Isomorphous Substitution of Si in Zeolite Single Crystals. Part II. On The Boron Distribution and Coordination In [B]-ZSM-5

619

P.A. Jacobs and R.A. van Santcn (Editors), Zeolites: Facts, Figures, Future 0 1989 Elsovier Science Publishcrs B.V., Amsterdam - Printed in T h e Netherlands

ISOMORPHOUS SUBSTITUTION OF Si IN ZEOLITE SINGLE CRYSTALS. PART 11. ON THE BORON DISTRIBUTION AND COORDINATION IN [B]-ZSM-5.

J.C. Jansen, R. d e Ruiter, E . Biron',

and H. van Bekkum

Department o f Organic Chemistry, Delft University of Technology, Jul i anal aan 136, 2628 BL Del ft, The Nether1 ands Abstract FTIR microscope measurements in the transmission mode on single crystal sheets of [B]-ZSM-5 have been used t o study the lattice boron distribution and coordination in the crystals. The infrared scans indicate that a homogeneous boron distribution is present in the [B]-ZSM-5 crystals, which contained 0-4 B/uc. A reversible hydrationldehydration experiment with a thin single crystal of [B]-ZSM-5 under the IR-Ticroscope l g f to further attribution o f the well known typical 1390 cm and 905 cm absorption bands in the [B]-ZSM-5 spectra. The 905 cm integrated band intensity was used for the quantitative determination of the latticelboron content, the reported asymmetric lattice vibration shift at 1085 cm cannot serve this purpose. [B]-ZSM-5 crystals of different form and morphology were used. Introduction Syntheses of ZSM-5 type zeolites in the presence o f T-atoms (B, Al, Ga) are well known (1). The characterization of the isomorphous substitution o f Si by T-atoms in the framework of zeolite powders (crystal diameter 2 pm) is mainly performed by three methods (2): (i) The unit cell contraction/expansion determination with XRD (3,4) ( i i ) The lattice vibration shifts measured with IR spectroscopy (5,6) ( i i i ) Multinuclear MAS NMR techniques (7,8). Neither XRD nor MAS NMR are suitable techniques t o study the distribution of lattice boron in the crystals. In the present work the lattice boron content and the lattice boron coordination were simultaneously studied with infrared spectroscopy. An infrared microscope was applied t o obtain maps of transmission/absorption

-

.............................

' Present

address: The Prime Minister's Office, Israel Institute for Biological Research, P.O. Box 19, Ness Ziona 70450, Israel.

680

s p e c t r a as a f u n c t i o n o f t h e i n f r a r e d beam s p o t p o s i t i o n on e i t h e r t h i n o r p o l i s h e d a s -s y n t h e s i z e d s i n g l e c r y s t a l s o r f l a k e s o f s i n g l e c r y s t a l s o f [B]-ZSM-5.

These

t e c h n i q u e s a l l o w e d t h e d e t e r m i n a t i o n o f t h e l a t t i c e boron

distribution. Experimental Single

crystals

o f [ B I - , [ A l l - and [Ga]-ZSM-5,

amounts, 23 < Si/T < l i n e d autoclave without chem ic als were: s i l i c a ,

c o n t a i n i n g d i f f e r e n t T-atom

( 0 - 4 T/uc), were o b t a i n e d b y u s i n g a 50 m l T e f l o n s t i r r i n g a t 180 O C d u r i n g 5 - 8 days (4). S t a r t i n g A e r o s i l 200 (Degussa), TPABr s u p p l i e d by CFZ, Zaltbommel, b o r i c a c i d , g a l l i u m t r i c h l o r i d e , and sodium h y d r o x i d e a l l reagent (o

grade chemicals f ro m Merck and sodiumaluminate f rom R i e d e l de Hahn. Two

crystal

forms,

cubic

and

prismatic,

of

[B]-ZSM-5,

were

obtained

depending on t h e f o r m u l a t i o n . I n b o t h cases boron c o u l d be b u i l t i n o v e r t h e 0-4 B/uc range.

