Low-Temperature Deep Oxidation of Aliphatic and ... - ACS Publications

Chapter 27

Low-Temperature Deep Oxidation of Aliphatic and Aromatic Hydrocarbons Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 17, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch027

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Mark T. Vandersall , Stephen G . Maroldo , William H . Brendley, J r . , Krzysztof Jurczyk , and Russell S. Drago 2

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Research Laboratories, Rohm and Haas Company, Spring House, PA 19477 Department of Chemistry, University of Florida, Gainesville, FL 32611

A new series of low temperature oxidation catalysts have been developed that comprise first-row transition metal oxides dispersed on a new class of synthetic carbonaceous supports, the Ambersorb adsorbents. Data are reported here demonstrating high conversion using these catalysts for the deep oxidation of aliphatic and aromatic compounds to C O and H O . The process temperatures required are only 175-250 °C, which is a substantial reduction over conventional oxidation systems. As such, these catalysts should offer cost advantages over the use of adsorption on activated carbon or thermal or catalytic oxidation for environmental control and remediation. 2

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P u b l i c a w a r e n e s s of the h a r m f u l effects of v o l a t i l e o r g a n i c c h e m i c a l ( V O C ) e m i s s i o n s a n d n o n - v o l a t i l e o r g a n i c c o m p o u n d s has l e d to i n c r e a s i n g l y stringent e n v i r o n m e n t a l regulations. T h e citizens of the w o r l d are d e m a n d i n g clean air, clean water, a n d p o l l u t i o n free l a n d to safeguard o u r b i o s p h e r e not o n l y for the present b u t also for centuries to come. These concerns h a v e b e e n h e a r d b y the c h e m i c a l i n d u s t r y i n g e n e r a l , w h i c h is p u r s u i n g the g o a l of cleaner, safer, a n d m o r e efficient plants. T h e c o n t r o l o f V O C e m i s s i o n s a n d d i f f i c u l t to d i s p o s e of b y - p r o d u c t s is o f k e y i m p o r t a n c e . W h i l e the p r i m a r y objective is the e l i m i n a t i o n o f processes p r o d u c i n g h a z a r d o u s w a s t e , the m a n a g e m e n t a n d c o n t r o l of h a r m f u l b y - p r o d u c t s are of i m m e d i a t e concern. T h i s p a p e r w i l l present the i m p o r t a n t roles p l a y e d b y the s y n e r g i s t i c association o f n e w , n o v e l carbonaceous adsorbents w i t h catalytic t e c h n o l o g y i n p r o t e c t i n g the e n v i r o n m e n t b y the d e e p o x i d a t i o n of a l i p h a t i c a n d a r o m a t i c hydrocarbon pollutants. A t present, t w o technologies are u t i l i z e d for the c o n t r o l of s e m i - v o l a t i l e a n d v o l a t i l e o r g a n i c c o m p o u n d emissions. B o t h have inherent d r a w b a c k s .

0097-6156/94/0552-0331$08.00/0 © 1994 American Chemical Society Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 17, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch027

