oxidative coupling of methane in a mecanismo

8/20/2019 Oxidative Coupling of Methane in a Mecanismo

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~ + ~ ~ ~ i A P P L I E D

: . , ,: ~ . ~ .~ C AT A L Y S I S

A GENERAL

E L S E V I E R Applied Catalysis A: General 130 (1995) 195-212

O xida t ive coupl ing of m ethane in am ixed cond uc t ing perovsk i te m emb rane reac tor

J .E. ten E lsh of , H.J .M. B ouw m eester, H. Ver wei jUn ivers i ty +~fTwen te . De pa r tm en t ¢~f Chem ica l Techno logy, Labora to ry o f Ino rgan ic M a te r i a l s Sc i ence ,

P.O . Box 217 , 7500 AE E nschede , Ne th e r l ands

Received 1 M arch 199 5; revised 19 April 1995: accepted 19 April 19 95

b s t r a c t

I o n i c - e l e c t r o n i c m i x e d - c o n d u c t i n g p e r o v s k i t e - t y p e o x i d e L a o 6 S r o . 4 C o o . s F e o 2 0 3 w a s a p p l i e d a s ad e n s e m e m b r a n e f o r o x y g e n s u p p l y i n a r e a c t o r f o r m e t h a n e c o u p l i n g . T h e o x y g e n p e r m e a t i o np r o p e r t i e s w e r e s tu d i e d i n t h e p o , - r a n g e o f 1 0 - 3 - 1 b a r at 1 0 7 3 - 1 2 7 3 K , u s i n g h e l i u m a s a s w e e p i n gg a s a t t h e p e r m e a t e s i d e o f th e m e m b r a n e . T h e o x y g e n s e m i - p e r m e a b i l i t y h a s a v a l u e c l o s e to 1 m m o lm ~ s ~ a t 11 7 3 K w i t h a c o r r e s p o n d i n g a c t i v a t i o n e n e rg y o f 1 3 0 - 1 4 0 k J / m o l . T h e o x y g e n f l u x i sl i m i t e d b y a s u r f a c e p r o c e s s a t t h e p e r m e a t e s i d e o f t h e m e m b r a n e . I t w a s f o u n d t h a t t h e o x y g e n f l uxi s o n l y s l i g h tl y e n h a n c e d i f m e t h a n e i s a d m i x e d w i t h h e l iu m . M e t h a n e i s c o n v e r t e d t o e t h a n e a n d

e t h e n e w i t h s e l e c t i v i t i e s u p t o 7 0 % , a l b e i t t h a t c o n v e r s i o n s a r e lo w, t y p i c a l l y 1 - 3 % a t 1 0 7 3 - 11 7 3 K .W h e n o x y g e n w a s a d m i x e d w i t h m e t h a n e r a t h e r t h a n s u p p l ie d t h r o u g h t h e m e m b r a n e , s e l e c ti v it i eso b t a i n e d w e r e f o u n d t o b e in th e r a n g e 3 0 - 3 5 % . S e g r e g a t i o n o f s t r o n t i u m w a s f o u n d a t b o t h s id e s o ft h e m e m b r a n e , b e i n g s e r i o u s l y a f f e c t e d b y t h e p r e s e n c e o f a n o x y g e n p r e s s u r e g r a d i e n t a c r o s s it . T h ei m p o r t a n c e o f a s u r f a c e l i m i t e d o x y g e n f l u x f o r a p p l i c a t i o n o f p e r o v s k i t e m e m b r a n e s f o r m e t h a n ec o u p l i n g i s e m p h a s i z e d .

Keywords. Perovskite; Lao~Sr. 4Coo sFeu ~O3; Conductivity (m ixe d): Oxygen permeation; Me thane coupling;Mem brane reactor

1 . I n t r o d u c t i o n

M a n y m i x e d o x i d e s o f t h e p e r o v s k i te - t yp e A M O 3 ) s h o w c a t a ly t ic a c ti v it y inoxida t ion reac t ions . The ca ta ly t ic proper t ies a re c lose ly re la ted to the na ture of theM -ca t ion , a s was show n fo r p ropane ox ida t i on [ 1 ] . Th e A -s i t e ca t i on i s in gene ra lca ta ly t ica l ly inac t ive , b u t the part ia l subs t i tu t ion of the A-s i te ca t ion by a l iova len ti ons ha s been u sed by s eve ra l au tho r s t o con t ro l bo th the va l ency o f t he M -e l em en t

* Corresponding author. E-mail [email protected]+ fax. ( + 31-53 ) 339546.

t)926 -860 X/9 5/$0 9.50 © 1995 Elsevier Science B.V. All rights reservedS S D I0 9 2 6 - 8 6 0 X ( 9 5 ) 0 0 0 9 8 - 4


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and the oxyg en vacan cy concen t ra t ion in the bu lk [2 ,3 ] . By th i s subs ti tu tion theM - O b o n d s t re n g t h a n d t h e m o b i l i t y o f th e o x y g e n a n i o n s a r e a ff e c te d . S i n ce t h eox yg en ion cond uc t iv i ty is genera l ly enhan ced by the par t ia l subs t i tu t ion o f A-s i t eca t ions by lower va len t ca t ions [4 ] , ox yg en ions in subsur face l ayers ma y be ab leto t ake par t in the ca ta ly t i c p rocess . The in t ra fac ia l mechanism as sugges ted byVo orh oev e fo r ox ida t ion reac t ions on perovsk i tes [ 5 ] ma y be p rom oted by th i s, a si t i s genera l ly be l i eved tha t the re i s a conn ec t ion be tw een th i s mec han ism and theease o f oxyg en ion migra t ion [ 6 ] . In add i t ion to the i r h igh ion ic condu c t iv i ty som eA-s i te subs t i tu ted perov sk i te - type ox ides w i th t rans i tion m eta l ions a t the M-s i teshow e lec t ron ic condu c t iv i ty [ 7 ]. A s an e xam ple , the par t ia l subs t i tu tion o f t riva len tlan than um in LaMO3 (M i s a tr ans i tion meta l ca t ion) b y d iva len t s tron t ium, resu l t -ing in the sol id solut ion La~ xSr~MO 3, s given by

SrMO3 --* SrLa'+ M M + 3 0 o x 1 )LaMO3

w h e r e t h e n o t a ti o n f o r d e fe c t s is f r o m K r 6 g e r a n d Vi n k [ 8 ] . D u e t o th e a b o v es u b s ti tu t io n t h e c o n c e n t r a t io n o f o x y g e n v a c a n c i es i s p r o m o t e d a c c o r d i n g t o t h edefec t reac t ion

2 M / .+ O ~ , ~ 2 M ~ + V o . + 1 / 2 0 2 (g ) . ( 2 )

Fur therm ore , M-s i t e t r ans it ion meta l ca t ions can sho w cha rge d i spropor t iona tion :

2 M h ~ M M + M r, ' (3)

I o n i c c o n d u c t i o n c a n t a k e p l a c e b y h o p p i n g o f o x y g e n i o n s t o n e i g h b o u r i n g

v a c a n t s i te s, w h e r e a s e l e c t ro n i c c o n d u c t i o n i s t h o u g h t t o o c c u r v i a M n + - o -M ~n+ ~ + co ndu c t ion pa i r s [9 ] .

