t o convert t h e hydrogen produced by r e a c t i o n t o t h a t a c t u a l l y present i n t h e f e e d . These c a l c u l a t i o n s a r e t h e average f o r t h e y i e l d s shown on F i g u r e 6. When t h e r a t i o i s equal t o u n i t y , e x a c t l y a s u f f i c i e n t amount o f oxygen i s i n t h e f e e d t o maintain t h e l e a d redox c y c l e . When t h e r a t i o o f needed-to-available oxygen i s s i g n i f i c a n t l y g r e a t e r than one, which i s t h e case a t 500/1 and no oxygen, then oxygen t o combust t h e H2 and t o produce CO2 must come from t h e PbO o f t h e c a t a l y s t . Hence t h e observed d e a c t i v a t i o n . Table I.

Oxygen Deficiency at High Feed GH4/O2 Ratio

CH /0 4

2

0 Needed/0 A v a i l a b l e 2

2

700°C

770°C

10/1

1.0

20/1 50/1

1.0 1.25

1.0 1.0 1.25

2.0

3.0

500/1

820°C 1.1 1.1

1.3 3.0


22.


WEISS E T A L

a.

S20

C

1050·

c ο1

ο

•

•

Φ


50-^

b.

770 CO : z 1050 LJ

C

Φ

504

oil

Φ

Φ

•

Φ Λ _

Λ c.

700

c

1050

φ •

Φ 50 25

35

2 -

65

THETA DEGREES

Figure 3. XRD spectra of the deactivated catalyst showing Pb° after reaction i n the absence of O at 820°C 770°C, and 700°C. Legend the same as Figure 1, plus C * Pb°. 2

f

2000 « REACTED AT 820 C CH« / O, - 10 / 1 TIME ON-UNE - 1260 MIN

CO

^1000· Lu

' Ï5 ' ' 25 ' ' 35 ' ' 45 ' ' 55 ' ' 65 ' ' 75 ' ' IB5

2 -

THETA DEGREES

Figure 4 . XRD spectra of the non-deactivated catalyst shoving PbO, but no Pb°, after reaction i n an excess of oxygen. Legend same as Figure 1·


249


250


a.

; R E D U C E D IN C H R E G E N E R A T E D IN

820 c

4

1050 H

• 50-

:

b.

: CO

770 c

- · J •·

1050 H

LU

: Δ

•

50-

:

c.

700

>

1050

-

C

• •

50· 25

1 1 1 1 I ι I 1 I I I ι ι 1 1 II?

35

2

-

45

55

THETA

65

DEGREES

F i g u r e 5. XRD s p e c t r a o f t h e regenerated d e a c t i v a t e d c a t a l y s t shoving PbO and no P b ° a f t e r r e a c t i o n i n t h e absence o f 0 a t 820°C, 770°C, and 700°C. Legend same as F i g u r e 1, p l u s Δ = α - A 1 0 . 2

2

3


22. WEISS ET AL.


The maximum yields o f C2 species were obtained at 10/1 methane/oxygen f e e d r a t i o and 820°C. Table I I l i s t s t h e e f f e c t s o f v a r y i n g space time i n t h e ranee o f 7.6 - 41.3 g-hr/mole. The maximum y i e l d s o f C2's a r e l i s t e d , along with t h e corresponding methane conversions and CO2 y i e l d . Table I I . The Effect of Température and Space Time on C2 Yield


Τ (°C)

W/F g-hr/mole

Conv (7.)

C2 Maximum Yield(7.)

Yiel3(7.)

7.6

15.2

11.7

3.6

20.3 41.3 7.6

16.8

13.3

17.3 9.9 10.8 11.4

13.7 6.1 6.2

3.6 3.6 3.9

820

700

20.3 41.3

^2^4/^2%

r a

5.5

COo

4.6 5.9

t i o s were c a l c u l a t e d a t e q u i l i b r i u m f o r t h e r e a c t i o n C H 2

6

C H 2

4 +

H

2

both a t 1 atm H2 and a t 0.1 atm H2. These values a r e p l o t t e d i n F i g u r e 7 v s . r e c i p r o c a l temperature. The experimental C2H4/C2HÇ r a t i o s were a l s o c a l c u l a t e d from t h e y i e l d curves o f F i g u r e 6 a t maximum conversion and p l o t t e d . The experimental values on F i g u r e 7 f a l l i n t o a range t h a t i s q u i t e low r e l a t i v e t o t h e e q u i l i b r i u m r a t i o s p o s s i b l e , l e s s than 17.. They show t h a t t h i s p a r t i c u l a r PbO-MgO-AI9O3 c a t a l y s t c a t a l y z e s n e i t h e r t h e dehydrogenation r e a c t i o n 01 ethane t o ethylene n o r t h e d i m e r i z a t i o n o f methane t o ethylene t o anywhere near t h e e q u i l i b r i u m p o s s i b i l i t y . I f t h e c a t a l y s t does not produce adsorbed ethane, ethane would have t o readsorb t o r e a c t t o ethylene. T h i s i s n o t t o o probable; and i t i s i n accord with o u r o b s e r v a t i o n o f very low ethylene t o ethane r a t i o ( a t best 1:1) r e l a t i v e t o t h e r a t i o s p o s s i b l e a t e q u i l i b r i u m (about 100:1). The mechanism o f t h e r e a c t i o n over t h i s c a t a l y s t appears t o be t h e redox c y c l e proposed by K e l l e r and Bhasin ( i ) , except t h a t t h e g r e a t e r p a r t o f t h e ethane probably desorbs rather than reacts further to ethylene. Conclusions When PbO-MgO-Al U3 i s degraded by c a l c i n a t i o n , t h e r e s u l t i s PbO i n a c a t a l y t i c form. Oxygen from PbO i s used t o convert t h e methane produced i n t h e r e a c t i o n s . I f t h i s oxygen i s not s u p p l i e d t o PbO 2