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ENVIRONMENTAL CATALYSIS

1. G r a n u l a r A c t i v a t e d C a r b o n ( G A C ) - G A C is u s e d as a c o n t r o l t e c h n o l o g y to a d s o r b V O C s f r o m air a n d w a t e r streams. (2) G A C is, h o w e v e r , d e p e n d e n t o n o p e r a t i n g c o n d i t i o n s to be effective. F o r the a d s o r p t i o n o f o r g a n i c c h e m i c a l s f r o m a i r , for e x a m p l e , r e l a t i v e h u m i d i t y c a n a d v e r s e l y affect the a d s o r p t i o n c a p a c i t y of the c a r b o n for these o r g a n i c s , w i t h the decrease b e c o m i n g v e r y significant as the r e l a t i v e h u m i d i t y o f t h e V O C s t r e a m is i n c r e a s e d above a p p r o x i m a t e l y 50%. I n a d d i t i o n , G A C is g e n e r a l l y r e b e d d e d after b r e a k t h r o u g h r e q u i r i n g t r a n s p o r t a t i o n o f the h a z a r d o u s c h e m i c a l c o n t a i n e d o n the G A C to i n c i n e r a t o r s l o c a t e d at specific sites. These l a r g e v o l u m e s of G A C are t r a n s p o r t e d as h a z a r d o u s w a s t e s for r e g e n e r a t i o n or d i s p o s a l , t h e r e b y i n c r e a s i n g costs a n d g e n e r a t i n g concerns o v e r safety issues. 2. T h e r m a l O x i d a t i o n - T h e r m a l o x i d a t i o n of V O C s is u s e d p r i m a r i l y w i t h a i r - s t r i p p i n g as a m e a n s of d e s t r u c t i o n o f h a z a r d o u s wastes.(2) T h i s m e t h o d of o p e r a t i o n is associated w i t h h i g h e n e r g y r e q u i r e m e n t s a n d o p e r a t i n g costs because of the h i g h t e m p e r a t u r e s r e q u i r e d for incineration. I n c o o p e r a t i o n w i t h the U n i v e r s i t y of F l o r i d a , w e h a v e d e v e l o p e d a n e w series o f l o w t e m p e r a t u r e catalysts that c o m p r i s e t r a n s i t i o n m e t a l o x i d e s d i s p e r s e d o n a n e w class of s y n t h e t i c carbonaceous a d s o r b e n t s ( A m b e r s o r b a d s o r b e n t s ; A m b e r s o r b is a r e g i s t e r e d t r a d e m a r k o f R o h m a n d H a a s C o m p a n y ) . (3) These patented adsorbents, sometimes c a l l e d c a r b o n m o l e c u l a r sieves ( C M S ) , offer a v a r i e t y o f p o r e s i z e d i s t r i b u t i o n s w i t h h i g h i n t e r n a l surface area a n d h i g h d i f f u s i v i t y of o r g a n i c m o l e c u l e s to a c t i v e m e t a l o x i d e sites.(4,5) In a d d i t i o n , these adsorbents f u n c t i o n as robust catalysis s u p p o r t s . P r e v i o u s w o r k has s h o w n that A m b e r s o r b a d s o r b e n t - s u p p o r t e d t r a n s i t i o n m e t a l o x i d e s are effective c a t a l y s t s for the d e e p o x i d a t i o n o f h a l o g e n a t e d h y d r o c a r b o n s at l o w temperatures.(3) T h e studies d e s c r i b e d i n t h i s p a p e r f u r t h e r d e m o n s t r a t e e x t r e m e l y h i g h c o n v e r s i o n s for the d e e p o x i d a t i o n o f a l i p h a t i c a n d a r o m a t i c h y d r o c a r b o n s to CO2 a n d H 2 O . T h e c a t a l y t i c process t e m p e r a t u r e s are l o w , t y p i c a l l y i n the r a n g e o f 170-250 ° C w h i c h is a s u b s t a n t i a l r e d u c t i o n o v e r c o n v e n t i o n a l t h e r m a l o x i d a t i o n systems.(2) P r e l i m i n a r y r e s u l t s w i t h b u t a n e , h e x a n e , a n d t o l u e n e are presented. Experimental T h e c a r b o n s u p p o r t s u s e d i n this s t u d y w e r e s y n t h e t i c c a r b o n a c e o u s a d s o r b e n t s f r o m R o h m a n d H a a s C o m p a n y ( A m b e r s o r b 563 a n d 572 adsorbents).(4,5) T h e A m b e r s o r b a d s o r b e n t s are m a d e b y the p a t e n t e d p y r o l y s i s of sulfonated styrene-divinylbenzene copolymers. The process p r o d u c e s a d s o r b e n t s w i t h t a i l o r a b l e a n d r e p r o d u c i b l e surface area a n d p o r e v o l u m e d i s t r i b u t i o n . T h e a d s o r b e n t s are s p h e r i c a l particles w i t h d i a m e t e r s

Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


27. VANDERSALL ET AL.

Oxidation of Aliphatic & Aromatic Hydrocarbons

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b e t w e e n 300 a n d 850 μ ι η . P r i o r to use, the carbons w e r e d r i e d at 120 ° C u n d e r v a c u u m for at least 18 h o u r s . C a t a l y s t s w e r e p r e p a r e d b y i m p r e g n a t i o n to i n c i p i e n t wetness.(6) M e t a l n i t r a t e salts w e r e p u r c h a s e d f r o m F i s h e r Scientific C o m p a n y a n d A l d r i c h C h e m i c a l C o m p a n y , Inc. I n a t y p i c a l p r e p a r a t i o n , a n a p p r o p r i a t e a m o u n t of the n i t r a t e salt of the d e s i r e d t r a n s i t i o n m e t a l {e.g., C o ( N 0 3 ) 2 - 6 H 2 0 ) w a s d i s s o l v e d i n t o a s o l u t i o n of 10% m e t h a n o l i n water. T h e a m o u n t o f s o l u t i o n u s e d w a s that r e q u i r e d to f i l l a l l of the pores of the adsorbent. T h i s m e t a l saltc o n t a i n i n g s o l u t i o n w a s then a d d e d d r o p w i s e to the adsorbent, w h i l e s t i r r i n g , o v e r the course o f about 15 m i n u t e s . A t the e n d of the a d d i t i o n , the adsorbent was d a m p i n appearance but no l i q u i d phase was present. This i m b i b e d s a m p l e w a s a l l o w e d to s t a n d for a p p r o x i m a t e l y f o u r h o u r s a n d t h e n w a s p l a c e d i n t o a 110 °C v a c u u m o v e n to d r y o v e r n i g h t . F o l l o w i n g the d r y i n g , the m e t a l - l o a d e d a d s o r b e n t w a s p l a c e d i n t o a 3.75 c m d i a m e t e r q u a r t z p y r o l y s i s t u b e a n d h e a t e d , u n d e r a n i t r o g e n a t m o s p h e r e , to 360 C for t w o h o u r s . T h e r m o g r a v i m e t r i c a n a l y s i s w i t h m a s s s p e c t r a l a n a l y s i s of the off gases i n d i c a t e d that these t e m p e r a t u r e s w e r e sufficient to v o l a t i l i z e the n i t r a t e g r o u p s , l e a v i n g the m e t a l o x i d e o n the c a r b o n surface. e

C a t a l y s t s w e r e tested i n a p y r e x glass reactor w i t h a n i n t e r n a l d i a m e t e r of 10 m m a n d w i t h a coarse glass frit as a b e d s u p p o r t . A t y p e J t h e r m o c o u p l e w a s t a p e d to the o u t s i d e o f the glass at a p o s i t i o n a p p r o x i m a t e l y 1 c m a b o v e the frit, a n d the entire reactor w a s w r a p p e d w i t h fiberglass tape a n d flexible h e a t i n g tape. The temperature was controlled w i t h a Controls and A u t o m a t i o n , L t d . M o d e l 9000 P I D c o n t r o l l e r u s i n g the t h e r m o c o u p l e as the sensor. T e m p e r a t u r e r e g u l a t i o n w a s quite g o o d , g e n e r a l l y w i t h i n 1 °C of the setpoint. T h e feed gases u s e d i n these tests w e r e either d e l i v e r e d f r o m a c y l i n d e r o r , for the case of hexane a n d toluene, f r o m p a s s i n g air t h r o u g h a n i m p i n g e r t u b e f i l l e d w i t h the o r g a n i c l i q u i d . I n s o m e e x p e r i m e n t s , w a t e r w a s also p r e s e n t i n the feed stream. T h i s w a s generated b y p a s s i n g the air t h r o u g h a M i l l i g a n jar f i l l e d w i t h w a t e r a n d then o n to the i m p i n g e r tube. F l o w rate w a s c o n t r o l l e d b y n e e d l e v a l v e s o r b y use of a P o r t e r I n s t r u m e n t C o . M o d e l V C D 1000 f l o w c o n t r o l l e r . A c t u a l f l o w rates w e r e m e a s u r e d w i t h a H u m o n i c s Inc. O p t i f l o w 520 d i g i t a l s o a p - f i l m flowmeter. P r o d u c t gases w e r e a n a l y z e d b y gas c h r o m a t o g r a p h y u s i n g t w o different t e c h n i q u e s . O n e t e c h n i q u e i n v o l v e d s a m p l i n g the gas t h r o u g h a s e p t u m w i t h a gas s y r i n g e a n d then injecting this s a m p l e i n t o a V a r i a n M o d e l 3700 gas c h r o m a t o g r a p h ( G C ) . In the other m e t h o d , o n - l i n e a n a l y s i s w a s a c h i e v e d b y s a m p l i n g the p r o d u c t gas stream w i t h a 6-port r o t a r y v a l v e a n d a 1 c m s a m p l e l o o p m o u n t e d o n a H e w l e t t - P a c k a r d M o d e l 5 8 8 0 A G C . F o r the latter case, a 1 m χ 1 / 8 " stainless steel 1% SP-1000 o n 6 0 / 8 0 C a r b o p a c k Β c o l u m n w a s u s e d . F l a m e i o n i z a t i o n detectors w e r e u s e d o n b o t h G C systems. C o n v e r s i o n s w e r e c a l c u l a t e d as the d i f f e r e n c e o f the i n l e t a n d o u t l e t concentrations, expressed as a percentage of the inlet concentration. 3



