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Models of The Direct Catalytic Partial Oxidation of Light Alkanes
G. Centi and F. Trifiro’ (Editors),New Developments in Selective Oxidation 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
405
MODELS OF THE DIRECT CATALYTIC PARTIAL OXIDATION OF LIGHT ALKANES J . G . McCARTY, A. B . McEWEN, and M. A. QUINLAN S R I I n t e r n a t i o n a l , 333 Ravenswood Avenue, Menlo P a r k , C a l i f o r n i a , USA 94025
SUMMARY
A p p l i c a t i o n of homogeneous k i n e t i c models t o methane a c t i v a t i o n i n d i c a t e s t h a t t h e h i g h e r hydrocarbon y i e l d may be l i m i t e d by homogeneous o x i d a t i o n of methyl r a d i c a l i n t e r m e d i a t e s . I n t h i s paper, w e d i s c u s s t h e development of a model t h a t d q s c r i b e s t h e homogeneous and heterogeneous c h e m i s t r y i n v o l v e d i n t h e s e l e c t i v e o x i d a t i o n of methane and l i g h t a l k a n e s and t h e impact of t h i s c h e m i s t r y on a l k e n e and h i g h e r a l k a n e y i e l d s . We a l s o p r e s e n t e x p e r i m e n t a l r e s u l t s f o r methane a c t i v a t i o n and e t h a n e dehydrogenation u s i n g s t a b l e n o n - v o l a t i l e c a t a l y s t s composed of a l k a l i n e and r a r e e a r t h c a r b o n a t e s s u p p o r t e d by r e f r a c t o r y complex o x i d e s . INTRODUCTION
There i s ample e v i d e n c e t h a t homogeneous r e a c t i o n s substantially contribute t o the c a t a l y t i c oxidative dimerization
of methane and t h e c a t a l y t i c o x i d a t i v e dehydro-genation t o ethene.
of e t h a n e
The p r o d u c t d i s t r i b u t i o n h a s o f t e n been
as
b e i n g c o n s i s t e n t w i t h homogeneous f r e e r a d i c a l c h e m i s t r y , b u t t h e d e f i n i t i v e e x p e r i m e n t s a r e t h o s e of Lunsford,
e t a 1 . , 4 - 6 who used
m a t r i x i s o l a t e d e l e c t r o n paramagnetic resonance ( M I E P R ) measurements t o d e t e r m i n e t h e d i s t r i b u t i o n of methyl r a d i c a l s downstream of a Li/MgO c a t a l y s t bed.
T h e MIEPR r e s u l t s of
Campbell and Lunsford6 show t h a t product e t h a n e forms downstream of t h e c a t a l y s t bed by homoaeneous methyl r a d i c a l recombination and
v e r i f y , w i t h i n a f a c t o r of two, t h e b i m o l e c u l a r recombination r a t e constant.
I s o t o p i c exchange experiments
7-10
a l s o s u p p o r t t h e view
t h a t t h e methyl r a d i c a l i s t h e primary i n t e r m e d i a t e i n t h e p r o d u c t i o n of e t h a n e .
Various r e c e n t l a b o r a t o r y r e s u l t s i n d i c a t e
t h a t d i r e c t c a t a l y t i c c o n v e r s i o n of premixed oxygen and methane i n t o h i g h e r hydrocarbons approaches a s i n g l e - p a s s
l i m i t of about
2 5 % y i e l d (on a C atom b a s i s ) r e g a r d l e s s of c a t a l y s t and r e a c t i o n c o n d i t i o n s ( F i g u r e 1) microreactors
11-14
.
F i n a l l y , o b s e r v a t i o n s t h a t empty
can s e l e c t i v e l y produce e t h a n e and e t h y l e n e
a f f i r m s t h e s i g n i f i c a n t r o l e of homogeneous k i n e t i c s i n a l l a s p e c t s of t h e r e a c t i o n .
These f i n d i n g s i n d i c a t e t h a t homogeneous
o x i d a t i o n k i n e t i c s p l a y an important r o l e i n t h e s e l e c t i v i t y of alkane p a r t i a l oxidation reactions
406 100
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This work Aika al al. (Tokyo). J. Cham SOC. Cham. Commun. 1986. 1210. Jones at al. (ARCO). Energy and Fuels 1,12 (1987). A IIo 01 al. (Texas A 6 M), J. Am. Cham. Soc. ~ . 5 0 S Z(1985). Olsuka el PI. (Tokyo), J. Cham. *.. Chom. Cornmun. 1986.586. V Hinsen ei al. (Berlin) 8th Int. Cang. Catal.. 1984. X Otsuka 01 al. (Tokyo), J. Caial. 1pe 353 (1986). Lunsford el al. (Tokyo). Texas A 6 M) (lo k publish& 1987). Lin 01al. (Texas A 6 M). J. Phyr. Cham. 9Q.534 (1986).
0 0
* +
0
KimMe and Kolls (Phillips Per.), Energy Prcgress 6.226 (1986). Lin el PI. (Texas A 6 M).J. Am. Cham. SOC. 1p9.4808 (1987). n Jones and Sofrank (ARCO), J. CaIal. 31 1 (1987).
u
Q
D 0 0 d
m,
lmai and Taaawa ITokvo). J. Cham. Soc. Cham. Commun. 1966.52. Deboy and Hicks Grace), J. Catal. U3,. 51 7 (1988). Gaffnay a1 ill. (ARCO), J. Catal. 422 (1988) Zhang at al. (Texas A 6 M). J. Catal. 366 (1988).
+.d.
m.
Fig.1. Laboratory fixed-bed catalytic oxidalive coupling of methane with premixed oxygen
RA-2614-9C
407
I n t h i s paper, we d e s c r i b e a model of c a t a l y t i c l i g h t alkane p a r t i a l o x i d a t i o n used t o e v a l u a t e t h e r e l a t i v e importance of i n d i v i d u a l homogeneous and heterogeneous r e a c t i o n s over a wide range of r e a c t i o n c o n d i t i o n s . The model i n c o r p o r a t e s key heterogeneous r e a c t i o n s t e p s i n t o a network of known g a s phase alkane f r e e r a d i c a l o x i d a t i o n r e a c t i o n s .
We a l s o r e p o r t t h e
a c t i v i t y and s e l e c t i v i t y of s t a b l e n o n - v o l a t i l e strontium-based complex oxide c a t a l y s t s for t h e d i r e c t o x i d a t i v e conversion of methane i n t o h i g h e r hydrocarbons and t h e d i r e c t o x i d a t i v e dehydrogenation of ethane t o e t h e n e . Comparison of t h e experimental r e s u l t s and model c a l c u l a t i o n s shows t h a t t h e c a t a l y s t s s e l e c t i v e l y o x i d i z e i n t e r m e d i a t e s such a s methanol and carbon monoxide a t r a t e s h i g h e r t h a n expected f o r heterogeneous hydrogen a b s t r a c t i o n r e a c t i o n s . METHODS The premise of our model i s t h a t most of t h e r e a c t i o n chemistry i n c l u d i n g by product formation o c c u r s b y homogeneous r e a c t i o n s i n t h e c a t a l y s t pores, c a t a l y s t bed void space, and p o s t - r e a c t o r volume. Our complete modells c o n s i s t s of 1 4 4 r e a c t i o n s , 134 r e v e r s i b l e homogeneous r e a c t i o n s and 1 0 r e a c t i o n s which i n v o l v e c a t a l y s t s u r f a c e s i t e s .
