Influence of Pore Structure on the Catalytic Behavior of Clay Compounds

Influence of Pore Structure on the Catalytic Behavior of Clay Compounds

E. Kikuchi and T. Matsuda

Department of Applied Chemistry. School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169. Japan

ABSTRACT

The effect of pore structure on the catalytic activity of pillared clays was investigated using alumina-pillared montmorillonite (Al-mont) with a microporous structure and alumina-pillared saponite (Al-sapo) having mesopores. The disproportionation and isomerization of trimethylbenzene, and the cracking of cumene were adopted as model reactions. The catalytic activity of pillared clays was affected by pore structure as well as by acidity: Al-mont having less acidity was more active for the disproportionation reaction than Al-sapo. although the effect of pore structure on the cracking and isomerization was small compared with the disproportionation. The microporosity is considered to enhance the concentration of reactant molecules at the acid sites, resulting in a high catalytic activity. It is suggested that the significant influence of pore structure on disproportionation is attributed to the second order kinetics of disproportionation which is a bimolecular reaction.

INTRODUCTION The activity advantage of zeolite catalysts over amorphous silica-alumina has well been documented. Weisz and his associates [ l ] reported that faujasite Y zeolite showed 103 t o 104 times greater activity for the cracking of n-hexane than silica-alumina. Wang and Lunsford et al. [2] also noted that acidic Y zeolites were active for the disproportionation of toluene while silica-alumina was inactive. The activity difference between zeolite and silica-alumina has been attributed to their acidic properties. It is, however, difficult to explain the superactivity o f zeolite relative to silica-alumina on the basis of acidity, since the number of acid sites of Y-type zeolite is only about 10 times larger than that of silica-alumina. To account for it, Wang et al. [2] proposed that the microporous structure of zeolite enhanced the concentration of reactant molecules at the acid sites. The purpose of the present work is to show that such a microporous effect is valid for pillared clay catalysts. Pillared clay is a new family o f molecular sieve materials obtained by

378 E. Kikuchi and T. Matsuda

exchanging charge compensating c a t i o n s between t h e s i l i c a t e l a y e r s o f c l a y w i t h large inorganic

polyoxycations

such

i n o r g a n i c o x i d e c l u s t e r s a r e formed:

as

[ A l l 304(OH)24(H20)1~]7t.

On h e a t i n g ,

t h e y p r o p open t h e c l a y l a y e r permanently

t o generate a microporous structure.

I t has r e c e n t l y been p o i n t e d o u t [ 3 , 4 ]

t h a t t h e a g g r e g a t i o n manner o f c l a y

l a y e r s d i f f e r e n t i a t e s t h e pore s t r u c t u r e o f p i l l a r e d c l a y s and t h a t t h i s h i g h l y dependent on t h e n a t u r e o f t h e c l a y .

Edge-to-face

o r edge-to-edge

is

layer

a g g r e g a t i o n g i v i n g mesopores competes f a v o r a b l y w i t h face t o f a c e a g g r e g a t i o n when t h e l a y e r s i z e i s s m a l l o r t h e l a y e r morphology i s l a t h - l i k e . w e l l ordered face t o face aggregation, zeolite-like

I n contrast,

which leads t o t h e formation o f a

r e g u l a r microporous structure,

o c c u r s i n t h e case o f c l a y s w i t h

l a r g e l a y e r s i z e o r p a n c a k e - l i k e morphology.

Small p a r t i c l e s a p o n i t e has a h i g h

tendency t o g i v e edge-to-edge

w i t h l a r g e p a r t i c l e s tends

o r edge-to-face to

aggregate

aggregation w h i l e montrnorillonite

i n the

face-to-face

manner.

These

a g g r e g a t i o n manners have been demonstrated b y e l e c t r o n microscopy and a d s o r p t i o n measurements [5,6].

I n t h i s study, we i n v e s t i g a t e t h e m i c r o p o r o u s e f f e c t u s i n g

p i l l a r e d m o n t m o r i l l o n i t e and s a p o n i t e .

EXPERIMENTAL Ca t a 1y s t The c l a y s used i n t h i s s t u d y were sodium-type

n a t u r a l m o n t m o r i l l o n i t e and

s y n t h e t i c s a p o n i t e o b t a i n e d f r o m Kunirnine I n d u s t r y Co. capacities

were

1.2

and

[Al,304(OH)24(H20)12]7t, addition o f

0.8 rneq./g,

respectively.

was p r e p a r e d f r o m

intercalating

since the addition of The

NaOH s o l u t i o n was

previously

The h y d r o l y z e d

u n t i l t h e p r e c i p i t a t e vanished, t o AlC13 s o l u t i o n

i n t e r c a l a t i o n method o f

[Al,30,(OH)24(H20)12]7+

agent,

h y d r o l y z e d AlC13 s o l u t i o n by

NaOH s o l u t i o n t o make t h e OH/A1 m o l a r r a t i o 2.5.

