Reactive CrâO Sites - ACS Symposium Series (ACS Publications)

Chapter 23

Reactive Cr—Ο Sites Catalytic Properties of Chromia-Pillared Montmorillonite and Preliminary Study Results Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 25, 2018 | https://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch023

1

1

1

1

Ahmad Moini , Thomas D. Brewer , Ming-Shin Tzou , Steven D. Landau , Boon-Keng Teo , and ThomasJ.Pinnavaia 2

1

1

Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824 Department of Chemistry, University of Illinois, Chicago, IL 60680 2

The catalytic and structural properties of two chromia-pillared montmorillonites were compared in an effort to establish structure-reactivity relationships in these materials. The basal spacings of pillared products, prepared by reaction of Na -montmorillonite with basehydrolyzed Cr(III) solutions, depended in part on the temperature used for the hydrolysis reaction. Catalytic studies on the derivatives with basal spacings of 23.5 and 13.7 Åshowed the intercalate with the larger gallery height to be considerably more reactive for the dehydrogenation of cyclohexane to benzene. Preliminary EXAFS results for the 23.5 Â intercalate showed local structural similarities between the chromia pillars and bulk Cr O . The relationship to chromium oxide was substantiated by the similar catalytic activities obtained for the 23.5 Â chromiapillared clay and Cr O supported on alumina. +

2

3

2

3

Among a l l l a y e r e d s i l i c a t e clays, the smectite family of 2:1 layer lattice structures are preeminent in their a b i l i t y to adsorb o r g a n i c molecules and to c a t a l y z e their chemical transformations. A l l metal oxides i n the soil environment may exhibit some degree of surface reactivity. However, the a d s o r p t i v i t y and r e a c t i v i t y of typical smectites are facilitated by their relatively high internal surface areas 700 m /g) and external surface areas (10-50 m / g ) . Solomon and Hawthorne (1) have p r o v i d e d a schematic summary o f t h e f u n c t i o n a l g r o u p r e a c t i v i t y o f smectites. More r e c e n t l y , i t was p o i n t e d o u t t h a t t h e t y p e s o f sites will depend on t h e state of hydration of the clay as 2

2

0097-6156/90/0415-0455$06.00/0 © 1990 American Chemical Society

Coyne et al.; Spectroscopic Characterization of Minerals and Their Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 25, 2018 | https://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch023

456

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

shown i n F i g u r e 1 ( Z i e l k e , R. C.; Pinnavaia, T. J.; Mortland, M. M. In Reactions and Movement of Organic Chemicals i n S o i l s : Sawhney, B. L., Ed.; S o i l S c i . Soc. Am.: Madison, i n press.) At low h y d r a t i o n l e v e l s , the g a l l e r i e s are c o l l a p s e d and i n a c c e s s i b l e f o r c a t a l y s i s , and the s t r e n g t h of the Lewis base s i t e s at the edges become more important. At high hydration s t a t e s , the g a l l e r i e s are swollen and a c c e s s i b l e , and the e x t e r n a l surfaces are more completely hydrated. This accessibility of i n t e r n a l surfaces, as w e l l as the s u r f a c e f u n c t i o n a l i t y of the c l a y , i s s t r o n g l y i n f l u e n c e d by h y d r a t i o n . The s t a t e of the smectite c l a y under i d e a l i z e d c o n d i t i o n s of dehydration i s represented at the bottom of Figure 1. The gallery exchange ions (M ) are coordinated to the oxygen atoms d e f i n i n g the d i - t r i g o n a l c a v i t i e s of a Kagome (2) network. Since the g a l l e r i e s are collapsed, no i n t e r l a m e l l a r adsorption or ion exchange i s p o s s i b l e unless the organic guest i t s e l f i s h i g h l y p o l a r and capable of s w e l l i n g the i n t e r l a y e r s . Thus, the surface f u n c t i o n a l groups of primary importance under the c o n d i t i o n s of low hydration are Lewis a c i d sites and hydroxyl groups l o c a t e d at e x t e r n a l edge positions. At higher degrees of hydration, however, the g a l l e r y ions may become aquated and provide a source of Bronsted acidity due to hydrolysis of exchange ions (3) . Regardless of the s t a t e of hydration of the surface, smectite c l a y s are capable of s h u t t l i n g e l e c t r o n s between s t r u c t u r a l redox centers and organic molecules adsorbed on the external or i n t r a c r y s t a l l i n e surfaces. For instance, Mossbauer s t u d i e s have demonstrated that i r o n centers i n c l a y s can be r e v e r s i b l y i n t e r c o n v e r t e d between +2 and +3 o x i d a t i o n s t a t e s by r e a c t i o n with simple o x i d i z i n g and reducing agents (A) . The above model of f u n c t i o n a l group reactivity a p p l i e s to most smectite c l a y s c o n t a i n i n g simple a l k a l i metal and a l k a l i n e e a r t h c a t i o n s . However, there e x i s t i n nature smectite d e r i v a t i v e s i n which the g a l l e r y c a t i o n s are e x t e n s i v e l y hydrolyzed polymeric species such as hydroxycations of aluminum and i r o n . These s o - c a l l e d "hydroxy-interlayered" (5-8) c l a y s have been recognized as p o s s i b l e c a t a l y s t s f o r the transformation of organic molecules i n the s o i l environment. Several hydroxyinterlayered smectites have been synthesized under c o n d i t i o n s approximating those i n nature. The products are t y p i c a l l y c h l o r i t i c i n s o f a r as the b a s a l spacings are s i m i l a r to the 14 Â observed f o r c h l o r i t e . Thus, the g a l l e r i e s are almost completely occupied ("stuffed") by the hydroxy c a t i o n s . More r e c e n t l y , polyoxocations have been used t o prepare p i l l a r e d forms of i n t e r l a y e r e d c l a y s t h a t are r e l a t e d to the h y d r o x y - i n t e r l a y e r e d analogs found i n n+