Severa l pe rovsk i te - type ox ides were a l ready inves t iga ted as ca ta lys t s fo r theme than e coupl ing reac t ion [ 10-15 ] . S om e LaCo O3-based perovsk i tes , pa r t i a llysubs t i tu ted wi th bar iu m and i ron on the A-and M-s i tes , r espec t ive ly, were foun d tobe ac t ive in the ox ida t ive coupl ing reac t ion whe n the reac tion was per fo rme d byt h e te m p e r a t u r e - p r o g r a m m e d r e d u c t io n ( T P R ) m e t h o d a n d c y c l ic o p e ra t io n [ 1 0 ] .A s tudy o fpero vsk i tes o f the fo rmula Ca~ _xSr~Tij- y M y O 3 ~( M = F e o r C o ; x = 0 -1 ; y = 0 -0 .6 ) ind ica ted tha t these ca ta lys t s showing b o th ion ic and p- type e lec t ron iccond uc t iv i ty g ive a h igh se lec t iv i ty to the d es i red p roduc ts [ I 1 ] . A c om binedM 6ssbau er and ca ta ly t i c s tudy of ACoo.8Feo .203 8 (A = Ba , Sr, Ca) show ed tha tthe bar ium and s t ron t ium perovsk i tes , which gave a h igh se lec t iv i ty towards C2p r o d u c t s ( C 2 in d i c a te s t h e s u m o f e th a n e a n d e t h e n e ) , h a v e a d i s o r d e r ed o x y g e nvaca ncy s t ruc ture [ 12] . In co n t ras t, the ca lc ium-c onta in in g sample adop ted theb r o w n m i l l er i t e s t ru c t u re , w h i c h i s k n o w n t o h a v e a n o r d e r e d a r r a n g e m e n t o f o x y g e nv a c a n c ie s . T h e p e r f o r m a n c e o f t h is c o m p o s i t i o n w a s c o n s i d e r a b l y le ss . C a - d o p e dBao.95Cao.osCoyFel yO3_ ~ (y >~ 0 .8 ) show ed even be t te r ca ta ly t i c p rop er t i es inc o m p a r i s o n w i t h t h e a b o v e m e n t i o n e d B aC % F e~_ y O 3 _ 6 [13] . I t was cons ideredby the au tho rs tha t the ox yg en def ic iency of BaCoyFel -yO3 was smal l and tha t i t


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J.E. ten Elshof et aL /Appli ed Catalysis A: General 130 1995) 195-212 197

l

P O ~

ll

V om

o S p o,ii

e

m

h

Fig. 1. Fluxes in mixed conducting membrane under the influence of a partial pressure gradient. Po/ is higheroxygen partial pressure, Po2 is lower oxygen partial pressure.

t e n d e d t o c o n t a i n m i x e d o x i d e s o f d i f f e r e n t s t ru c t u re s . B y d o p i n g a s m a l l a m o u n tof Ca the perovsk i te s t ruc tu re was s tab i l i zed and i ts non s to ich io me t ry inc reased ,wh ich was th oug ht to con t r ibu te to the imp rov ed C2 se lec t iv i ty.

T h e s i m u l t a n e o u s o c c u r r e n c e o f i o n i c a n d e l e c t r o n i c co n d u c t i v i t y m a k e s t h a t

t h e s e m a t er i a ls c a n b e a p p l i e d a s o x y g e n s e m i p e r m e a b l e m e m b r a n e s i n m e m b r a n ereac tors . Teraoka e t a l . were the f i r s t to measure s teady-s ta te oxygen permea t iont h r o u g h m i x e d - c o n d u c t i n gL a l _ x A x C o l _ y M y 0 3 _ ~m e m b r an e s ( A = S r , C a;M = F e , C o , C u , N i ) d i r e c tl y [ 1 6 , 1 7 ] . H i g h e r o x y g e n f l u x e s w e r e o b s e r v e d w i t hincreas ing leve l o f A-s i t e subs t i tu t ion and reduc ib i l i ty o f the perov sk i tes [ 18] .S ince these in i ti a l s tud ies , co mp os i t ions o f La l _xSr~Co~y F e y O 3 _ ~ h a v e b e e nc l a i m e d i n p a t e n t s f o r u se a s o x y g e n s e p a r a ti o n m e m b r a n e , e .g . f o r t h e p r o d u c t i o no f o x y g e n [ 1 9 ] .

T h e p r i n c ip l e o f a m e m b r a n e , s e m i p e r m e a b l e t o o x y g e n , i s sh o w n i n F i g. 1. B ya p p l y i n g a n o x y g e n p a r ti a l p r e ss u r e g r a d i e n t a cr o s s th e m e m b r a n e , o x y g e n i s d r i v e nf rom the h igh to the low par t i a l p ressure s ide . Loca l charg e neu t ra l i ty i s main ta inedb y t h e j o i n t d i f f u s i o n o f o x y g e n v a c a n c i e s a n d e l e c t r o n s o r e l e c t ro n h o l es . T h e n e tf lu x i s d e t e r m i n e d b y t h e s p e c i es w i t h t h e s m a l l e s t c o n d u c t iv i t y. W a g n e r ' s t h e o r yo f o x i d e f i l m g r o w t h [ 2 0 ,2 1 ] o n m e t a l s i s g e n e r a l l y u s e d t o d e s c r ib e t h e o x y g e nf lu x a c r o ss a m e m b r a n e .

P 2 ¢

J _ RTo 2 4 2 F 2 L ft i t e l ° ' t o t a | dIn Po2

P02 tr

4 )

He re Jo2 i s the ox yg en p erm ea t ion f lux in too l c m -2 s - ~, t~ and te~ a re the ion icand e lec t ron ic t rans fe ren ce num bers , O 'tota= O'io q- T e l ) i s the to tal con duc t iv i ty( / 2 - 1 c m - I ) , P o 2 ' a n d P o 2 a r e th e o x y g e n p a r t ia l p r e s su r e s a t t h e h i g h a n d t h e l o wp a r ti a l p r e s su r e s i d e o f t h e m e m b r a n e , r e s p e c t iv e l y ; L is t h e m e m b r a n e t h i c k n e s s( c m ) . R , F a n d T re f e r t o t h e g a s c o n s t a n t ( J m o l - ~ K - ~ ), F a r a d a y ' s c o n s t a n t ( Cm o l - ~ a n d t h e t e m p e r a t u r e ( K ) , r e s p e c t iv e l y.

T h e o x y g e n p e r m e a b i l it y o fL a o . 6 S r o . 4 C o O 3 w a sf o u n d t o b e r o u g h l y p r o p o r t i o n a lto it s ion ic cond uc t iv i ty [22 ] , wh ich i s in ag reem ent wi th the fac t tha t it s e lec t ron icc o n d u c t i v i t y i s s e v e r a l o r d e rs o f m a g n i t u d e l a rg e r. F o r s e v e r al o t h e rL a j - x S r x C o l_ y F e y O 3 _ ~p e r o v s k i t e s t h e e l e c t ro n i c c o n d u c t i v i t y w a s a l so f o u n d t o


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198 J .E . ten E l sho f e t a l. /App l i e d Ca ta ly s is A : Gene ra l 130 1995) 195 -21 2

cont ro l the to ta l condu c t iv i ty [4 ,23] . Th is imp l ies tha t tel~ 1 so tha t the p rod uc tti/elOrtotal redu ces to t r ion and the ox yg en f lux b eco m es prop ort iona l to the ionicconduc t iv i ty. On the assumpt ion tha t a l l oxygen vacanc ies a re fu l ly ion ized , ane x p r e s s io n f o r i o n i c c o n d u c t i o n c a n b e g i v e n o n t h e b a si s o f t h e N e r n s t - E i n s t e i nrela t ion:

4F [VO ]DvO ' io n - - ( 5 )

R T V m

[ V 0 ] i s t h e o x y g e n v a c a n c y m o l e f r a ct i o n , D v t h e o x y g e n v a c a n c y d i ff u s i o ncoeff ic ien t (cm 2 s - 1 ) and Vm the m ola r vo lum e of the perov sk i te( c m 3mol 1).

( E q . 4 ) i s v a li d o n l y i f t h e o x y g e n f l u x t h ro u g h t h e m e m b r a n e i s c o n t r o ll e d b yb u l k d i f f u s i o n t h r o u g h t h e o x i d e l a tt ic e . R e d u c t i o n o f t h e m e m b r a n e t h i c k n e s s Lwi l l in tha t case l ead to h igher ox yg en f luxes . Teraoka m ent ion ed the in f luence o fs i n te r in g t e m p e r a t u r e o n o x y g e n s e m i p e r m e a b i l i t y [ 1 6 ] , w h i c h m a y i n d i c a t e th a tg ra in bou nda ry d i ffus ion p lays a s ign i f i can t ro le . In add i t ion , the oxy gen f lux ma ybe in f luence d or l imi ted by sur face exch ang e k ine t ics [2 4] . A charac te r i s t i c th ick-ness Lc has been iden t i fi ed , i. e. the m em bran e th ickness a t which the ox yg en f luxi s e q u a l ly d e t e r m i n e d b y s u r f a c e e x c h a n g e k i n e t ic s a n d b u l k t ra n s p or t. T h e v a l u eo f L c c a n b e c a l c u l at e d p r o v i d e d t h a t t h e s u r f a c e o x y g e n e x c h a n g e r a te a n d t h eox yg en d i ffus iv i ty a re kno wn , e .g . f rom ~80-~60 i so top ic excha ng e techn iques . Ina p rev ious paper [25] we rep or ted on the use o f La~ _xSrxFeO3 m em bran es in theh i g h t e m p e r a t u r e o x i d a ti o n o f c a rb o n m o n o x i d e . I t w a s s h o w n t h a t th e o x y g e nperm ea t ion ra te i s l imi ted by the red uc t ion p rocess a t the su r face in the lo wer par t ia lp r e s s u re c o m p a r t m e n t .