AlC13 s o l u t i o n was aged f o r about 12 h a t 50°C precipitate.

An

T h e i r c a t i o n exchange

sodium-type

described

in

yielded

clay with

detail

[7].

a

The

i n t e r c a l a t e d p r o d u c t was c a l c i n e d a t a g i v e n t e m p e r a t u r e i n t h e range 400-600°C f o r 4 h.

P i l l a r e d m o n t m o r i l l o n i t e and s a p o n i t e t h u s o b t a i n e d a r e a b b r e v i a t e d as

Al-mont and Al-sapo,

respectively.

Apparatus and procedures The

disproportionation

and

isornerization

of

trimethylbenzene(TrM6)

s t u d i e d a t 2OO0C u s i n g a c o n t i n u o u s f i x e d bed r e a c t o r . d i l u t e d w i t h n i t r o g e n i n a m o l a r r a t i o of c a r r i e d o u t a t 4OO0C u s i n g a p u l s e r e a c t o r .

1:9.

were

The r e a c t a n t TrMB was

The c r a c k i n g o f curnene was

The c a t a l y s t was t r e a t e d i n a

stream o f n i t r o g e n f o r 1 h a t a d e s i r e d t e m p e r a t u r e i n t h e range 400-600°C

prior

Pore Structure and Catalysis of Clay Compounds 379

t o reaction. Temperature programmed d e s o r p t i o n measurements The number o f a c i d s i t e s o n p i l l a r e d c l a y s was d e t e r m i n e d b y means o f temperature programmed d e s o r p t i o n (TPD) o f ammonia.

I n each TPD experiment,

a

sample weighing about 0.5 g was t r e a t e d in vacuo f o r 1 h a t a g i v e n temperature i n t h e range 400

-

60OoC.

Amnonia was adsorbed a t a d e s i r e d temperature (100-

30OoC) f o r 30 min and evacuated f o r 30 min. a r a t e o f 10°C/min detector.

T h i s sample was heated t o 7OO0C a t

and desorbed ammonia was m o n i t o r e d by thermal c o n d u c t i v i t y

As water was desorbed s i m u l t a n e o u s l y w i t h ammonia,

spectrum was o b t a i n e d by p o i n t - b y - p o i n t

t h e ammonia TPD

s u b t r a c t i o n o f t h e water d e s o r p t i o n

spectrum o b t a i n e d w i t h t h e sample which had n o t adsorbed ammonia.

RESULTS AND DISCUSSION F i g u r e 1 shows t h e ammonia TPD s p e c t r a o b t a i n e d w i t h Al-mont c a l c i n e d a t 40OoC.

Al-sap0

was more a c i d i c

t h a n Al-mont.

and Al-sap0

It i s generally

assumed t h a t t h e a c i d s i t e s on p i l l a r e d c l a y s a r e a t t r i b u t a b l e e i t h e r t o t h e s i l i c a t e layer o f clays or t o the p i l l a r s .

I t was shown p r e v i o u s l y [8,9] t h a t

t h e a c i d i t y i n c r e a s e d w i t h i n c r e a s i n g number o f p i l l a r s .

The number o f p i l l a r s ,

however, cannot s e r v e t o e l u c i d a t e t h e d i f f e r e n c e i n a c i d i t y between Al-mont and Al-sap0 because more a c i d i c Al-sap0 has s m a l l e r number o f p i l l a r s than Al-mont. being

2.20

and

3.3 mnol/g, r e s p e c t i v e l y .

Many

investigators

have proposed

m

8

H L

v 0 l

W

u 4-

0

c

0

F

&I

I 9 U

c W c u

0

0

100 Desorptlon temperature/°C Fig. 1. Ammonia TPD s p e c t r a o f Al-mont(--.-) Fig. 2. Ammonia TPD s p e c t r a o f Al-mont. 25OoC (---- ), and 3OO0C(---).

2OOOC (---),

200 300 400 k s o r p t Ion temperature/%

and Al-sapo(-)

500

c a l c i n e d a t 40OoC.

Ammonia was adsorbed a t 100°C(-),

:M(I E. Kikuchi and T. hlatsuda

-

10.