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Results and Discussion Surface areas a n d p o r e v o l u m e d i s t r i b u t i o n s o f the c a r b o n s u p p o r t s w e r e determined by nitrogen porosimetry using a Micromeritics ASAP-2400 p o r o s i m e t e r . T y p i c a l v a l u e s for these m a t e r i a l s are s h o w n i n T a b l e I. F o r c o m p a r i s o n p u r p o s e s , d a t a are a l s o s h o w n for a s t a n d a r d v a p o r - p h a s e g r a n u l a r activated c a r b o n ( C a l g o n C a r b o n C o r p . T y p e B P L , 12x30 mesh). T h e p o r e v o l u m e s l i s t e d w e r e calculated b y the p o r o s i m e t e r u s i n g the B J H m e t h o d . T h i s m e t h o d uses the H a l s e y e q u a t i o n for the a d s o r b e d layer thickness a n d the K e l v i n e q u a t i o n for the c a p i l l a r y c o n d e n s e d v a p o r a n d is w i d e l y - u s e d to characterize meso a n d m a c r o p o r o u s solids.(7) T h e pore size classification u s e d c o r r e s p o n d s w i t h I U P A C - r e c o m m e n d e d definitions.(8) T h e B J H p o r e v o l u m e d i s t r i b u t i o n s for b o t h u n d o p e d A m b e r s o r b 572 adsorbent a n d 5% (by w e i g h t ) C o O o n A m b e r s o r b 572 a d s o r b e n t are s h o w n i n F i g u r e 1. T h e n i t r o g e n p o r o s i m e t r y data i n d i c a t e that there is r o u g h l y a 15% loss i n surface area a n d the t-plot m i c r o p o r e v o l u m e of the s u p p o r t d u e to i n c o r p o r a t i o n of the m e t a l oxide.

T a b l e I. T y p i c a l properties of the c a r b o n s u p p o r t s A m b e r s o r b 563 A m b e r s o r b 572 C a l g o n T y p e B P L 12x30 adsorbent adsorbent 1100 1100 550 B E T surface area ( m / g ) 0.47 0.41 0.23 Micropore volume (cm /g) 0.07 0.19 0.14 Mesopore volume (cm /g) 0.24 0.03 0.23 Macropore volume (cm /g) 0.48 0.49 0.53 Bulk density ( g / c m ) Property

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T o d e t e r m i n e the h y d r o p h o b i c i t y of the carbonaceous s u p p o r t s u s e d i n this s t u d y , w a t e r v a p o r a d s o r p t i o n i s o t h e r m s w e r e m e a s u r e d b y e x p o s i n g p r e d r i e d , w e i g h e d s a m p l e s of the adsorbents to a t m o s p h e r e s of c o n t r o l l e d h u m i d i t y for t w o w e e k s . P r e v i o u s experience h a d s h o w n that this t w o - w e e k p e r i o d w a s sufficient for each s a m p l e to reach e q u i l i b r i u m w i t h the headspace w a t e r v a p o r c o n c e n t r a t i o n . T h e r e l a t i v e h u m i d i t y for e a c h s a m p l e w a s established b y u s i n g s t a n d a r d sulfuric a c i d solutions for w h i c h the h u m i d i t y as a f u n c t i o n of s o l u t i o n c o m p o s i t i o n is w e l l k n o w n . ( 9 ) T h e specific u p t a k e of w a t e r w a s c o m p u t e d for each adsorbent at each relative h u m i d i t y , a n d these data are s h o w n g r a p h i c a l l y i n F i g u r e 2. In general, the A m b e r s o r b adsorbents are m o r e h y d r o p h o b i c t h a n t y p i c a l G A C s ( c o m p a r e w i t h C a l g o n T y p e B P L carbon) a n d , i n p a r t i c u l a r , A m b e r s o r b 563 a d s o r b e n t is s u b s t a n t i a l l y m o r e h y d r o p h o b i c , h a v i n g o n l y about 25% of the water u p t a k e o f C a l g o n T y p e B P L carbon. C o n v e r s i o n of h e x a n e w a s m e a s u r e d u s i n g a f l o w r a t e of 3 c m / m i n hexane i n air. F o r these e x p e r i m e n t s , a catalyst w e i g h t of 1.0 g w a s u t i l i z e d , 3