23. MOINI ET AL.

Reactive Cr-O Sites

457

γγγγίγγγγ;" Hydrated Galleries

Μ(Η 0) 2

χ

vvvwvvvv M

H

< 2°»x

3+

Al

Anhydrous Galleries " "

ΛΛΛΛΛΛΛΛΛ (M )

(M

)

wwwwv /wwww\ Al «4

F i g u r e 1. Schematic functional groups on of smectite clay.

illustration of the dehydrated and hydrated

:B

surface states



458


nature. The t h e r m a l d e h y d r a t i o n and d e h y d r o x y l a t i o n of these hydroxy-interlayered derivatives converts them to metal oxide pillared forms which are stable to high temperatures. These m a t e r i a l s exhibit novel properties as catalysts for a wide range of chemical conversions ( 9 - 1 1 ) , many o f w h i c h a r e r e l a t e d t o p r o c e s s e s o c c u r r i n g in the s o i l environment. Although hydroxy-interlayered clays found i n nature are generally c h l o r i t e - l i k e , s o m e f o r m s may r e s e m b l e the hydroxy-interlayered precursors to metal oxide pillared clays. Figure 2 illustrates the structure of a t y p i c a l c h l o r i t e i n which the g a l l e r i e s are o c c u p i e d by b r u c i t e l i k e sheets of edge-shared Mg(OH) and A l ( O H ) octahedra. In c o n t r a s t , pillared hydroxy-interlayered clays contain more robust polyoxocations such as the Al 0 (OH)24 (H20) " (12) which has the isomeric K e g g i n s t r u c t u r e as shown i n F i g u r e 3 . Since the latter hydroxy cations are l a t e r a l l y spaced i n the gallery of the host clay, their thermal dehydroxylation to metal oxide aggregates, as i l l u s t r a t e d i n E q u a t i o n 1, results in acidic galleries which have two-dimensional microporous structures for the a d s o r p t i o n and catalytic conversions of guest molecules. 6

( 7

1 3

4

+ x

1 2

2

x ) +

x

Α1 3θ4(ΟΗ)24^χ(Η 0)ΐ2-χ(7-χΗ 1

6

>

6.5 "A1 0 " + (7-x) 2

3

H+

(1)

