The Carbonium Ion Mechanism of Catalytic Cracking - American

19

Downloaded by OHIO STATE UNIV LIBRARIES on October 14, 2014 | http://pubs.acs.org Publication Date: June 3, 1983 | doi: 10.1021/bk-1983-0222.ch019

The Carbonium Ion Mechanism of Catalytic Cracking H E R V E Y H. V O G E

1

Sebastopol, CA 95472

Development of the carbonium ion mechanism of catalytic cracking in 1940-1950 is reviewed. Studies of the thermal and catalytic cracking of pure hydrocarbons played a key role, and were complemented by earlier hypotheses of Whitmore regarding the probable carbonium ion intermediates in low-temperature acid-catalyzed hydrocarbon reactions. Workers in several laboratories arrived at similar conclusions at about the same time. C a t a l y t i c cracking of h i g h - b o i l i n g petroleum f r a c t i o n s i s the largest i n d u s t r i a l c a t a l y t i c process i n terms of m a t e r i a l processed and c a t a l y s t required. I t has become the c e n t r a l process i n petroleum r e f i n i n g . The h i s t o r y of c a t a l y t i c cracking shows that i t developed l a r g e l y e m p i r i c a l l y , and benefited from basic understanding only i n l a t e r stages. Modern c a t a l y t i c cracking was preceded by an e a r l y cracking process employing as a c a t a l y s t . This process was t r i e d i n 1913-1915 by McAfee ( 1 ) but had no extensive use. The chemistry of cracking with was not examined c l o s e l y . The present c a t a l y t i c cracking process arose from c l a y treatments of petroleum f r a c t i o n s . A strong impetus was given by the French chemist Eugene Houdry about 1924-28 (2). In a s e r i e s of exploratory experiments Houdry observed that the gasoline produced when h i g h e r - b o i l i n g petroleum f r a c t i o n s were heated i n the presence of c e r t a i n a c i d - t r e a t e d c l a y s gave improved performance i n racing cars. This led eventually to the Houdry fixed-bed cracking process, f i r s t put i n t o use i n a plant i n 1936. Then, as now, the advantage of c a t a l y t i c cracking did not derive from the increased rate of cracking, but rather from the greater value of the products as compared to those from the older thermal cracking processes. The Houdry process was

AICI3

AICI3

1

Retired from Shell Development Company

0097-6156/83/0222-0235$06.00/0 © 1983 American Chemical Society In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

236

HETEROGENEOUS

CATALYSIS

promoted i n the USA by the Houdry Process Corp. However the competitive c r e a t i v i t y of the O i l Industry r a p i d l y modified the process, and the fixed-bed process was f a i r l y soon replaced by moving-bed and f l u i d i z e d - b e d processes.


Research at S h e l l In the e a r l y development of c a t a l y t i c cracking s e v e r a l l o o s e l y cooperating industry groups were formed. At S h e l l we c o l l a b o r a t e d with Standard O i l Development Co., The Texas Co., Standard O i l Co. of Indiana, U n i v e r s a l O i l Products and the M. W. Kellogg Co. This group was f i r s t organized as the " C a t a l y t i c R e f i n i n g Agreement , and l a t e r operated c o o p e r a t i v e l y under "Recommendation 41 of the Petroleum Administrator for War". The c o l l a b o r a t i o n ended s h o r t l y a f t e r the war, but by that time the f l u i d i z e d - b e d cracking process had been well e s t a b l i s h e d (3.). A leading r o l e i n the c r e a t i o n of t h i s process was played by Standard O i l Development Co. (a part of what i s now Exxon). Because of the basic c a t a l y s i s research that had been done e a r l i e r at S h e l l , e s p e c i a l l y by 0. Beeck, A. Wheeler, M. W. Tamele, and others, i t was suggested about 1939 that S h e l l should look i n t o the more fundamental aspects of c a t a l y t i c cracking. B. S. Greensfelder and I concentrated on the reactions of c a t a l y t i c cracking; G. M. Good joined us l a t e r . Petroleum above the gasoline b o i l i n g range i s composed of very many hydrocarbon compounds, i n c l u d i n g some that also contain s u l f u r and nitrogen atoms. To a s c e r t a i n the nature of the cracking r e a c t i o n s , Greensfelder wisely chose to study the behavior of representative pure compounds. In our j o i n t work more than 50 pure hydrocarbons were tested over a t y p i c a l s o l i d cracking c a t a l y s t , a s i l i c a - z i r c o n i a - a l u m i n a material that had been made experimentally by UOP. Later various s i l i c a - a l u m i n a s were used with s i m i l a r r e s u l t s . Comparisons were also made with thermal cracking, and with cracking over c a t a l y s t s of decidedly d i f f e r e n t character, such as a c t i v a t e d carbon. From the modern viewpoint these researches were done using p r i m i t i v e a n a l y t i c a l methods, and much care and expense was devoted to i d e n t i f y i n g r e a c t i o n products. The pure hydrocarbon studies were extended by using mixtures of s e v e r a l compounds and by studying the e f f e c t s of oxygen, n i t r o g e n , and s u l f u r compounds. These studies were valuable for commercial a p p l i c a t i o n s of c a t a l y t i c cracking, aiding the s e l e c t i o n of feedstocks, choice of operating c o n d i t i o n s , use of r e c y c l e cracking or staged cracking, e t c . But we were e s p e c i a l l y anxious to know what was taking place on the c a t a l y s t surface. 11