Most of t h e gas phase
r e a c t i o n parameters were o b t a i n e d from t h e review compilations of Frenklach16, Warnatz17, or Tsang
18
.
The primary source of e t h a n e
i n o u r mechanism of methane co-oxidative coupling i s from t h e gas phase recombination of methyl r a d i c a l s , .CH3 + .CH3 ====> C2H6
(1)
while e t h e n e i s produced from e t h a n e by thermal ( 2 ) and o x i d a t i v e ( 3 ) dehydrogenation. + M ====> C2Hq
,C2H5
C2H4
+
*H
+
*OZH
O2 ====> A fundamental q u e s t i o n is t o what degree deep o x i d a t i o n r e s u l t s from g a s phase or s u r f a c e chemistry.19 The presence of '2 H 5
+
premixed oxygen, although necessary t o provide a s i n k f o r hydrogen and t h e thermodynamic d r i v i n g f o r c e f o r t h e coupling p r o c e s s , u n f o r t u n a t e l y l e a d s t o undesired oxygenated by-products, C02,
e . g . CO,
and formaldehyde. The f i r s t s t e p i n t h e c a t a l y t i c c y c l e i n v o l v e s t h e a c t i v a t i o n
of methane by a s u r f a c e oxygen atom.
The heterogeneous H
408 a b s t r a c t i o n s t e p can be g e n e r a l i z e d t o i n c l u d e s c i s s i o n of any C-H bond by an Eley-Rideal
r e a c t i o n with s u r f a c e oxygen (0 ) t o form a
s
gas phase a l k y l r a d i c a l and a hydroxyl s u r f a c e s i t e ( H O S ) , RH
+
OS
>
====
*R
where RH = ( 4 a ) C H 4 ;
CH20.
+
HOs
(4b) C2H6;
( 4 c ) C2H4;
(4n) ( 4 d ) CH30H; and ( 4 e )
The a c t i v a t i o n e n e r g i e s used f o r t h e r e a c t i o n of 0
s
with
o t h e r C-H bonds ( e . g . C 2 H 6 ) r e f l e c t t h e i r bond s t r e n g t h s r e l a t i v e t o methane.
The r a t e d e t e r m i n i n g s t e p i n t h e o x i d a t i v e c o u p l i n g
of methane over Li/MgO was shown by Cant e t a1.''
t o be methane C-H
bond s c i s s i o n , CH4 + Os ====> *CH3 + (4a) based on a l a r g e , p o s i t i v e ( 1 . 5 ) deuterium i s o t o p e e f f e c t .
Amorebieta and Colussi21 showed a t low p r e s s u r e
to
atm)
t h a t methane c o n v e r s i o n over Li/MgO i s h a l f o r d e r i n oxygen and f i r s t o r d e r i n methane.
T h i s r e s u l t s u g g e s t s t h a t methane r e a c t s
w i t h atomic s u r f a c e oxygen.
Labinger e t a l . , 2 2 ' 2 3 r e p o r t t h a t w i t h
t h e Na/Mg/Mn c a t a l y s t , CZH6 c o n v e r t s 1 . 9 t i m e s f a s t e r t h a n CH4
.
We have a d j u s t e d t h e r a t e c o n s t a n t s f o r C 2 H 6 t o g i v e t h i s r a t i o (4b/4a = 1 . 9 ) a t 1 0 0 0 K .
Rate c o n s t a n t s f o r t h e o t h e r r e a c t a n t s
(H-C2H3, H-CH OH, and H-CHO) were determined by f i x i n g t h e i r 2 frequency f a c t o r s t o t h a t of e t h a n e and a d j u s t i n g t h e i r a c t i v a t i o n
e n e r g i e s r e l a t i v e t o methane i n p r o p o r t i o n t o t h e d i f f e r e n c e s i n reaction enthalpy. The r e a c t i o n of t h e methyl r a d i c a l with s u r f a c e oxygen can s i g n i f i c a n t l y a l t e r t h e s e l e c t i v i t y p r e d i c t e d by t h e model. Labinger and O t t 2 '
a n a l y z e d t h e i r r e s u l t s and concluded t h a t t h e
o x i d a t i o n of methyl r a d i c a l s w i t h t h e Na/Mg/Mn c a t a l y s t
(without
f e e d g a s oxygen) was 2 7 0 0 t i m e s t h e r a t e of methane a c t i v a t i o n . The r a t i o of heterogeneous o x i d a t i o n t o homogeneous c o u p l i n g of methyl r a d i c a l s i s t h e e s s e n t i a l f a c t o r governing t h e s e l e c t i v i t y a t low c o n v e r s i o n .
T h e r e f o r e , t h e second key premise of o u r model
i s t h a t a l k y l r a d i c a l s i r r e v e r s i b l y react i n a n o n - a c t i v a t e d s t e p with s u r f a c e oxygen t o form adsorbed complexes t h a t a r e p r e c u r s o r s t o oxygenates. .R + Os =====>
ROs (5) The heterogeneous r a t e p a r a m e t e r s i n v o l v i n g s u r f a c e s i t e s
were optimized t o f i t e x p e r i m e n t a l r e s u l t s f o r Na/CaO a t 1 0 7 3 K. The v a r i a b l e , $s,
represents the i n i t i a l active s i t e density
( e s s e n t i a l l y t h e sum of 0
S
and U ) . S
For a s p e c i f i c s i t e d e n s i t y ,
t h e a c t i v a t i o n energy for C-H bond a c t i v a t i o n was t h e n a d j u s t e d t o
409
o b t a i n e x p e r i m e n t a l l y observed conversion r a t e s . Reaction r a t e and product r a t i o s were i n v e s t i g a t e d a s a f u n c t i o n of t h e a c t i v e s i t e d e n s i t y a t 1073 K with a methane t o oxygen r a t i o of 1 0 ( F i g . 2 When t h e Os c o n c e n t r a t i o n and i s high ( i . e . Os = 10- t o methane conversion i s high and o x i d a t i o n t o CO i s t h e dominant p r o c e s s e s . A t lower s u r f a c e s i t e c o n c e n t r a t i o n s (4, < the conversion r a t e i s lower and t h e C2 s e l e c t i v i t y i s h i g h e r . The 2).
h i g h e s t C2 y i e l d was found t o b e @ s = lo-’. These optimized independent parameters t h a t a f f e c t t h e product s e l e c t i v i t y were used for a l l subsequent c a l c u l a t i o n s
(OS
=
lo-’ and an a c t i v a t i o n
energy (E,) for t h e r a t e determining a b s t r a c t i o n s t e p ( r e a c t i o n 4a) of 6 3 . 6 k J mol-l) .