Metal oxide p i l l a r e d clays are relatively complex m a t e r i a l s and t h e i r s t r u c t u r a l c h a r a c t e r i z a t i o n has been a challenging experimental problem. Recent magic angle s p i n n i n g NMR s t u d i e s of alumina-pillared clays (13-15) suggest that the Keggin i o n - l i k e s t r u c t u r e i s r e t a i n e d at temperatures below 350°C. G r a f t i n g of the p i l l a r to the host layers may or may not occur depending on the constitution of the 2:1 layered silicate structure. Chemical studies of the surface organometallic chemistry of pillared clays suggest that the reactivity of the surface hydroxyl groups of the pillars toward metal cluster c a r b o n y l compounds i s s i m i l a r to the reactivity f o u n d on t h e s u r f a c e s o f b u l k a l u m i n a (IS.)· In an e f f o r t t o more f u l l y e l u c i d a t e the structure and reactivity of metal oxide pillared clays, we have been i n v e s t i g a t i n g the s t r u c t u r e - r e a c t i v i t y p r o p e r t i e s of chromia-pillared derivatives (12). In the following sections, we provide an example of the structurecatalytic reactivity properties of chromia-pillared montmorillonites. A l s o , we r e p o r t o u r i n i t i a l e f f o r t s to s t r u c t u r a l l y c h a r a c t e r i z e the i n t e r c a l a t e d chromia aggre gates by Extended X - r a y A b s o r p t i o n F i n e S t r u c t u r e (EXAFS) Spectroscopy. Unlike previously reported metal oxide pillared clays, chromia-pillared clay exhibits strong Kedge a b s o r p t i o n and f i n e structure suitable for deter mination of metal-oxygen bond d i s t a n c e s i n the p i l l a r s .



23. MOINI ET AL.

459

Reactive Cr-O Sites

14 A

F i g u r e 2. Structure of a typical chlorite-like h y d r o x y - i n t e r l a y e r e d clay i n which the g a l l e r i e s of a 2:1 s m e c t i t e s t r u c t u r e a r e f i l l e d , or n e a r l y so, with brucite-like sheets of mainly edge-shared Mg(OH) octahedra. Aluminum occasionally substitutes for magnesium i n the b r u c i t e sheet to provide the charge balance necessary for e l e c t r i c a l neutrality.



460


F i g u r e 3. Structure of a typical polyoxocation, namely A 1 0 (OH) 4 + ( H 0 ) "" ' , used to form a pillared hydroxy-interlayered clay. Twelve aluminum ions occupy positions in octahedral interstices and one aluminum occupies a tetrahedral position in the center to form the Keggin s t r u c t u r e . When χ = 0, the terminal vertices of the octahedra are occupied by water, w h e r e a s t h e s h a r e d v e r t i c e s a r e o c c u p i e d b y OH groups. The v e r t i c e s o f the central tetrahedron are o c c u p i e d by oxygen atoms. The d o t t e d lines in the center of the figure define the t e t r a h e d r a l s i t e and the three octahedra which are p a r t i a l l y blocked from view. ( 7

1 3

4

2

x

2

1 2

x

+

x


23.

Structure-Reactivity Clays


461

Reactive Cr-0 Sites

MOINIETAL.

R e l a t i o n s h i p s f o r Chromia

Pillared

The p r o p e r t i e s of hydroxychromium i n t e r l a y e r e d c l a y s as p r e c u r s o r s t o c h r o m i a - p i l l a r e d c l a y s depend c r i t i c a l l y on the nature of the hydroxychromium oligomers which i n i t i a l l y occupy the g a l l e r i e s of the host c l a y . In general, pillaring solutions are prepared by basic h y d r o l y s i s of chromium (III) s o l u t i o n s at temperatures above 60°C. The base/metal r a t i o s (n) of these s o l u t i o n s are v a r i e d between 1.0 and 2.0. The r e a c t i o n of Na m o n t m o r i l l o n i t e with these p i l l a r i n g agents (n = 1.0-2.0) r e s u l t s i n the formation of hydroxychromium d e r i v a t i v e s with b a s a l spacings i n the range 20-28 Â under a i r - d r i e d conditions. Conversely, the b a s i c h y d r o l y s i s of chromium s o l u t i o n s at 25°C r e s u l t s i n the formation of s m a l l e r oligomers which react with Na -montmorillonite t o form d e r i v a t i v e s with b a s a l spacings i n the range 14-18 Â. Table I compares the chemical and physical properties of two hydroxychromium montmorillonites prepared by reaction of a Na -exchanged Wyoming montmorillonite (unit cell formula 0.6t 3.23 0.42 90.47] °20 < > 4> chromium p i l l a r i n g s o l u t i o n s prepared by h y d r o l y s i s at 95°C and 25°C and η value of 2.0 (17) . The i n t e r c a l a t e obtained from the chromium reagent hydrolyzed at 95°C c o n t a i n s 3.53 Cr per O ( O H ) unit c e l l . Henceforth, this material will be referred to as Cr montmorillonite. In comparison, the chromium s o l u t i o n prepared at 25°C a f f o r d s a hydroxychromium i n t e r l a y e r product c o n t a i n i n g 1.88 Cr per O (OH) unit cell, henceforth abbreviated Cr -montmorillonite. Both m a t e r i a l s undergo a r e d u c t i o n i n b a s a l spacing upon dehydration and dehydroxylation at temperatures up t o 500°C under a n i t r o g e n atmosphere. Upon dehydration at 350°C, the b a s a l spacing of the C r - m o n t m o r i l l o n i t e (23.5 Â) i s substantially larger than the spacing observed f o r the C r derivative (14.2 Â) . The dehydrated Na -montmorillonite has a b a s a l spacing of 9.7 Â which corresponds t o the van der Waals t h i c k n e s s of a montmorillonite layer. The f r e e g a l l e r y h e i g h t s f o r the 3.£3 * C r - m o n t m o r i l l o n i t e s are t h e r e f o r e 13.8 and 4.5 A, r e s p e c t i v e l y , a f t e r degassing at 350°C. The l a r g e d i f f e r e n c e i n g a l l e r y h e i g h t s f o r C r and C r - m o n t m o r i l l o n i t e s leads t o dramatic d i f f e r e n c e s i n c a t a l y t i c r e a c t i v i t y (12) · F i g u r e 4 i l l u s t r a t e s the conversion of cyclohexane t o benzene over both m a t e r i a l s at 550°C as a f u n c t i o n of r e a c t i o n time. Both c a t a l y s t s were pre-reduced under H i n a continuous flow r e a c t o r at 500°C, f o l l o w e d by r e a c t i o n with cyclohexane (weight h o u r l y space v e l o c i t y = 3, contact time = 6 sec, He c a r r i e r gas.) The c l a y remains i n t a c t at these r e a c t i o n temperatures as evidenced from the thermal data (12) · +