Mechanism In the e a r l y years of c a t a l y t i c cracking there was

In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


19.

VOGE

The Carbonium Ion Mechanism

237

considerable speculation about the mechanism. The c a t a l y s t s then used were n o n - c r y s t a l l i n e s o l i d s of high surface area (200 - 500 m^/g). U s u a l l y they were composed of silicon dioxide plus 13 - 25% aluminum oxide. They were not obviously a c i d i c , and i t was only much l a t e r that d i r e c t evidence of the a c i d i c nature of the surface a f t e r proper dehydration was obtained. One e a r l y proposal ascribed cracking to strong adsorption of adjacent carbon atoms on the s o l i d surface. Another ascribed a c t i v i t y to a c t i v e carbon formed on the c a t a l y s t surface. There were other t h e o r i e s , u s u a l l y rather vague and without much evidnece to back them up. As we learned more about the hydrocarbon r e a c t i o n s , some s i m i l a r i t i e s to reactions catalyzed by strong acids at much lower temperatures became evident. An a d d i t i o n a l and d i f f e r e n t impetus to understanding came from an i n t e r l u d e of thermal cracking s t u d i e s . At the time we were i n t e r e s t e d i n the thermal cracking of normal p a r a f f i n s (waxes) f o r production of alpha olefins. Thermal cracking of n-hexadecane gave products i n c l o s e agreement with the Rice-Kosiakoff theory of f r e e - r a d i c a l chain reactons (4). According to that theroy a r e a c t i v e (and isomerizing) f r e e - r a d i c a l intermediate propagates the chain, and through progressive breakdown stages about 4 moles of product hydrocarbons form from each mole of hexadecane that cracks. In c a t a l y t i c cracking we had observed 3.5 moles of hydrocarbon products per mole of hexadecane cracked, and the number was almost independent of conversion level from 11 to 70% conversion. This suggested progressive breakdown in a chain r e a c t i o n i n v o l v i n g a r e a c t i v e intermediate. The papers of Whitmore ( 5 ) proposed a carbonium ion as the intermediate in low-temperature a c i d - c a t a l y z e d r e a c t i o n s such as alkylation and polymerization; the same intermediate could perhaps be involved in c a t a l y t i c c r a c k i n g . Some of the s i m i l a r i t i e s between c a t a l y t i c cracking and low-temperature acid-catalyzed reactions include: high r e a c t i v i t y of o l e f i n s compared to p a r a f f i n s , removal of complete side chains from alkylaromatics, high r e a c t i v i t y of compounds containing tertiary carbon atoms, selective formation of i s o p a r a f f i n s , o l e f i n isomerization r e a c t i o n s , hydrogen t r a n s f e r r e a c t i o n s , and hydrogen-deuterium exchange r e a c t i o n s . There are of course important d i f f e r e n c e s , most of which can be explained by the d i f f e r e n t thermodynamic e q u i l i b r i a that p r e v a i l at 500°C and 20°C. As soon as we applied the concept of the carbonium ion intermediate, many previous observations could be c o r r e l a t e d . A few simple r u l e s about formation, i s o m e r i z a t i o n , and cracking of the hypothesized ions explained most of the experimental data on hydrocarbon cracking. Quantitative p r e d i c t i o n of the products from n-hexadecane cracking was p o s s i b l e with the a i d of only one a d d i t i o n a l assumption, as noted i n the paper of Greensfelder, Voga and Good (6). For d e t a i l s of the carbonium ion mechanism