Additionally,

in

the

case o f [B]-ZSM-5 m o r p h o l o g i c a l l y m o d i f i e d p r i s m a t i c

s i n g l e c r y s t a l s ( s h e l f - t y p e w i t h 1 - 4 B/uc) were s y n t h e s i z e d by u s i n g a d i l u t e s y n t h e s i s m i x t u r e a t r e l a t i v e l y l o w pH (see F i g u r e 1).

a)

b)

c)

,

F i g u r e 1. SEM photographs o f p r i s m a t i c and s h e l f - t y p e c r y s t a l s w i t h 0.1 B/uc (a ), 1.9 B/uc ( b ) and 3.1 B/uc, r e s p e c t i v e l y . Table

1 gives

the

type

o f [B]-ZSM-5 c r y s t a l s o b t a i n e d t o g e t h e r w i t h t h e

corre s ponding s y n t h e s i s f o r m u l a t i o n .

68 1 Table 1. [B]-ZSM-5 c r y s t a l f o r m and m o l e c u l a r s y n t h e s i s f o r m u l a t i o n . reag e nt s

c r y s t a l form cubic p r i s m a t i c (9) 0-.5 12 20 20 2000

%3 Na TPa20 HO,

6

prismatic

0-3 56 9 9 2000

+

shelf

2.5 12 4. 5 4.5 2000

0

+

I.

The

elemental a n a l y s i s was done on a P e r k i n Elmer Plasma I 1 ICPAES (Induced

Coupled

Plasma

Atomic

selected

and

dissolved

Emission Spectroscopy)

measured

i n p l a s t i c v e s s e l s . F u r t h e r c h a r a c t e r i z a t i o n was made w i t h SEM and

in

1-4% aqueous

apparatus.

C r y s t a l s were

HF. The s o l u t i o n s were s t o r e d and

XRD.

For t h e i n f r a r e d measurements a s - s y n t h e s i z e d t h i n (2 pm) s i n g l e c r y s t a l s and crystal

coupes

were

used,

as

w e l l as c r y s t a l s which were p o l i s h e d a f t e r

w h i l e being embedded i n epoxide/amine r e s i n . The

crystals under

stage

were

p l a c e d on

a computer c o n t r o l l e d s t e p p i n g mot or sample

an I R microscope adapted t o a Bruker FTIR IFS-66 spect romet er.

The microscope m a g n i f i c a t i o n used was 360 x o r 720 x, t h e beam s p o t s i z e was 8,

13.5

or

pm and

20

scanning

intervals o f

5

pm were a p p l i e d . A l l

measurements were performed i n t h e t r a n s m i s s i o n mode. A s h e l f - t y p e [B]-ZSM-5 c r y s t a l sheet was c a l c i n e d a t 450 O C ; t h e h e a t i n g and c o o l i n g r a t e was temperature

1

controlled

OC/min. sample

The

calcined

stage

(max.

c r y s t a l sheet was p l a c e d on a 180

OC)

under t h e microscope.

S pec t ra were t h en recorded as a f u n c t i o n o f t h e temperature. R e s u l t s and d i s c u s s i o n I n our

IR

s pec t r o s c o p i c s t u d y o f [B]-ZSM-5,

w i t h 23 < Si/B <

m,

attention

c e n t r e d on ( i ) t h e gradual s h i f t o f t h e asymmetric s t r e t c h l a t t i c e v i b r a t i o n at

-

1085 c m-l and ( i i ) t h e 6 - 0 - S i l a t t i c e v i b r a t i o n a t

The asymmetric s t r e t c h l a t t i c e v i b r a t i o n a t

-

-

905 an-'.

1065 cm-'

Powder The s h i f t o f t h e 1085 cm-l band as a f u n c t i o n o f T-atom s u b s t i t u t i o n i s w e l l documented high

(5 )

f o r z e o l i t e s w i t h l o w Si/T r a t i o s . Though n o t s e n s i t i v e f o r

s i l i c a z e o l i t e s , t h i s s h i f t has been a p p l i e d r e c e n t l y a t a f u n c t i o n o f

T-atom i n c o r p o r a t i o n f o r [B]-ZSM-5 and [Gal-ZSM-5 (6,lO).