+

+

Na

A1

Fe

M

A1

0H

w

i

t

h

7 8

20

4

3

20

x

5 3

4

88

3 5 3

x

8 8

+

Cr

a n c

1 8 8

3

1 8 8

2


5 3

462


Table I.

Hydroxychromium M o n t m o r i l l o n i t e s

B a s a l Spacings,


Sample

#

Cri

25°

350°

500°

m /g

a

27.6

23.5

21.0

353

b

16.8

14.2

13.7

280

53-Mont.

# 8 8

-Mont.

Surface

Â

Area

2

T h i s sample, which contained 3.53 Cr per 0 q (OH) unit cell, was prepared by reaction of Na m o n t m o r i l l o n i t e with a C r s o l u t i o n hydrolyzed at 95°C and η = 2.0. a

4

2

+

3 +

° The pillaring solution used to prepare d e r i v a t i v e was hydrolyzed at 25°C and η = 2.0. c

Outgassing of samples was c a r r i e d out the absence of oxygen. The surface determined by n i t r o g e n BET a d s o r p t i o n .

this

at 350°C i n areas were

80 |-

7θΙ

0

20

40

60

80

100

120

MINUTES F i g u r e 4. Conversion of cyclohexane t o benzene over chromia pillared montmorillonites at 550°C: (A) Cr -montmorillonite, (B) Cr -montmorillonite. The" cyclohexane contact time was 6.0 s and the weight h o u r l y space v e l o c i t y was i n the range 1.0-3.0. Each sample was heated under a flow of hydrogen at 550°C p r i o r t o use of the c a t a l y s t . 3 β5 3

1

m88


c

23. MOINIETAL,

463

Reactive Cr-0 Sites

Little or no reactivity is observed for C r montmorillonite, but the C r derivative with a large gallery height i s much more r e a c t i v e . A l s o , i t s h o u l d be noted that, as e x p e c t e d , pure Na -montmorillonite showed no c a t a l y t i c activity. The d e c r e a s e i n r e a c t i v i t y with increasing reaction time for Cr -montmorillonite most likely arises from the formation of coke and the concomitant occlusion of g a l l e r y pores. 1

