November 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
(23) Thomas, C. L., and Ahlberg, J. E. (tcJ Universal Oil Products Co.), U. S. Patent 2,229,353 (Jan. 21, 1941): 2,285,314 (June 2, 1942); 2,329,307 (Sept. 14, 1943). 5 (24) Thomas, C. L., and Bloch, H. 8. (to Universal Oil Products C o . ) , U. S. Patent 2,242,553 (May 20, 1941). (25) Ibid.,2,333,903 (Nov. 9, 1943). (26) Ibid., 1,416,965-6 (Mar. 4, 1947). (27) Thomas, C. L., and Danforth, J. D. (to Universal Oil Products Go.), U. 8. Patent 2,287,917 (June 30, 1942). (28) Thomas, C. L., Hoekstra, J., and Pinkston, J. T.,J . Am. Chem. Soc., 66,1694 (1944).
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(29) Voge, H. H., Good, G. N., and Greensfelder, B. S., IND.ENQ CHEM., 38, 1033 (1946). (30) Whitmore, F. C., Chem. Eng. News, 26, 668 (1948). (31) Whitmore, F. C., J . Am. Chem. Sec., 54, 3274 (1932). (32) Whitmore, F. C., and StahIy, E. E., Ibid.,55, 4153 (1933). RECEIVEDNovomber 15, 1948. This paper is taken from which was part of a technioal information exchange ordered ieum Administrator for War in Recommendation 41. The formed at the Riverside, Ill., laboratories of Universal Company.
a 1945 report by the Petrowork was perOil Products
Catalytic and Thermal Cracking of Pure Hvdrocarbons J
MECHANISMS OF REACTION B. S. GREENSFELDER, H. H. VOGE, AND G. M. GOOD Shell Development Company, E m e r y d l e , Calif. T h e primary cracking of pure hydrocarbons both with and without catalysts has been studied in terms of the distribution by carbon number of the cracked fragments to allow arriving a t a mechanism of molecular disintegration. The secondary reactions of the cracked fragments have been followed by analyses of the product fractions to allow a further definition of the nature of the cracking system. On the basis of this work, cracking systems are assigned to two fundamental classes; each class is described by a set of characteristic reactions covering both the primary cracking and the secendary reactions. Correspondingly, two types of reaction mechanisms are proposed, one a free radical (thermal type) mechanism based on the Rice-Kossialroff theory of cracking, the other a carbonium ion (acid-activated type) mechanism
derived from the work of Whitmore and others on the properties of carbonium ion systems. Cracking catalysts are available for either type of reaction mechanism; those which accelerate free radical type reactions are nonacidic, and those which accelerate carbonium ion type reactions are acidic. Commercial acid-treated clay and synthetic silica-alumina cracking catalysts belong to the latter class. Activated carbon, a highly active, nonacidic catalyst, gives a unique product distribution which is explained as a quenched free radical type of cracking. Activated pure alumina has weakly acidic properties and produces moderate catalysis of both types of reaction mechanism, the primary cracking corresponding to a free radical mechanism and the secondary reactions of product olefins following a carbonium ion mechanism.
P
mental unity is thus established for a number of important hydrocarbon catalytic reaction systems. Thermal cracking and cracking over nonacidic catalysts have also been studied. Mechanisms are also proposed for these systems for comparison with those of the industrial or conventional catalytic cracking process, Despite the wide variety of products obtained in the cracking of different hydrocarbons either thermally or by any catalytic process, i t has become increasingly evident that certain characteristic severances of carbon-carbon bonds and secondary reactions of olefins are always obtained thermally and over certain nonacidic catalysts, whereas another set of reactions prevails consistently in the presence of acidic oxide cracking catalysts. The principal contrasting reactions are shown here with respect to specific hydrocarbons or hydrocarbon types which have been tested. Comparisons between classes refer to hydrocarbons with the same number of carbon atoms (Table A). Both the hydrocarbon class and the isomeric form of a given hydrocarbon control the primary products obtained. Because of uniformity and simplicity of structure, normal paraffins (and olefins) were given preferred study. The use of a relatively large aliphatic hydrocarbon assists identification of important secondary reactions because of its extensive fragmentation.