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F i g u r e 1. B J H p o r e v o l u m e d i s t r i b u t i o n for A m b e r s o r b 572 adsorbent a n d for 5% C o O o n A m b e r s o r b 572 adsorbent.

572

563

Relative humidity

(%)

F i g u r e 2. W a t e r v a p o r a d s o r p t i o n isotherms for the carbonaceous supports.



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c o r r e s p o n d i n g to a n a p p r o x i m a t e space v e l o c i t y (SV) of 150 h " . C a t a l y s t c o m p o s i t i o n a n d r e a c t i o n c o n d i t i o n s , a l o n g w i t h the i n i t i a l h y d r o c a r b o n c o n v e r s i o n , are s h o w n i n Table II. In general, c o n v e r s i o n fell w i t h t i m e , w i t h d e a c t i v a t i o n t a k i n g place m o r e q u i c k l y at l o w e r temperatures. F o r the r u n s at 250 °C, the decrease i n c o n v e r s i o n w a s o n l y a f e w percent at 100 h o u r s t i m e o n stream, w h e r e a s for the r u n s at 175 °C, c o n v e r s i o n fell to the 50% l e v e l i n the s a m e t i m e p e r i o d . T h e d e a c t i v a t i o n c u r v e for o n e of the catalysts tested is s h o w n i n F i g u r e 3. T h e catalysts c o u l d , h o w e v e r , be reactivated b y s w i t c h i n g the feed s t r e a m to air (no h y d r o c a r b o n s ) a n d i n c r e a s i n g the t e m p e r a t u r e to 250 °C for 15 h o u r s , w h e r e u p o n the c o n v e r s i o n r e t u r n e d to 99.8%.

T a b l e II. C o n v e r s i o n u s i n g hexane feed s t r e a m Initial Temperature Dopant Dopant Support c o n v e r s i o n (%) level (%) ( Ό 99.9 5 250 572 MnO 99.8 5 200 MnO 572 99.7 5 250 MnO 563 99.9 5 175 MnO 563 99.9 250 5 CoO 572 99.8 200 5 572 CoO 99.9 5 ZnO 572 175 99.9 10 175 ZnO 563