W e used t h e Chemkin k i n e t i c modeling program t o s o l v e t h e s e t of non-linear d i f f e r e n t i a l e q u a t i o n s . I n l i n k i n g t h e heterogeneous r e a c t i o n s t o t h e homogeneous r e a c t i o n network, we used a c o n s t a n t s u r f a c e t o volume r a t i o and c a l c u l a t e d t h e s u r f a c e s i t e concentration.
The f r a c t i o n of a c t i v e c e n t e r s on t h e s u r f a c e
of s u r f a c e oxide c a t i o n s . r e a c t i o n i s not s u r f a c e t r a n s p o r t l i m i t e d i n o u r model calculations. ( @ s ) was normalized t o t h e amount
The
REACTIVE CENTER CONCENTRATION (ML)
Fig. 2. Effect of reactive oxygen center surface concentralion on melhane conversion and higher hydrocarbon selectivity at 1073 K vrilh 0.3 atrn methane and 0.03 atm oxygen.
410
RESULTS Once t h e heterogeneous parameters were e s t a b l i s h e d , t h e temporal c o n c e n t r a t i o n s of co-oxidation products were determined f o r various reaction conditions
.
The r e s u l t s o b t a i n e d a t 1 0 7 3 K
with CH4 and O2 c o n c e n t r a t i o n s of 0 . 3 and 0 . 0 3 atm., r e s p e c t i v e l y ( F i g 3 ) , show t h a t ethane i s t h e major carbon c o n t a i n i n g product, followed by CO and e t h y l e n e . Other s i g n i f i c a n t p r o d u c t s a r e methanol and formaldehyde, which d e c r e a s e i n r e l a t i v e importance w i t h increased reaction t i m e .
The r e l a t i v e importance of s e v e r a l
gas phase r e a c t i o n s a t v a r i e s with i n i t i a l p a r t i a l p r e s s u r e s and r e a c t i o n time (Table 1 ) . A t low p r e s s u r e t h e main source of methyl r a d i c a l s i s t h e heterogeneous a c t i v a t i o n s t e p , while a t high p r e s s u r e two a d d i t i o n a l gas phase s o u r c e s of methyl r a d i c a l s a r e r e a c t i o n s i n v o l v i n g t h e hydrogen and hydroxyl r a d i c a l s . Higher hydrocarbon s e l e c t i v i t y i n t h e methane coupling p r o c e s s i s very dependent on oxygen p a r t i a l p r e s s u r e . T h e e f f e c t oxygen p a r t i a l p r e s s u r e on methane conversion and product s e l e c t i v i t y was s y s t e m a t i c a l l y examined ( F i g . 4 ) f o r f i x e d methane
9
.1 L
CONTACTTIME($1
Fig. 3. Calculated product distribution vs. contact time for methane coupling at 1073 K with 0.3 atrn methane and 0.03 atm oxygen.
-
TABLE 1
Reaction Rates for T 1.Oe-5
-
1073 K, PCH4
CH3+02-CH302 CH302=CH3+02 CH4+MEOS-CH3+MEOHS 2.05E-06 MEOHS+MEOHS-H20+MES+MEOS CH3+CH3-C2H6 MES+MES+OZ-MEOStMEOS CH4+OH-CH3+H20 CH3+02-CH20+OH CH3+H02-CH30+OH 9.6E-07 CH30+02-CH20+H02 CH30+CH4-CH3+CH30H CH4+H-CH3+H2 CH3+MEOS-CH3MOS CH3MOStMEOS-CHZO+MEOHS+MES HCO+02-H02+CO CH2O+MEOS-HCWMEOHS CH4+H02-CH3+H202 CH3+H202-H02+CH4 C2H5-C2H4+H CHZO+CH3=HCO+CH4 HCO+M=H+CO+M CZH6+MEOS-C2H5+MEOHS CH302+CH3-CH30+CH30 CH3O+M-CHZO+H+M C2H6+CH3-CZHS+CH4 C2H5+02-C2H4+H02 MEOS+MEOS-MES+MES+02
-
411
- 0.3 atrn, PO2 - 0.03, Phi -
Eak
Bate
2.175E-05 CH4+H-CH3+H2 2.155E-05 CH4+MEOS-CH3+MEOHS 1.964E-05MEOHS+MEOHS-H20+MES+MEOS
3,~ O E - O ~ 3.04E-06
1.04BE-05 C2H5-C2H4+H 9.74E-06 CH3+HZ-CH4+H 5.673-06 CH3+CH3-C2H6 2.84E-06 HCO+M-H+CO+M 1.47E-06 CZH6+H-C2H5+H2 1.46E-06MES+MES+02-MEOS+MEOS
2.01E-06 1.98E-06 1.94E-06 1.41E-06 1.13E-06
B.4E-07 7.6E-07 7.3E-07 6.73-07 6.7E-07 5.OE-0 7 3.9E-07 3.6E-07 3.2E-07 3. OE-07 2.9E-07 2.6E-07 2.2E-07 2.OE-07 1. CIE-O~ 1.4E-07 1.2E-07 1.1E-07
5,BE-07 5.7E-07 5.5E-07 5.3E-07 4.4E-07 4.2E-07 4.OE-07 3.4E-07 3.1E-07 2.9E-07 2.7E-07 2.4E-07 1.9E-07 1.9E-07 1.9E-07 1.8E-07 1.7E-07 1.5E-07
Fig. 4. Methane coupling conversion and higher hydrocarbon selectivity vs. oxygen partial pressure at 1073 K with 0.3 atm methane.
412
p a r t i a l p r e s s u r e ( 0 . 3 atm) and f i x e d a c t i v e s i t e c o n c e n t r a t i o n ( $ s = The methane conversion i n c r e a s e d , t h e C2+ s e l e c t i v i t y decreased, w h i l e t h e e t h y l e n e t o ethane r a t i o i n c r e a s e d t o a c o n s t a n t l e v e l w i t h i n c r e a s i n g oxygen p a r t i a l p r e s s u r e . S e v e r a l homogeneous methyl r a d i c a l o x i d a t i o n pathways a r e i m p o r t a n t . Under high temperature and low p r e s s u r e c o n d i t i o n s t h e r e a c t i o n of methyl r a d i c a l s w i t h hydrogen peroxy r a d i c a l s i s t h e prominent o x i d a t i o n pathway, *OCH3 + *OH '(6) '02H ====' w h i l e a t high p r e s s u r e and low temperature t h e r e a c t i o n of methyl
+
*CH3
r a d i c a l s w i t h methyl peroxy r a d i c a l s i s prominent. The major s o u r c e s of -O2H a r e hydrogen a b s t r a c t i o n r e a c t i o n s of u n s t a b l e r a d i c a l s such a s -CHO and *C2H5 with diatomic oxygen. Conversion w i t h Although a l k a l i promoted c a t a l y s t s have g r e a t e r s e l e c t i v i t y t h a n unpromoted a l k a l i n e e a r t h and r a r e e a r t h oxides, t h e r e i s some concern about s t a b i l i t y of t h e s e c a t a l y s t s given t h e high vapor p r e s s u r e s of a l k a l i under r e a c t i o n c o n d i t i o n s . The v o l a t i l i t y of a l k a l i under t y p i c a l a l k a n e a c t i v a t i o n c o n d i t i o n s i s due t o t h e high vapor p r e s s u r e of t h e a l k a l i hydroxide molecules i n t h e presence of steam and oxygen, although t h e s o l i d phase i s l i k e l y t o be a l k a l i c a r b o n a t e . Thermochemically s t a b l e c a r b o n a t e s a l t s w i t h t h e low v o l a t i l i t y i n steam are S r C 0 3 and BaC03. Perovskite-supported a l k a l i n e e a r t h c a r b o n a t e s , Sr/SrZrOg and Ba/SrZr03 are s e l e c t i v e and s t a b l e methane a c t i v a t i o n c a t a l y s t s ( F i g u r e 51, comparable or s u p e r i o r i n t h i s r e s p e c t t o Li-promoted MgO and Na-promoted CaO.