251


252


TIME.

MINUTES

F i g u r e 6 . Y i e l d s o f C H , C H , and C 0 v s . t i m e on stream a t 820°C. Parameters o f C H / 0 r a t i o a t W/F = 7 . 6 g - h r / m o l e . 2

6

2

4

2

4

2

6.00

CD X

CM 3.50 Η

Ο \ X

1.00

CM Ο CH«/0 - 10/1 CH /0, - 20/1 CH«/0, - 50/1 CH /0, - 500/1 - NO 0 · PRESENT 7.6g-hr/mole VF a

C -1.50

H

4

EXPERIMENTAL

4

-4.00 0.0007

I I I I

0.0008

0.0009

0.0010

TEMPERATURE

I

I I I I I I I I I |

0.0011 1/T )

(

0.0012

Figure 7. Calculated equilibrium C H / C H r a t i o s at b o t h 0 . 1 and 1.0 atm H and e x p e r i m e n t a l C H / C H r a t i o s v s . r e c i p r o c a l temperature. 2

2

4

2

6

2

4

2

6


22. WEISS ET AL

Redox Cycle During Oxidative Coupling ofMethane 253

from t h e gas phase d u r i n g r e a c t i o n , t h e PbO i s converted t o m e t a l l i c l e a d . The r e s u l t i s r a p i d c a t a l y s t d e a c t i v a t i o n . The P b ° can be r e a d i l y r e o x i d i z e d back t o t h e c a t a l y t i c s t a t e . T h i s redox behavior was observed i n t h i s study by x-ray d i f f r a c t i o n . The ethane produced i n t h e system i s o n l y dehydrogenated t o about one percent o f i t s thermodynamic p o t e n t i a l , suggesting, even though product ethane t o ethylene r a t i o s a r e about 1:1, t h a t dehydrogenation does not proceed on t h e c a t a l y s t .


Acknowledgments

T h i s work i s p a r t o f t h e US-Bulgarian Cooperative Research Program. The authors extend t h e i r a p p r e c i a t i o n both t o t h e U n i t e d S t a t e s Army f o r L t . Cook's and Capt. Holmes' support and t o t h e N a t i o n a l Science Foundation f o r i t s grant No. INT-8810539.

Literature Cited 1. Keller, G. E.; Bhasin, M. M. J. Catal. 1982, 73(1), pp. 9-15. 2. Ito,T.; Wang, J.; Lin, C . ; Lunsford, J. J. Am. Chem. Soc. 1985, 107, pp. 5062-5068. 3. Otsuka, K.; Sekiyu Gakkaishi 1987, 30(6), pp. 385-396. 4. Sofranko, J . Α.; Leonard, J. J.; Jones, C. Α.; Gaffney, A. M.; Withers, H. P., Preprints, Symposium on Hydrocarbon Oxida tion, ACS New Orleans Meeting, August 30-September 4, 1987, pp. 763-769. 5. Bytyn, W.; Baerns, M. J. Appl. Catal. 1986, 28, pp. 199-207. 6. Hinsen,W.;Bytyn,W.;Baerns, Μ., Proceedings of Eighth International Congress on Catalysis, Toyko, Japan, 1984, pp. 581-592. 7. Asami, K.; Shikada, T.; Fujimoto, K.; Tominaga, H. Ind. Eng. Chem. Res. 1987, 26, pp. 2384-2353. 8. Session on Methane Conversion, Proceedings of Ninth Inter national Congress on Catalysis, Calgary, Alberta, 1988, pp. 883-989. 9. Iwamatsu, E.; Moriyama, T . ; Nobhiro, T . ; Aika, K. J. Chem. Soc., Chem. Commun., 1987, 1, pp. 19-20. 10. Lee, J. S.; Oyama, S. T. Catal. Rev. - Sci. Eng., 1982, 30, pp. 249-280. 11. Reichle, W. T. ChemTech, 1986, 16, pp. 58-63. RECEIVED May 9, 1990


Novel Materials in Heterogeneous Catalysis - American Chemical

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