682 I n a s e r i e s o f experiments, u s i n g t h e KBr p e l l e t t echnique, t h e s h i f t o f t h e band a t

1085

cm-l

of

powdered

a s - s y n t h e s i zed

[B]-ZSM-5

crystals

was

i n v e s t i g a t e d . No c o r r e l a t i o n c o u l d be f o u n d between t h e boron c o n t e n t o f t h e c r y s t a l s and t h e s h i f t o f t h e 1085 cm-l band, see T a b l e 2. T able 2. T y p i c a l w T-0-T band p o s i t i o n as a f u n c t i o n o f d i f f e r e n t 1 - Z S M - 5 z e o l i t e s i f i s t h e a s - s y n t h e s i z e d and c a l c i n e d forms, r e s p e c t i v e l y . as - s y n t h

Si/T Silicali t e [B]-ZSM-5 [B]-ZSM-5 [ B] - E M - 5 [ B] -ZSM-5 [B]-ZSM-5 [B]-ZSM-5 [Al]-ZSM-5 [Gal-ZSM-5 Moreover,

1090 1084 1087 1088 1085 1084 1084 1088 1086

m

320 160 95

50

30 23 11 46 the

.

calcined 1103 1100

1099 1099

band p o s i t i o n o f t h e asymmetric s t r e t c h v i b r a t i o n o f t h e as-

s y n t h e s i z e d s i l i c a l i t e h a r d l y d i f f e r s f r o m t h a t o f T-atom s u b s t i t u t e d ZSM-5 c r y s t a l s . Upon c a r e f u l c a l c i n a t i o n t h e samples o f T able 2 show, r e g a r d l e s s t h e T-atom t y p e and c o n t e n t , an u n d i f f e r e n t i a t e d s h i f t o f t h e band f r o m 1085 c m - l t o 1100 cm-'.

It

i s t o be concluded t h a t t h e s h i f t o f t h e asymmetric s t r e t c h v i b r a t i o n i s n o t s u i t a b l e f o r t h e q u a n t i t a t i v e d e t e r m i n a t i o n o f boron ( n o r o f A1 and Ga) in the ZSM-5 framework. The d i f f e r e n c e s i n wave number between as-sy n t h es iz ed

and c a l c i n e d samples m i g h t be r e l a t e d t o t h e f o r m a t i o n o f an

improved l a t t i c e upon c a l c i n a t i o n . Sinqle crystal I n an at t emp t

to

single

flake

the in

crystal

transmission

study

t h e was T-0-T band i n more d e t a i l a wedge-shaped

o f [B]-ZSM-5 was measured under t h e IR-microscope i n

mode. T h i s f l a k e , w i t h 2.6 B/uc,

v a r i e d f rom 3.5 t o 12 pm

t h i c k n e s s . A t t h e l a t t e r t h i c k n e s s t h e c r y s t a l was s t i l l t r a n s m i t t i v e i n

t h e was T-0-T h i g h i n t e n s i t y band r e g i o n . t r a n s m i s s i o n scan - i n d u p l o - was made o f a p a r t o f t h e c r y s t a l as

A

i n d i c a t e d i n F i g u r e 2a. The a p e r t u r e chosen was 13.5 pm and t h e s t e p w i d t h 5 pm. S u r p r i s i n g l y , t h e asymmetric s t r e t c h v i b r a t i o n was observed a t 1060 cm-l

i n t h e 3.5 pm l a y e r area, as shown i n t h e spectrum i n F i g u r e 3. Furthermore,

683 I

YO,

T-0-T

[cm-11

F i g u r e 2a. Schematic drawing o f t h e s i n g l e c r y s t a l f l a k e o f [B]-ZSM-5 w i t h 2.6 B/uc. Scan p o i n t s a r e i n d i c a t e d by ( t ) . T-0-T band p o s j l i o n as a f u n c t i o n o f t h e TPA' i n t e g r a t e d band 2b. u i i f t e n s i t y (1450 cm ) , e x p r e s s i n g t h e c r y s t a l t h i c k n e s s . the

scans

position,

show see

integrated cm-'. an

shift

Figure

TPA

2b.

