Methanol Conversion on Ceramic Honeycombs Coated with Silicalite

Chapter

31

Methanol Conversion on Ceramic Honeycombs Coated with Silicalite M. D. Patil and I. M. Lachman

Downloaded by CORNELL UNIV on May 13, 2017 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch031

Corning Glass Works, SP-DV-19, Corning, NY 14831 Silicalite, a pentasil type zeolite was washcoated on a cordierite ceramic honeycomb using a silica binder derived from prehydrolyzed ethyl-orthosilicate. Binder content in the washcoat was varied from 10% to 40% to determine the effect on blocking the zeolite active sites. Methanol conversions on H-silicalite/honeycombs were conducted on four samples with different binder contents. Percentage of methanol conversion was calculated per gram of zeolite present in the washcoat. Highest methanol conversions for 10% and 20% binder in washcoat indicate maximum availability of active sites of the zeolite; whereas, lower methanol conversion was observed for 30% and 40% binder content indicating the blockage of some active sites of the zeolite. A washcoat with 20% and 30% binder content gave higher loading than 10% binder and good binding to the cordierite surface. The washcoat made with 40% binder content gave the highest loading and excellent binding to the honeycomb substrate. Thus 20-30% binder in the washcoat combines excellent adhesion, high loading of catalyst with high catalytic activity. A methanol conversion was also conducted on extruded silicalite honeycomb with alumina binder. C a t a l y s t s i n t h i n - w a l l honeycomb form o f f e r the advantages o f low p r e s s u r e drop, h i g h g e o m e t r i c s u r f a c e a r e a , and s h o r t d i f f u s i o n d i s t a n c e as compared to c o n v e n t i o n a l p e l l e t s and beads i n f i x e d bed r e a c t o r s ( 1 ) . A c t i v e z e o l i t e c a t a l y s t s may be e x t r u d e d i n the form o f a honeycomb s t r u c t u r e o r they may be washcoated on c e r a m i c honeycomb s u b s t r a t e s . The l a t t e r t e c h n i q u e has been w i d e l y used i n a u t o m o t i v e e m i s s i o n s c o n t r o l ( 2 ) , woodstove combustors ( 3 ) , c o n t r o l o f v o l a t i l e o r g a n i c e m i s s i o n s from o r g a n i c s o l v e n t s ( 4 ) , ozone abatement i n j e t a i r c r a f t p a s s e n g e r c a b i n s ( 5 ) , and Ν 0 abatement χ

0097-6156/88/0368-0492$06.00/0 © 1988 American Chemical Society

Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


31.

PATIL & LACHMAN

Methanol Conversion on Ceramic Honeycombs 493

from i n d u s t r i a l e m i s s i o n s ( 6 ) . However, washcoats o f z e o l i t e c a t a l y s t s have not y e t been e x p l o i t e d i n the c h e m i c a l p r o c e s s industries. A p e n t a s i l type z e o l i t e (ZSM-5) i s b e i n g used i n a methanol to g a s o l i n e (MTG) c o n v e r s i o n p r o c e s s (7) i n a f i x e d bed r e a c t o r . T h i s type o f z e o l i t e when washcoated on c e r a m i c honeycomb s u b s t r a t e s s h o u l d o f f e r the advantages o f lower p r e s s u r e d r o p and h i g h g e o m e t r i c s u r f a c e a r e a i n the r e a c t o r . Such washcoated z e o l i t e c a t a l y s t s must be s t r o n g l y bound to the honeycomb s u b s t r a t e and s h o u l d have the maximum number o f a c t i v e s i t e s f o r h i g h e s t c a t a l y t i c a c t i v i t y f o r chemical process a p p l i c a t i o n s . Thus the o b j e c t i v e o f t h i s s t u d y i s to f i n d the b e s t c o m b i n a t i o n o f b i n d e r and z e o l i t e c o m p o s i t i o n f o r w a s h c o a t i n g s i l i c a l i t e ( a p e n t a s i l type z e o l i t e ) on a c e r a m i c honeycomb s u b s t r a t e , and to e v a l u a t e the c a t a l y t i c a c t i v i t y f o r methanol c o n v e r s i o n . T h i s w i l l demonstrate the u t i l i t y o f z e o l i t e honeycomb c a t a l y s t s f o r chemical processing. S i l i c a l i t e honeycombs were a l s o p r e p a r e d by e x t r u s i o n and s i m i l a r l y evaluated. Experimental S t a n d a r d corçjierite honeycombs ( C o r n i n g code 9475) w i t h 400 channels/in. were c o r e - d r i l l e d ( 1 " d i a m e t e r ) and c u t i n t o 1" l e n g t h p i e c e s . These p i e c e s were a c i d l e a c h e d ( 8 ) i n 1.5 Ν HNO^ s o l u t i o n f o r 2 hours a t 95°C i n p r e p a r a t i o n f o r w a s h c o a t i n g . A l e a c h e d c o r d i e r i t e s u r f a c e p r o v i d e s b e t t e r b o n d i n g f o r the subsequent washcoat than the a s - f i r e d s u r f a c e . A f t e r l e a c h i n g and washing, the samples were d r i e d i n an oven a t 100°C f o r 16 h o u r s . The p e r c e n t a g e weight l o s s i s g i v e n i n T a b l e 1.