238

HETEROGENEOUS

CATALYSIS


as a p p l i e d to cracking the reader i s r p f e r r e d to a review chapter by the present author (7)· Greensfelder, i n another chapter (8), extended the b a s i s for the theory by c a l c u l a t i n g r e l a t i v e energies of formation and r e a c t i o n of various simple gaseous carbonium ions, using thermodynamic data f o r hydrocarbon d i s s o c i a t i o n energies, and i o n i z a t i o n p o t e n t i a l s for gaseous free r a d i c a l s . It has to be recognized that the carbonium ions of cracking e x i s t adsorbed on the surface of a s o l i d , but there i s reason to expect some r e l a t i o n s h i p between r e l a t i v e energies there and i n the gaseous s t a t e . A c i d i c Nature of C a t a l y s t s B e l a t e d l y good evidence for the strong a c i d i t y of s u i t a b l y dehydrated c a t a l y s t was obtained. Tamele (9), r e p o r t i n g work done in c o l l a b o r a t i o n with 0. Johnson, L. B. Ryland, and Ε. E. Roper, noted that evidence for a c i d i t y was obtained for s i l i c a - a l u m i n a from ammonia adsorption, and that the c a t a l y s t a c i d i t y could be t i t r a t e d with butylamine in benzene. Still l a t e r , Benesi (10), using the Hammett i n d i c a t o r s , showed that s i l i c a - a l u m i n a c a t a l y s t had an acid strength greater than that of 90% s u l f u r i c a c i d . Another sign of the importance of c a t a l y s t a c i d i t y was the strong i n h i b i t i o n of cracking by small amounts of nitrogen bases. In the cracking of hexadecane, an amount of q u i n o l i n e s u f f i c i e n t to cover only 2% of the c a t a l y s t surface reduced the cracking to a small percentage of the u n i n h i b i t e d . Our f i r s t observation of the nitrogen base e f f e c t came from a c c i d e n t a l contamination. We tried the effect of b i c y c l i c aromatic compounds on the cracking of hexadecane and found strong inhibition. But the main cause of the i n h i b i t i o n was traced to small amounts of nitrogen compound impurities in the aromatics. Work of Others In 1948-50 the time was s u r e l y r i p e for r e c o g n i t i o n of p o s s i b l e carbonium ion mechanisms f o r c a t a l y t i c cracking. At least five groups of workers proposed carbonium ion intermediates f o r cracking. There were v a r i a t i o n s i n the hypotheses and u s u a l l y not much evidence or d e t a i l . Since there was a c e r t a i n amount of intercommunication i t i s hard to name any one as c l e a r l y first. Carbonium ion mechanisms were proposed by Bremner (11), Hansford (12), and C i a p e t t a , Macuga and Leum ( 13). Most s i g n i f i c a n t was the work of C. L. Thomas (14). At UOP he had done e a r l y work on the cracking of pure hydrocarbons. He had noted the hydrogen t r a n s f e r r e a c t i o n and other c h a r a c t e r i s t i c r e a c t i o n s . His paper (14) was published simultaneously with that of Greensf e l d e r , Voge, and Good (6). His r u l e s f o r the reactions of carbonium ions on the c a t a l y s t surface are very s i m i l a r to ours, but he did not make numerical


19.

