14 Ahlborn Wheeler-Catalytic Scientist G. A L E X MILLS
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University of Delaware, Center for Catalytic Science and Technology, Newark, DE 19711
Ahlborn Wheeler i s widely recognized i n the cata l y t i c community for his pioneering contributions to catalysis and p a r t i c u l a r l y his development of an understanding of the quantitative relationship between the physical properties of c a t a l y s t s and t h e i r a c t i v i t y and s e l e c t i v i t y . The fundamental r e l a t i o n ships which he developed have stood the test of time. They are recognized as his personal intellectual contributions. At the same time, he worked c l o s e l y with others and published the results of outstanding research with some f i f t e e n d i f f e r e n t collaborators while at Princeton University, Shell O i l Co., the DuPont Co., Houdry Process Co., Aerojet-General Corp., and Lockheed M i s s i l e and Space Co. Wheeler began his c a t a l y t i c career at an unusual time and place - i n the early 1930's at Princeton ( B . S . , M . S . and P h . D . ) . Princeton was at that time famous as the e x c i t i n g U.S. center for c a t a l y s i s . The concept of active s i t e s had been proposed by Taylor and were discussed vigorously. Deuterium had just been discovered by Urey at Columbia. Wheeler published a remarkable series of papers with Eyring, Pease, and others on the r e l a t i v e rates of combinat i o n of H and D with ethylene and acetylene as well as the absolute rates of combination of H with halogens. S i g n i f i c a n t l y , he turned his attention to questions on physical adsorption of gases on surfaces. He published the results of his work on r e l a t i v e adsorption of H and D and, with Otto Beeck and E . A. Smith, on the activated adsorption of nitrogen on iron and the significance of t h i s to NH synthesis. Research on catalyst surfaces at Shell was at a period of time of great excitement, fostered greatly by the enthusiasm of Beeck. Studies on the detailed 2
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0097-6156/83/0222-0179$06.00/0 © 1983 American Chemical Society Davis and Hettinger; Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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s t r u c t u r e o f metal s u r f a c e s u t i l i z e d a new t e c h n i q u e o f examining the r e l a t i o n s h i p o f the o r i e n t a t i o n o f atoms i n metal f i l m s and t h e i r c a t a l y t i c b e h a v i o r . T h i s new concept was t o open the way f o r a s e r i e s o f r e s e a r c h i n v e s t i g a t i o n s on the n a t u r e o f s o r p t i o n o f H on n i c k e l and on the s i g n i f i c a n c e o f h e a t s of adsorption . F o l l o w i n g a decade a t S h e l l , Wheeler i n 1946 j o i n e d the DuPont Co. where f o r the next e i g h t y e a r s he plunged i n t o p r a c t i c a l r e s e a r c h on a v a s t a r r a y o f c a t a l y t i c problems concerned w i t h the a p p l i c a t i o n o f c a t a l y s i s to organic reactions. However, i t was at t h i s time i n the e a r l y 1950's t h a t he made what many r e g a r d as h i s g r e a t e s t i n d i v i d u a l c o n t r i b u t i o n - h i s original and comprehensive treatise on "Reaction Rates and S e l e c t i v i t y i n C a t a l y s t Pores". Fortunately, the r e s u l t s o f h i s i n t e l l e c t u a l and original t o u r - d e - f o r c e was p u b l i s h e d i n two c l a s s i c a l book series, Advances in Catalysis, (1951) e d i t e d by Frankenburg, Komarewsky and R i d e a l , and i n C a t a l y s i s , (1955) e d i t e d by Emmett. At t h i s time i t had become p o s s i b l e t o determine e x p e r i m e n t a l l y t o t a l s u r f a c e a r e a and the d i s t r i b u t i o n o f s i z e s and t o t a l volume o f p o r e s . Wheeler s e t f o r t h t o p r o v i d e the t h e o r e t i c a l development of c a l c u l a t i n g the r o l e of t h i s pore s t r u c t u r e i n d e t e r m i n i n g c a t a l y s t performance. In a v e r y slow r e a c t i o n , r e a c t a n t s can d i f f u s e t o the c e n t e r o f the c a t a l y s t p e l l e t b e f o r e they r e a c t . On the o t h e r hand, i n the case of a very active catalyst containing small p o r e s , a r e a c t a n t m o l e c u l e w i l l r e a c t (due t o c o l l i s i o n w i t h pore w a l l s ) b e f o r e i t can d i f f u s e v e r y d e e p l y i n t o the pore s t r u c t u r e . Such a f a s t r e a c t i o n f o r which d i f f u s i o n i s slower than r e a c t i o n w i l l use o n l y the o u t e r pore mouths o f a c a t a l y s t p e l l e t . An i m p o r t a n t r e s u l t of the t h e o r y i s t h a t when d i f f u s i o n i s slower than r e a c t i o n , a l l the important k i n e t i c q u a n t i t i e s such as a c