Perspectives in Molecular Sieve Science - American Chemical Society

Chapter

39

Methanol-to-Gasoline Process Reaction Mechanism Clarence D. Chang

Downloaded by EMORY UNIV on June 17, 2014 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch039

Central Research Laboratory, Mobil Research and Development Corporation, Princeton, NJ 08540

Mobil's Methanol-to-Gasoline (MTG) Process [1] is the first new synfuels process to be commercialized since the Fischer-Tropsch process of the 1920's. The MTG process was chosen by New Zealand to convert natural gas from their extensive offshore Maui field into gasoline via methanol. Started up in late 1985, the plant at Motonui produces 14,500 BPD of premium gasoline, which is one third of New Zealand's total demand. The successful development and scale-up of the MTG process [2] was a triumph of modern catalytic and reaction engineering. Ironically the detailed chemistry of methanol transformation to hydrocarbons remains unknown to this date, and is a widely debated issue. Indeed, some two dozen more or less distinct mechanistic proposals may be found in the current literature. This paper presents an overview, focussing on several of the more popular concepts, as well as the sparse experimental evidence pro and con the various schemes. GENERAL CHARACTERISTICS AND REACTION PATH Hydrocarbon formation from methanol is catalyzed by Bronsted acids. The general reaction path for hydrocarbon formation from methanol over zeolite ZSM-5 [3], the catalyst of choice [4], was defined in early Mobil studies [lb], and is represented by: -nM0 yl2CHOH^CH30CH 4 H0| * n2n *n(CH) where (CH) » average formula of a paraffin-aromatic mixture. 2

C H

3

3

2

2

2

0097-6156/88/0368-0596$06.00/0 © 1988 American Chemical Society

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

39. C H A N G

Methanol-to-Gasoline Process

597

Methanol r a p i d l y e q u i l i b r a t e s t o a dimethyl ether (DME) a n d w a t e r m i x t u r e . Further dehydration affords l i g h t o l e f i n s , which undergo subsequent r e a c t i o n t o f o r m g a s o l i n e - r a n g e a r o m a t i c s a n d p a r a f f i n s . The r e a c t i o n i s exothermic [ l b ] and a u t o c a t a l y t i c [ 5 - 7 ] . The mechanisms o f e t h e r i f i c a t i o n a n d o l e f i n aromatization a r ewell-understood [4,8]. The c o n t r o v e r s y c e n t e r s o n t h e mechanism o f f o r m a t i o n o f t h e f i r s t C-C bond f r o m a s i n g l e - c a r b o n s o u r c e . The q u e s t i o n whether e t h y l e n e i s t h e " f i r s t " o l e f i n has a l s o b e e n d e b a t e d [ 4 ] , b u t w i l l n o t be c o n s i d e r e d i n t h i s paper.


MECHANISM OF I N I T I A L C-C BOND FORMATION V i r t u a l l y every p o s s i b l e r e a c t i v e i n t e r m e d i a t e has b e e n i n v o k e d t o e x p l a i n t h e c r u c i a l s t e p o f i n i t i a l C-C bond f o r m a t i o n f r o m methanol/DME. P r o p o s e d mechanisms c a n be b r o a d l y c l a s s i f i e d a s c a r b e n i c , c a r b o c a t i o n i c , y l i d e , and f r e e r a d i c a l . I n some p r o p o s a l s s e v e r a l o f t h e s e c a t e g o r i e s a r e combined. C a r b e n e s were t h e e a r l i e s t p r o p o s e d C l i n t e r mediates. T h e s e were c o n s i d e r e d t o be g e n e r a t e d v i a ae l i m i n a t i o n o f water from methanol i t s e l f [9,10], o r from c a t a l y s t s u r f a c e methoxyls [11]. The r e a c t i o n may be a s s i s t e d b y c o o p e r a t i v e a c t i o n o f a c i d a n d b a s e s i t e s on t h e c a t a l y s t [10]. O l e f i n s w o u l d be f o r m e d by p o l y m e r i z a t i o n o f t h e ( f r e e ) :CIÎ2. T o overcome t h e h i g h e n e r g e t i c requirements o f carbene g e n e r a t i o n (vide i n f r a ) , a n d t h e l o w p r o b a b i l i t y o f :CH2 s e l f - c o n d e n s a t i o n , Chang a n d S i l v e s t r i [ l b ] p r o p o s e d a c o n c e r t e d mechanism o f c a r b e n e g e n e r a t i o n w i t h s p 3 i n s e r t i o n i n t o t h e C-H bond o f DME t o f o r m MeOEt.

