The Mechanism of Catalytic Cracking - Advances in Chemistry (ACS

The Mechanism of Catalytic Cracking B. S. GREENSFELDER

Downloaded by CORNELL UNIV on November 24, 2012 | http://pubs.acs.org Publication Date: January 1, 1951 | doi: 10.1021/ba-1951-0005.ch002

Shell Development Co., Emeryville, Calif.

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics.

In little more than half of the 25 years covered b y this symposium, catalytic cracking has been developed from its first acceptance to a major industrial process. I t has served to increase the amount and octane rating of gasoline and the amounts of valuable C3 and C4 gas components obtainable from petroleum feed stocks over those from thermal cracking alone. I t is therefore of interest to seek an explanation of the nature of the products obtained i n catalytic cracking i n terms of the hydrocarbon and catalyst chemistry which has been developed within the past 25 years. The first object of this paper is to set forth the basic product distributions obtained i n the catalytic cracking of the major classes of pure hydrocarbons, which will serve to demonstrate the action of the cracking catalyst. The second object is to assemble these data into patterns having common denominators, to arrive at a consistent mechanism of hydrocarbon cracking which can be specifically related to the chemical nature of the cracking catalyst. T h e third is to review the experimental data available on the structural energy relationships within and among hydrocarbons to provide real support for the proposed mechanism of catalytic cracking. The proposed mechanism is then utilized to explain a number of important secondaiy reactions encountered i n catalytic cracking operations and to characterize the nature of catalytic gasoline to which both primary and secondary reactions contribute. The study of the catalytic cracking of pure hydrocarbons as a key to the interpretation of the catalytic cracking of petroleum fractions is predicated on the belief that most of the hydrocarbons present i n petroleum can be allocated to relatively few simple classes. This belief is supported particularly b y the accumulated results of A P I Project 6, originally titled " T h e Separation, Identification, and Determination of the Constituents of Petroleum" (now retitled "Analysis, Purification, and Properties of Hydrocarbons"). For the sake of consistency of experimental conditions, the data reported are those of the 3

In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

4

ADVANCES IN CHEMISTRY SERIES


Shell Development C o . (5-7). F u l l acknowledgment of the work of others reported extensively i n the literature is made thankfully, as it was of much assistance. Accordingly, work has been done on series of η-paraffins,* isoparafhns, naphthenes, aromatics, and naphthene-aromatics which have been chosen as representative of the major components of petroleum. I n addition, olefins, cyclo-olefins, and aromatic olefins have been studied as a means of depicting the important secondary reactions of the copious amounts of unsaturates produced i n the majority of catalytic cracking reactions. A silica-zirconia-alumina catalyst was used principally; i t resembles closely i n cracking properties typical commercial synthetic silica-alumina catalysts. The cracking of four important classes of petroleum hydrocarbons is surveyed, using the gaseous hydrocarbon products as a basic index of the nature of the cracking process. Table I gives the complete gas analyses. The uniformity of cracking of n-paraffins at 500° C . may be seen from the simplified mole percentage gas compositions shown below by carbon number on a hydrogen-free basis. η-Paraffin C

7

C12

Cie C4 2

Ci

C

5 9 2 5

C

2

16 9 6 5

C*

a

42 42 45 40

37 40 47 50

These gaseous products comprise from 41 to 86 weight % of the total feed reacted. Their striking over-all consistency indicates that a uniform mode of cracking must prevail. A very similar pattern is given b y the cracking of η-olefins, which are shown next as mole percentage gas composition. The gaseous product is now 12 to 35 weight % of the total feed reacted for Cie and C , respectively, and is low because of the low cracking temperature, 400° C. Although rarely present i n petroleum, olefins are important p r i mary products of cracking. 8

η-Olefin

Ci

C

Ce Cie

1 5

2 2

C

2

3

C*

30 29

67 64

A t 500° C , the gaseous products from cetene and cetane become indistinguishable and the former represents 40 weight % of the feed reacted (see Table I I ) . The cracking at 500° C . of both monocyclic and bicyclic cyclohexane-type naphthenes, which are important components of petroleum, again displays uniformity i n gas composi tions, approaching that of the η-paraffins. The gaseous products shown here as mole percentage amount to 26 to 52 weight % of the total feed reacted. Naphthene