Good ethene s e l e c t i v i t y was a l s o shown
f o r co-oxidative dehydrogenation of e t h a n e (Figure 6 ) . Unlike t h e alkali-promoted a l k a l i n e e a r t h c a t a l y s t s , SrZrO o p e r a t e d 20 hours 3 a t 1 1 7 3 K with no evidence of evaporation o r c o r r o s i v e a t t a c k on our q u a r t z r e a c t o r s . These c a t a l y s t s appear t o achieve hydrocarbon s e l e c t i v i t y approaching t h e o r e t i c a l y i e l d s based on l a b o r a t o r y r e a c t i o n c o n d i t i o n s and p r e d i c t e d homogeneous o x i d a t i o n rates. DISCUSSION As a r e s u l t of o u r a n a l y s i s with t h e heterogeneoushomogeneous model, we conclude t h a t methane co-ox coupling p r o c e s s e s with premixed oxygen and methane may be l i m i t e d t o a
413
Fig. 5. Methane conversion and Cp selectivity for SrZQ versus reaction temperature. Conditions: 0.29atm CH4, 0.029-atm Q,3.3 mL s-1 (NTP) flow at 1-atm pressure.
60
-
Elhene Selectlvlty
/ 40-
20
-
0,
L
800
900
TEMPERATURE (K)
Fig. 6. Ethane partial oxidation on SrZrO3 with 0.03 atm oxygen.
1000
414
maximum h i g h e r hydrocarbon y i e l d of about 30 mol% (carbon b a s i s ) and a maximum ethene/ethane r a t i o of about 2 . C a t a l y s t s t h a t can f a v o r a b l y i n f l u e n c e t h e s e l e c t i v i t y by combining high t u r n o v e r r a t e s f o r a l k y l r a d i c a l g e n e r a t i o n p e r r e a c t i v e s i t e with a very low s u r f a c e c o n c e n t r a t i o n of r e a c t i v e c e n t e r s ( o p t i m a l l y one p a r t p e r l o 5 s i t e s ) . Formation of a s u r f a c e o r bulk b a s e metal carbonate l a y e r s may be one way of reducing t h e d e n s i t y of r e a c t i v e oxygen c e n t e r s t o l e v e l s t h a t avoid t h e r a p i d o x i d a t i o n of a l k y l r a d i c a l s a t t h e c a t a l y s t s u r f a c e , and indeed o u r temperature programmed d e s o r p t i o n experiments show t h a t t h e s u r f a c e s of t h o s e s o l i d - s t a t e b a s i c oxide c a t a l y s t s h i g h l y s e l e c t i v e f o r l i g h t alkane dehydrogenation and methane coupling a r e c o n s i s t e n t l y covered with a t l e a s t one monolayer of c a r b o n a t e . Homogeneous r e a c t i o n s can f u l l y account f o r t h e p r o d u c t i o n of hydrocarbon p r o d u c t s and t h e s e l e c t i v i t y between coupling p r o d u c t s and COX, b u t p r e d i c t h i g h e r y i e l d s of CH30H, CH20, and H 2 , and g r e a t e r CO/C02 product r a t i o s t h a n observed e x p e r i m e n t a l l y . Heterogeneous o x i d a t i o n r e a c t i o n s a r e e v i d e n t l y r e s p o n s i b l e f o r s i g n i f i c a n t o x i d a t i o n of oxygenate products, t h e s u b s t a n t i a l conversion of product hydrocarbons o r r a d i c a l s , and p o s s i b l y d i r e c t o x i d a t i o n of e t h y l e n e t o COP and H 2 0 i n co-oxidation p r o c e s s e s by c a t a l y s t s with poor i n h e r e n t s e l e c t i v i t y . ACKNOWLEDGEMENT
The a u t h o r s g r a t e f u l l y acknowledge t h e support of t h e Methane Reaction Science Program d i r e c t e d by Dr. Daniel A . S c a r p i e l l o f o r t h e Gas Research I n s t i t u t e and a s s o c i a t e d I n d u s t r i a l A f f i l i a t e cosponsors. REFERENCES 1. I t o , T., Wang, J . - X . ,
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L i n t C.-H, and Lunsford, J. H . , J . Am. Chem. SOC. (1985) 107, 5062. Sofranko, J . A . , Leonard, J . L.., and Jones, C. A . , J . C a t a l . (1987) 103, 302. Kimble, J . B.,and Kolts, J. H . , Energy P r o g r e s s (1986) 6 ( 4 ) , 226. D r i s c o l l , D . J . , M a r t i r , W . , Wang, J . - X . , and Lunsford, J . H . , J . Am. Chem. SOC. (1985) 107, 58. Campbell, K . D . , Morales, E., and Lunsford, J . H . , J . Amer. Chem. SOC. (1987) 109, 7900. Campbell, K.,and Lunsford, J . H . , J. Phys. Chem. (1988) 92, 5792. Nelson, P . F., Lukey, C . A . , and Cant, N.W., J. Phys. Chem. (1988) 92, 6176.
415 8. 9. 10.