F r o m t h e a b o v e i t i s cl e a r t h a t m i x e d - c o n d u c t i n g p e r o v s k i t e m e m b r a n e s m a y b eu s e d a s o x y g e n s u p p l y i n g m e m b r a n e a n d m e t h a n e c o u p l i n g c a t a l y s t a t t h e s a m et ime . Otsuk a was the f i r st to use separa ted feeds o f a i r and m ethan e in a s tudy inw h i c h o x y g e n w a s e l e c t r o c h e m i c a l l y p u m p e d i n to t h e m e t h a n e s t re a m b y u s i n g a ny t t r i a -s tab i l ized z i rcon ia (Y SZ ) e lec t ro ly te ce l l [26 ] . T h is approach has beena t t em p t e d b y s e v e ra l o th e r s a n d w a s r e v i e w e d r e c en t l y [ 2 7 ] . I t w a s s h o w n t h a te l e c t r o c h e m i c a l l y p u m p e d o x y g e n c o u p l e d m o r e s e l e c t iv e l y t h a n g a s e o u s o x y g e n .S ince the measured ac t iva t ion energ ies fo r C2 and CO2 format ion d i ffe red wheno x y g e n w a s s u p p l i e d b y e l e c t r o c h e m i c a l p u m p i n g r a t h e r t h a n b y p r e m i x i n g o fmo lecu la r oxy gen in the feed s t ream [ 28] , i t has been sugges ted tha t the ac t ivespec ies is d i ffe ren t in these two cases. Bes ides tha t l a t t ice oxy gen m ay co uplem e t h a n e m o r e s e l e ct i v e ly th a n g a s e o u s o x y g e n , a i r m a y b e u s e d a s t h e o x i d a n ti n s te a d o f m o r e e x p e n s i v e o x y g e n .

Th e ob serva t ions tha t were m ade in the CO oxida t ion s tudy on La~ _xSrxFeO3m e m b r a n e s [ 2 5 ] a r e o f i m p o r t a n c e f r o m a t e c h n o l o g i c a l p o i n t o f v i e w, s i n c e t h e ys u g g e s t t h a t u n d e r a c t u a l o p e r a ti n g c o n d i t i o n s t h e d e p t h o f r e d u c t i o n o f t h e o x i d emembrane sur face can be kep t l imi ted . I t impl ies tha t cons t ra in t s regard ing thes t ruc tura l in tegr i ty and chemica l s t ab i l i ty in reduc ing env i ronments o f po ten t ia lm e m b r a n e c a n d i d a t e s c a n b e r e l a x e d . T h e r e f o r e ,Lao.6Sro.4Coo.8Feo.203_8m e m -


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J .E. ten Elsh of e t a l . /A pp l ie d Ca ta lys is A: General 130 1995) 195-212 199

b r a n e s a r e u s e d a s o x y g e n s e p a r at i n g m e m b r a n e a n d m e t h a n e c o u p l i n g c a t a ly s t inthe p resen t s tudy. Coba l t was par t i a l ly subs t i tu ted by Fe to inc rease the chemica ls t ab i li ty o f t h e p e r o v s k i t e i n r e d u c i n g a t m o s p h e r e s . T h e r e su l ts a r e c o m p a r e d w i t ht h o s e o b s e r v e d w h e n t h e m e m b r a n e is p l a c e d in a s i n g l e - c h a m b e r r e a c t o r a n do x y g e n i s a d m i x e d w i t h t h e m e t h a n e f e e d s tr e am .

2 Experimental

2.1. Me mb rane preparat ion

Nit ra tes o f the cons t i tuen t meta l s (M erck , p .a . qua l i ty ) w ere separa te ly d i s so lvedi n Q 2 - w a t e r a n d t h e i r c o n c e n t r a t io n s w e r e d e t e r m i n e d b y t it ra t io n w i t h E D T A .S t o i c h i o m e t r i c a m o u n t s o f t h e s o l u ti o n s w e r e t a k e n a n d m i x e d . A s o l u ti o n o fa m m o n i a / E D TA w a s a d d e d t o t h e m i x e d m e t a l s o lu t io n , t h e f in a l c o n c e n t r a t i o n o fE D TA b e i n g 1 .5 t i m e s t h e t o ta l m e t a l c a t io n c o n c e n t r a t io n . T h e p H o f t h e s o l u ti o nw a s a d j u s t e d t o 8 - 9 b y u s i n g a m m o n i a . T h e s o l u t i o n w a s m i x e d f o r s ev e r a l h o u r su n d e r m o d e r a t e h e a ti n g t o c o m p l e t e c o m p l e x a t i o n a n d t h e n p y r o l y s e d in a s t o v e a t500 K. The resu l t ing powder was mi l l ed fo r severa l hours in a p lane ta ry mi l l inace ton e and a f te r d ry ing ca lc ined a t 1200 K in s tagnan t a i r fo r 12 h . Ca lc ina t ion a tl o w e r t e m p e r a t u r e s a ls o r e s u l t ed i n p e r o v s k i t e f o r m a t i o n b u t i n t h e s e c a s e s X - r a yd i f f r ac t i o n ( X R D ) r e v e a l e d t ra c e s o f S rC O 3 . A f t e r re p e a t in g t h e m i l l in g p r o c e d u r ed i s k s o f 2 5 m m d i a m e t e r w e r e o b t a i n e d b y u n i a x i a l p r es s in g a t 1 .5 b a r, f o l l o w e dby i sos ta t i c p ress ing a t 4000 bar. Th e d i sks we re s in te red in P t - l ined boa t s a t 1 450 -

1 5 0 0 K f o r 24 h i n a p u r e o x y g e n a t m o s p h e r e . M e m b r a n e s o f 1 5.2 m m d i a m e t e ra n d 0 . 5 - 2 m m t h i c k n e s s w e r e c u t f r o m t h e s i n t e re d d is k s . P r io r to u s e t h e m e m -b r a n e s w e r e p o l i s h e d w i t h 1 0 0 0 M E S H S i C a n d u l t ra s o n i c a ll y c le a n e d i n e th a n o l .A n A r c h i m e d e s m e t h o d w a s u s e d t o d e t e r m i n e th e d e n s i ty o f t h e m e m b r a n e s .

2.2. Exp erim ental set up

O x y g e n p e r m e a t i o n m e a s u r e m e n t s w e r e p e r f o r m e d in a q u a r t z r e a ct o r [ 2 5 ] w i t ha r e a c t o r v o l u m e o f a p p r o x i m a t e l y 3 m l . C a t a ly t i c e x p e r i m e n t s w e r e p e r f o r m e d ina quar tz reac tor dep ic ted in F ig . 2 . S up rem ax g lass r ings ( Scho t t Ne der lan d B.V. )w e r e u s e d t o s e a l th e m e m b r a n e i n t o t h e q u a r t z r e a c to r a t 1 31 0 - 1 3 3 0 K i n s t a g n a n ta i r, thus c rea t ing two i so la ted chamb ers .

