Chapter 12
New Tubular Silicate-Layered Silicate Nanocomposite Catalyst Microporosity and Acidity Todd A. Werpy, Laurent J. Michot, and Thomas J. Pinnavaia Department of Chemistry, Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824
The microporosity of a new tubular s i l i c a t e -layered s i l i c a t e nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded a l i q u i d micropore volume of ~0.20 cm g , as determined by both the t-plot and the Dubinin -Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller l i q u i d pore volume (~0.11 cm g ), most l i k e l y due to incomplete f i l l i n g of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid s i t e s . The TSLS complex was shown to be active for the acid -catalyzed dealkylation of cumene at 350°C, but the complex was less reactive than a conventional alumina p i l l a r e d montmorillonite. +
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Imogolite i s a n a t u r a l l y o c c u r r i n g a l u m i n o s i l i c a t e mineral with a unique t u n n e l - l i k e or t u b u l a r structure . The e x t e r n a l and i n t e r n a l van der Waals diameters of the tubes are approximately 25Â and 10Â r e s p e c t i v e l y ' . The tube dimensions are l a r g e r f o r the s y n t h e t i c form than f o r the n a t u r a l l y o c c u r r i n g d e r i v a t i v e owing t o a d i f f e r e n c e i n the number of repeat u n i t s d e f i n i n g the tube w a l l s . The molecular s i e v i n g p r o p e r t i e s of imogolite already have been demonstrated " . The adsorption o f small molecules occurs r e a d i l y on the i n t e r n a l surfaces, but l a r g e r species with k i n e t i c diameters > 10Â, such as p e r f l u r o t r i b u t y l a m i n e , are 1
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excluded . The p o t e n t i a l f o r s h a p e - s e l e c t i v e c a t a l y s i s by imogolite also has been recognized, but the thermal instability o f the t u b u l a r structure is a limiting factor ' " . We have been i n v e s t i g a t i n g the use o f i m o g o l i t e as a pillaring agent f o r smectite c l a y s with layer lattice structures "^. The r e g u l a r i n t e r c a l a t i o n o f t h e tubes w i t h i n t h e l a y e r e d host r e s u l t s i n the formation of a t u b u l a r s i l i c a t e - l a y e r e d s i l i c a t e (TSLS) complex. These new nanocomposite m a t e r i a l s may be viewed as p i l l a r e d c l a y s i n which the pillars themselves are microporous. S i g n i f i c a n t l y , the TSLS s t r u c t u r e i s thermally s t a b l e up t o 450°C when montmorillonite i s s e l e c t e d as t h e l a y e r e d h o s t . A schematic representation o f a TSLS complex i s provided i n F i g u r e 1. On the b a s i s o f p r e l i m i n a r y XRD and s t o c h i o m e t r i c s t u d i e s , i t appears t h a t t h e i m o g o l i t e tubes are i n van der Waals contact, most l i k e l y i n a l o g - j a m - l i k e array i n the l a y e r s i l i c a t e g a l l e r i e s . Although the tubes s t u f f t h e g a l l e r i e s , two unique adsorption environments are a v a i l a b l e , namely, the i n t r a - a n d i n t e r - t u b e pores designated A and Β i n F i g u r e 1. In t h e present work we examine the m i c r o p o r o s i t y o f a TSLS complex formed from s y n t h e t i c i m o g o l i t e and n a t u r a l montmorillonite. Nitrogen adsorption and desorption isotherms are reported and analyzed i n terms of microporous volume and surface area. Also, the adsorption isotherm f o r an organic adsorbate, m-xylene, i s reported. Preliminary FTIR r e s u l t s f o r the chemisorption o f p y r i d i n e and c a t a l y t i c s t u d i e s o f t h e d e a l k y l a t i o n of cumene suggest t h a t TSLS complexes are promising microporous acids f o r shape s e l e c t i v e chemical conversions. 2