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Chem. SOC., Chem. Commun. ( 1 9 8 8 ) 7 9 7 . 11. v a n K a s t e r e n , W . M . H . , Geerts, W . M . H . , a n d v a n der Wiele, K . , Proceedinas of t h e -, 2, p . 930, P h i l l i p s , M . J . , a n d T e r n a n , M . , e d s . , T h e Chemical I n s t i t u t e of C a n a d a ( 1 9 8 8 ) . 1 2 . Lo, M . - Y . , Agarwal, S. K., a n d M a r c e l i n , G . , J . C a t a l . ( 1 9 8 8 ) 112, 1 6 8 . 13. L a n e , G. S . , a n d W o l f , E . E . , J . C a t a l . ( 1 9 8 8 ) 113, 1 4 4 . 1 4 . A s a m i , K., O m a t a , K . , F u j i m o t o , K . , a n d Tominaga, H . , E n e r g y a n d F u e l s (1988) 2, 574. 1 5 . McEwen, A . , B . , Q u i n l a n , M. A . , a n d McCarty, J . G . , ( t o be published, 1989). 1 6 . F r e n k l a c h , M. , ( p r i v a t e c o m m u n i c a t i o n ) 17. Warnatz, J., i n , G a r d i n e r , W. C . , J r . , e d . , Chpt. 5, p . 1 9 7 , S p r i n g e r - V e r l a g , (1984) 1 8 . T s a n g , W., J . P h y s . a n d Chem. R e f . Data ( 1 9 8 6 ) 15, 1 0 8 7 . 1 9 . D r i s c o l l , D . J . , Campbell, K. D . , a n d L u n s f o r d , J . H . , Adv. C a t a l . ( 1 9 8 7 ) 35, 1 3 9 . Lukey, C . A . , N e l s o n , P . F . , a n d T y l e r , R . J . , J . 20. Cant, N.W., C h e m . SOC. C h e m . Commun. ( 1 9 8 8 ) 7 6 6 . 2 1 . Amorebieta, V. T . , a n d C o l u s s i , A . J . , J . P h y s . C h e m . ( 1 9 8 8 ) , 92, 4 5 7 6 . 2 2 . L a b i n g e r , J. A . , a n d O t t , K. C . , J . P h y s . Chem. ( 1 9 8 7 ) 91, 2682, 2 3 . L a b i n g e r , J. A . , Mehta, S., O t t , K . C . , R o c k s t a d , H . K . , a n d Zoumalan, S., i n W s i s 1 9 8 7 , Ward, J . W . , e d . , p . 513, E l s e v i e r (1988).
:
.
.
405
MODELS OF THE DIRECT CATALYTIC PARTIAL OXIDATION OF LIGHT ALKANES J . G . McCARTY, A. B . McEWEN, and M. A. QUINLAN S R I I n t e r n a t i o n a l , 333 Ravenswood Avenue, Menlo P a r k , C a l i f o r n i a , USA 94025
SUMMARY
A p p l i c a t i o n of homogeneous k i n e t i c models t o methane a c t i v a t i o n i n d i c a t e s t h a t t h e h i g h e r hydrocarbon y i e l d may be l i m i t e d by homogeneous o x i d a t i o n of methyl r a d i c a l i n t e r m e d i a t e s . I n t h i s paper, w e d i s c u s s t h e development of a model t h a t d q s c r i b e s t h e homogeneous and heterogeneous c h e m i s t r y i n v o l v e d i n t h e s e l e c t i v e o x i d a t i o n of methane and l i g h t a l k a n e s and t h e impact of t h i s c h e m i s t r y on a l k e n e and h i g h e r a l k a n e y i e l d s . We a l s o p r e s e n t e x p e r i m e n t a l r e s u l t s f o r methane a c t i v a t i o n and e t h a n e dehydrogenation u s i n g s t a b l e n o n - v o l a t i l e c a t a l y s t s composed of a l k a l i n e and r a r e e a r t h c a r b o n a t e s s u p p o r t e d by r e f r a c t o r y complex o x i d e s . INTRODUCTION
There i s ample e v i d e n c e t h a t homogeneous r e a c t i o n s substantially contribute t o the c a t a l y t i c oxidative dimerization
of methane and t h e c a t a l y t i c o x i d a t i v e dehydro-genation t o ethene.
of e t h a n e
The p r o d u c t d i s t r i b u t i o n h a s o f t e n been
as
b e i n g c o n s i s t e n t w i t h homogeneous f r e e r a d i c a l c h e m i s t r y , b u t t h e d e f i n i t i v e e x p e r i m e n t s a r e t h o s e of Lunsford,
e t a 1 . , 4 - 6 who used
m a t r i x i s o l a t e d e l e c t r o n paramagnetic resonance ( M I E P R ) measurements t o d e t e r m i n e t h e d i s t r i b u t i o n of methyl r a d i c a l s downstream of a Li/MgO c a t a l y s t bed.
T h e MIEPR r e s u l t s of
Campbell and Lunsford6 show t h a t product e t h a n e forms downstream of t h e c a t a l y s t bed by homoaeneous methyl r a d i c a l recombination and
v e r i f y , w i t h i n a f a c t o r of two, t h e b i m o l e c u l a r recombination r a t e constant.
I s o t o p i c exchange experiments
7-10
a l s o s u p p o r t t h e view
t h a t t h e methyl r a d i c a l i s t h e primary i n t e r m e d i a t e i n t h e p r o d u c t i o n of e t h a n e .
Various r e c e n t l a b o r a t o r y r e s u l t s i n d i c a t e
t h a t d i r e c t c a t a l y t i c c o n v e r s i o n of premixed oxygen and methane i n t o h i g h e r hydrocarbons approaches a s i n g l e - p a s s
l i m i t of about
2 5 % y i e l d (on a C atom b a s i s ) r e g a r d l e s s of c a t a l y s t and r e a c t i o n c o n d i t i o n s ( F i g u r e 1) microreactors
11-14
.
F i n a l l y , o b s e r v a t i o n s t h a t empty
can s e l e c t i v e l y produce e t h a n e and e t h y l e n e
a f f i r m s t h e s i g n i f i c a n t r o l e of homogeneous k i n e t i c s i n a l l a s p e c t s of t h e r e a c t i o n .
These f i n d i n g s i n d i c a t e t h a t homogeneous
o x i d a t i o n k i n e t i c s p l a y an important r o l e i n t h e s e l e c t i v i t y of alkane p a r t i a l oxidation reactions
406 100
80
$
f
60
? W
rn
+
u" s
40
,a
20
osrco,
0
OCaO
0 MgO
CaO
0 CaO I
0
1
I
20
0
I
I
I
I
60
40
I 80
I
I 100
Ye CHI CONVERSION
rn
This work Aika al al. (Tokyo). J. Cham SOC. Cham. Commun. 1986. 1210. Jones at al. (ARCO). Energy and Fuels 1,12 (1987). A IIo 01 al. (Texas A 6 M), J. Am. Cham. Soc. ~ . 5 0 S Z(1985). Olsuka el PI. (Tokyo), J. Cham. *.. Chom. Cornmun. 1986.586. V Hinsen ei al. (Berlin) 8th Int. Cang. Catal.. 1984. X Otsuka 01 al. (Tokyo), J. Caial. 1pe 353 (1986). Lunsford el al. (Tokyo). Texas A 6 M) (lo k publish& 1987). Lin 01al. (Texas A 6 M). J. Phyr. Cham. 9Q.534 (1986).
0 0
* +
0
KimMe and Kolls (Phillips Per.), Energy Prcgress 6.226 (1986). Lin el PI. (Texas A 6 M).J. Am. Cham. SOC. 1p9.4808 (1987). n Jones and Sofrank (ARCO), J. CaIal. 31 1 (1987).
u
Q
D 0 0 d
m,
lmai and Taaawa ITokvo). J. Cham. Soc. Cham. Commun. 1966.52. Deboy and Hicks Grace), J. Catal. U3,. 51 7 (1988). Gaffnay a1 ill. (ARCO), J. Catal. 422 (1988) Zhang at al. (Texas A 6 M). J. Catal. 366 (1988).