Ci

C

C» Cu

8 10 4 9

5 7 11 8

C12 Cie

C

2

C*

3

35 36 37 36

52 47 48 47

I n the case of aromatics, an entirely different gas composition pattern is found. O n selecting some of the outstanding examples of this behavior among the alkylbenzenes (6), the striking fact emerges that the predominant gas component as mole percentage i n each case corresponds exactly in structure to the original a l k y l group on the benzene ring. Aromatic

Ci

C2

Ci

n-d

88

2

Iso-Ci 2

7

80

12

86


5

GREENSFELDER—THE MECHANISM OF CATALYTIC CRACKING

ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—r

CARBON NUMBER


Figure 1.

Catalytic Cracking Product Distribution

I n a l l these examples, the total benzene plus the corresponding C or total C4 gas components amounts to 8 9 % of the feed reacted. 3

Basic Cracking Patterns F r o m these simple gas products, which correspond to a very large portion of the reacted feed stock, two basic cracking patterns are postulated ; the first is applicable to aliphatics and alicyclics (I) (thus including paraffins, olefins, and naphthenes), the second to sub stituted aromatics (II). These two basic patterns are best illustrated b y Figures 1 and 2, which show the molar distribution of the principal cracked products according to the number of carbon atoms i n the fragments, per 100 moles of feed stock cracked, for selected representatives of the four major hydrocarbon classes, allât 500° C. (see Table I I for experimental conditions and product analyses). Table I. (Mole % ;

Comparative Gas Analyses in Catalytic Cracking

1 atmosphere;

silica-zirconia-alumina catalyst, Universal Oil Products Co. Type B ; 1-hour process period) Paraffins Olefins n-Hexadecene Parowax n-Hexadecane η -Dodecane (ca. n-tetracosane) n-Octenes (cetene) (cetane) b

n-Heptane°

400° C , about 7 moles per liter per hour

500° C , 13.2 to 14.2 moles per liter per hour

C4H10

6.5 4.3 8.3 6.5 25.8 14.2 6.4 10.6 17.4

6.5 8.8 5.1 3.2 25.8 13.7 8.7 10.4 17.8

4.9 1.5 3.4 2.6 32.8 10.3 10.6 15.8 18.1

6.1 4.9 2.3 2.2 30.5 7.3 11.9 18.4 16.4

1.7 0.9 0.7 1.3 26.6 2.4 17.5 26.5 22.4

Run No.

C-86

C-103

C-578

C-160

C-174

H, CH CH CHe CHe CHe 4

4

I80-C4H8

n-CHe

Isopropylcyclohexane

Naphthenes AmylTriethylcyclohexanes cyclohexanes

Amyl decalins

500° C , about 13 moles per liter per hour Η, CH CH CHe C,H CHe Iso-CHe n-CHe CHu>

12.0 6.9n. 3.7 0.9 19.5 11.1 4.8 8.0 33.1

Run No.

C-146

4

4

e

17.6 η 8.2 4.3 1.7 19.5 9.9 4.4 8.3 26.1

tm

C-156

550° C . A t 6.8 moles per liter per hour. 500° C . * 400° C . α

b

c

.λ

2.2 4.7 1.4 1.0 23.6 4.9 11.7 22.7 27.8 C-46

Aromatics η-Propylsec-Butyl ieri-Butyl benzene benzene^ benzene** 400° and 500° C , 12.3 to 13.7 moles per liter per hour 6

0.6

14.4 3.0 6.7 3.0 20.7 10.7 3.8 8.4 29.3

19.3 ο 7.3 4.2 2.1 22.1 7.2 4.8 9.6 23.4

5.6 - ί> 3.5 2.0 1.7 76.2 7.4 1.0 1.0 1.6/

4.4 Λ Λ 0.0 0.3 0.3 5.4 1.1 73.4 3.6 11.5/

76.7 10.6*

C-145

C-144

C-150

C-542

C-540

- '

· C i - C saturates. / Assume iso/n = 3. ο Assume iso/n ~ 7


Λ

β

0!2

1.1 3.5

e

7.3

6


Table II.