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Experimental S e c t i o n ; An i m o g o l i t e p i l l a r i n g s o l u t i o n was prepared by the a c i d h y d r o l y s i s o f 30mM t e t r a e t h y l o r t h o s i l i c a t e and 60mM aluminum sec-butoxide f o r 2 days at 98° C according t o t h e method o f F a r m e r . The r e a c t i o n s o l u t i o n was d i a l y z e d 4 days against d i s t i l l e d water t o remove i o n i c and molecular by-products prior t o being used as a pillaring reagent f o r montmorillonite. I n t e r c a l a t i o n of i m o g o l i t e i n t o sodium montmorillonite was achieved by r e a c t i n g -1.0 wt% aqueous suspensions o f the two reagents according t o p r e v i o u s l y d e s c r i b e d methods ' . Nitrogen adsorption/desorption isotherms were determined at -196°C oh a Quantachrome Autosorb Sorptometer. U l t r a h i g h p u r i t y n i t r o g e n was used as t h e adsorbate and u l t r a h i g h p u r i t y helium was the c a r r i e r gas. The adsorption isotherm f o r m-xylene was obtained on a McBain balance equipped with quartz g l a s s springs and buckets. The samples were outgassed at 325° C under vacuum f o r about four hours p r i o r t o adsorption. The FTIR spectrum f o r chemisorbed p y r i d i n e was obtained on an IBM IR44 spectrometer. The TSLS sample was prepared 10
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by e v a p o r a t i n g a n aqueous s u s p e n s i o n o f t h e complex o n t o a silicon wafer a t room temperature and subsequently o u t g a s s i n g t h e f i l m a t 350°C p r i o r t o p y r i d i n e a d s o r p t i o n a t 25°C. P h y s i c a l l y a d s o r b e d p y r i d i n e was removed b y pumping u n d e r vacuum a t room t e m p e r a t u r e . C a t a l y t i c d e a l k y l a t i o n o f cumene was c a r r i e d o u t i n a fixed bed reactor operated a t 350°C and atmospheric pressure. T h e c o n t a c t t i m e was 1.5 s e c a n d t h e WHSV was 0.4g cumene/g/hr. The c o n v e r s i o n o f cumene was d e t e r m i n e d u s i n g a H e w l e t t P a c k a r d 5890 g a s C h r o m a t o g r a p h e q u i p p e d w i t h a S u p e l c o wide b o r e c a p i l l a r y column. R e s u l t s and D i s c u s s i o n Isotherms f o r t h e a d s o r p t i o n and d e s o r p t i o n o f n i t r o g e n ( k i n e t i c diameter, 3 . 6 Â ) were o b t a i n e d f o r t h e i m o g o l i t e m o n t m o r i l l o n i t e TSLS complex a t -196°C (see F i g u r e 2 ) . The shape o f t h e a d s o r p t i o n i s o t h e r m below a p a r t i a l p r e s s u r e o f 0.5 was s i m i l a r t o t h e c l a s s i c a l t y p e I isotherm f o r a m i c r o p o r o u s ( •β
1
60
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20
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ο
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t(A) Figure
4.
t-Plot
forN
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a d s o r p t i o n on t h e TSLS
complex.
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WERPYET AL.
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100.
3
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ε ο >
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Ο
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(Α
5 20
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Adsorption 25°C.
o f m-xylene
b y t h e TSLS
complex a t
-0.90 -0.92 ^
I ]
-0.94 _
•
• -0.96 .
•
-0.98 _ -1.00 _ -1.02 -1.04 . -1.06
i
-1.08 _ -1.10
1
0
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2 2
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[log (P/Po)] Figure
6.
m-Xylene a d s o r p t i o n d a t a p l o t t e d the Dubinin-Radusikevich equation.