H Ζ

Θ Hi

OR

VI

CH

/J 2

Η

s

φ

-HZ

CH CH OR' + ROH 3

2

CH OR' 2

/^-Elimination affords ethylene. I n d i r e c t evidence i n support o f carbene intermediacy was o b t a i n e d t h r o u g h i s o t o p e l a b e l l i n g s t u d i e s o f t h e i n t e r a c t i o n o f MeOH w i t h p r o p a n e d i l u e n t o v e r HZSM-5 [11]. MTG h y d r o c a r b o n s a r e r i c h i n i s o p a r a f f i n s a l o n g with the aromatics. F o r example, t h e r a t i o o f i s o - t o n o r m a l b u t a n e ( i / n ) i s t y p i c a l l y g r e a t e r t h a n 2. I n


PERSPECTIVES IN MOLECULAR SIEVE SCIENCE


598

the p r e s e n c e o f propane d i l u e n t under c o n d i t i o n s w h e r e p r o p a n e i t s e l f was u n r e a c t i v e , however, t h i s r a t i o d r o p s f r o m 4 ( w i t h He d i l u e n t ) t o 1, a p p r o a c h i n g t h e t h e r m o d y n a m i c r a t i o , 0.75. When i s o t o p i c a l l y l a b e l l e d MeOH ( 9 0 % 1 3 C , 10% 12 C) was u s e d , t h e p r o d u c t b u t a n e s had a nonrandom i s o t o p e d i s t r i b u t i o n h i g h i n s i n g l y s u b s t i t u t e d butane ( F i g u r e l a ) . Furthermore, t h e i / n r a t i o o f s i n g l y - l a b e l l e d b u t a n e s was c a . 1, w h i l e t h a t o f t h e f u l l y - l a b e l l e d b u t a n e s was c a . 3 ( f r o m s e l f r e a c t i o n o f 13 C MeOH), ( F i g u r e l b ) . The p r e s e n c e o f d o u b l y and t r i p l y - l a b e l l e d b u t a n e s i n d i c a t e s some isotope scrambling. These o b s e r v a t i o n s w e r e t a k e n a s e v i d e n c e t h a t t h e propane d i l u e n t served t o i n t e r c e p t t h e r e a c t i v e C^ i n t e r m e d i a t e f r o m MeOH t o f o r m b u t a n e , and t h a t t h i s i n t e r m e d i a t e was c a r b e n i c , inserting i n t o t h e C-H bond o f p r o p a n e . Such i n s e r t i o n s a r e random b e c a u s e o f t h e h i g h r e a c t i v i t y o f c a r b e n e . A c a r b o c a t i o n i c i n t e r m e d i a t e would tend t o g i v e i s o b u t a n e predominantly, v i a the r e l a t i v e l y stable t - b u t y l c a t i o n [12].

:CH

CH

+ C H

2

3

C H

C H

2

+ CH CH CH

3

3

2

3

^

—

CH CH CH CH 3

2

2

CH CHCH

3

3

3

3

A c a r b e n i c mechanism was a l s o f a v o r e d b y L e e a n d Wu [ 1 3 ] , who p r o p o s e d t h a t t h e c a r b e n e i n t e r m e d i a t e w o u l d be s t a b i l i z e d by t h e z e o l i t e framework ( F i g u r e 2). They a t t e m p t e d t o model t h e r e a c t i o n b y s t u d y i n g the i n t e r a c t i o n o f s i n g l e t methylene (from CH2N2) w i t h v a r i o u s s u r f a c e s , i n c l u d i n g ZSM-5. A l t h o u g h o l e f i n s were o b s e r v e d e v e n i n t h e p r e s e n c e o f " i n e r t " m a t e r i a l s such as Vycor, hydrocarbon y i e l d s i n c r e a s e d s u b s t a n t i a l l y i n the presence of a c i d i c surfaces.

H Ο CH N 2

+

2

Ο

0

\ / + \ _ 0—Si Al 0

-N

Ο

2

1 . O—Si

Al

I

0

0

/ \ 0

0

-N | 2

CH N 2

2

Η CH CH

I ο

0

ο /+\

\ 0—

2

SI

I

+

Al—

r/

I

0Η =0Η 2

2

0—

1

Si

Al

1

/ \

0 0

3

ο



39. CHANG

Methanol-to-Gasoline Process

599

13

Figure 1 Butanes from Interaction of C H O H with C H over HZSM-5 3

8

Si

Si /

0

3

N)