Catalytic Cracking of Representative Hydrocarbons (500° C , 1 atmosphere; siKca-zirconia-alumina catalyst)

Feed Process period, min. LHSV Moles/liter/hour Material balance, wt. % charge, no-loss basis Gas Liquid below original boiling point Remaining liquid Coke Moles/100 moles cracked Ci C C C C Ce 2

3

4

B


CT

Ce Ce Cio Cu Cl2 Cl3 Cu Cl5 Total H

Cetene

Amyl Decalins

Cumene

60 4.0 13.6

15 7.2 25.0

60 3.0 12.8

60 1.9 13.7

21.0 17.9 60.0 1.1

38.3 56.0 3.7 2.0

18.4 51.6 28.7 1.3

25.0 54.4 16.4 4.2»

4 5 77 98 49 38 18 9 7 6 4

11 9 42 55 43 32 18 10 12 10 9 8 3 2

3 2 79 2

4 16 112 116 43 38 7 8 7 4 3 1) 1 lj 367 14

2

Run N o . β

Cetane

C-578

4 2 321 5 C-1104

100

264 28

186 2

C-144

C-131

Carbon only.

T o obtain the first clue to the reaction mechanism, two hydrocarbons may be con sidered: (1) 1-hexadecene (cetene), representing group I , a n d (2) isopropylbenzene (cumene), representing group I I . W h a t common property of the catalyst will explain the cracking patterns of both, i n conformity with what is known of the chemical reac tions of carbon compounds? CH3

V

CH3

6 100

Figure 2.

90 - f 78 moles Catalytic Cracking of Cumene

Based on much evidence (both from the literature and these laboratories), i t is predi cated that the cracking reactions of cetene and cumene are directly related to the wellknown low temperature liquid or vapor phase acid-catalyzed reactions of olefin poly merization and the alkylation of aromatics with olefins, respectively. The reciprocal relationship of olefin polymerization and cracking is best demonstrated b y the cracking of diisobutenes to give a gaseous product containing 73 mole % isobutene and that of triisobutenes to give a gaseous product containing 81 mole % isobutene. T h e extension of this reciprocal relationship of polymerization and cracking to straight-chain olefins creates an apparent difficulty, because n-olefins polymerize to branched products. H o w ever, the same mechanistic rules which predict the structures of such polymers also govern the catalytic cracking of η-olefins. I n the same sense, aromatic alkylation b y olefins using acid catalysts and the cracking of a l k y l aromatics have a corresponding reciprocal relationship. T o illustrate, the cracking of cumene gave benzene and propyl ene, the original components, to the extent of about 86 weight % of feed cracked. I t is concluded that the correlation between hydrocarbon cracking patterns and acidic catalysts (both proton and "Lewis a c i d " types) is sufficient to justify further exploration of their relationship. In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


7

The acid-catalyzed reactions of olefin polymerization and aromatic alkylation b y ole fins have been very well explained b y the carbonium ion mechanism developed b y W h i t more (21). This mechanism provides the basis of the ensuing discussion, which is de voted to the application of such concepts (7,17) to catalytic cracking systems and to the provision of much added support i n terms of recently developed structural energy rela tionships among hydrocarbons and new experimental evidence. Study of the primary cracking step of the four major hydrocarbon classes leads to an important generalization, which m a y be seen from the following type reactions :


Paraffin Olefin Naphthene Aromatic

—> —> —> —>

paraffin + olefin olefin + olefin saturate + olefin or olefin + olefin aromatic + olefin