T a b l e 1: P e r c e n t a g e weight l o s s o f c o r d i e r i t e honeycomb p i e c e s ( 1 " d i a m e t e r χ 1" l e n g t h ) a f t e r l e a c h i n g i n 1.5 Ν HN0 a t 95°C f o r 2 hours 3

Sample No.

Wt. b e f o r e l e a c h i n g (g)

Wt. a f t e r l e a c h i n g (g)

%

1

5.15

4.90

4..85

2

5.24

4.98

4,,96

3

5.16

4.91

4..84

4

5.31

5.05

4..90

5

5.73

5.46

4..71

6

5.47

5.20

4..94

Wt,

C o r d i e r i t e j s C o m i n g ' s C e l c o r s u b s t r a t e code 9475 w i t h s q u a r e s / i n . , 0.16 mm w a l l .

400



494

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE

For washcoat p r e p a r a t i o n s i l i c a l i t e z e o l i t e powder (S-115 from Union C a r b i d e Corp.) was d r i e d a t 100°C i n an oven o v e r n i g h t . P r e h y d r o l y z e d e t h y l - o r t h o s i l i c a t e c o n t a i n i n g 19.5% s i l i c a ( S i l b o n d H-5 from S t a u f f e r Chemical Co.) was the s o u r c e o f the s i l i c a b i n d e r . A b s o l u t e e t h a n o l (from U.S. I n d u s t r i a l Chemicals Co.) and a b s o l u t e methanol ( r e a g e n t grade from J . T. Baker Company) were used w i t h o u t f u r t h e r p u r i f i c a t i o n f o r the l i q u i d p o r t i o n of the washcoat s l u r r y and f o r the methanol c o n v e r s i o n r e a c t i o n , r e s p e c t i v e l y . Reagent grade ammonium n i t r a t e (from J . T. B a k e r ) was used f o r ion-exchange purposes. The washcoat s l u r r y was prepared by s l o w l y a d d i n g a known amount o f s i l i c a l i t e to a s t i r r e d s o l u t i o n of a known amount o f S i l b o n d H-5. A f t e r complete a d d i t i o n o f s i l i c a l i t e the m i x t u r e was s t i r r e d f o r another 15-30 minutes. S l u r r y m i x t u r e s w i t h d i f f e r e n t s o l i d to l i q u i d r a t i o s were made by d i l u t i n g the m i x t u r e s w i t h absolute ethanol. Leached c o r d i e r i t e honeycomb p i e c e s were immersed c o m p l e t e l y i n t o s t i r r e d washcoat s l u r r i e s and a f t e r removal the channels were c l e a r e d of excess s l u r r y by b l o w i n g w i t h compressed a i r . Samples were then d r i e d i n a i r f o r 3-5 hours. T h i s c o a t i n g procedure was r e p e a t e d , g e n e r a l l y two more t i m e s , u n t i l s u f f i c i e n t c o a t i n g was a c h i e v e d . A f t e r c o a t i n g , samples were d r i e d i n an oven at 100°C f o r 16 hours and f i r e d a t 500°C f o r 6 hours. The percentage o f washcoat l o a d i n g , T a b l e 2, was c a l c u l a t e d by w e i g h i n g the samples b e f o r e and after coating.