0/

Figure 2. Surface-Stabilized Singlet Carbene




600

Theoretical j u s t i f i c a t i o n for framework-stabili z a t i o n o f c a r b e n e was p r o v i d e d by ab i n i t i o (MINDU3) c a l c u l a t i o n s o f Drenth e t a l . [14]. The z e o l i t e a c i d s i t e was s i m u l a t e d u s i n g H 0 A 1 0 , HOF, and H as e l e c t r o n a c c e p t o r s o f i n c r e a s i n g s t r e n g t h , w i t h two hydroxyl anions as e l e c t r o n donors. The i n t e r a c t i o n o f C H 2 w i t h t h i s a s s e m b l a g e was examined ( F i g u r e 3 ) . I t was f o u n d t h a t I^O"*" p r o t o n a t e d C H 2 w h i l e t h e weaker a c i d s tended t o r e t a i n t h e i r proton. Calculated s t a b i l i z a t i o n e n e r g i e s were 284-313 k J / m o l . The h e a t o f f o r m a t i o n o f C H 2 f r o m MeOH i s 349 k J / m o l ; t h u s , f r a m e w o r k - s t a b i l i z a t i o n C H 2 i s not unreasonable. C a r b o c a t i o n i c mechanisms were p r o p o s e d by Uno and M o r i [15] a n d o t h e r s [ 1 6 , 1 7 ] . A c c o r d i n g t o Ono and M o r i , s u r f a c e m e t h o x y l s may behave a s i n c i p i e n t m e t h y l c a t i o n s , w h i c h a d d t o t h e C-H o f DME, f o r m i n g a p e n t a v a l e n t carbonium t r a n s i t i o n s t a t e . MeOEt i s f o r m e d upon p r o t o n l o s s . CH + + C H 3 O R

^CHoOR

3

•CH CH OR + H 3

2

+

CH,

Since HC1 addition d i d not poison the reaction, t h e s e w o r k e r s c o n c l u d e d t h a t b a s i c s i t e s were n o t involved. Whether t h e C-H bond o f DME o r MeOH i s s u f f i c i e n t l y n u c l e o p h i l i c t o u n d e r g o s u b s t i t u t i o n b y CH3"*" i s debatable. The g a s p h a s e r e a c t i o n ο f C H w i t h MeUII was s t u d i e d b y S m i t h a n d F u t r e l l [18] u s i n g i o n c y c l o t r o n resonance techniques. Hydride a b s t r a c t i o n f o r m i n g C H 4 a n d C H 2 0 H + were s e e n t o be t h e p r e d o m i n a n t reaction (85-90%). +

?

CHj + CH3OH

CH —Ο—Η

[CH 0H]

3

2

+

+ CH

4

(80%)

+ ΔΗ = -255 kJ/mol

CHj

+ CH3OH ·

CHjOH

2

+ CH

(7%)

2

ΔΗ = 84 kJ/mol

CHj

+ CH3OH

|CH OHJ+ + C H 2

4

(5-10%)


Methanol-to-Gasoline

39. C H A N G

Process

601

Τ R1


ο—H

c;

I R1

οΙ

1

Η -R2Optimized bonding energy of C H 2 , Δ Ε , and bond lengths (in pm) Acid

HOAIO HOF H30+

Varying R 3

R 3 90 pm «1

R2

ΔΕ (kJmoh 1 )

R3a

190 190 190

275 270 240

255 267 485

105 115 132

(kJmoh 1 ) 284 313 c

Value of R 3 in the C H 2 complex. R 1 and R 2 as in this Table; R 3 optimized both in the complex and in the CH 2 -free model. c Variation of R 3 does not yield a meaningful Δ Ε since in the CH 2 -free model, the proton is transferred to one of the O H - groups.

a

b

Fig. 3. Zeolite model for ab initio molecular orbital calculations.



602


I n t e r e s t i n g l y , p r o t o n t r a n s f e r f r o m CH3 t o MeOH f o r m i n g CH2 o c c u r s t o a s i g n i f i c a n t e x t e n t ( 7 % ) , b u t s p e c i e s were n o t d e t e c t e d . I n s u p e r a c i d s y s t e m s , m e t h y l a t i o n o f DME g i v e s t r i m e t h y l o x o n i u m (TMO) i o n [ 1 9 ] . Mechanisms i n v o k i n g h y p e r v a l e n t c a r b o n i n t e r m e d i a t e s t h e r e f o r e seem unlikely. The most p o p u l a r mechanisms a t p r e s e n t i n v o k e oxonium y l i d e s as i n t e r m e d i a t e s . van den Berg e t a l . [20] p r o p o s e d t h a t DME i s p r o t o n a t e d by a B r o n s t e d s i t e , and t h e r e s u l t a n t i o n s u f f e r s n u c l e o p h i l i c a t t a c k by a s e c o n d m o l e c u l e o f DME t o f o r m TMO w i t h r e l e a s e o f MeOH. The TMO i o n i s t h e n d e p r o t o n a t e d by a b a s i c s i t e to form t h e dimethyloxonium m e t h y l i d e , which undergoes a Stevens-type rearrangement t o g i v e m e t h y l e t h y l o x o n i u m i o n . MeOEt i s s u b s e q u e n t l y f o r m e d upon βelimination. No e x p e r i m e n t a l e v i d e n c e was o f f e r e d i n s u p p o r t o f t h e scheme.

CH

CH

3

Ο H

®

+CH3OCH3

HC

Η

3

CH

3

ι + CH OCH H C 3

3

3

ν

3

ι /

Perspectives in Molecular Sieve Science - American Chemical Society

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