T h e e x p e r i m e n t a l s e t -u p i s s h o w n i n F i g . 3 . In t h e h i g h e r o x y g e n p a rt ia l p r e s s u rec o m p a r t m e n t t h e o x y g e n p a rt ia l p r e ss u r e w a s m o n i t o r ed b y a n Y S Z - b a s e d o x y g e ns e n so r. T h e o x y g e n p a r t ia l p r e s s u r e in t h i s c o m p a r t m e n t c o u l d b e a d j u s te d i n t h er a n g e 1 0 - 2 - 1 b a r b y a d m i x i n gN2,a i r and 02 . The to ta l f low ra te in th i s comp ar t -m e n t w a s k e p t h i g h [ 1 0 0 - 1 8 0 m l / m i n ( S T P ) ] t o p r e v e n t m a s s t r a n s fe r l i m i ta t io n sf r o m t h e g a s p h a se . B r o o k s 5 8 0 0 m a s s f l o w c o n t r o l le r s w e r e u s e d t o c o n t r o l t h eg a s f l o w r a te s t h r o u g h b o t h c o m p a r t m e n t s . A Va r i a n 3 3 0 0 g a s c h r o m a t o g r a p h a n d


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200 J .E . t en E l sho f e t a l. /App l i e d Ca ta ly s i s A : Gen e ra l 130 1995) 195 -212

t h e r m o c o u p l e

~u2t ~Ul.~14et

/ \

gl s s se l mem br ne

in le t0 2 N 2

• ~ t h e r m o c o u p l eFig. 2 . Schemat ic d iagram of the cata lyt ic reactor.

a L D C / M i l t o n R o y C L - 1 0 i n t eg r a to r w e r e u s e d f o r a n a l y si s o f t h e c o m p o s i t i o n s a tbo th the in le t and ou t l e t o f the lower oxy gen pa r t ia l p ressure com par tm ent o f thereac to r. A mo lecu la r s i eve 13X w as used fo r separa t ion o f H2 , 02 , N2 , CH4 andC O . A P o r a p a k - N c o l u m n w a s u s e d f o r s ep a r a ti o n o f C O 2 , h y d r o c a r b o n s a n d H 2 0 .The to ta l gas f lows a t the in le t and ou t l e t o f the r eac to r w ere sepa ra te ly mea suredb y a B r o o k s V o l u m e t e r.

2 3 Permeat ion and catalytic measurements

In the oxygen pe rmea t ion measurements the oxygen pa r t i a l p ressure a t the l eans i d e o f t h e m e m b r a n e w a s v a r i e d b y a d j u s t in g t h e H e ( 4 . 6 N ) f l o w r a te i n t he r a n g e


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J .E . t en E l sho f e t a l. / App l i ed Ca ta ly s i s A : G ene ra l 130 1995) 195 -212 201

A i r N 2 0 2

NeO CN4

: L , . V e n t .- 0 2I - s e n s o r

R e a c t o r

L : t , : 0 2~ V e n t i s e n s o r

Fig. 3. Schem atic diagram of the experim ental set-up.

5-70 ml/ mi n (STP) . The gas tightness of membrane and seal was checked by GCdetection of nitrogen in the effluent stream, since in case of a leak free membraneno N 2 will be present there. The contribution of leakage to the total oxygen fluxcould be calculated from the measu red nitrogen concentration and was found to beless than 3 in all cases, and typically well below 1 . Calculated oxygen perme-ation fluxes were corrected for this contribution.

In the catalytic measurements the feed consisted of He and CH 4. The permeationwas calculated from the outlet flow rate and the concentrations of all oxygenconta ining species (O2, CO, CO2 and H 2 0 ) . Selectivities were calculated from thetotal amounts of products f ormed. The carbon balance was in all cases in the range99 + 2 . O xygen fluxes and reaction rates were normalized with respect to thegeometric surface area exposed to the metha ne/ hel ium atmosphere.

To compar e the results with the effect of cofed oxygen, measurem ents were alsoperformed in a single-chamber reactor with similar geometr y and dimensions. Anamount of oxygen was admixed with methane which was equal to the amount ofpermeated oxygen measured under similar conditions. These experiments will bereferred to as 'cofeed mode', as distinct from the 'membrane mode' described

above.

2.4. XRD SEM EDX and AES

Sample characterisation was performed by taking XRD spectra (Phili~psPW 1710) from crush ed sintered membranes using Ni-filtered Cu Ka~ ( 1.5406 A)radiation. LaB6 was added as an internal standard and the samples were measur edwith a 20 scan f rom 20 ° to 140 ° with steps o f 0.018 °. The in tensity was collectedduring 5-10 s.


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202 J .E . t en E l sho f e t a l. /App l i ed Ca ta ly s i s A . Gene ra l 130 (1995) 195 -212

T h e m e m b r a n e s u r f a c e m o r p h o l o g y w a s e x a m i n e d b y h i g h r e so l u t io n s c a n n i n ge l e c tr o n m i c r o s c o p y ( H R - S E M ) ( H i t a c h i S - 8 0 0 f ie ld e m i s s i o n m i c r o s c o p e ) .P h a s e s p r e s e n t a t t h e s u r fa c e w e r e d e t e r m i n e d b y X R D . S u r f a c e e l e m e n t a n a l y s isw a s p e rf o r m e d b y en e rg y di s pe r s iv e X - r ay ( E D X ) ( K e v e x D e l ta R a n g e ) a n d b ya n a ly t ic a l e l e c tr o n s p e c t r o s c o p y ( A E S ) ( P e r k i n E l m e r P H I 6 0 0 ) . A n A u g e r d e p thprof i le w as o b ta ined by spu t t e r ing wi th a 1 mm 2 3 .5 keV Ar ÷ bea m on 30 ° ti lt edsamp les wi th a spu t t e r r a te o f 32 .5 A m in J ( + 10% ) . The inc iden t ang le be tw eent h e b e a m a n d t h e s a m p l e w a s 4 5 ° an d t h e s u r f a c e a r e a a n a l y z e d w a s 1 5 0 × 1 2 0~m 2

3 R e s u l t s a n d d i s c u s s i o n

3 . 1 . X R D

T h e X R D s p e c tr a o f p o w d e r s o b t a i n e d f r o m c r u s h e d f r e s h ly p r ep a r e d m e m b r a n e sshowed a s ing le -phase pe rovsk i t e . Al l peaks cou ld be indexed on the bas i s o f arhom bohe dra l ly d i s to r t ed cub ic ce l l wi th l a tt i ce pa ram ete r s a = 5 .44 50( 9) A andc = 13 .2553 (3 ) A (hexa gona l se t t ing) , in c lose ag reem ent w i th c rys ta l lograph icda ta on Lao.6Sro.4CoO 3 [29] . On the bas i s o f the un i t ce l l vo lume , the dens i t ie s o fm e m b r a n e s u s e d i n th is s t u d y w e r e c a l c u la t e d to b e i n th e r a n g e o f 9 6 . 5 - 9 8 . 0 % o ftheoret ica l .

3 .2 . O x y g e n p e r m e a t i o n m e a s u r e m e n t s

The A r rhen ius cu rve s o f the oxygen f lux in an a i r /H e g rad ien t a re show n inF ig . 4 fo r two sam ples o f d i ff e ren t th ickness . Af te r sea l ing the sam ples we re coo leddown to t empera tu res be tween 1223-1273 K and kep t a t th i s t empera tu re un t i l t hep e r m e a t i o n b e c a m e s t a b le ( 1 0 - 1 5 h ) . A t t h e H e s i d e a n o x y g e n p a r ti al p r e s s u r e o f0 .017 ba r was m ain ta ined . As can be seen f rom the f igure the two curves co inc idewel l in abso lu te magn i tude .

In con t ras t, t he s t ab i li za t ion t imes o f m em bran es w hich were im m edia te ly coo ledd o w n f r o m t h e te m p e r a t u r e a t w h i c h s e a l in g w a s p e r f o r m e d t o 11 0 0 - 1 2 0 0 K w e r econs ide rab ly longer and the i r final pe rmea b i l i ty wa s a f ac to r o f 3 -4 lowe r than i s

£~ -6.5

_~ 7.o ---~ l-6.5

• 0 .5 5m m m ~ -7 .0_ . . m 0.98 mm 7.5

0 .75 0 .8 0 .85 0 9

1000/1 ( l /K)

Fig. 4. Temperature dependence of oxygen permeation flux in air/He (0.017 bar O~) gradient for samples ofdifferent thicknesses.