,V

I

V v) W

% 12.5

c

c

-8

7,

‘m

I

01

7

P

\

c

a

U

U v)

c

c

3

5.

c

0

V

U W

5.0

U 01

4

m

rY

2

7,5

v)

0

g

10.0

\

U

rY

*

**

2,5

z

0 0.2 0,4 0.6 Concentration o f a c i d s i es adsorbing ammonia a t 2000C/nmol g-

F

Coricerilratioii of a c i d sites/imno1 (J-1

F i g . 3. R e l a t i o n between t h e c r a c k i n g a c t i v i t y o f Al-mont and t h e c o n c e n t r a t i o n o f a c i d s i t e s . Ammonia was adsorbed a t 100°C( 0 ) .2OO0C( A ) , and 25OoC( 0 ). Fig. 4. The c r a c k i n g a c t i v i t y o f Al-mont( 0 ) and Al-sapo( A t h e c o n c e n t r a t i o n o f a c i d s i t e s a d s o r b i n g ammonia a t 20OoC.

)

as a f u n c t i o n o f

[ l o - 1 2 1 t h a t a c i d i t y i s g e n e r a t e d by t h e decomposition o f p i l l a r s as f o l l o w s .

+

[A11304(OH)24(H20)12]7t

I f t h i s i s t h e case, capacity (CEC).

+

7 Ht

+

20.5 H20

p i l l a r e d c l a y would have c o r r e s p o n d i n g c a t i o n exchange

The C E C v a l u e s o f t h e s e c l a y s d e c r e a s e d t o z e r o on

i n t e r c a l a t i o n w i t h [Al,304(OH)24(H,0)12]7t a l t h o u g h a f t e r c a l c i n a t i o n a t 40OoC. 0.13 meq.g-l

6.5 A1203

o f CEC.

respectively.

cations. Al-sapo

C a l c i n a t i o n i n c r e a s e d CEC,

and A l - m o n t

showed 0.35

and

Thus, t h e d i f f e r e n c e i n t h e a c i d i t y observed

between Al-mont and Al-sapo seems t o be a t t r i b u t a b l e p r e d o m i n a n t l y t o t h e number o f c a t i o n exchangeable s i t e s g e n e r a t e d on decomposition o f p i l l a r s . As expected from t h e TPD r e s u l t s , Al-sapo was more a c t i v e f o r t h e c r a c k i n g o f cumene on a p e r w e i g h t o f c a t a l y s t b a s i s t h a n Al-mont. t h e c a t a l y t i c a c t i v i t y on a b a s i s o f a c t i v e s i t e s , a c t i v e s i t e s on these c a t a l y s t s . temperature F i g . 2.

o f ammonia a d s o r p t i o n .

By i n t e g r a t i n g

I n o r d e r t o compare

we e v a l u a t e d t h e number o f

TPD s p e c t r a were measured w i t h v a r y i n g t h e Typical

these spectra,

r e s u l t s on Al-mont the concentration o f

a r e shown

in

acid sites

c o r r e s p o n d i n g t o d i f f e r e n t s t r e n g t h o f a c i d i t y can be determined. The t y p i c a l c r a c k i n g a c t i v i t y o f Al-mont. constant,is

expressed by t h e f i r s t o r d e r r a t e

shown i n Fig. 3 as a f u n c t i o n o f t h e c o n c e n t r a t i o n of a c i d s i t e s

t h u s determined.

Here, t h e c o n c e n t r a t i o n o f a c i d s i t e s was changed by c a l c i n i n g

Pore Structure and Catalysis of Clay Compounds 381

t h e p i l l a r e d c l a y a t 400,

500,

and 600OC.

C a l c i n a t i o n a t h i g h e r temperature

It i s obvious f r o m these r e s u l t s t h a t t h e

c o n s i d e r a b l y decreased t h e a c i d i t y .

a c i d s i t e s a d s o r b i n g ammonia a t 2OO0C a r e r e s p o n s i b l e f o r t h e c r a c k i n g o f cumene. F i g u r e 4 compares t h e c r a c k i n g a c t i v i t i e s o f Al-mont function o f the concentration o f acid sites. concentration o f a c t i v e sites, t h a n Al-sapo.

Al-mont

and A l - g a p 0

as a

When compared a t a d e f i n i t e

exhibited o n l y s l i g h t l y higher a c t i v i t y

The a c t i v i t y o f an a c i d c a t a l y s t i s a f f e c t e d by t h e n a t u r e o f t h e

a c i d i t y as w e l l as b y t h e number o f a c i d s i t e s .