+.d.
m.
Fig.1. Laboratory fixed-bed catalytic oxidalive coupling of methane with premixed oxygen
RA-2614-9C
407
I n t h i s paper, we d e s c r i b e a model of c a t a l y t i c l i g h t alkane p a r t i a l o x i d a t i o n used t o e v a l u a t e t h e r e l a t i v e importance of i n d i v i d u a l homogeneous and heterogeneous r e a c t i o n s over a wide range of r e a c t i o n c o n d i t i o n s . The model i n c o r p o r a t e s key heterogeneous r e a c t i o n s t e p s i n t o a network of known g a s phase alkane f r e e r a d i c a l o x i d a t i o n r e a c t i o n s .
We a l s o r e p o r t t h e
a c t i v i t y and s e l e c t i v i t y of s t a b l e n o n - v o l a t i l e strontium-based complex oxide c a t a l y s t s for t h e d i r e c t o x i d a t i v e conversion of methane i n t o h i g h e r hydrocarbons and t h e d i r e c t o x i d a t i v e dehydrogenation of ethane t o e t h e n e . Comparison of t h e experimental r e s u l t s and model c a l c u l a t i o n s shows t h a t t h e c a t a l y s t s s e l e c t i v e l y o x i d i z e i n t e r m e d i a t e s such a s methanol and carbon monoxide a t r a t e s h i g h e r t h a n expected f o r heterogeneous hydrogen a b s t r a c t i o n r e a c t i o n s . METHODS The premise of our model i s t h a t most of t h e r e a c t i o n chemistry i n c l u d i n g by product formation o c c u r s b y homogeneous r e a c t i o n s i n t h e c a t a l y s t pores, c a t a l y s t bed void space, and p o s t - r e a c t o r volume. Our complete modells c o n s i s t s of 1 4 4 r e a c t i o n s , 134 r e v e r s i b l e homogeneous r e a c t i o n s and 1 0 r e a c t i o n s which i n v o l v e c a t a l y s t s u r f a c e s i t e s .
Most of t h e gas phase
r e a c t i o n parameters were o b t a i n e d from t h e review compilations of Frenklach16, Warnatz17, or Tsang
18
.
The primary source of e t h a n e
i n o u r mechanism of methane co-oxidative coupling i s from t h e gas phase recombination of methyl r a d i c a l s , .CH3 + .CH3 ====> C2H6
(1)
while e t h e n e i s produced from e t h a n e by thermal ( 2 ) and o x i d a t i v e ( 3 ) dehydrogenation. + M ====> C2Hq
,C2H5
C2H4
+
*H
+
*OZH
O2 ====> A fundamental q u e s t i o n is t o what degree deep o x i d a t i o n r e s u l t s from g a s phase or s u r f a c e chemistry.19 The presence of '2 H 5
+
premixed oxygen, although necessary t o provide a s i n k f o r hydrogen and t h e thermodynamic d r i v i n g f o r c e f o r t h e coupling p r o c e s s , u n f o r t u n a t e l y l e a d s t o undesired oxygenated by-products, C02,
e . g . CO,
and formaldehyde. The f i r s t s t e p i n t h e c a t a l y t i c c y c l e i n v o l v e s t h e a c t i v a t i o n
of methane by a s u r f a c e oxygen atom.
The heterogeneous H
408 a b s t r a c t i o n s t e p can be g e n e r a l i z e d t o i n c l u d e s c i s s i o n of any C-H bond by an Eley-Rideal
r e a c t i o n with s u r f a c e oxygen (0 ) t o form a
s
gas phase a l k y l r a d i c a l and a hydroxyl s u r f a c e s i t e ( H O S ) , RH
+
OS
>
====
*R
where RH = ( 4 a ) C H 4 ;
CH20.
+
HOs
(4b) C2H6;
( 4 c ) C2H4;
(4n) ( 4 d ) CH30H; and ( 4 e )
The a c t i v a t i o n e n e r g i e s used f o r t h e r e a c t i o n of 0
s
with
o t h e r C-H bonds ( e . g . C 2 H 6 ) r e f l e c t t h e i r bond s t r e n g t h s r e l a t i v e t o methane.
The r a t e d e t e r m i n i n g s t e p i n t h e o x i d a t i v e c o u p l i n g
of methane over Li/MgO was shown by Cant e t a1.''
t o be methane C-H
bond s c i s s i o n , CH4 + Os ====> *CH3 + (4a) based on a l a r g e , p o s i t i v e ( 1 . 5 ) deuterium i s o t o p e e f f e c t .
Amorebieta and Colussi21 showed a t low p r e s s u r e
to
atm)
t h a t methane c o n v e r s i o n over Li/MgO i s h a l f o r d e r i n oxygen and f i r s t o r d e r i n methane.
T h i s r e s u l t s u g g e s t s t h a t methane r e a c t s
w i t h atomic s u r f a c e oxygen.
Labinger e t a l . , 2 2 ' 2 3 r e p o r t t h a t w i t h
t h e Na/Mg/Mn c a t a l y s t , CZH6 c o n v e r t s 1 . 9 t i m e s f a s t e r t h a n CH4
.
We have a d j u s t e d t h e r a t e c o n s t a n t s f o r C 2 H 6 t o g i v e t h i s r a t i o (4b/4a = 1 . 9 ) a t 1 0 0 0 K .
Rate c o n s t a n t s f o r t h e o t h e r r e a c t a n t s
(H-C2H3, H-CH OH, and H-CHO) were determined by f i x i n g t h e i r 2 frequency f a c t o r s t o t h a t of e t h a n e and a d j u s t i n g t h e i r a c t i v a t i o n
e n e r g i e s r e l a t i v e t o methane i n p r o p o r t i o n t o t h e d i f f e r e n c e s i n reaction enthalpy. The r e a c t i o n of t h e methyl r a d i c a l with s u r f a c e oxygen can s i g n i f i c a n t l y a l t e r t h e s e l e c t i v i t y p r e d i c t e d by t h e model. Labinger and O t t 2 '
a n a l y z e d t h e i r r e s u l t s and concluded t h a t t h e
o x i d a t i o n of methyl r a d i c a l s w i t h t h e Na/Mg/Mn c a t a l y s t
(without
f e e d g a s oxygen) was 2 7 0 0 t i m e s t h e r a t e of methane a c t i v a t i o n . The r a t i o of heterogeneous o x i d a t i o n t o homogeneous c o u p l i n g of methyl r a d i c a l s i s t h e e s s e n t i a l f a c t o r governing t h e s e l e c t i v i t y a t low c o n v e r s i o n .