I n every case a n olefin is one of the products of the primary cracking step. N o w by considering each reaction i n reverse, a common denominator for a l l the designated crack ing systems can be found i n the chemistry of olefins. T h e answer lies i n the character of the olefinic double bond, which comprises the normal valence pair electrons, and i n addition two extra or " p i " electrons, which endow the double bond with the ability to attract positively charged groups, especially protons. This ability is expressed quanti tatively b y the "proton affinity," which is shown below for propylene and isobutylene : AHm, K g . - C a l . / M o l e Propylene

C H = C H — C H , + H+

Isobutylene

CH = C—CH, + H

> CH,—CH—CH,

2

-181

+

CH,

CH, > CH,—C—CH,

+

2

-195

+

These energy values are calculated from thermochemical tables (11) and the ionization potentials of hydrocarl>ons obtained b y Stevenson (15) using mass spectrometric methods. The union of an olefin and a proton from a n acid catalyst leads to the formation of a positively charged radical, called a "carbonium i o n . " T h e two shown above are secpropyl and ter£-butyl, respectively. [For addition to the other side of the double bond, Ai/298 = —151.5 and —146 kg.-cal. per mole, respectively. F o r comparison, reference is made to the older (4) values of E v a n s and Polanyi, which show differences of — 7 and —21 kg.-cal. per mole between the resultant n - and s-propyl and iso-and tert-butyl ions, respectively, against —29.5 and —49 kg.-cal. per mole here. These energy differences control the carbonium ion isomerization reactions discussed below. ] Such an ion may m turn combine with a second olefin : CH CH —CH-CH 3

3

H

3

+ CH2=CH—CH — > H C — i — C H — d — C H 3

3

A * 2

3

which is the basic reaction of acid-catalyzed olefin polymerization. B y release of a proton, the larger ion becomes the olefin polymer. T h e heats of addition of the most important carbonium ions to a n olefinic double bond m a y be represented b y the following figures, derived from ionization potential data of Stevenson (14,15) and thermochemical data (11) for the reactions a l k y l ion ( R ) + ethylene, for two alternative cases: +

R+ + H C = C H

2

R+ + H C = C H

2

2

2

—> R—CH —CH + 2

—>

R—CH—CH

(Reaction 1)

2

(Reaction 2)

3

+ ΔΗμ, Ion, R +

CH,+

C H

Reaction 1 Reaction 2 Reaction 3

-58.5 -88 -93

-22 -60.5 -58

2

6

+

K g . - C a l . per Mole n-C,H e-C,H 7

+

-21 -59.5 -57

7

+

feri-C«H

+7 -31.5 -29


9

+

+21.5 -17 -13.5

8


Clearly, Reaction 2 is favored over Reaction 1. I n extension of Reaction 2 to higher homologs, Reaction 3 corresponds to the union of the a l k y l ion, R , with propylene or higher r*-alpha-olefins (1-C»H »), to give the secondary ion, R — C H 2 — Ç H — ( C H ) « - 3 CH3, with the approximate energy values listed above. These heats of reaction also apply to the reverse reactions, which represent the cracking of carbonium ions. I t is evident that on a n energy basis i t is much more difficult to obtain methyl or ethyl ions as fragments than to obtain s-propyl or ferJ-butyl ions. Furthermore, the release of s-propyl ion from an olefin is favored over that of n-propyl b y about 28 kg.-cal. per mole and the release of tert-butyl ion over that of η-butyl to an even greater degree—45 kg.-cal. per mole (estimated values, H). These structural energy relationships provide the basis for the lack of Ci and C and the predominance of C and C i n the gaseous products of catalytic cracking. A l l values derived from mass spectroscopic measurements of ionization potentials are indeed considered to be significant measures of the energy relationships among the ionic reaction intermediates. However, further qualifications are necessary before these values may be applied to the calculation of rates of reaction i n a specific catalytic system. These qualifications are yet to be developed. +

2

2

2


8

4

Formation and Reaction of the Carbonium Ion Intermediate T o complete this picture, i t is necessary to show how the carbonium ion intermediate is formed i n the catalytic cracking of hydrocarbons. F o r olefins, i t is the reaction of proton addition: H C=CH—(CH )„—CH + H + — > H C — C H — ( C H ) — C H 2

2

3

3

+

2

n

3

F o r paraffins and naphthenes, the important reaction of hydride ion exchange (2) is postulated, which is i n turn initiated b y carbonium ions derived from small amounts of thermally produced olefins i n the cracking system.