T a b l e 2: S i l i c a l i t e c o n c e n t r a t i o n i n washcoated ( s i l i c a b i n d e r ) or e x t r u d e d ( a l u m i n a b i n d e r ) honeycomb p i e c e s , 1" ( d i a m e t e r ) χ 1" (length)

Sample Code

Zeolite/ Binder

Wt. B e f o r e Coating(g)

Wt. a f t e r %Vt. coating(g) loading

A Β

90:10 90:10

5.16 5.19

5.81 5.84

11.2 11.1

0.58 0.58

C D

80:20 80:20

5.15 5.29

6.30 6.08

18.2 13.0

0.92 0.63

Ε F

70:30 70:30

5.21 5.20

6.53 6.48

20.2 19.7

0.92 0.90

G H

60:40 60:40

5.12 5.16

7.31 7.44

30.0 30.6

1.31 1.37

I (Extruded)

84:16

Silii (g)

3.80

Samples A to H were washcoated on l e a c h e d c o r d i e r i t e honeycombs and sample I was e x t r u d e d w i t h 16% a l u m i n a b i n d e r . A l l washcoat s l u r r y m i x t u r e s had 30% (wt.) o f s o l i d s .


31. PATIL & LACHMAN


S e l e c t e d samples were immersed i n 10% ammonium n i t r a t e s o l u t i o n o v e r n i g h t at room temperature to exchange N H ^ f o r N a i o n s i n the s i l i c a l i t e . The immersed samples were washed w i t h d i s t i l l e d w a t e r , d r i e d , and f i r e d at 5 0 0 ° C f o r 6 hours to decompose the NH^* i o n to H i o n and thus g i v e a H - s i l i c a l i t e c o a t e d honeycomb. E x t r u d e d s i l i c a l i t e z e o l i t e honeycombs were a l s o p r e p a r e d by w e l l known p r o c e d u r e s ( 9 , 1 0 ) . S i l i c a l i t e z e o l i t e powder ( U n i o n C a r b i d e C o r p . S-115) was e x t r u d e d w i t h a boehmite a l u m i n a ( V i s t a C h e m i c a l C a t a p a l D) which forms a gamma a l u m i n a b i n d e r phase a f t e r heat t r e a t m e n t . The f i n a l c o m p o s i t i o n i s 84% s i l i c a l i t e + 16% gamma a l u m i n a . These e x t r u d e d honeycomb samples were i o n - e x c h a n g e d and e v a l u a t e d i n the same way as the washcoated o n e s . The methanol c o n v e r s i o n r e a c t i o n was c a r r i e d out o v e r t h e s e H - s i l i c a l i t e / h o n e y c o m b samples in a r e a c t o r set up as shown in F i g u r e 1. A b s o l u t e methanol was i n j e c t e d i n t o an e v a p o r a t o r at the r a t e o f 0.49 m l / h o u r . Dry h e l i u m , a d i l u e n t g a s , was passed i n t o the e v a p o r a t o r at a r a t e o f 45 m l / m i n u t e . Gaseous p r o d u c t s were d i r e c t l y passed v i a a heated l i n e through a s i x p o r t V a l c o v a l v e w i t h a 0.25 ml sample l o o p . The p r o d u c t s were a n a l y z e d by a V a r i a n 6000 GC w i t h a Chromosorb-102 column. For each sample, r e a c t i o n runs were made f o r at l e a s t 50 hours and s t e a d y s t a t e c o n v e r s i o n d a t a were c o l l e c t e d . Methanol c o n v e r s i o n was m o n i t o r e d at d i f f e r e n t catalytic r e a c t i o n t e m p e r a t u r e s : 1 0 0 ° C , 1 5 0 ° C , and 2 5 0 ° C . However, p e r c e n t methanol c o n v e r s i o n s were c a l c u l a t e d o n l y f o r the 1 0 0 ° C and 1 5 0 ° C runs. M e t h a n o l c o n v e r s i o n , c a l c u l a t e d per gram of z e o l i t e i n the washcoat, was p l o t t e d ( F i g u r e 2) v s . p e r c e n t a g e b i n d e r c o n t e n t .