RIOR work on the catalytic cracking of pure hydrocarbons has led t o a general characterization of the rates of cracking and product distributions of the principal classes of petroleum hydrocarbons (10-13). I n addition, a number of secondary reactions of olefins have been investigated and the effects of structural isomerism on the rates of cracking of several types of hydrocarbons were examined (9,54). Consistent mechanisms of reaction are now proposed, based on the primary hypothesis that any hydrocarbon reacting over this type of catalyst is transformed into a carbonium ion (33,which then cracks or undergoes secondary reactions according to definite rules. This hypothesis is directly coupled with the requirement that the acidic oxide type of cracking catalyst must make available reactive positive hydrogen ions (protons) capable of producing carbonium ions on contact with the hydrocarbon feed. A similar type of approach was proposed independently by Thomas (52). The properties of carbonium ions, which are postulated to represent the reactive form of the hydrocarbon in conventional catalytic cracking, also determine the mechanism of reaction and the type of product in many other acid-catalyzed hydrocarbon reactions, such a8 the isomerization, polymerization, parafKn alkylation, and hydrogen transfer reactions of olefins, the isomerization of paraffins, and the alkylation of aromatics. Funda-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Therefore, much study was devoted to the cracking of n-hexadecane (cetane), a representative normal paraffin in the gas-oil boiling range. TERMINOLOGY AND PROCEDURE. The definitions and terminology used here correspond to those in a previous paper (10); extent of cracking, conversion, and percentage decomposed are used interchangeably t o include gas, liquid boiling below the original, and coke-redefined (34) t o include carbon and hydrogen-all summed on a no-loss basis (9). T h e apparatus and procedure used in this work have heen described (9, 15) with certain modifications for thermal craclcing experiments (33); analytical methods have been amplified t o include deteimination of paraffin isomers by infrared and aromatics by ultraviolet spectrometry. Liquid products from aliphatic feed stocks were fractionated by carbon number (9); those from other hydrocarbons were separated into distillxte fractions comprising significant boiling ranges. THERMAL CRACKING
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the amount of ethylene, but i t still reniains the major product. By successive recracking, the radicals ultimately are reduced t o methyl or ethyl fragments. These radicals then react with feed stock molecules t o produce new fIee radicals and are themselves converted t o methane or ethane. Thus, cracking is propagated ass a chain reaction. T o start the chain and to compensate for the loss of chain carriers by side reactions, it may be assumed t h a t a few highly activated molecules decompose spontaneously or a t the wall. The R-K theory also concerns the manner of formation of a iadical fiom the original paraffin. .;2 primary hydrogen atom is more securely bonded and is removed less readily than asecondary hydrogen atom, and a numerical value of 2000 calories is assigned as the difference in activation eneigies, which corresponds to a relative rate of removal of 1 to 3.66 a t 500" C. Tertiary hydrogen is still more easily removed, 13.4 times as fast as primary hydrogen, but this does not enter into the cracking of normal paraffins, since no skeletal isomerization appears to take place. Radical isomerization presumably occurs through a coiled configuration of a single radical, in which the hydrogen donor and acceptor carbon atoms must closely approach each other. This last restriction affects the calculations for cetane vcry littlc ( 3 3 ) . A schematic representation of cetane cracking IS as follows:
The most successful present explanation of thermal cracking of hydrocarbons is the Rice free radical theory (2b-27) as modified b y Kossiakoff and Rice (19). This will be called the "RKtheory" and is summarized from another paper (33) as follows to explain the cracking of a normal paraffin: 1. Small radical, such as CH8, from prior cycle or from initial The normal paraffin molecule loses a hydrogen atom by collihydrocarbon rupture, combines with an I1 atom in cetane to give sion and reaction with a small free hydrocarbon radical or a free a cetyl radical and methane: hydrogen atom, thereby becoming a free radical itself. H H I1 H H H H €I H H H €I H H H H This radical may immediately crack or may undergo CHa HC -C-C-C-C-C-C-C-C-C-C-C-C-C-C-CH radical isomerization prior to cracking. Radical isom€I H H . H H H H H H H H H H H H erization is a change of the position of a hydrogen 2. Cetyl radical cracks beta to free valence t o give, say, n atom, usually to yield a more stable radical. Cracking of pentene-1 and undecyl-1 radical: either the original or isomerized radical then takes place a t a carbon-carbon bond located in the beta position t o the carbon H H H H H H H H H H H H H H H H atom lacking one hydrogen atom. Cracking at the beta posiHC-C-C-C=C and C-C-C-C-C-C-C-C-C--C-CH H H H H H H H H H H H H H H H tion gives directly an alpha olefin and a primary radical (lacking one hydrogen atom on a primary carbon atom); in this 3. Undecyl-1 radical cracks beta to free valence t o give ethylstep there is no change of position of any hydrogen atom with ene and nonyl-1 radical; repeat process t o give ethylene and respect t o the carbon skeleton. heptyl-1 radical; repeat process to give ethylene and amyl-l radical; repeat process to give ethylene and propyl-1 radical; The primary radical derived from this step may immediately repeat process t o give ethylene and methyl radical, which then recrack a t the beta bond t o give ethylene and another primary reacts as in step 1 t o continue the chain reaction. radical, or i t may first isomerize. I n the absence of radical isomn4. Alternatively, some of the radicals in step 3 may isomerize erization, only primary radicals are derived from the cracking to secondary forms, for example, reactions of normal paraffins; primary radicals thus give only H H H H ethylene as the olefinic product. Radical isomerization rrduces HC-C-C--C--R H . H H
+
-
TABLE A Hydrocarbon n-Hexadecane (cetane) Alkyl aromatics Normal olefins Olefins
Iiaphthenos
'
Thermal Cracking Major product is CP with much C i a n d Ca; much Ci to CIS n-a-olefins. few branched sliphatiis Cracked within side chain Double bond shifts slowly; little skeletal isomerization Hydrogen transfer is a minor reaction and is nonselective for tertiary olefins Crack a t about same rate a s corresponding pariaffins Crack a t lower r a t e t h e n paraffins
Catalytic Cracking Major product is Ca to Cs; few n-a-olefins above C4; aliphatics mostly branched Cracked next to ring Double bond shifts rapidly: extensive skelet a l isomerization Hydrogen transfer is a n important reaction and is selective for tertiary olefins Crack a t much higher rate than corresponding. paraffins Crack a t about same rate a s paraffins with equivalent etructural groups
Crack a t lower r a t e than paraffins
Crack at higher rate than paraffins
Small amounts of aromatics formed a t 500° C.
Large amounts of aro; matica formed a t 500 C.
(9)
Alkyl aromatics (with propyl or larger substituents) Aliphatics
~~
~~
~~
~~
~~
which gives propylene on cracking beta to the free valence. The final radical in either step 3 or 4 may be ethyl instead of methyl, which also reacts as in step 1t o continue the chain reaction.