It has been shown [7,13]

Al-sapo i s f a r more Bronsted a c i d i c t h a n Al-mont,

due t o t h e presence o f Si-0-A1

linkages i n t h e tetrahedral layer.

that

Thus, a c i d i t y cannot e l u c i d a t e t h e a c t i v i t y

d i f f e r e n c e between A l - m n t and Al-sapo.

As proposed by Wang and L u n s f o r d [ 2 ] t o

e x p l a i n t h e s u p e r i o r i t y o f HY over s i l i c a - a l u m l n a ,

t h e a c t i v i t y o f an a c i d s i t e

seems t o be a f f e c t e d by t h e c o n c e n t r a t i o n o f r e a c t a n t around t h e s i t e .

I n that

case, t h e a c t i v i t y should depend on t h e pore s t r u c t u r e o f t h e c a t a l y s t s i n c e t h e c o n c e n t r a t i o n o f r e a c t a n t molecules i s considered t o be structure o f a solid.

s e n s i t i v e t o t h e pore

We deduce t h a t t h e h i g h c r a c k i n g a c t i v i t y o f Al-mont

is

due t o i t s r e g u l a r microporous s t r u c t u r e p e r m i t t i n g a h i g h c o n c e n t r a t i o n o f cumene a t t h e a c i d s i t e s . The same e f f e c t should be expected i n o t h e r c a t a l y t i c r e a c t i o n s . confirm

this,

disproportionation

0

0.1

and

O,2

isomerization

0,3

t

I n order t o

reactions

0,4

Concentration f acid s i es adsorbing anmonia a t 2508C/nmol g-

o)

and Al-sapo( A ) as Fig. 5 . The d s p r o p o r t i o n a t i o n a c t i v i t y o f Al-mont( a f u n c t on o f t h e c o n c e n t r a t i o n o f a c i d s i t e s a d s o r b i n g ammonia a t 25OOC.

were

382 E. Kikuchi and T.Matsuda

I smer iza t ion

c I

u

0)

m c I

ml

5

4

\

c, K

m m c

c,

0

u

W c,

2

m

E

r-

5? 0

0,2 0,4 0,6 Concentration o f a c i d s i es adsorbing a n o n i a a t 200O~/mn01 g-

0

0-1

0,2

0,3 0,4 Concentratlon o f a c i d s i f e s adsorbing

f

a m n i a a t Z S O O C / ~ I g-

Fig. 6. The i s o m e r i z a t i o n and d i s p r o p o r t i o n a t i o n a c t i v i t i e s o f Al-mont( Al-sapo( A ) as a f u n c t i o n o f t h e c o n c e n t r a t i o n o f a c i d s i t e s . investigated.

F i g u r e 5 shows t h e r e l a t i o n s h i p between t h e a c t i v i t y f o r

d i s p r o p o r t i o n a t i o n o f 1.2.4-trimethylbenzene o f acid sites, 25OoC.

o ) and

(1,2,4-TrMB)

and t h e c o n c e n t r a t i o n

which was determined from t h e amounts o f ammonia adsorbed a t

F o r t h i s r e a c t i o n , a b e t t e r l i n e a r r e l a t i o n was o b t a i n e d w i t h t h e

r e s u l t s o f TPD o f ammonia adsorbed a t 25OoC.

Al-mont was f a r more a c t i v e f o r

t h i s r e a c t i o n t h a n Al-sapo. I t i s n o t e d t h a t t h e microporous e f f e c t was g r e a t e r i n t h e d i s p r o p o r t i o n a t i o n

o f 1.2.4-TrMB paper [14],

t h a n i n t h e c r a c k i n g o f cumene. t h e d i s p r o p o r t i o n a t i o n o f 1.2.4-TrMB

As shown i n t h e p r e v i o u s a t 2OO0C p r o c e e d s v i a a

b i m o l e c u l a r t r a n s i t i o n s t a t e and obeys t h e second o r d e r k i n e t i c s ,

I n contrast,

t h e c r a c k i n g o f cumene i s t h e f i r s t o r d e r k i n e t i c s w i t h r e s p e c t t o cumene concentration. significantly