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J .E . ten E l sho f e t a l. /Ap p l i e d Ca ta ly s i s A : G ene ra l 130 1995) 195 -21 2

Table 1Activat ion energies of o xyg en semi-p ermea bi l i ty of Lao.6Sro.4Coo.8Feo.203 in an air / He gradient

203

T h i c k n e s s ( m m ) T K) EaCt kJ/mol)

0.55 1123-1 273 138 ± 100 .98 1123-1223 128± 8

2 o / A ~ Y ~~ ~ • • 1223 K

~ 0 0 ~L , ~- 2 . 6 - 2 . 2 - 1 . 8 - 1 . 4

lo P o / b a r )Fig . 5 . Depen dence o f ox ygen f lux on pa r ti a l p ressure in lower pa r t ia l p ressure compar tment a t d i ff e ren t t emper-atures.

exp ec ted on the bas i s o f the r e su lt s show n in F ig . 4 . Up on inc reas ing the t emp era tu ret o 1 2 0 0 - 1 3 0 0 K , t h e f l u x e s t h r o u g h t h e s e m e m b r a n e s i n c r e a s e d i r r e v e r s i b l y a n dthe i r pe r fo r ma nces b eca m e s imi la r to those o f the h igh- temp era tu re s t ab i l ized sam-pies .

The ac t iva t ion energ ies ob ta ined f rom the Ar rhen ius p lo t s in F ig . 4 a re g iven inTab le 1 . I toh e t a l. ca lcu la ted an ac tiva t ion energy o f 117 kJ /m ol fo r the ion icc o n d u c t i v i t y f r o m o x y g e n p e r m e a t i o n d a t a o fL a o . 6 S r o . 4 C o O 3 [ 3 0 ] .The au thorsu s e d a s i m p l e l o g a r it h m i c d e p e n d e n c e f o r th e o x y g e n f l ux , w h i c h c a n b e d e r i v e df rom (Eq . 4 ) a s suming a cons tan t ion ic conduc t iv i ty. I t i s p laus ib le tha t the ac t i -v a t i o n e n e rg y f o rL a o . 6 S r o . 4 C o o . 8 F e o . 2 0 3i s s l igh t ly h igher due to the p resence o firon.

F i g . 5 s h o w s t h e o x y g e n p e r m e a t i o n f l u x i n an a i r / H e g r a d ie n t a s a f u n c ti o n o fthe oxygen pa r t i a l p ressure in the lower pa r t i a l p ressure compar tment . The meas -u r e m e n t s w e r e p e r f o r m e d o n t w o d i f f e r e n t s a m p l e s o f 1 . 0 m m t h ic k n e s s a t d i f f er e n tt e m p e r a t u re s . S t a b il iz a t io n w a s p e r f o r m e d a t t h e t e m p e r a t u r e o f m e a s u r e m e n t . T h ed i ffe renc e in pe rm ea t ion ra te s co r respo nd w e l l wi th the obse rve d ac tiva t ion energy.A s t rong ind ica t ion fo r a su r face con t ro l l ed f lux emerges f rom the dependence o ft h e o x y g e n f lu x o n t h e pa r ti a l p r e ss u r e . F o r a l l m e m b r a n e s m e a s u r e d , t h e o x y g e nf lux can be re l a t ed to the oxyg en pa r t ia l p ressure in the He-con ta in ing cham ber v ia

Jo2 ~ P~2 (6 )

w h e r e n = 0 . 2 2 _ 0 . 0 1 . W h e n a m a s s a c ti o n t y p e r e la ti on i s a p p li e d t o ( E q . 2 ) ,t ak ing in to accoun t tha t the oxyg en s to ich iom et ry is de f ined by [ O~ ] = 3 - [V o ] ,the fo l lowing re la t ion ho lds fo r the oxygen vacancy concen t ra t ion :

3[M~a] 2[ Vo ] - [Mi~]2 K2[M~a]2p~/2 ( 7 )


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2 0 4 J.E. ten Elshof et al. / Applied Catalysis A: General 130 1995) 195-212

w h e r eK2 i s t h e e q u il ib r i u m c o n s ta n t . T h e p o w e r d e p e n d e n c e o f t he o x y g e n v a c a n c yconcen t ra t ion on oxy gen pa rt ia l p ressure wi l l t hus va ry be tw een - 1 /2 and 0,depend ing on oxyge n pa r ti a l p ressure and tempera tu re . F ro m Eqs . (4 ) , (5 ) and(7) the same pow er depende nce i s expec ted fo r the oxy gen f lux . A pos i t ive n -va lueas is exper imen ta l ly fo und he re ma y ind ica te tha t a p rocess o the r than bu lk d i ffus ionl imi t s the oxygen f lux . Combined wi th the da ta p resen ted in F ig . 4 , a s d i scussedabove , w e t ake th is a s ev idenc e o f a su r face -ex change con t ro l l ed f lux .

Measurements a t 1153 K were pe r fo rmed in which the pa r t i a l p ressure in theh igher pa r ti a l p ressure compa r tme nt was va r i ed (0 .01-0 .21 ba r ) , w h i l e the oxy genconcen t ra t ion a t the He s ide was kep t a t a cons tan t va lue o f 5200 ppm. In thesecases the va lue o f n is c lose to ze ro . The exp er imen ta l oxyg en f luxes can ber e p r e s en t e d b y

J o 2 ( m m o i m 2 s ~) = 1 . 0 5 4 + 0 . 5 0 6 l o g p o 2 ' ( 8 )

I t has been show n [ 31 ] tha t the Po2 depe nden cy i s s t rong ly depen den t on theapp l i ed g rad ien t. Th i s a l so bec om es c lea r whe n (Eq . 7 ) i s cons ide red . The oxy genvaca ncy concen t ra t ion w i ll be d i ffe ren t on bo th s ides o f the mem brane . Thus , thed i ffe rences in dependenc ies a s found he re do no t necessa r i ly ind ica te a d i ffe rencein the na tu re o f the r a t e -de te rmin ing s t ep o f the pe rm ea t ion p rocess .

3 3 Ca ta ly ti c measurem ents

The t empera tu re dependence o f the ca ta ly t i c ac t iv i ty and C2 se lec t iv i ty wasd e t e r m i n e d b y f e e d i n g 0 .2 5 b a r m e t h a n e a t a t o ta l f lo w o f 1 6 .4 m l / m i n ( S T P )

(2 .79 /~m ol s - J CH 4) . Ai r was supp l i ed a t the h igh oxy gen pa r ti a l p ressure s ide o fthe me mb rane . The resu l ts a re sh ow n in F ig . 6 . Af te r an in it ia l inc rease o f these lec t iv i ty be tw een 1073 and 1123 K to 62% , a s ligh t dec rease i s obse rved be tw een1123 and 1173 K, fo l low ed by a s teep dec rease abo ve 1173 K to f ina l ly r each 41%at 1223 K. The se lec t iv i ty d rop can be a t t ribu ted to a s t rong inc rease o f the p ro -duc t ion ra te o f CO2. S ince th is r a t e inc rease i s accom pan ie d by a s t rong inc reaseof the H2 produc t ion ra te i t i s l ike ly tha t above 1173 K ca rbon depos i t ion occursb y t h e rm a l d e c o m p o s i t i o n o f m e t h a n e , f o l l o w e d b y o x i d a ti o n w i th u n r e a c t e d p e r -m e a t e d o x y g e n . T h e a c ti v a ti o n e n e rg y f o r o x y g e n p e r m e a t i o n w a s d e t e rm i n e d t obe 169 + 6 kJ /m ol . Al thoug h th is va lue i s abou t 35 kJ /m ol h igher than de te rmine d

7 5 - 6 5

4 5 0 i A C 2 s e l e c t i v it y \ 5 5 rJ )e 01 rate '~ " ~

E 3 0 0 ~ 5 0 ~=L ImH rate

"C" 150 • v C2rate f ~ r L 4 ~

o ~ o1 0 7 0 11 2 0 11 7 0 1 2 2 0

T (K)

F i g . 6 . R e a c t i o n r a t e s a n d s e l e c t i v i t y t o e t h a n e / e t h e n e a s f u n c t i o n o f t e m p e r a t u r e . PC H, = 0 . 2 5 b a r . To t a l f l o w r a tei s 16 .4 m l / m i n ( S T P ) .