Cracking then proceeds b y the reverse of olefin polymerization, ultimately producing relatively noncrackable C3, C4, and C5 carbonium ions from the larger carbonium ion intermediates. These small ions revert to olefins b y loss of a proton, which is the reverse of the proton addition reactions illustrated previously, or become small paraffins b y the hydride ion exchange reaction. T h e factors governing the size of the accompanying olefinic fragment are discussed later. Aromatics are i n a sense unique i n their catalytic cracking reactions. The aromatic ring contains the equivalent of six double bond or p i electrons, which are, however, mutually stabilized b y strong resonance energy. We may postulate an association be tween a carbonium ion and these electrons i n a generalized sense:

in which the forward reaction represents the alkylation of a n aromatic and the reverse represents the cracking of an aromatic over an acid catalyst. T h e energies of combina tion of alkyl carbonium ions and aromatics are not known. Based on experimental results and b y analogy with the reaction of carbonium ion and olefin, the same or similar relative energy differences appear to govern the alkylation and cracking of aromatics. In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

9


Thus, the ease of cracking alkyi aromatics increases i n the order methyl, ethyl, isopropyl, and ter^butylbenzene (6). This is i n exact agreement with the decreasing energies of combination of the corresponding carbonium ions with the ethyienic double bond i n ethylene or propylene. Confirmatory evidence for this mechanism has been obtained by Roberts and Good b y examining the cracking of alkyl aromatics i n which the electron density at the alkyl-aryl bond was changed in a specific manner (13). A s portrayed above, no aromatic carbonium ion is formed as such. Rather, one positive group is expelled as the other one enters. However, other schemes have been suggested, such as that shown below (12,17), as well as more complicated ones which i n volve several resonance structures. H


+

H

+

I n a n y of these cases, a n analogy of the initiatory mechanism to that encountered in olefin cracking is clear; thus, association with a proton, rather than hydride ion removal (as required for paraffins and naphthenes), normally constitutes the first step i n the cracking of both aromatics and olefins. I n summary, the two basic types of reaction intermediates and their products are: For paraffins, olefins, and naphthenes: C—C—C—R — > C—C=C + R

+

For aromatics:

I n each case, the resultant carbonium ion, R , if large, will tend to recrack. I n general, the final ion may separate into an olefin and a proton, or especially i n the cracking of saturates, may remove a hydride ion from a neutral molecule to form a small paraffin and a new carbonium ion. Therefore, two mechanisms are seen for the propagation of catalytic cracking: (1) proton transfer, wherein a proton is returned to the catalyst or donated to another molecule to regenerate the cycle; a n d (2) hydride ion exchange, wherein a new carbonium ion is formed b y release of a hydride ion to a n existing carbonium ion. A l l group I hydrocarbons (paraffins, olefins, and naphthenes) crack to give a n olefin and a carbonium ion by the generalized mechanism: +

C—C—C—R — > C—C=C + R +

+

I t is noteworthy that the charged carbon atom of the intermediate becomes part of the resultant olefin. The extremely important isomerization reactions of carbonium ions determine the position of the charged atom and therefore both the size and isomeric form of the olefinic fragment i n the primary cracking step (see 7, p. 2580, for more detailed explanation). These isomerization reactions are governed b y the same energy relationships which enter into the proton-olefin and carbonium ion-oiefin combination energies shown above. Thus, whenever possible, primary carbonium ions will rearrange to secondary ions prior to cracking, so that the smallest olefin produced b y the simplest possible type of cracking will be propylene, as shown i n the example above. Other isomeric, secondary ions will yield larger olefins. If rearrangement to tertiary ions takes place prior to cracking, the smallest olefin will then be isobutylene, b y the same princ pie. The designated mode of cracking at the carbon-carbon bond once removed from the charged carbon atom is the simplest possible mechanism; additionally, the ionic partner,