t i v i t y , s e l e c t i v i t y , temperature c o e f f i c i e n t and k i n e t i c r e a c t i o n o r d e r become dependent on the pore s i z e and p e l l e t s i z e w i t h which a p e l l e t i s prepared. T h i s i s because pore s i z e and p e l l e t s i z e determine the degree t o which d i f f u s i o n a f f e c t s reaction rates. Wheeler saw t h a t u n l i k e many a s p e c t s o f heterogeneous c a t a l y s i s , the e f f e c t s o f pore s t r u c t u r e on c a t a l y s t b e h a v i o r can be put on quite a rigorous basis, making p r e d i c t i o n s from t h e o r y r e l a t i v e l y a c c u r a t e and r e l i a b l e . Wheeler examined the mechanism o f d i f f u s i o n and f l o w i n c a t a l y s t pores and t h e i r consequence on r e a c t i o n rates i n pores. He always was concerned and
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Davis and Hettinger; Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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anxious t o t r a n s l a t e the r e s u l t s o f h i s s o p h i s t i c a t e d mathematical c a l c u l a t i o n s i n t o s i g n i f i c a n t p r a c t i c a l terms. The a t t a c h e d f i g u r e i l l u s t r a t e s h i s c l a s s i c a l estimate f o r the % o f i n t e r n a l c a t a l y s t surface a v a i l a b l e f o r a number o f important i n d u s t r i a l r e a c tions. The i n f l u e n c e o f pore s t r u c t u r e on o t h e r s i g n i f i c a n t events was c a l c u l a t e d , such as p r e s s u r e and temperature g r a d i e n t s t o be expected i n c a t a l y s t pellets. S p e c i a l a t t e n t i o n was drawn t o t h e e f f e c t on a c t i v i t y and s e l e c t i v i t y o f c a t a l y s t p o i s o n s which adsorb s e l e c t i v e l y on pore mouth l o c a t i o n s .
TRANSFER AND LIQUID PHASE REACTIONS ON 1/8* PILLS f LESS THAN 20%. l }
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Dependence of fraction of surface available on the ratio
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Figure 1. Wheeler's c l a s s i c a l estimate f o r the percent of i n t e r n a l c a t a l y s t surface a v a i l a b l e f o r a number of important industrial reactions.
Davis and Hettinger; Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Wheeler s r e s e a r c h a t Houdry i n v o l v e d him i n petroleum r e f i n i n g and p e t r o c h e m i c a l s . Again, he became i n v o l v e d e a r l y i n a new f i e l d , namely the homogeneous a c t i v a t i o n o f m o l e c u l a r hydrogen (by now hydrogen was an o l d f r i e n d ) . The unusual c a p a b i l i t y of aqueous s o l u t i o n s o f c o b a l t c y a n i d e t o a c t as a c a t a l y s t s e r v e d t o broaden the f i e l d o f c a t a l y t i c a c t i o n o f metal i o n s and s e r v e d t o c o n s t r u c t a b r i d g e to metal s u r f a c e c a t a l y s i s . In more r e c e n t y e a r s Wheeler c o n t i n u e d h i s c o n c e r n t o e s t a b l i s h the fundamental r e l a t i o n s h i p between the p h y s i c a l system and c a t a l y t i c behavior. Research published while at A e r o j e t - G e n e r a l and Lockheed d e a l w i t h i n t e r m o l e c u l a r f o r c e s i n gases, r a t e s o f s u b l i m a t i o n , t r a c e contami n a n t s a d s o r p t i o n and s o r b e n t r e g e n e r a t i o n as w e l l as the performance o f f i x e d bed o f c a t a l y s t r e a c t i o n w i t h p o i s o n s i n the f e e d . Each r e s e a r c h endeavor i s c h a r a c t e r i z e d by a deep concern f o r the fundamental phenomena o c c u r r i n g a t s u r f a c e and what can be der i v e d based on c h e m i c a l p r i n c i p l e s . As mentioned, A h l b o r n made i n d i v i d u a l c o n t r i b u t i o n s by l o o k i n g deeply i n t o mathematical relations h i p s between the t h e o r y o f p h y s i c a l events and t h e i r consequence to c a t a l y s i s . At the same time he enjoyed discussions with everyone concerned with a p p l i e d c a t a l y s i s , and he was happy t o c o o p e r a t e . A congenial co-worker, "Bud" Wheeler conducted his arguments w i t h gentlemanly p e r s i s t e n c e . As t o p e r s o n a l s t y l e , i t was w e l l known t h a t i t was almost i m p o s s i b l e t o get him i n t o the l a b o r a t o r y on time i n the morning and l i k e w i s e n e a r l y i m p o s s i b l e t o get him t o l e a v e i n the evening. When he was i n t e r e s t e d i n a problem, i t became a l l p e r v a s i v e t o him. Perhaps the one e x c e p t i o n noted a t Gordon Research C o n f e r ences, were p e r i o d i c absences d u r i n g t r o u t f i s h i n g season. RECEIVED November 17,
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Davis and Hettinger; Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