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J.E. ten Elsho f et al. /A pp lie d Catalysis A: General 130 (1995) 195- 212 205

250 ~ C H 6 CO2 0 0 4 A C 2 H 4 C O

~ H 2 ~ ~ / ~ /

15 0 i ~

0 0 2 0 4 0 6 0 8

PcH bar)

Fig. 7. Reaction rates at 1153 K as function of the methane partial pr essur e at a total flow rate of 20.5 ml /m in(STP),

f rom a i r /He measurements , the d i ffe rence i s l ike ly to be due to the f ac t tha ts t ab i l i za t ion was pe r fo rmed a t 1073 K on ly.

T h e a c t iv a t io n e n e rg y f o r m e t h a n e c o n v e r s i o n w a s 2 1 7 + 1 2 k J / m o l , w h i l e t h ato f C 2 f o rm a t i o n w a s c a l c u la t e d t o b e 1 75 + 1 0 k J / m o l . I n t h e a f o r e m e n t i o n e d

t e m p e r a t u r e r a n g e , t h e C 2 y i e l d v a r ie d b e t w e e n 0 . 2 5 % a n d 3 . 1 % . F r o m t h e s e r es u l ts1153 K was se lec ted as s t andard t empera tu re fo r fu r the r exper imen t s , s ince bo threasona b le se lec t iv i t i e s and ox yge n f luxes are found he re .

The m ethane pa r t i a l p ressure depen denc e a t a to ta l f low ra te o f 20 .5 m l /m in(S T P) i s show n in F ig . 7 . Ai r wa s aga in sup p l i ed in the h igh pa r t ia l p ressurec h a m b e r. T h e o x y g e n f lu x w a s f o u n d t o b e 0 . 8 4 + 0 . 0 5 m m o l m - 2 s - 1 a n d w a s n o tin f luenced by chang ing the me thane pa r t i a l p ressure . At me thane p ressures above0 .5 b a r p r o d u c t i o n o f H 2 i s s ee n , p r o b a b l y d u e t o t he r m a l d e c o m p o s i t i o n o f m e t h an e .T h e r a t e o f c a r b o n m o n o x i d e f o r m a t i o n w a s v i r tu a l ly i n d e p e n d e n t o f th e m e t h a n epar t ia l p ressure , wh i l e the r a t e s o f fo rm at ion o f e thane , e thene and ca rb on d iox ides tead i ly inc reased . The methane convers ion ranges be tween 2 .6 and 1 .1%, to bec o m p a r e d w i t h 0 . 4 5 - 0 . 3 5 % f r o m b l a n k e x p e r i m e n t s .

Methane convers ion , se lec t iv i t i e s and re la t ive oxygen consumpt ion a re g iven inTa b l e 2 . T h e o x y g e n c o n s u m p t i o n X o : ( d e f i n e d a s t h e to t al a m o u n t o f o x y g e nc o n s u m e d b y r e a c t io n d i v i d e d b y t h e t o ta l a m o u n t o f o x y g e n p e r m e a t e d ) i n c r e as e sf rom 44% to 75% upon inc reas ing the me thane pa r t i a l p ressure f rom 0 .15 ba r to0 . 9 3 b a r. T h u s , g a s e o u s o x y g e n i s o b s e r v e d in t h e m e t h a n e c o n t a i n in g c o m p a r t m e n t ,and Po2 va r i e s the re be tweenl 0 - 2 2 t and 10 -2 .55 ba r. Co mp ar ing the ob se rv edoxy gen f lux wi th the r e su l t s show n in F ig . 5 c l a r if i e s tha t the oxyg en f lux i s no t o ro n l y s l i g h tl y e n h a n c e d b y t h e p r e s e n c e o f m e t h a n e .

Table 2Methane conversion, selectivities and relative oxygen consu mption as a function of pert4. T= 1153 K; flow rate is20.5 ml/min (STP)

peru (bar) XcH4 ( ) Sco ( ) Sco2 ( ) Sc2ru ( ) SC~,H6 ( ) Xo~ ( )

0.15 2.6 14.5 40.5 14.8 30. l 43.80.30 1.9 I 1.6 32.3 22.8 31.6 49.90.60 1.3 8.3 29.1 28.0 31.9 66.60.93 l. l 5.0 27.8 31.4 31.2 74.7


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2 0 6 J .E , t en E l sho f e t a l . / App l i ed Ca ta ly s is A : Gene ra l 130 1995) 195 -212

4I • 002

| 3 AC2H6 J ~i vC2H . ~-

2 ~

00 0.2 0.4 0.6 0 8

Pc.(bar)

F i g . 8. R a t i o s o f r e a c ti o n r a te s i n m e m b r a n e m o d e a n d c o f e e d m o d e a t 11 5 3 K a s f u n c t i o n o f m e t h a n e p a r t i a lp r e s s u re . F l o w r a te i s 2 0 . 5 m l / m i n ( S T P ) .

Fig . 8 shows the ra t ios o f the p roduc t ion ra te s o f C2 p roduc t s and CO2 as af u n c t io n o f P c m w h e n o p e r a t i n g in th e m e m b r a n e m o d e a n d c o f e e d m o d e . N op r o d u c t i o n o f c a r b o n m o n o x i d e w a s o b s e r v e d i n t h e c o f e e d m o d e . I t i s s ee n t h att h e s e le c t iv i t y t o e t h a n e / e t h e n e i s m u c h h i g h e r i n t h e m e m b r a n e m o d e . N o t o n l yt h e p r o d u c ti o n r a te o f c a rb o n d i o x i d e i s 4 0 - 5 0 % o f t ha t in t h e c o f e e d m o d e , t h era tes o f e thane and e thene fo rma t ion a re cons ide rab ly h igher. At PCH4= 0.93 bar,the r a te s o f e thane and e thene fo rm at ion equa l 1 .4 and 3 .2 times those in the co feedm o d e , r e s p e c t i v e l y. T h e p r o d u c t i o n r a t e s o b t a i n e d u s i n g m e m b r a n e a n d c o f e e dm o d e , a s w e l l a s th o s e f r o m b l a n k e x p e r i m e n t s ( w i t h o u t c a t a l y s t ) , a r e g iv e n i nTab le 3 . The p resence o f the ca ta lys t l eads to a l a rge inc rease in the p roduc t ionra te o f CO2. At low m ethane pa r ti a l p ressures the p roduc t ion ra te s o f e thane ande thene a re subs tan t i a l ly h igher in co fee d m ode than in the b lank exper im en t s , bu ta t 0 .93 ba r th is d i ff e rence i s a lmos t neg l ig ib le . Al thoug h the rat e o f CO2 produc t ioni s a l so h igh in me mb rane m ode , the r a t e o f C2 p rodu c t ion bec om es h igher than inthe oth er tw o ca ses fo r h igh PCH4.

TO compare the C2 se lec t iv i ty a s a func t ion o f the me thane convers ion in bo thm ode s o f opera t ion , m e thane was fed to the r eac to r wi th a pa rt ia l p ressure o f 0 .40bar, whi l e the convers ion was va r i ed by chang ing the to tal f low ra te. The resu l tsa re show n in F ig . 9 . The se lec t iv i ty o f the me m brane p roce ss i s cons ide r ab ly h igherthan tha t o f the co fee d p rocess . These resu l ts m ay ind ica te tha t the na tu re o f theca ta lys t is d i ff e ren t in the two cases . H ow eve r, due to the h igh t em pera tu res a pp l i edin these exper imen t s , the ro le o f the gas phase reac t ions can n o t be underes t ima ted .There fo re , the re su l t s ma y a l so be a t t r ibu ted to a s imple m ass t r anspor t e ffec t . S incethe concen t ra t ion p ro f i l e o f oxyg en th roug hou t the r eac to r wi l l be d i ffe ren t in the

Ta b l e 3

P r o d u c t i o n r a t e s at d i ff e r e n t m e t h a n e p a r t i a l p re s s u r e s i n m e m b r a n e m o d e a n d c o f e e d m o d e . ' B l a n k ' r e f e rs t oc o f e e d m o d e m e a s u r e m e n t s w i t h p o l i s h e d q u a r t z m e m b r a n e . F l o w i s 2 0 .5 m l / m i n ( S T P )

P CH 4 ( b a r ) C O 2 ( / ~ m o l m - 2 s i ) C 2 H r ( / x m o l m 2 s l ) C2H 4 ( /xm ol m - 2 s i )

M e m b r a n e C o f e ed B l a n k M e m b r a n e C o f e e d B l a n k M e m b r a n e C o f e e d B l a n k

0 .15 128 305 8 48 51 19 23 27 00 . 4 5 1 7 4 3 7 0 1 2 8 8 7 3 4 4 7 3 3 6 1 60 .93 215 437 17 121 86 77 122 37 38


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J .E . ten E l sho f e t a l. /App l i ed Ca ta ly s i s A : Gen e ra l 130 1995) 195 -212 207

1 0 0 ]~ • c o f e e d m o d e

=

60 i ~t~ -~ -~ -- ~,t I

2 0 0 ] ~ L ~ . . . . ~ . _ _

0 1 2 3 4 5 6

x (%)

Fig. 9. C2 selectivities versus methane conve rsion at 1153 K for membra ne mode and cofeed mode. PeR, at inletis 0.40 bar.