Hexane (ppmv) 2000 1000 1000 1000 2000 1000 1500 1000

T h i s p a t t e r n o f d e a c t i v a t i o n is s u g g e s t i v e of a " c o k i n g " m e c h a n i s m w h e r e b y , at l o w e r t e m p e r a t u r e s , d e p o s i t i o n of c a r b o n o r o t h e r p a r t i a l l y o x i d i z e d organics at the active sites or i n the t r a n p o r t pores leads e v e n t u a l l y to loss o f a c t i v i t y . H e a t i n g the catalyst to h i g h e r t e m p e r a t u r e s w i t h a n air feed can e v i d e n t l y o x i d i z e a n d burnoff these coke p r o d u c t s a n d recover the activity. T h e a p p a r e n t i n d u c t i o n p e r i o d s h o w n i n F i g u r e 3 suggests that b u i l d u p of a s u b s t a n t i a l a m o u n t of c a r b o n a c e o u s deposits m a y be necessary before a n y r e d u c t i o n i n reaction rate is observed. A d d i t i o n a l w o r k is necessary, h o w e v e r , to further u n d e r s t a n d the details of the deactivation m e c h a n i s m . Butane conversion was measured i n a similar fashion u s i n g a flowrate of 3 c m / m i n b u t a n e i n air a n d 1.0 g o f catalyst ( S V 150 r r ) . C a t a l y s t c o m p o s i t i o n a n d r e a c t i o n c o n d i t i o n s , a l o n g w i t h the i n i t i a l h y d r o c a r b o n c o n v e r s i o n , are s h o w n i n T a b l e III. T h e catalysts t e n d e d to deactivate m o r e q u i c k l y w i t h the b u t a n e feed than they d i d w i t h hexane. A c t i v i t y at 175 °C is also a p p a r e n t l y l o w e r as s h o w n b y the i n i t i a l c o n v e r s i o n o f 5 1 % for 5% M n O o n A m b e r s o r b 572 adsorbent. T o l u e n e c o n v e r s i o n w a s also m e a s u r e d i n a s i m i l a r f a s h i o n u s i n g a f l o w r a t e of 3 c m / m i n toluene i n air b u t w i t h a charge o f 2.0 g o f catalyst ( S V 75 h - l ) . C a t a l y s t c o m p o s i t i o n a n d reaction c o n d i t i o n s , a l o n g w i t h the i n i t i a l h y d r o c a r b o n c o n v e r s i o n , are s h o w n i n T a b l e I V . F o r these r u n s , v i r t u a l l y n o 3

1

3





F i g u r e 3. D e a c t i v a t i o n of 5% Z n O o n A m b e r s o r b 572 a d s o r b e n t at 175 °C w i t h a feed stream of 1500 p p m v hexane.


337

338



d e a c t i v a t i o n of the catalyst w a s seen d u r i n g 105 h o u r s o f t i m e o n s t r e a m . S p e c i a t i o n of the p r o d u c t gases i n d i c a t e d that the major p r o d u c t gas w a s CO2.

Butane (ppmv) 30000 2000 5000

T a b l e ΙΠ. C o n v e r s i o n u s i n g b u t a n e feed stream Dopant Initial Dopant Temperature Support c o n v e r s i o n (%) l e v e l (%) (°C) 5 572 99.8 MnO 250 5 50.8 MnO 572 175 5 CoO 572 99.8 250

Toluene (ppmv)

T a b l e I V . C o n v e r s i o n u s i n g toluene feed stream Dopant Initial Temperature Dopant Support l e v e l (%) c o n v e r s i o n (%) ( Ό

600 1300

5 5

CoO CoO

572 563

99.0 99.9

210 250

A d d i t i o n a l tests w e r e r u n at a m u c h h i g h e r c o n c e n t r a t i o n o f t o l u e n e v a p o r u s i n g w a t e r as a co-feed (22000 p p m v of each species) a n d w i t h a f l o w r a t e of 50 c m / m i n a n d 1.0 g o f catalyst ( S V 2500 h " ) . F o r these r u n s , three catalysts w i t h v a r y i n g l e v e l s of C o O w e r e u s e d . T h e c o n v e r s i o n s are s h o w n i n T a b l e V w h e r e it is clear that, despite the short contact time, the h i g h c o n c e n t r a t i o n , a n d the l o w temperature, g o o d c o n v e r s i o n is a c h i e v e d . These data suggest that, u n d e r these c o n d i t i o n s , m e t a l l o a d i n g levels as l o w as 1% b y w e i g h t are q u i t e effective. T h e surface areas, p o r e v o l u m e s , a n d t o l u e n e c o n v e r s i o n s o f these catalysts d e c r e a s e d as the m e t a l l o a d i n g l e v e l w a s i n c r e a s e d . These changes m a y be d u e to either p o o r d i s p e r s i o n o f the m e t a l o x i d e ( a g g r e g a t i o n i n the m i c r o p o r e s ) o r to a d e c r e a s e i n the t o l u e n e d i f f u s i v i t y because of transport p o r e constriction. A d d i t i o n a l w o r k is p l a n n e d to characterize the C o O d i s p e r s i o n a n d to m e a s u r e the effective d i f f u s i v i t y o f t o l u e n e i n these catalysts w h i c h s h o u l d be h e l p f u l i n u n d e r s t a n d i n g the o b s e r v e d changes. 3

1

T a b l e V . C o n v e r s i o n u s i n g toluene feed stream at short contact times Toluene Initial Dopant Dopant Temperature Support (ppmv) l e v e l (%) c o n v e r s i o n (%) (°C) 1 22000 CoO 572 95.5 175 22000 2 572 80.6 CoO 175 22000 10 77.5 572 CoO 175


27.