10


R , may rearrange to a secondary or tertiary ion during the cracking of the "activated complex." I n reverse, these same rules successfully predict that branched-chain olefin polymers will be obtained from either straight-chain or branched monomers. W i t h little modification, the structures of paraffin-olefin alkylates from acid catalysts may be predicted i n the majority of cases. The preferential release of C and C4 as the smallest fragments is a relative matter; ethylene, ethane, and methane can be produced under more drastic experimental conditions, and are produced i n small amounts i n ordinary catalytic cracking. The conventional process operates under conditions which maximize the desired type of splitting to the more useful gaseous products. T o demonstrate the application of theory to practice, the predicted and experimental curves for the cracking of cetane (7) are shown in Figure 3. +


3

C A R B O N NO.

Figure 3.

Catalytic Cracking of Cetane

There remains no doubt that the ionic reaction pattern of hydrocarbons is firmly related to the presence of acid catalysts. Recent work at the Shell Development Co. (1) and the H o u d r y Process Corp. (8) on the acid-catalyzed hydrogen-deuterium exchange and isomerization reactions of small paraffins has brought forth strong confirmation of the mechanistic pattern already applied to catalytic cracking. Furthermore, the work of Thomas (17, 18), Tamele (16), and M i l l i k e n , M i l l s , and Oblad (9), among others, has established the conventional cracking catalyst to be a n acid catalyst, capable of acting both as a proton donor and as a strongly polar Lewis acid. T h e valences of silicon, aluminum, and oxygen are so distributed that additional cations, such as protons, are required for electrostatic neutrality. The physical reality of the postulated carbonium ion intermediates is then indeed a question worthy of discussion. Free a l k y l ions are produced under electron impact i n the high vacuum of a mass spectrometer. They are the ions recorded b y this instrument at the cathode as an " i o n current." I n the presence of a n acid catalyst i n a heterogeneous system containing gaseous or liquid hydrocarbons their free existence is difficult to establish, as their negative partners must be close at hand at the surface of the catalyst. A t the San Antonio, Tex., Southwest Regional M e e t i n g of the AMERICAN C H E M I C A L SOCIETY, December 1950,

in a Symposium on Carbonium Ions, Matsen and coworkers (S) presented cryoscopic evidence of the existence of carbonium ions formed from 1-octene i n sulfuric acid solution. I n addition, they indicated that the carbonium ion from 1-octene was detectable by characteristic maximum absorption i n the ultraviolet region a t 3000 to 3200 A . i n acidic media such as sulfuric acid, phosphoric acid, and complexes such as aluminum chloride-fer£-butyl chloride and boron trifluoride-rc-propyl chloride. The reality and the mode of existence of carbonium ions are most interesting topics for further research i n the field of hydrocarbon chemistry.

Applications F r o m the general principles of carbonium ion systems, a host of applications m a y be made to important reactions of the catalytic cracking system. Some of these follow: In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

11

GREENSFELDER—THE MECHANISM OF CATALYTIC CRACKING Preferential Saturation of Isobutylene vs. n-Butylenes

Δ#29β Kg.-Cal./Mole (14)

c

c

C—i=C+H

+

C — C = C — C + H+

—>

C—i—C

-195

—>

C—C—C—C

+

-186.5

+

Self-Saturation of Olefins

c

c

c

c

C-L_c—C + C — C = C — > C— coke (via further reactions) +

+

Desulfurization of Mercaptans (Thiols)