200 - - = 1 . 2• C~He • J ° 2 ~ C -

O 0 r 3_E ~ J ~ ~ o 6 o~ 100~ ~ ~ . _501 ~ - ~ : ~ ~ ' - ~ o . a =7

0 1o0.01 0.1 1

Po, (bar)

Fig. 10. Reaction rates and oxygen flux at l 153 K as function of oxygen partial pr essure in the oxygen-suppl yingcompartment. Flow rate is 20.5 m l/ min (STP) . Pen4 = 0.40 bar.

100

75

v 50o)

2 5

05 1 0 1 5 2 0 2 5

t i m e h )

Fig. 11. Selectivities to C 2 and CO and conversion rate of methane at 1153 K as function of time in reductionexperiment. Flow rate is 20.5 m l/ mi n ( STP). PcH, = O. 15 bar.

1 6

~CO rCH4 ~Sc / L12 ~

2 / o3. : / ~0.8 ~

~ .4 ~

t w o m o d e s o f o p e r a ti o n , i t c o u l d b e s u g g e s t e d t h a t t h e d i f f e r e n c e i n s e l e c ti v i tyr e s u lt s f r o m a d i f f e re n t C H 4 / O : r a ti o i n t h e v i c i n it y o f t h e s u rf a c e. I n t h e m e m b r a n em o d e t h e c o n c e n t r a t i o n o f m e t h a n e m a y b e v e r y l o w c l o s e to t h e s u r f a c e, d u e t ot h e c o n t i n u o u s f e e d i n g o f m o l e c u l a r o x y g e n f r o m t h e s u r fa c e , w h e r e a s in t h e c o f e e dm o d e , i t w o u l d r e m a i n h i g h .

T h e d e p e n d e n c e o f t h e o x y g e n p a rt ia l p r e s s u re i n th e o x y g e n s u p p l y i n g c o m -p a r t m e n t o n t h e o x y g e n f l u x a n d r e a c t io n r a t es i n t h e l o w e r p a rt ia l p r e s s u re c h a m b e r,w h e n o p e r a t i n g i n m e m b r a n e m o d e , i s s h o w n i n F i g . 1 0. R o u g h l y, a li n e a r r e l at i o nbe tween Jo2 and log Po2 i s seen , in agreement wi th the resu l t s ob ta ined f rom thep e r m e a t i o n e x p e r i m e n t s . I n c r e a s i n g t h e o x y g e n p a r t ia l p r e s s u r e i n t h e h i g h p a r t ia lp r e s s u re c h a m b e r m a i n l y c o n t r i b u t e s to t h e c o n v e r s i o n o f m e t h a n e t o C O a n d C O 2 .T h e r a te o f C 2 f o r m a t i o n r e m a i n s n e a r l y c o n s t a n t .

T h e i m p o r t a n c e o f a s u r f a c e - e x c h a n g e l i m i t ed o x y g e n f l u x ca n b e i ll u s tr a te d b ya n e x p e r i m e n t i n w h i c h a m e m b r a n e w a s f ir st e q u i l ib r a t e d at 11 5 3 K i n a 1 % 0 2 /H e a t m o s p h e r e a n d s u b s e q u e n t l y r e d u c e d in 1 5%CH4/He( 2 0 m l / m i n ( S T P ) ; 2 . 0


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208 , I.E . ten E l sho f e t aL /App l i ed Ca ta ly s is A : Ge ne ra l 130 1995) 195 -21 2

/ zmol s -~ CH4). T h e e x p e r i m e n t w a s p e r f o r m e d i n a s i n g l e - c h a m b e r r e a c t o r a sused in the co feed mo de exper im en t s , bu t wi th no add i t iona l ox yge n supp l i ed a f t e requi l ibra t ion. T he resul ts are sh ow n in Fig . 11. In i t ia l ly, a C2 se lect iv i ty of over30% and a h igh me thane con vers ion ra te a re r eached . In the course o f the exper i-men t , the p roduc t ion o f C2 stops . CO pro duc t ion i s seen a f t e r 4 h , accom pan ied byforma t ion o f hydrogen (no t show n in the f igure ) . Af te r 20 h the se lec t iv i t ie s toca rbon m ono xide and H2 reach 100% and 93%, respec t ive ly. At the same t ime themethan e co nvers ion ra te inc reases s t eep ly. These resu l ts ind ica te tha t r educ t ion o fthe pe rovsk i t e to e i the r bas ic ox ides o r me ta l l i c Co and /o r Fe l eads to an ac t iveca ta lys t fo r syngas p roduc t ion . T hus , in case o f a bu lk d i ffus ion con t ro l l ed oxyg enf lux the su r face would become reduced by methane , un t i l t he membrane th icknesshas dec rease d to such an ex ten t tha t the inc rease o f the oxy gen f lux , due to thed e c r e a s e d m e m b r a n e t h ic k n e s s, c o u n t e r b a la n c e s t h e c o n s u m p t i o n o f o x y g e n b ymethane .

The ca ta ly t i c pe rmea t ion exper imen t s tha t were desc r ibed in th i s paper werep e r f o r m e d o n s a m p l e s t h a t s h o w e d n o d e a c t i v a t i o n , e v e n a f t e r s e v e r a l w e e k s o fcon t inuous opera t ion under v a ry ing cond i t ions . Tak ing the re su l ts o f the r educ t ionexper imen t desc r ibed above in to accoun t , th i s conf i rms tha t a su r face exchangeprocess l imi t s the oxygen pe rmea t ion p rocess . Moreover, the r educ t ion exper imen tshow s tha t it may b e poss ib le to f ine -tune the ca ta ly t ic p roper t i e s o f the me mb raneby t r ea tment o f the me m brane su r face in a r educ ing a tm osphere [ 32] o r by ad jus t -me n t o f the amoun t o f oxy gen tha t i s f ed to the ca ta lys t su r face , e i the r by cofeed ingor by pe rmea t ion .

3 4 Stront ium segregat ion

E D X a n a l ys i s o f s a m p l e s a f te r t r e at m e n t i n an a i r / H e a n d / o r a i r / H e , C H 4 g r a d i e n tshow ed an enr i chmen t in S r at bo th h igh and low par t ia l p ressure s ides o f them e m b r a n e . X R D a n a ly s i s o f t h e s a m p l e s u r f a c e s in d i c a te d t h e p r e s e n c e o f S r C O 3and SrSO4. The o r ig in o f su lphur i s no t im me dia te ly c l ea r, bu t i t m ay b e no ted tha tthe ca ta ly t i c ac t iv i ty o fSrSO4 i s k n o w n tO be smal l [33] . To inves t iga te thesegrega t ion in mo re de ta il , two sam ples wer e t r ea ted fo r 48 h a t 1153 K in 0 .25 ba rCH 4 a t a to t al f lo w o f 2 0 .5 m l / m i n ( S T P ) a n d ra p i d ly c o o l e d d o w n t o r o o mtempera tu re . One sample was ob ta ined a f t e r opera t ion in the membrane mode , theo the r a f t e r opera t ion in the co feed mode under comparab le cond i t ions . F ig . 12s h o w s d e p t h p r o fi le s m a d e b y A u g e r a n a l y s is o n t h e m e m b r a n e s i de s th a t h ad b e e nexp osed to me thane . The p ro f i l e s o f Fe and C o a re no t show n in th i s f igure , bu tthe i r beha v iour i s s imi la r to tha t a s obse rved fo r La . Su r face enr i chmen t o f S r occursin bo th cases . The segrega t ion dep th i s abou t 15-30 nm fo r the sample ob ta inedf r o m t h e c o f e e d m o d e , w h i l e i t e x t e n d s o v e r 6 0 - 8 0 n m i n th a t f r o m t h e m e m b r a n emo de . Th i s sho ws tha t no t on ly segrega t ion occurs in bo th mo des o f opera t ion , bu ta l so tha t segrega t ion migh t be a ffec ted by the p resence o f an oxygen po ten t i a lg rad ien t ac ross the ma te r i a l , a s occurs dur ing opera t ion in the membrane mode .