+

Results

and

+

Discussion

A u t o m o t i v e e m i s s i o n c o n t r o l c a t a l y s t s and woodstove combustor c a t a l y s t s a r e washcoated on h i g h l y porous c o r d i e r i t e honeycomb s u b s t r a t e s . Gamma a l u m i n a i s the p r i n c i p l e b i n d i n g phase f o r t h e s e commercial p r o d u c t s . S i l i c a l i t e z e o l i t e c a t a l y s t s i n t h i s s t u d y were washcoated s i m i l a r l y onto porous c o r d i e r i t e honeycomb s u b s t r a t e s but w i t h a s i l i c a b i n d e r . In p r e p a r a t i o n f o r w a s h c o a t i n g , the c o r d i e r i t e honeycomb s u b s t r a t e s were a c i d l e a c h e d (8) f o r two hours a t 9 5 ° C w i t h 1.5 Ν HNO^ because l e a c h i n g s e l e c t i v e l y removes a l u m i n a and magnesia from c o r d i e r i t e c r y s t a l l i t e s and l e a v e s a h i g h s u r f a c e a r e a m i c r o p o r o u s s i l i c a phase. A c i d l e a c h i n g o f c o r d i e r i t e honeycomb s u b s t r a t e s under these c o n d i t i o n s ( T a b l e 1) shows a weight l o s s o f 5%. E a r l i e r r e s u l t s by Elmer ( 8 ) , suggest a s u r f a c e a r e a o f 20-40 m2/g f o r s i m i l a r l y l e a c h e d honeycombs, which s t i l l r e t a i n 80% o f t h e i r o r i g i n a l c r u s h i n g s t r e n g t h . A s u r f a c e of microporous s i l i c a i s d e s i r a b l e for washcoating with z e o l i t e s u s i n g a s i l i c a binder because s t r o n g b i n d i n g appears to d e v e l o p between the c o a t i n g and the l e a c h e d s u b s t r a t e . S i l i c a l i t e washcoat r e s u l t s g i v e n i n T a b l e 2 i n d i c a t e the weight l o a d i n g s a t t a i n e d w i t h d i f f e r e n t b i n d e r c o n t e n t s i n the washcoat s l u r r y . G e n e r a l l y weight l o a d i n g i n c r e a s e s as the b i n d e r c o n t e n t i n the s l u r r y i n c r e a s e s . Sometimes l o a d i n g w i t h the same c o m p o s i t i o n s l u r r y may not be r e p r o d u c i b l e ( e . g . , samples C and D) because of c h a n n e l b l o c k i n g w i t h the s l u r r y . G e n e r a l l y a c o n t i n u o u s l y mixed s l u r r y g i v e s c o n s i s t e n t l o a d i n g o f the washcoat on the c o r d i e r i t e s u b s t r a t e . A t h i c k v i s c o u s s l u r r y ( w i t h h i g h