T h e r e f o r e , t h e second key premise of o u r model
i s t h a t a l k y l r a d i c a l s i r r e v e r s i b l y react i n a n o n - a c t i v a t e d s t e p with s u r f a c e oxygen t o form adsorbed complexes t h a t a r e p r e c u r s o r s t o oxygenates. .R + Os =====>
ROs (5) The heterogeneous r a t e p a r a m e t e r s i n v o l v i n g s u r f a c e s i t e s
were optimized t o f i t e x p e r i m e n t a l r e s u l t s f o r Na/CaO a t 1 0 7 3 K. The v a r i a b l e , $s,
represents the i n i t i a l active s i t e density
( e s s e n t i a l l y t h e sum of 0
S
and U ) . S
For a s p e c i f i c s i t e d e n s i t y ,
t h e a c t i v a t i o n energy for C-H bond a c t i v a t i o n was t h e n a d j u s t e d t o
409
o b t a i n e x p e r i m e n t a l l y observed conversion r a t e s . Reaction r a t e and product r a t i o s were i n v e s t i g a t e d a s a f u n c t i o n of t h e a c t i v e s i t e d e n s i t y a t 1073 K with a methane t o oxygen r a t i o of 1 0 ( F i g . 2 When t h e Os c o n c e n t r a t i o n and i s high ( i . e . Os = 10- t o methane conversion i s high and o x i d a t i o n t o CO i s t h e dominant p r o c e s s e s . A t lower s u r f a c e s i t e c o n c e n t r a t i o n s (4, < the conversion r a t e i s lower and t h e C2 s e l e c t i v i t y i s h i g h e r . The 2).
h i g h e s t C2 y i e l d was found t o b e @ s = lo-’. These optimized independent parameters t h a t a f f e c t t h e product s e l e c t i v i t y were used for a l l subsequent c a l c u l a t i o n s
(OS
=
lo-’ and an a c t i v a t i o n
energy (E,) for t h e r a t e determining a b s t r a c t i o n s t e p ( r e a c t i o n 4a) of 6 3 . 6 k J mol-l) .
W e used t h e Chemkin k i n e t i c modeling program t o s o l v e t h e s e t of non-linear d i f f e r e n t i a l e q u a t i o n s . I n l i n k i n g t h e heterogeneous r e a c t i o n s t o t h e homogeneous r e a c t i o n network, we used a c o n s t a n t s u r f a c e t o volume r a t i o and c a l c u l a t e d t h e s u r f a c e s i t e concentration.
The f r a c t i o n of a c t i v e c e n t e r s on t h e s u r f a c e
of s u r f a c e oxide c a t i o n s . r e a c t i o n i s not s u r f a c e t r a n s p o r t l i m i t e d i n o u r model calculations. ( @ s ) was normalized t o t h e amount
The
REACTIVE CENTER CONCENTRATION (ML)
Fig. 2. Effect of reactive oxygen center surface concentralion on melhane conversion and higher hydrocarbon selectivity at 1073 K vrilh 0.3 atrn methane and 0.03 atm oxygen.
410
RESULTS Once t h e heterogeneous parameters were e s t a b l i s h e d , t h e temporal c o n c e n t r a t i o n s of co-oxidation products were determined f o r various reaction conditions
.
The r e s u l t s o b t a i n e d a t 1 0 7 3 K
with CH4 and O2 c o n c e n t r a t i o n s of 0 . 3 and 0 . 0 3 atm., r e s p e c t i v e l y ( F i g 3 ) , show t h a t ethane i s t h e major carbon c o n t a i n i n g product, followed by CO and e t h y l e n e . Other s i g n i f i c a n t p r o d u c t s a r e methanol and formaldehyde, which d e c r e a s e i n r e l a t i v e importance w i t h increased reaction t i m e .
The r e l a t i v e importance of s e v e r a l
gas phase r e a c t i o n s a t v a r i e s with i n i t i a l p a r t i a l p r e s s u r e s and r e a c t i o n time (Table 1 ) . A t low p r e s s u r e t h e main source of methyl r a d i c a l s i s t h e heterogeneous a c t i v a t i o n s t e p , while a t high p r e s s u r e two a d d i t i o n a l gas phase s o u r c e s of methyl r a d i c a l s a r e r e a c t i o n s i n v o l v i n g t h e hydrogen and hydroxyl r a d i c a l s . Higher hydrocarbon s e l e c t i v i t y i n t h e methane coupling p r o c e s s i s very dependent on oxygen p a r t i a l p r e s s u r e . T h e e f f e c t oxygen p a r t i a l p r e s s u r e on methane conversion and product s e l e c t i v i t y was s y s t e m a t i c a l l y examined ( F i g . 4 ) f o r f i x e d methane
9
.1 L
CONTACTTIME($1
Fig. 3. Calculated product distribution vs. contact time for methane coupling at 1073 K with 0.3 atrn methane and 0.03 atm oxygen.
-
TABLE 1
Reaction Rates for T 1.Oe-5
-
1073 K, PCH4
CH3+02-CH302 CH302=CH3+02 CH4+MEOS-CH3+MEOHS 2.05E-06 MEOHS+MEOHS-H20+MES+MEOS CH3+CH3-C2H6 MES+MES+OZ-MEOStMEOS CH4+OH-CH3+H20 CH3+02-CH20+OH CH3+H02-CH30+OH 9.6E-07 CH30+02-CH20+H02 CH30+CH4-CH3+CH30H CH4+H-CH3+H2 CH3+MEOS-CH3MOS CH3MOStMEOS-CHZO+MEOHS+MES HCO+02-H02+CO CH2O+MEOS-HCWMEOHS CH4+H02-CH3+H202 CH3+H202-H02+CH4 C2H5-C2H4+H CHZO+CH3=HCO+CH4 HCO+M=H+CO+M CZH6+MEOS-C2H5+MEOHS CH302+CH3-CH30+CH30 CH3O+M-CHZO+H+M C2H6+CH3-CZHS+CH4 C2H5+02-C2H4+H02 MEOS+MEOS-MES+MES+02
-
411
- 0.3 atrn, PO2 - 0.03, Phi -
Eak
Bate
2.175E-05 CH4+H-CH3+H2 2.155E-05 CH4+MEOS-CH3+MEOHS 1.964E-05MEOHS+MEOHS-H20+MES+MEOS
3,~ O E - O ~ 3.04E-06
1.04BE-05 C2H5-C2H4+H 9.74E-06 CH3+HZ-CH4+H 5.673-06 CH3+CH3-C2H6 2.84E-06 HCO+M-H+CO+M 1.47E-06 CZH6+H-C2H5+H2 1.46E-06MES+MES+02-MEOS+MEOS
2.01E-06 1.98E-06 1.94E-06 1.41E-06 1.13E-06
B.4E-07 7.6E-07 7.3E-07 6.73-07 6.7E-07 5.OE-0 7 3.9E-07 3.6E-07 3.2E-07 3. OE-07 2.9E-07 2.6E-07 2.2E-07 2.OE-07 1. CIE-O~ 1.4E-07 1.2E-07 1.1E-07
5,BE-07 5.7E-07 5.5E-07 5.3E-07 4.4E-07 4.2E-07 4.OE-07 3.4E-07 3.1E-07 2.9E-07 2.7E-07 2.4E-07 1.9E-07 1.9E-07 1.9E-07 1.8E-07 1.7E-07 1.5E-07
Fig. 4. Methane coupling conversion and higher hydrocarbon selectivity vs. oxygen partial pressure at 1073 K with 0.3 atm methane.