of

t h e vas T-0-T band as a f u n c t i o n o f t h e spot

The

spot

p o s i t i o n s have been expressed as t h e

(tetrapropylammonium i o n ) a b s o r p t i o n band i n t e n s i t y a t 1450

Assuming e s s e n t i a l l y complete f i l l i n g o f t h e z e o l i t e w i t h TPA t h i s i s undependent measure o f

lattice the

a

c r y s t a l t h i c k n e s s . A t t h e same t i m e a c o n s t a n t

boron c on t e n t was observed ( v i d e i n f r a ) on t h e s e spot p o s i t i o n s . On

basis

uas T -0-T

t h e above o b s e r v a t i o n s i t i s concluded t h a t t h e s h i f t o f t h e

of band

i n t h i s p a r t i c u l a r experiment i s a f u n c t i o n o f t h e c r y s t a l

t h i c k n e s s and n o t o f t h e boron c o n t e n t . T h i s c o n c l u s i o n was f u r t h e r v e r i f i e d on a s i l i c a l i t e s i n g l e c r y s t a l sheet, w i t h a t h i c k n e s s between 3 and 15 pm, which showed under s i m i l a r c o n d i t i o n s t h e same uas T-0-T band s h i f t . Though band

not shift

angles.

The

i n the is

scope o f t h i s paper, i t i s supposed t h a t t h e uas T-0-T

c o r r e l a t e d t o i n c r e a s i n g T-0 d i s t a n c e s and d e c r e a s i n g T-0-T

question

remains

microstructure o f the crystals.

how t h i s

phenomenon

i s correlated t o the

684

The B-0-Si lattice vibration at

4000

-

905 cm-l

2000

3000

WAVENUMBER

1000

CM-'

Figure 3. IR transmission spectrum of [B]-ZSM-5. Crystal thickness: 3.5 pn. Infrared spectra of [B]-ZSM-5 are known to show (6) an absorption band at 905 cm'l, which is not present in silicalite. This band, which is apparently due to lattice boron, was studied as a function of the temperature. A stack plot of infrared spectra was recorded of a calcined single crystal of [B]-ZSM-5, with 4 B/uc. According to the interference wave present on the base line of the recorded spectra the crystal was about 18 pn thick. The crystal was positioned with the ac plane perpendicular to the beam direction. The absorption spectra of Figure 4 show upon cooling down from 180 to 20 'C: (i) an in intensity increasing sharp band at 3660 cm-', ( i i ) a decreasing band intensity at 1380 cm-l and ( i i i ) simultaneously increasing and decreasing absorption at 905 cm-l and 860 cm-', respectively. The 1380 cm-' band has been attributed to three-coordinated boron in the lattice and the 905 cm-l band to a B-0-Si vibration (11). According to "B MAS NMR results (12) trigonally coordinated boron converts reversibly into tetrahedrally coordinated boron upon hydration. The reversible hydration process was performed on the heatable sample stage under the microscope. Temperatures were chosen between 20 and 180 O C . On the analogy of the NMR

685 3660

Absorbance

"C

1000

2000 3000 Wavenumber cm-1

4000

F i g u r e 4. S t a c k p l o t o f i n f r a r e d s p e c t r a o f a [B]-ZSM-5 c r y s t a l w i t h 4 B/uc as a f u nc t i o n l o f t h e temperature. The asymmetric s t r e t c h v i b r a t i o n at 1085 cm has been o m i t t e d f o r c l a r i t y .

-

Scheme I. I n t e r p r e t a t i o n o f FTIR and MAS NMR r e s u l t s (12) on [B]-ZSM-5 on t h e basis o f r e v e r s i b l e hydration. Temp. (OC)

Boron c o o r d i n a t i o n i n the l a t t i c e

180

tr igonal

It

It

"B

MAS NMR (12)

H

M \-0

20

tetrahedral

FTIP cm

*.,

6

1

0

Si

0 **.