VOGE

The Carbonium Ion Mechanism

239


interpretations of products. Thomas gave an i l l u m i n a t i n g explanation of the a c i d i t y of s i l i c a - a l u m i n a . He pointed out that when t e t r a - v a l e n t s i l i c o n and t r i - v a l e n t aluminum atoms are both t e t r a h e d r a l l y coordinated with oxygen atoms i n a l a t t i c e , then an extra p o s i t i v e ion i s necessary to complete the structure. By s u i t a b l e preparation t h i s p o s i t i v e ion i s a hydrogen ion and an a c t i v e c a t a l y s t r e s u l t s . He showed a f a i r l y good correspondence between measured a c i d i t i e s and a c t i v i t i e s for a s e r i e s of s i l i c a - a l u m i n a s covering a wide composition range. Maximum a c t i v i t y and t i t r a t a b l e a c i d i t y were observed at A l / S i r a t i o of about one. Carbonium Ions i n Other Media The idea of carbonium ions i s quite o l d i n organic chemistry. Olah has traced the e a r l y h i s t o r y ( 15). In 1902 Von Baeyer wrote of carbonium s a l t s i n explaining the deep c o l o r formed when triphenylmethyl c h l o r i d e was d i s s o l v e d i n s u l f u r i c a c i d . Carbonium ions as r e a c t i o n intermediates were proposed by Meerwein i n 1922, and much used by Ingold, Hughes, and others i n England soon t h e r e a f t e r . F. C. Whitmore i n the USA from 1932 on showed how carbonium ions as r e a c t i o n intermediated could explain the acid-catalyzed reactions of alkylation, polymerization, and i s o m e r i z a t i o n . His studies were summarized in a review a r t i c l e i n 1948 (_5). More r e c e n t l y , of course, there have been many spectroscopic studies of stable carbonium ions formed i n h i g h l y a c i d i c s o l u t i o n s at low or moderate temperatures, as, f o r example, i n the works o f N. C. Deno and G. A. Olah. Unfortunately the t r a n s i e n t carbonium ions presumably formed in catalytic cracking cannot be e a s i l y observed. Furthermore, there remain many questions about degrees of adsorption bonding involved with these intermediates, as w e l l as need of information about s t r u c t u r e s and p o s s i b l e a l t e r n a t i v e r e a c t i o n paths. In general, however, t h i s lack of exact knowledge i s c h a r a c t e r i s t i c of almost a l l r e a c t i o n k i n e t i c s , c a t a l y t i c or otherwise. Later Developments Very s i g n i f i c a n t improvements are s t i l l being made i n c a t a l y t i c cracking, both from the engineering side and from the chemistry. Among the former are r i s e r cracking and hightemperature regeneration. Chemically, c a t a l y s t improvements are s t i l l being made. The i n t r o d u c t i o n of the z e o l i t i c cracking c a t a l y s t s , l a r g e l y by Mobil workers about 1964 ( 1 6 ) was a major advance that brought increased c a t a l y s t a c t i v i t y and markedly improved s e l e c t i v i t y . This advance could have been suggested by C. L. Thomas, who, i n h i s 1949 paper s a i d : "To obtain maximum a c t i v i t y t h e s e c a t a l y s t s s h o u l d be made i n s p e c i a l ways. S i l i c a -



240

HETEROGENEOUS CATALYSIS

alumina c a t a l y s t s of maximum a c t i v i t y probably cannot be prepared by forming a s i l i c a hydrogel and depositing alumina on it." The l a t e r method of preparation, or even less s o p h i s t i c a t e d methods of modifying n a t u r a l c l a y s , had been widley used e a r l i e r . There i s c e r t a i n l y need for b e t t e r evidence on the d e t a i l s of the carbonium ion mechanism, p a r t i c u l a r l y as regards the i n i t i a t i o n of η-paraffin cracking, which has been much debated. There i s always room f o r better c a t a l y s t s that w i l l g r e a t l y reduce coke formation, improve octane number, or permit r e a c t i o n s to be more s p e c i f i c f o r p a r t i c u l a r products. The future will undoubtedly bring improvements and better understanding i n c a t a l y t i c cracking and i n the c l o s e l y r e l a t e d reactions of hydrocracking, residue treatment, coal l i q u e f a c t i o n , shale o i l r e f i n i n g , and the processing of t a r from sands.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

McAfee, A. M. Ind. Eng. Chem. 1915, 7, 737. Houdry, E . ; Joseph, A. Bull Assoc. Franc. Technicians Petrole 1956, 117, 177. Murphree, Ε. V. Advances in Chemistry 1951, No. 5, 30. Voge, H. H.; Good, G. M. J. Am. Chem. Soc. 1949, 71, 593. Whitmore, F. C. J . Am. Chem. Soc. 1932, 54, 3274; Chem. Eng. News 1948, 26, 668. Greensfelder, B. S.; Voge, H. H.; Good, G. M. Ind. Eng. Chem. 1949, 41, 2573. Voge, H. H. "Catalysis", Emmett, P. H. Ed. Reihhold 1958; Vol. VI, 407. Greensfelder, B. S. "The Chemistry of Petroleum Hydrocarbons", Brooks, B. T. Ed. Reinhold 1955; Vol. II, 137. Tamele, M. W. Faraday Soc. Disc. 1950, No. 8, 270. Benesi, H. A. J . Am. Chem. Soc. 1956, 78, 5490. Bremner, J . G. M. Research 1948, 1, 281. Hansford, R. C. Ind. Eng. Chem. 1947, 39, 849. Ciapetta, F. G.; Macuga, S. J.; Leum, L. N. Ind. Eng. Chem. 1948, 40, 2091. Thomas, C. L. Ind. Eng. Chem. 1949, 41, 2564. Olah, G. A. Chem. Eng. News 1967, March 27, 77. Plank, C. J.; Rosinski, E. J.; Hawthorne, W. P. Ind. Eng. Chem. Prod. Res. Develop. 1964, 3, 165.

RECEIVED November 17, 1982


The Carbonium Ion Mechanism of Catalytic Cracking - American

Recommend Documents