412
p a r t i a l p r e s s u r e ( 0 . 3 atm) and f i x e d a c t i v e s i t e c o n c e n t r a t i o n ( $ s = The methane conversion i n c r e a s e d , t h e C2+ s e l e c t i v i t y decreased, w h i l e t h e e t h y l e n e t o ethane r a t i o i n c r e a s e d t o a c o n s t a n t l e v e l w i t h i n c r e a s i n g oxygen p a r t i a l p r e s s u r e . S e v e r a l homogeneous methyl r a d i c a l o x i d a t i o n pathways a r e i m p o r t a n t . Under high temperature and low p r e s s u r e c o n d i t i o n s t h e r e a c t i o n of methyl r a d i c a l s w i t h hydrogen peroxy r a d i c a l s i s t h e prominent o x i d a t i o n pathway, *OCH3 + *OH '(6) '02H ====' w h i l e a t high p r e s s u r e and low temperature t h e r e a c t i o n of methyl
+
*CH3
r a d i c a l s w i t h methyl peroxy r a d i c a l s i s prominent. The major s o u r c e s of -O2H a r e hydrogen a b s t r a c t i o n r e a c t i o n s of u n s t a b l e r a d i c a l s such a s -CHO and *C2H5 with diatomic oxygen. Conversion w i t h Although a l k a l i promoted c a t a l y s t s have g r e a t e r s e l e c t i v i t y t h a n unpromoted a l k a l i n e e a r t h and r a r e e a r t h oxides, t h e r e i s some concern about s t a b i l i t y of t h e s e c a t a l y s t s given t h e high vapor p r e s s u r e s of a l k a l i under r e a c t i o n c o n d i t i o n s . The v o l a t i l i t y of a l k a l i under t y p i c a l a l k a n e a c t i v a t i o n c o n d i t i o n s i s due t o t h e high vapor p r e s s u r e of t h e a l k a l i hydroxide molecules i n t h e presence of steam and oxygen, although t h e s o l i d phase i s l i k e l y t o be a l k a l i c a r b o n a t e . Thermochemically s t a b l e c a r b o n a t e s a l t s w i t h t h e low v o l a t i l i t y i n steam are S r C 0 3 and BaC03. Perovskite-supported a l k a l i n e e a r t h c a r b o n a t e s , Sr/SrZrOg and Ba/SrZr03 are s e l e c t i v e and s t a b l e methane a c t i v a t i o n c a t a l y s t s ( F i g u r e 51, comparable or s u p e r i o r i n t h i s r e s p e c t t o Li-promoted MgO and Na-promoted CaO.
Good ethene s e l e c t i v i t y was a l s o shown
f o r co-oxidative dehydrogenation of e t h a n e (Figure 6 ) . Unlike t h e alkali-promoted a l k a l i n e e a r t h c a t a l y s t s , SrZrO o p e r a t e d 20 hours 3 a t 1 1 7 3 K with no evidence of evaporation o r c o r r o s i v e a t t a c k on our q u a r t z r e a c t o r s . These c a t a l y s t s appear t o achieve hydrocarbon s e l e c t i v i t y approaching t h e o r e t i c a l y i e l d s based on l a b o r a t o r y r e a c t i o n c o n d i t i o n s and p r e d i c t e d homogeneous o x i d a t i o n rates. DISCUSSION As a r e s u l t of o u r a n a l y s i s with t h e heterogeneoushomogeneous model, we conclude t h a t methane co-ox coupling p r o c e s s e s with premixed oxygen and methane may be l i m i t e d t o a
413
Fig. 5. Methane conversion and Cp selectivity for SrZQ versus reaction temperature. Conditions: 0.29atm CH4, 0.029-atm Q,3.3 mL s-1 (NTP) flow at 1-atm pressure.
60
-
Elhene Selectlvlty
/ 40-
20
-
0,
L
800
900
TEMPERATURE (K)
Fig. 6. Ethane partial oxidation on SrZrO3 with 0.03 atm oxygen.
1000
414
maximum h i g h e r hydrocarbon y i e l d of about 30 mol% (carbon b a s i s ) and a maximum ethene/ethane r a t i o of about 2 . C a t a l y s t s t h a t can f a v o r a b l y i n f l u e n c e t h e s e l e c t i v i t y by combining high t u r n o v e r r a t e s f o r a l k y l r a d i c a l g e n e r a t i o n p e r r e a c t i v e s i t e with a very low s u r f a c e c o n c e n t r a t i o n of r e a c t i v e c e n t e r s ( o p t i m a l l y one p a r t p e r l o 5 s i t e s ) . Formation of a s u r f a c e o r bulk b a s e metal carbonate l a y e r s may be one way of reducing t h e d e n s i t y of r e a c t i v e oxygen c e n t e r s t o l e v e l s t h a t avoid t h e r a p i d o x i d a t i o n of a l k y l r a d i c a l s a t t h e c a t a l y s t s u r f a c e , and indeed o u r temperature programmed d e s o r p t i o n experiments show t h a t t h e s u r f a c e s of t h o s e s o l i d - s t a t e b a s i c oxide c a t a l y s t s h i g h l y s e l e c t i v e f o r l i g h t alkane dehydrogenation and methane coupling a r e c o n s i s t e n t l y covered with a t l e a s t one monolayer of c a r b o n a t e . Homogeneous r e a c t i o n s can f u l l y account f o r t h e p r o d u c t i o n of hydrocarbon p r o d u c t s and t h e s e l e c t i v i t y between coupling p r o d u c t s and COX, b u t p r e d i c t h i g h e r y i e l d s of CH30H, CH20, and H 2 , and g r e a t e r CO/C02 product r a t i o s t h a n observed e x p e r i m e n t a l l y . Heterogeneous o x i d a t i o n r e a c t i o n s a r e e v i d e n t l y r e s p o n s i b l e f o r s i g n i f i c a n t o x i d a t i o n of oxygenate products, t h e s u b s t a n t i a l conversion of product hydrocarbons o r r a d i c a l s , and p o s s i b l y d i r e c t o x i d a t i o n of e t h y l e n e t o COP and H 2 0 i n co-oxidation p r o c e s s e s by c a t a l y s t s with poor i n h e r e n t s e l e c t i v i t y . ACKNOWLEDGEMENT
The a u t h o r s g r a t e f u l l y acknowledge t h e support of t h e Methane Reaction Science Program d i r e c t e d by Dr. Daniel A . S c a r p i e l l o f o r t h e Gas Research I n s t i t u t e and a s s o c i a t e d I n d u s t r i a l A f f i l i a t e cosponsors. REFERENCES 1. I t o , T., Wang, J . - X . ,
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