905

686 results

we

ascribe

the

905 cm-' and t h e 860 cm-l band t o t e t r a h e d r a l and

t r i g o n a l l a t t i c e boron, r e s p e c t i v e l y (see Scheme 1). The 905 cm-l and o r 1380 c m - l band, depending upon t h e p a r t i c u l a r sample conditions,

may

serve

to

characterize

the

tetrahedral

and

trigonal

c o o r d i n a t e d l a t t i c e boron d i s t r i b u t i o n i n [B]-ZSM-5 c r y s t a l s . Single

crystals

under

the

infrared

of

microscope w i t h

the

ac

plane

perpendicular t o the

beam d i r e c t i o n . The c r y s t a l s were advanced under t h e microscope on

an e l e c t r o n i c a l l y spot

[El-ZSM-5 w i t h a t h i c k n e s s up t o 20 pm were p o s i t i o n e d

infrared

size

recording

c o n t r o l l e d sample stage; t h e s t e p w i d t h was 5 pm and t h e

8 o r 20 pm. The framework boron d i s t r i b u t i o n was i n v e s t i g a t e d by

absorption

s p e c t r a as a f u n c t i o n o f t h e beam s p o t p o s i t o n on t h e

crystal

f ac e. A t y p i c a l a b s o r p t i o n s t a c k p l o t i s shown i n F i g u r e 5, t o g e t h e r w i t h a scheme o f t h e row o f s p o t p o s i t i o n s chosen on t h e c r y s t a l .

Absor

J 4000

3000 2000 wavenumber cmi

1000

F i g u r e 5. S t a c k p l o t o f i n f r a r e d s p e c t r a o f a p o l i s h e d [El-ZSM-5 c r y s t a l w i t h 4.1 B/uc as a f u n c t i o n o f t h e beam s p o t p o s i t i o n on t h e c r y s t a l . The scan p o i n t s a r e i n d i c a t e d by (x) on t h e c r y s t a l ac plane. The band

rims

of

intensity

t h e measured c r y s t a l s show a s l i g h t decrease i n t h e 905 cm- 1 compared

to

the

inner

part

of

t h e c r y s t a l s . It could,

however, e m p i r i c a l l y be e s t a b l i s h e d t h a t a s m a l l p a r t o f t h e d i r e c t beam was bent

at

the

c r y s t a l edge because o f d i f f r a c t i o n i n t o t h e a p e r t u r e r e g i o n .

T h i s f r e e pathway o f t h e s m a l l p a r t o f t h e i n f r a r e d beam s l i g h t l y reduced t h e l a t t i c e band i n t e n s i t i e s a t t h e p e r i p h e r y o f t h e c r y s t a l compared t o i n n e r p a r t o f t h e c r y s t a l (13).

687 Taking

this

phenomenon

i n t o account i t appears t h a t , a c c o r d i n g t o t h e 905

cm-l band i n t e n s i t y i n t h e s t a c k p l o t o f F i g u r e 5, an e s s e n t i a l l y homogeneous lattice

boron d i s t r i b u t i o n i s p r e s e n t i n t h e scanned c r y s t a l . Moreover, t h e

framework

boron

microscope between

amount

beam can

the

905/800

i n the

be

crystal

volume measured by t h e i n f r a r e d determined. A l i n e a r r e l a t i o n s h i p

quantitatively

c m - l i n t e g r a l band i n t e n s i t y r a t i o and t h e b u l k boron

c o n t e n t r e s u l t s i s observed ( 1 4 ) .

vuc

1

4.04

+

++

3.

++ + + + + + +

+

+

++

++

+

+ + + + + ++

+

+

+

vuc

+ +++

2D]

++ + +

+

+ +

++++++

+++

+ +

+ +

a

F i g u r e 6. The boron c o n t e n t expressed as B/uc, as a f u n c t i o n o f t h e spot p o s i t i o n i n t h e c - d i r e c t i o n on t h e ac c r y s t a l p l a n e o f t h r e e d i f f e r e n t [B]-ZSM-5 c r y s t a l forms. The c r y s t a l s ( l e f t ) p r o d u c t s o f t h e second s y n t h e s i z e m i x t u r e , a r e g rowt h i n h i b i t e d i n c o n t r a s t t o t h e c r y s t a l ( r i g h t ) a product o f t h e t h i r d synthesis mixture. This