496


s o l i d s c o n t e n t ) g i v e s a n o n - u n i f o r m and i n c o n s i s t e n t l o a d i n g , whereas a t h i n f l u i d s l u r r y w i t h a low s o l i d s c o n t e n t g i v e s a low but c o n s i s t e n t l o a d i n g . Thus a compromise i n the s o l i d s c o n t e n t o f the s l u r r y i s n e c e s s a r y to get good w a s h c o a t i n g w i t h c o n s i s t e n t l o a d i n g . A s o l i d s c o n t e n t o f 30-40% by weight i n the s l u r r y gave b e s t r e s u l t s i n t h i s case f o r u n i f o r m i t y and c o n s i s t e n t l o a d i n g . F i g u r e 3 shows t h r e e o p t i c a l m i c r o g r a p h s : ( a ) a n uncoated c o r d i e r i t e honeycomb s u b s t r a t e and two washcoated honeycomb s u b s t r a t e s , (b) 20% s i l i c a b i n d e r , and (c) 40% s i l i c a b i n d e r . Figure 3(c), 40% s i l i c a b i n d e r , shows s h r i n k a g e c r a c k s , whereas i n F i g u r e 3(b), 20% s i l i c a b i n d e r , t h e r e are no apparent c r a c k s . Higher d r y i n g and f i r i n g s h r i n k a g e i s expected i n case o f 40% s i l i c a b i n d e r c o n t e n t as compared to the 20% s i l i c a b i n d e r . The washcoats were u n i f o r m f o r a l l the samples and appeared s t r o n g l y bound to the honeycomb s u b s t r a t e s . A l l washcoated samples and e x t r u d e d honeycomb samples were exchanged w i t h NH^ i o n to get H - s i l i c a l i t e a f t e r f i r i n g a t 500°C f o r 6 h r s . H - s i l i c a l i t e c o a t e d or e x t r u d e d honeycombs were l o a d e d i n t o a 1" d i a m e t e r r e a c t o r as shown i n F i g u r e 1. Methanol c o n v e r s i o n was m o n i t o r e d by gas c h r o m a t o g r a p h i c a n a l y s i s of gaseous p r o d u c t s . P e r c e n t methanol c o n v e r s i o n s were c a l c u l a t e d o n l y f o r the 100°C and 150°C temperature r u n s . There was v i r t u a l l y no c o n v e r s i o n at 100°C. I t i s assumed o n l y a c t i v e z e o l i t e s i t e s c o n v e r t methanol to h y d r o c a r b o n s and the degree of c o n v e r s i o n a t 150°C i s t h e r e f o r e a f u n c t i o n o f t o t a l a v a i l a b l e a c t i v e z e o l i t e s i t e s . Thus p e r c e n t methanol c o n v e r s i o n i s c a l c u l a t e d per gram o f z e o l i t e i n the honeycomb sample i n o r d e r to compare the b l o c k a g e o f a c t i v e s i t e s . Methanol c o n v e r s i o n ( F i g u r e 2) per gram o f z e o l i t e d e c r e a s e s as the p e r c e n t a g e b i n d e r c o n t e n t i n c r e a s e s i n the washcoat. The washcoated sample w i t h 90% z e o l i t e and 10% s i l i c a b i n d e r has the h i g h e s t methanol c o n v e r s i o n , 37.6%, whereas the lowest methanol c o n v e r s i o n , 15.3%, i s o b s e r v e d f o r the sample w i t h 60% z e o l i t e and 40% s i l i c a b i n d e r . T h i s i n d i c a t e s the a v a i l a b l e a c t i v e z e o l i t e s i t e s d e c r e a s e as the b i n d e r c o n t e n t i n the washcoat i n c r e a s e s . B l o c k a g e from the b i n d e r i s the most l i k e l y cause o f the d e c r e a s e i n these a c t i v e z e o l i t e s i t e s . These r e s u l t s i n d i c a t e about 60 p e r c e n t o f the a c t i v e s i t e s a r e b l o c k e d i n sample G (40% s i l i c a b i n d e r ) as compared to sample A (10% s i l i c a b i n d e r ) . F o r t h i s c a l c u l a t i o n i t i s assumed t h a t t h e r e i s no b l o c k a g e i n sample A, even though a s m a l l amount must be p r e s e n t . A low 12% methanol c o n v e r s i o n per gram o f z e o l i t e was observed f o r e x t r u d e d H - s i l i c a l i t e w i t h 16% a l u m i n a b i n d e r . A comparison o f 12% methanol c o n v e r s i o n a c t i v i t y to 37.6% f o r the washcoated z e o l i t e w i t h 10% s i l i c a b i n d e r i n d i c a t e s about 68% b l o c k a g e or impedance to the a c t i v e s i t e s under the g i v e n t e s t c o n d i t i o n s . +