CH —CH —CH SH + H


3

2

2

—>

+

CH —CH=CH 3

CH —CH —CH + 3

2

+ H

2

2

+ H S—> 2

+

Isomerization of Naphthenes

H C

/

Η C +

\

2

Hè

CH kn

2

2

Η C

/

X

H C

2

2

>

H

2

/

/

CH

i CH \ / " CH

Η C

X

H C

2

2

*

2

2

CH

H C CH " \ * / CH 2

/

2

CH Η / C

3

\

H C

CH

2

2

3

H,

H

Double Bond Shift of Olefins

C=C-C—C + H

— > C—C—C—C — > C—C=C—C + H+

+

Isomerization of Olefins

C c = c — C — C + H+ — > c — c — c — c — > c—h—c +

+

c — > c = c — C + H+

Despite much recent progress, the energetic relationships and specific mechanistic steps involved i n these reactions require more detailed experimental examination to provide explanation of all the observed facts and to enable more reliable prediction of new reactions. Likewise, the specific interaction between cracking catalyst and hydrocarbon, which also has been the subject of recent work (8, 9), is a promising field for mechanistic studies.

Liquid Products The liquid products of catalytic cracking (obtained i n accordance with the described principles) have been omitted from consideration thus far, except i n the case of the a l k y l aromatics. T o the refiner, the liquid obtained is of prime importance, both as gasoline and heavier intermediate oils. Paraffins produce mostly C and C liquid product, principally olefins and paraffins. Based on feed reacted, n-Ci gave 49, n-Ci gave 44, and n-C gave 57 weight % liquid product (20), under conditions given i n Table I . Monocyclic naphthenes give relatively more cracked liquid than paraffins, primarily because of the partial retention of rings after the cracking of side chains and because of some dehydrogenation to aromatics. Based on feed reacted, Cu gave 59, C i gave 68, and Cie gave 73 weight % liquid product (20), under conditions as i n Table I . Bicyclic aromatics and naphthenes are important components of cracking feed stocks. The former, after cracking i n the side chains to gasoline and gas, will remain as smaller bicyclic aromatics in the cracked gas oil. The latter will be converted to naphthenes and aromatics distributed i n both the gasoline and gas oil, together with aliphatic gas and gasoline components. 6

2

6

6

24

2




Olefins are usually absent from petroleum feed stocks, but they occupy a position of great importance i n determining the character of the gasoline. I n general, the catalytic cracking of any hydrocarbon gives at least one olefinic fragment. Such olefins are both rapidly transformed and considerably equilibrated thermodynamically b y a number of the ionic reactions illustrated, including double bond shift, skeletal isomerization, poly merization, and cracking (19). F o r this reason, the gasolines produced i n the catalytic cracking of a wide variety of petroleum feed stocks have notable similarities i n composi tion, physical properties, and engine performance. This is i n contrast to the gasolines obtained by thermal cracking; the absence of a catalyst capable of promoting the trans formations of olefins makes many of the properties of thermal gasolines much more dependent upon the composition of the feed stock and the exact conditions of cracking. The general relationship of the cracking of pure paraffins, naphthenes, and aromatics to that of petroleum fractions was given recently b y Voge, Good, and Greensfelder (20) at the T h i r d World Petroleum Congress. I n general, i t was demonstrated that gasoline yields are capable of reasonably close prediction from hydrocarbon-type analysis of the feed stock. The presence of aromatic nitrogen bases, which specifically poison the catalyst as shown b y M i l l s , Boedeker, and Oblad (10), makes these predictions inappli cable unless the nitrogen compounds are extracted; their data also provide new e v i dence of the acidic nature of the cracking catalyst.

Summary Fundamental studies of catalytic cracking have led to the conclusion that the chief characteristics of the products may be traced t o the primary cracking of the hydro carbons i n the feed stock and to the secondary reactions of the olefins produced; both correspond to the ionic reaction mechanisms of hydrocarbons i n the presence of acidic catalysts. The chemistry of both the hydrocarbons and catalysts dealt with here has advanced rapidly i n the last decade. Nevertheless, much further exploration is required with respect to the nature of the catalyst and the properties of the hydrocarbons under going reaction. A promising field lies ahead for future research.

Acknowledgment The assistance of G . M . Good, D . P . Stevenson, and H . H . Voge, and of M r s . A . Carruth of the Shell Development C o . i n the preparation of this paper is gratefully acknowledged.

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The Mechanism of Catalytic Cracking - Advances in Chemistry (ACS

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