integral

plotted crystal

band

intensity

ratio

i s expressed as B/uc i n F i g u r e 6 and

a g a i n s t beam s p o t p o s i t i o n s as i n d i c a t e d by t h e d o t t e d l i n e s on t h e ac planes. Three t h i n a s - s y n t h e s i zed c r y s t a l s o f [B]-ZSM-5 w i t h

shelf-type

morphology (15) ( l e f t ) and p r i s m a t i c t y p e morphology ( r i g h t ) a r e

d e p i c t e d i n F i g u r e 6. The p a r t i c u l a r measurements show a homogeneous l a t t i c e boron d i s t r i b u t i o n ( k .5 B/uc) which was observed i n a l l [B]-ZSM-5 c r y s t a l s examined.

688 Concl us ions (i) The

lattice

boron

FTIR-microscopy ( i i ) The

.

uas T-0-T

content

band

shift

and

d i s t r i b u t i o n can

i n MFI

be s t u d i e d u s i n g

frameworks i s a p p a r e n t l y due t o

framework r e l a x a t i o n and n o t t o isomorphously s u b s t i t u t e d T-atoms. (iii)The

905 c m-l band i n t h e [B]-ZSM-5 i n f r a r e d s p e c t r a was a t t r i b u t e d t o

t e t r a h e d r a l l a t t i c e boron. ( i v ) The

tetragonal

lattice

boron

distribution

i n prismatic

and

m o r p h o l o g i c a l l y m o d i f i e d p r i s m a t i c [B]-ZSM-5 c r y s t a l s i s homogeneous.

References

1. P.A. Jacobs and J.A. Martens, Stud. S u r f . S c i . C a t a l . 33, 1987, E l s e v i e r

Amsterdam, and r e f e r e n c e s c i t e d t h e r e i n . T i e l e n and P.A. Jacobs, Proc. I n t . Symp. Z eol. Cat al. , S i o f o k , Hungary, 1985, p. 1. 3. B.L. Meyers, S.R. Ely, N.A. K u t z , J.A. Kaduk, and E . van den Bossche, J. C a t a l . 9l, 1985, 352. 4. J.C. Jansen, E. B i r o n , and H. van Bekkum, Stud. S u r f . S c i . C a t a l . 37,

2. M.

1988, 133.

5 . E.M.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

F la nigen, i n J.A. Rabo ( e d i t o r ) , Z e o l i t e Chemist ry and C a t a l y s i s , ACS Monograph 171, Am. Chem. SOC., Washington D.C., 1976, p. 80. 6. Co udurie r and J.C. Vedrine, i n Y. Murakami, A. Lyima and J.W. Ward (editors), Proc. 7th Int. Conf. on Zeolites, Tokyo, 1986, Kodankska/Elsevier. D. 643. J. K l i n o w s k i , Ann.'Rev. Mater. S c i . 18, 1988, 189. E. Brumer, H . E r n s t , D. Freude, M. Hunger, and H. Pf e f e r , Stud. Surf. S c i . C a t a l . 37, 1988, 155. M.R. K l o t z , U . S . P a t e n t 4,268,420, 1981. J.M. Thomas and Xin-Sheng L i u , J. Phys. Chem. B, 1986, 4843. G. Co udurie r, A. Anrouse, J.C. Vedrine, R.D. Farke, L. Abrams, and R.D. Shamon, J. C a t a l . 108, 1987, 1. K.F.M.G.J. S c h o l l e and W.S. Veeman, Z e o l i t e s 5. 1985, 1 8. Private communication o f d r . .G. Zachmann o f . B r u k e r A n a l y t i s c h e Messtechnik GmbH, K a r l s r u h e . R. de R u i t e r , communicated a t t h e EUCHEM Conference on C a t a l y s i s by Z e o l i t e s , Lyon, France, October 1988, t o be p u b l i s h e d . J.C. Jansen, C.W.R. Engelen, and H. van Bekkum, accepted f o r p u b l i c a t i o n i n t h e ACS Monograph o f t h e 196th ACS N a t i o n a l Meeting, Los Angeles,

1988.