3

5

8

8

3

+


3 i

5 3

The high dehydrogenation activity observed for Cr -montmorillonite almost certainly arises from the facile accessibility of the chromium oxide aggregates supported i n the clay g a l l e r y . Substantial contribution to the observed activity due to active sites at the external surfaces of the mineral is precluded by the virtual absence of activity for the C r derivative. Thus, Cr - m o n t m o r i l l o n i t e behaves catalytically much l i k e b u l k chromium o x i d e s u p p o r t e d on a l u m i n a ( H ) . Evidence that difference in reactivity for C r and Cr -montmorillonite is due largely to differences in gallery accessibility is p r o v i d e d by the adsorption d a t a i n F i g u r e 5. The C r derivative, which retains a basal spacing near 21 Â after reaction, is capable of rapidly adsorbing cyclohexane. However, the C r d e r i v a t i v e w i t h a b a s a l s p a c i n g near 13.7 Â adsorbs ve'ry l i t t l e cyclohexane. 3

5

3

1

3

1

8

5

8

8

3

5

3

1

8

8

8

3

EXAFS

>

3

Studies

of

Chromia

#

5

3

Pillars

The similarities in catalytic reactivity between C r m o n t m o r i l l o n i t e and c h r o m i a s u p p o r t e d on a l u m i n a s u g g e s t that the structure of the intercalated chromia particles may r e s e m b l e the structure of the bulk oxide. In order to obtain structural information for the chromia aggregates in pillared clays, we have initiated structural studies of these materials. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is being r e c o g n i z e d as an e f f e c t i v e t o o l f o r d e t e r m i n i n g t h e local structure of a variety of materials. The basic principles and utility of this technique have been d i s c u s s e d e l s e w h e r e (18.) . X-ray absorption studies of Cr -montmorillonite were c a r r i e d out at the C o r n e l l H i g h E n e r g y S y n c h r o t r o n Source (CHESS), Cornell University. The measurements were p e r f o r m e d a t room t e m p e r a t u r e on t h e C - 2 EXAFS beam line. T h e s a m p l e s t u d i e d was t h e 2 3 . 5 Â h e a t e d chromiapillared montmorillonite, and the measurements w e r e made at the chromium Κ absorption edge. The raw data, c o l l e c t e d as a f u n c t i o n o f photon energy ( Ε ) , was reduced to photoelectron wavevector (k) s p a c e , a n d t h e b a c k g r o u n d was r e m o v e d (18,19). T h i s was f o l l o w e d b y r e m o v a l o f the high-frequency noise and the truncation of the data at 4.0 and 12.0 A' . The resulting EXAFS modulation is shown i n F i g u r e 6. The F o u r i e r t r a n s f o r m o f t h e s e data, 3

3 e

5 3

1


5

3

464



2.0 1.8 1.6 1.4

< Ο

1.2 1.0 0.8 0.6 0.4 0.2 0.0 10

12

1 / 2

\f\, MIN

F i g u r e 5. Uptake of cyclohexane at 25°C and p / p = 0.95 by chromia p i l l a r e d m o n t m o r i l l o n i t e s : (A) C r 53montmorillonite, (B) Cr -montmorillonite. The uptake curves were determined for both materials after use as a catalyst for the conversion of cyclohexane to benzene at 550°C. 0

3

x

8 8



23. MOINI ET AL.

Reactive Cr-O Sites

4

6

465

8

10

12

k (A-i) F i g u r e 6. Cr-edge montmorillonite.

EXAFS

spectrum

for

r

C 3.53~

shown i n F i g u r e 7, gave r i s e t o two m a j o r p e a k s at 1.6 and 2.6 Â before the phase shift correction. No other s i g n i f i c a n t peaks were o b s e r v e d a t l o n g e r b o n d distances. These maxima i n t h e Fourier transform data, which c o r r e s p o n d to the d i f f e r e n t chromium c o o r d i n a t i o n shells, were isolated using a filter window function. The inverse t r a n s f o r m o f e a c h p e a k was g e n e r a t e d and fitted using a n o n - l i n e a r least squares program. The a m p l i t u d e and phase functions were obtained from the theoretical curves reported by Teo and Lee (20.) . The parameters w h i c h were r e f i n e d i n c l u d e d a s c a l e factor, the DebyeWaller factor, the interatomic distance, and the t h r e s h o l d energy d i f f e r e n c e . This process led to refined d i s t a n c e s o f 1.97(2) and 2.73(2) Â which were attributed t o C r - 0 and C r - C r d i s t a n c e s , respectively. Our i n a b i l i t y to resolve s e c o n d n e a r e s t n e i g h b o r C r - C r d i s t a n c e s may b e a consequence of the l i m i t e d domain s i z e of the p i l l a r s . Both of the c o r r e c t e d values agree with the range of C r - 0 and C r - C r d i s t a n c e s observed f o r the oxides (21,22). Crystallographic studies on

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