Thus,

i t seems t h a t

t h e microporous e f f e c t

i s e x e r t e d more

i n t h e second o r d e r r e a c t i o n ( d i s p r o p o r t i o n a t i o n )

f i r s t order reaction (cracking)

than

i n the

i f pore s t r u c t u r e p l a y s an i m p o r t a n t r o l e i n

l o c a l i z i n g c o n c e n t r a t i o n o f r e a c t a n t molecules. F u r t h e r p r o o f was o b t a i n e d b y c o m p a r i n g t h e c a t a l y t i c a c t i v i t y f o r t h e d i s p r o p o r t i o n a t i o n and i s o m e r i z a t i o n o f t r i m e t h y l benzene. i s expected t o obey f i r s t o r d e r k i n e t i c s ,

As t h e i s o m e r i z a t i o n

t h e microporous e f f e c t would appear

more i n d i s p r o p o r t i o n a t i o n t h a n i n i s o m e r i z a t i o n . trimethylbenzene (1.2.3-TrMB) isomerization

Here,

1.2.3-

was used as r e a c t a n t i n s t e a d o f 1,2,4-TrMB, s i n c e

conversion o f 1,2,4-TrMB

was t o o small t o d i s c u s s

t h e change i n

Pore Structure and Catalysis of Clay Compounds 383

Table. 1. Rate c o n s t a n t s f o r d i s p r o p o r t i o n a t i o n and i s o m e r i z a t i o n o f 1.2.3-TrMB a t 2OO0C. Catalyst

Al-mont

Al-sapo

kdis/mOl g - l

St3C-l

5.0

10-7

2.1

10-7

kiso/mol

sec-’

3.7

10-7

3.5

10-7

g-l

0.60

1.35

kdis/kiso

F i g u r e 6 shows t h e r e l a t i o n s h i p b e t w e e n t h e a c t i v i t y f o r

the a c t i v i t y .

d i s p r o p o r t i o n a t i o n and i s o m e r i z a t i o n o f 1.2.3-TrMB sites.

and t h e c o n c e n t r a t i o n o f a c i d

The a c i d s i t e s adsorbing ammonia a t 2OO0C were r e s p o n s i b l e f o r t h e

i s a n e r i z a t i o n o f 1,2,3-TrMB.

When compared a t a g i v e n c o n c e n t r a t i o n o f a c i d

s i t e s , Al-mont

e x h i b i t e d about 5 t i m e s h i g h e r a c t i v i t y f o r d i s p r o p o r t i o n a t i o n

t h a n Al-sapo.

I n contrast,

times h i g h e r t h a n Al-sapo.

t h e i s o m e r i z a t i o n a c t i v i t y o f Al-mont was about 2 T h i s d i f f e r e n c e i s a t t r i b u t e d t o t h e second o r d e r

k i n e t i c s o f d i s p r o p o r t i o n a t i o n , which i s a b i m o l e c u l a r r e a c t i o n .

Table 1 shows

t h e r a t e c o n s t a n t a t 2OO0C a n d t h e r a t i o f o r d i s p r o p o r t i o n a t i o n t o isomerization,

namely,

isomerization.

the

selectivity

Microporous Al-mont

for

disproportionation

against

exhibited high selectivity f o r

d i s p r o p o r t i o n a t i o n compared w i t h Al-sapo.

coNcLusIoN Al-mont h a v i n g z e o l i t e - l i k e r e g u l a r micropores i s a c t i v e f o r t h e c r a c k i n g o f cumene, and t h e d i s p r o p o r t i o n a t i o n and i s o m e r i z a t i o n o f TrMB compared f a v o r a b l y w i t h Al-sapo w i t h mesopores, a l t h o u g h Al-mont

i s l e s s a c i d i c t h a n Al-sapo.

The

microporous s t r u c t u r e i s considered t o enhance t h e c o n c e n t r a t i o n o f r e a c t a n t molecules a t t h e a c i d s i t e s ,

resulting i n

high catalytic activity.

The

microporous e f f e c t i s e x e r t e d more s i g n i f i c a n t l y i n d i s p r o p o r t i o n a t i o n than i n cracking

and

isomerization.

Thus,

the

catalytic

selectivity

for

d i s p r o p o r t i o n a t i o n and i s o m e r i z a t i o n i s a l s o a f f e c t e d by t h e m i c r o p o r o s i t y .

Al-

mont e x h i b i t s h i g h e r s e l e c t i v i t y f o r d i s p r o p o r t i o n a t i o n a g a i n s t i s o m e r i z a t i o n t h a n Al-sapo.

These p r o p e r t i e s a r e a t t r i b u t a b l e t o t h e second o r d e r k i n e t i c s o f

d i s p r o p o r t i o n a t i o n , which i s a b i m o l e c u l a r r e a c t i o n .

381

E. Kikuchi and T.Matsuda

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