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1200 T

15 ~ 46 ~

epth n m )

a )

75 0 15 30 45

e p t hnm)

b )

Fig. 12. Auger depth profiles of samples after 48 h treatment at 1153 K. PCH4=0.25 bar. ( a) Membrane mode,highPo.~ = 0.21 bar. (b ) Cofeed mode, Po2 = 0.01 bar at inlet.

T h e i n f l u e n c e o f t h e o x y g e n p a r t ia l p r e s s u r e g r a d i e n t o n s e g r e g a ti o n c a n p o s s i b l yb e d e s c r i b e d i n t e r m s o f k i n e t i c d e m i x i n g [ 3 4 ] , b u t t h e p r e c i s e m e c h a n i s m i sunc lea r.

A S E M p i c t u re o f a c r o s s - se c t i o n o f a s a m p l e e x p o s e d o v e r 2 0 0 h t o C H 4 ( H e )a t 1153 K a f te r va r ious ca ta ly t i c expe r imen ts in the me mb rane mo de i s shown inFig . 13 . As can be seen the perovsk i te s truc tu re i s p rese rved . Ho wev er, the p resen ceo f m o l e c u l a r o x y g e n i s r eq u i r ed . I n e x p e r i m e n t s w h e r e 0 2 w a s a b s en t , t h e rm a ld e c o m p o s i t i o n o f m e t h a n e o c c u r r e d e a s i ly. A t t h e t e m p e r a t u r e s u s e d h e r e , S r r e a ct sw i t h t h e d e p o s i t ed c a r b o n t o f o r m S rC O 3. T h i s r e a c t io n p r o b a b l y e n h a n c e s f u r t h e r

Fig. 13. SEM photograph of cross-sect ion of membrane after treatment at 1153 K u nder varying conditions.


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J .E . t en E l sho f e t a l. /Ap p l i e d Ca ta ly s i s A : Gene ra l 130 1995) 195 -21 2 211

t h e o r e ti c a l v a l u es . P e r m e a t i o n f l u x e s a r e t y p i c a l ly 1 m m o l m - 2 s - 1 a t 11 73 K w i t ha n a c t iv a t i o n e n e rg y i n th e r a n g e o f 1 3 0 - 1 4 0 k J / m o l . S i n c e th e o x y g e n f lu xr e m a i n e d c o n s t a n t w i t h d e c r e a s i n g m e m b r a n e t h i c k n e s s a n d i n c r e a s e d w i t h i n cr e a s -i n g s u r f a c e a re a a t th e H e - s i d e o f t h e m e m b r a n e , i t is c o n c l u d e d t h a t th e o x y g e nf lu x i s l im i t e d b y t h e r e le a s e o f o x y g e n a t t h e H e s i d e o f th e m e m b r a n e .

W h e n t h e m e m b r a n e w a s a p p l i e d a s a n o x y g e n s u p p l ie r an d m e t h a n e c o u p l i n gc a t al y s t, n o e n h a n c e m e n t o f t h e o x y g e n f lu x c o u l d b e o b s e r v e d i n c o m p a r i s o n w i t ht h e f l u x e s o b s e r v e d i n a i r / H e g r a d ie n t s. F o r m e t h a n e c o u p l i n g p u r p o s e s i t is i m p o r-tan t tha t the oxygen f lux i s l imi ted by sur face exchange k ine t ics , s ince a bu lkd i f f u s i o n c o n t r o l le d f l u x w o u l d l e a d to r e d u c t i o n o f t h e m e m b r a n e s u r fa c e , l o w e r i n gthe overa l l se lec t iv i ty towards C2 produc ts .

A t t e m p e r a t u r e s a b o v e 11 7 0 K , s u b s t an t ia l p r o d u c t i o n o f h y d r o g e n w a s s e e n ,w h i c h i s l ik e l y d u e t o t h e r m a l d e c o m p o s i t i o n o f m e t h a n e . T h e C 2 s el e c ti v i tydecreased s t rong ly above th i s t empera tu re . At 1153 K, C2 se lec t iv i t i e s inc reasedwi th inc reas ing metha ne part i al p ressure , up to 67% a t PCH4 0 .93 bar. Inc reas ingt h e o x y g e n p a r ti a l p re s s u r e in t h e o x y g e n s u p p l y i n g c h a m b e r w a s s h o w n t o c o n -t r ibu te main ly to the fo rmat ion of CO and CO2.

W h e n m o l e c u l a r o x y g e n w a s c o f e d w i t h m e t h a n e r a t h e r t h a n s u p p l i e d v i a t h em e m b r a n e , t h e C 2 s e l e c t i v i t i e s w e r e c o n s i d e r a b l y l o w e r ( 2 5 - 3 5 % ) , d u e t o ai n c r e a s e d ra t e o f C O 2 f o r m a t i o n a n d l o w e r r a t es o f e t h a n e a n d e t h e n e f o r m a t i o n .T h i s m a y i n d i c at e t h at t h e n a t u r e o f t h e c a t a ly s t is d i f f e r e n t i n t h e m e m b r a n e m o d ea n d c o f e e d m o d e o f o p e r at i o n , p o s s ib l y d u e t o t h e p r e s e n c e o f d i f f e r e n t t y p e s o fa c t i v e o x y g e n s p e c i e s a t t h e s u r f a c e . S e g r e g a t i o n o f S r w a s f o u n d b y E D X a n dA E S . T h e d e p t h o f s e g r e g a t io n i s a fa c t o r o f 2 - 3 l a rg er in t h e m e m b r a n e m o d e .

S u b s t a n ti a l i m p r o v e m e n t s o f t h e m e t h a n e c o n v e r s i o n a n d s e l e c t iv i t y m a y b eexpec ted f rom sur face modi f ica t ion , e .g . the app l ica t ion o f th in porous l ayers o fw e l l - k n o w n m e t h a n e c o u p l i n g c a t a l y s t s s u c h a s L i / M g O o r S r / L a 2 0 3 . T h e t o t a ls u p p l y o f o x y g e n t o t h e m e t h a n e s t r ea m m a y b e i n c r e a s e d b y e n l a rg e m e n t o f t h esur face a rea o f the mem bran e . For indus t r i a l app l ica tions , the use o f ce ram ic tubesins tead of p lanar mem bran es wi l l be ad van tageo us , s ince l a rge ac t ive sur face a reasp e r u n i t v o l u m e c a n b e a c h i e v e d i n s u c h g e o m e t r i es . H a z b u n [ 3 6 ] p r o p o s e d s e v e ra lm u l t i - tu b e c o n f i g u r at i o n s, b o t h w i t h c r o s s - fl o w s a n d c o u n t e r - f lo w s o f t h e m e t h a n eand a i r s t reams .

cknowledgements

T h e s u p p or t o f th e C o m m i s s i o n o f t h e E u r o p e a n C o m m u n i t ie s in t h e f r a m e w o r ko f t h e J o u l e p r o g r a m m e , s u b - p r o g r a m m e E n e rg y f r o m F o s si l R e s o u r c es , H y d r o -c a r b o n s, i s g r a t e f u ll y a c k n o w l e d g e d . T h e a u t h o rs w o u l d l i k e t o t h a n k G . M o l l e n -hors t and R. v an den Bl ink fo r the i r he lp in the des ign an d fabr ica t ion o f the ca ta ly t i creactor.


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