VANDERSALL ET AL.


339


Conclusion T h e s e p r e l i m i n a r y results c l e a r l y s h o w that f i r s t - r o w t r a n s i t i o n m e t a l s , w h e n d i s p e r s e d o n A m b e r s o r b a d s o r b e n t s u p p o r t s , are effective catalysts for the d e e p o x i d a t i o n o f a l i p h a t i c a n d a r o m a t i c c o m p o u n d s at r e l a t i v e l y l o w t e m p e r a t u r e s . A l t h o u g h the flowrates u s e d i n this w o r k are g e n e r a l l y l o w e r t h a n those t y p i c a l for c o m m e r c i a l t h e r m a l o r c a t a l y t i c o x i d a t i o n s y s t e m s , it s h o u l d be p o s s i b l e to o p t i m i z e the catalyst c o m p o s i t i o n to r e d u c e the r e q u i r e d contact t i m e w h i l e e n s u r i n g near 100% h y d r o c a r b o n d e s t r u c t i o n efficiency. Because these catalysts are effective at l o w o p e r a t i n g temperatures, the energy r e q u i r e m e n t s for a catalytic o x i d a t i o n u n i t u s i n g these m a t e r i a l s are e x p e c t e d to be s i g n i f i c a n t l y l o w e r t h a n for c o n v e n t i o n a l o x i d a t i o n systems. A s a result, the o p e r a t i n g costs of s u c h a s y s t e m s h o u l d be l o w e r a n d therefore y i e l d significant savings o v e r the lifetime of a n e n v i r o n m e n t a l r e m e d i a t i o n project. A s c a t a l y s t s u p p o r t s , the A m b e r s o r b a d s o r b e n t s a p p e a r to offer a u n i q u e c o m b i n a t i o n of advantages. T h e h i g h surface area of these adsorbents, i n c o n j u n c t i o n w i t h s i g n i f i c a n t m e s o - a n d m a c r o p o r o s i t y , s h o u l d a l l o w the synthesis of h i g h l y d i s p e r s e d catalysts w i t h h i g h d i f f u s i v i t y . C o n s e q u e n t l y , as s h o w n a b o v e , e x c e l l e n t a c t i v i t y c a n be o b t a i n e d at r e l a t i v e l y l o w process temperatures. I n a d d i t i o n , the h y d r o p h o b i c n a t u r e of the s u p p o r t s s h o u l d be b e n e f i c i a l i n processes w h e r e h i g h w a t e r v a p o r c o n c e n t r a t i o n s are present, either b y offering r e d u c e d s e n s i t i v i t y to w a t e r i n h i b i t i o n o f active sites, o r b y enhanced h y d r o t h e r m a l stability under process conditions w h e r e silica or a l u m i n a s u p p o r t s w o u l d be unsuitable.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Graham, J.; Ramaratnam, M. Chemical Engineering, February 1993, 6-12. Agarwal, S.K.; Spivey, J.J. Appl. Catal. A:General,1992, 81, 239-55. Petrosius, S.C.; Drago, R.S. J. Chem. Soc. Chem. Commun., 1992, 4, 344-5. Neely, J.W.; Isacoff, E.G. Carbonaceous Adsorbents for the Treatment of Gro and Surface Waters; Marcel Dekker: New York, NY, 1982. Maroldo, S.G.; Betz, W.R.; Borenstein, N. U.S. Patent 4 839 331, 1989. Satterfield, C.N. Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980; p 82. Barrett, E.P.; Joyner, L.G.; Halenda, P.P. J. Am. Chem. Soc., 1951, 73, 373-380. Sing, K.S.W.; Everett, D.H.; Haul, R.A.W.; Moscou, L.; Pierotti, R.A.; Rouquérol, J.; Siemieniewska, T. Pure & Appl. Chem., 1985, 57, 603-19. Weast, R.C. (ed.) Handbook of Chemistry and Physics, 51st Ed.; Chemical Rubber Co.: Cleveland, OH, 1970; p E-40.

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November 1, 1993


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