according t o
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I n an e f f o r t t o i d e n t i f y t h e n a t u r e o f a c i d s i t e s i n t h e TSLS c o m p l e x and t h e p o t e n t i a l f o r a c i d c a t a l y s i s , we investigated the chemisorption of pyridine by FTIR spectroscopy. T h i s a d s o r b a t e i s known t o e x h i b i t a r o m a t i c ring stretching frequencies c h a r a c t e r i s t i c of pyridinium ion (Bronsted a c i d i t y ) and of coordinated p y r i d i n e molecules (Lewis a c i d i t y ) ' ^ . The F T I R s p e c t r u m shown i n F i g u r e 7 was o b t a i n e d a f t e r o u t g a s s i n g t h e TSLS c o m p l e x a t 350°C, a d s o r b i n g p y r i d i n e a t 25°C and P / P ° = 1.0, and s u b s e q u e n t l y outgassing to remove much of the physically adsorbed pyridine. B o t h B r o n s t e d and L e w i s a c i d s i t e s were c l e a r l y e v i d e n t i n the FTIR spectrum. An i n t e n s e r i n g stretching mode c h a r a c t e r i s t i c o f p y r i d i n e c o o r d i n a t e d t o L e w i s acid s i t e s was f o u n d a t 1450 cm"" , whereas a weaker b a n d due t o t h e N-H d e f o r m a t i o n o f p y r i d i n i u m i o n was o b s e r v e d a t 1540 cm" . Also, a ring stretching band of intermediate i n t e n s i t y due t o b o t h B r o n s t e d and L e w i s acid sites was f o u n d a t 1490 cm"" . The acidic functionality of the TSLS complex was demonstrated i n t h e c a t a l y t i c d e a l k y l a t i o n o f cumene. For t h i s m o d e l r e a c t i o n a t 350°C t h e c o n v e r s i o n o f cumene t o b e n z e n e was m o n i t o r e d as a f u n c t i o n o f t i m e on s t r e a m (see F i g u r e 8) . Included i n the study f o r comparison purposes 1 8
1
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1
1
1.0
Lu Ο
L
0.8-J
< QD m
ο
CO CO
0.7H
< 0.6H
0.5 1700
1600
1500
1400
1300
WAVENUMBERS Figure
7.
FTIR spectrum (1700 a d s o r b e d on t h e TSLS
1
- 1300 cm"" ) complex.
for
pyridine
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WERPY ET AL.
New Tubular Silicate Nanocomposite
Catalyst
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100
Time on Stream (min) Figure
8.
C a t a l y t i c d e a l k y l a t i o n o f cumene a t 350°C o v e r t h e i m o g o l i t e - m o n t m o r i l l o n i t e TSLS complex a n d a l u m i n a p i l l a r e d m o n t m o r i l l o n i t e (APM).
was a typical alumina pillared montmorillonite (APM) p r e p a r e d b y t h e r e a c t i o n o f aluminum c h l o r h y d r a t e a n d t h e Na exchange form o f t h e c l a y . F o r both c a t a l y s t s t h e c o n v e r s i o n d e c r e a s e d w i t h i n c r e a s i n g t i m e on s t r e a m , most l i k e l y due t o t h e f o r m a t i o n o f c o k e a n d r e s t r i c t e d a c c e s s e s to the acid s i t e s . S i g n i f i c a n t l y , t h e TSLS complex i s l e s s a c t i v e than APM. I n i t i a l c o n v e r s i o n s were a p p r o x i m a t e l y 55% a n d 88% f o r the TSLS a n d APM, r e s p e c t i v e l y . This difference i n r e a c t i v i t y was n o t e s p e c i a l l y s u r p r i s i n g , b e c a u s e t h e c h a r g e on t h e m o n t m o r i l l o n i t e component o f t h e TSLS complex i s b a l a n c e d by weakly a c i d i c N a i o n s , whereas i n APM t h e c h a r g e - b a l a n c i n g c a t i o n s a r e more h i g h l y a c i d i c aluminum c a t i o n s and/or hydrogen i o n s . Thus, t h e a c i d i t y observed b o t h b y p y r i d i n e a d s o r p t i o n a n d b y cumene c r a c k i n g most l i k e l y r e s u l t e d from t h e i n t r i n s i c a c i d i t y o f t h e i m o g o l i t e component o f t h e TSLS complex. Future c a t a l y t i c studies w i l l f o c u s on t h e e f f e c t o f t h e e x c h a n g e c a t i o n on TSLS catalytic activity. +
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Acknowledgments The financial support of the National Science F o u n d a t i o n t h r o u g h g r a n t DMR-8903579 a n d t h e M i c h i g a n S t a t e University Center f o r Fundamental M a t e r i a l s Research i s g r a t e f u l l y acknowledged.
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RECEIVED May 9, 1990