Conclusions Z e o l i t e c a t a l y s t s , e i t h e r i n e x t r u d e d honeycombs o r washcoated on honeycomb s u b s t r a t e s , show good c a t a l y t i c a c t i v i t y f o r methanol conversion. A v a i l a b i l i t y o f a c t i v e z e o l i t e s i t e s i s important f o r c a t a l y t i c r e a c t i o n s . The d a t a show t h a t a c t i v e z e o l i t e s i t e s i n washcoated honeycombs may be o p t i m i z e d by the amount o f b i n d e r i n the washcoat.


31. PATIL & LACHMAN


- VENT TO HOOD METHANOL INJECTOR ZL

SS REACTOR VALCO VALVE

It

RECORDER

FURNACE HEATER

GC


I20°C EVAPORATOR HONEYCOMB CATALYST

' HEATED LINES HELIUM GAS Figure

1.

Schematic

of reactor

system.

25 PERCENT METHANOL CONVERSION PER GRAM SILICALITE

20+

15

20 PERCENT Figure versus

30 BINDER

I N WASHCOAT

2. P e r c e n t m e t h a n o l c o n v e r s i o n p e r g r a m o f s i l i c a l i t e percent b i n d e r i n washcoat.




498

F i g u r e 3. O p t i c a l m i c r o g r a p h s o f u n c o a t e d and washcoated c o r d i e r i t e honeycomb s u b s t r a t e s . (a) Uncoated honeycomb, (b) 18.2% washcoat (80% s i l i c a l i t e + 20% s i l i c a b i n d e r ) , and ( c ) 30% washcoat (60% s i l i c a l i t e + 40% s i l i c a b i n d e r ) .


31. PATIL & LACHMAN

Methanol Conversion on Ceramic Honeycomb

Extruded zeolite catalysts offer higher zeolite content per unit volume, which increases the overall catalytic activity. These results demonstrate that zeolite catalysts perform well in honeycomb form. This is similar to the well established use of noble metals/gamma alumina washcoats on cordierite honeycombs for automotive emission control. This geometrical form offers the advantages of low pressure drop, high geometric surface area, and short diffusion distance in a fixed bed reactor. References


1)

Deluca, J. P., and Campbell, L. L., Thin Wall Ceramics as Monolithic Catalyst Supports; Advanced Materials in Catalysis, Academic Press, Inc., 1977; Chapter 10. 2) Howitt, J. S., Ceramic Industry 1975, 104, 19; Thin Wall Ceramics as Monolithic Catalyst Supports; SAE Tech Paper No. 800082 (1980). 3) Vara, J . , Voodburning; The Catalytic Combustor comes of Age; Country Journal, Nov. 1982, pp 92-99. 4) Kenson, R. E., Chemical Engineering Progress. Nov. 1985, 57. 5) Bonacci, J. C., Egbert, V., Collions, M. F., and Heck, R. M., Proceedings of the 6th Annual Conference held at Newport Beach CA, 1982; Precious Metals 1983, pp 29-44. 6) Harrison, B., Diwell, A. F., andWyatt,Μ., Platinum Metals Rev. 1985, 29, (2) 50-56. 7) Chang, C. D., Kuo, J. C. W., Lang, W. Η., Jacob, S. Μ., Wise, J. J . , and Silvestri, A. J . , Ind. Eng. Chem. Process Res. Dev. 1978, Vol. 17, 225. 8) Elmer, Τ. Η., Ceramic Eng. and Sci. Proceedings 1986, Vol. 7, No. 1-2, 40-51. 9) Lachman, I. M., Bagley, R. D., and Lewis, R. M., Amer. Ceramic Soc. Bull. 1981, 60, No. 2, p. 202-5. 10) Lachman, I. M., and Nordlie, L. Α., U.S. Patent No. 4,631,267, Dec. 23, 1986. RECEIVED January 25, 1988


Methanol Conversion on Ceramic Honeycombs Coated with Silicalite

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