CRACKING OF CETANE, CETENE,PARAFFIN WAX, AND ISODOThe authors' work on the thermal cracking of cetane a t 500 C. and 1 atmosphere gave the products shown in Table I, columns 1 and 2 (with quartz chips as inert filler in the rpactor), as repeated in simplified form from another paper (33). The agreement with the product distribution worked out by the rules of the R-K theory (33) is considered good. To summarize, long normal paraffins crack thermally to give ( a ) the complete sequence of normal alpha olefins (899); ( b ) large amounts of ethylene and propylene by successive beta cracking of the resultant primary or isomerized radicals; and (e) fairly large amounts of methane and ethane as end products of radical d e composition. Of great importance is the absence of secondary reactions, especially of olefins. Considerable weight may be placed on the results (23)of cracking a paraffin wax (averaging n-Cz6Hsa) as further evidence for the rules cited here. The products corresponded with those expected from the R-K theory, and of particular interest were the liquid fractions, Ce to Cia, which were about 90% olefinic by bromine number. UECAXE. O
November 1949
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INDUSTRIAL AND ENGINEERING CHEMISTRY
From Gault (8) the over-all product distribution from the thermal cracking of cetene a t 450 to 550 C. (11)closely resembles that from cetane. The first order cracking rate constants are also similar, Keetane = 0.003 (33) and Koetene = about 0.007 second-1 (11,recalculated from 8) a t 500' C.; this demonstrates that the double bond has no large effect on either the mechanism or rate of cracking. For further verification of the R-K theory, the product distribution was calculated by the authors (33) for isododecane (presumably chiefly 2,2,4,6,6-~entamethyIheptane), which cracks very differently from its isomer, n-dodecane (Table V of 10). As may be seen in another work (33) the correspondence of the calculated and experimental results is considered fairly good in view of the complex structure of this hydrocarbon and the approximate nature of the parameters used in the R-K theory. CRACKINGOF NAPHTHENESAND AROMATICS.Naphthenes and aromatics are of much importance in petroleum fractions. Most of these cyclic hydrocarbons in petroleum are alkyl substituted; their cracking behavior is determined by the combined effect of the cyclic group, the alkyl groups attached thereto, and the nature of the bonds linking the side groups to the ring. D a t a on the thermal cracking of pure naphthenes are scanty; Decalin (30) appears to crack like a branched paraffin, accompanied by ring dehydrogenation to aromatics. The available data lead to the conclusion that aromatic production via dehydrogenation is an important reaction of cyclohexane type naphthenes, but t h a t otherwise there is no departure from the general prin-ciples of the R K theory implied in the observed results. Information is lacking on the liquid products of thermally cracking cyclopentane type naphthenes, which cannot dehydrogenate to aromatics without prior ring isomerization. One publication (18) indicates t h a t scant aromatics came from cyclopentane or methylcyclopentane. Aromatic rings are stable under thermal cracking conditions. Therefore, the cracking of petroleum aromatics is essentially confined t o the cracking of attached carbon chains which may be alkyl or cycloalkyl groups, or naphthenic portions of condensed ring systems. These chains may be expected to tend t o crack within themselves in accordance with the rules of the R-K theory. I n thermal cracking, there is considerable reluctance to crack a t the bond next t o the aromatic ring. Thus, n-propyl and isopropyl benzenes give chiefly toluene and styrene, respectively ( 5 ) . CATALYTIC CRACKING
The study of the catalytic cracking of pure hydrocarbons was undertaken to explore the chemistry of the industrial process. The present commercial catalysts are effective agents for accelerating those cracking and secondary reactions which lead t o a product distribution of considerable economic value t o the petroleum refiner. The majority of the authors' published experiments have been made with a synthetic silica-zirconiaalumina catalyst designated as UOP cracking catalyst Type B; this has virtually the same cracking characteristics as the synthetic silica-alumina catalysts in general commercial use. Catalysts prepared from natural clays, such as acid-treated Californian bentonitic montmorillonite, give a fairly similar product distribution and are also in commercial use. I n general, the statements regarding acidity of porous solids made herein rest on the findings of Tamele (31)which are supported by the work of Thomas (32)and refer particularly to the type of acidity denoted by the term "proton availability"; this means t h a t protons (hydrogen ions) are present and available for reaction with even weak bases and with suitable hydrocarbons. This acidity is measured not only as pH of the material in contact with water, but also by the reaction of the dry solid with ammonia, a basic gas. On the whole, good qualitative correspondence between acidity so considered and catalytic cracking activity has been established.
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AND OBSERVED PRODUCTS IN THE TABLE I. CALCULATED THERMAL AND CATALYTIC CRACKING OF CETANE
(Temperature, 500' C.; pressure, atmospheric) Quartz Activated Carbon UOP-B Run C-590 C-708 C-579 L.h.s.v.a ... 0.05 . .. 10.0 .. 10.0 Conversion, wt. % ' 31.5 .. 26.6 ... 2 4 . 2 Moles product/100 Calcd. Obsvd. moles cracked Calcd. Obsvd. Calcd. Obsvd. 61 53 4" 11 c1 0 5 139 130 13 22 0 12 CZ 50 60 21 23 Ca 95 97 27 23 17 17 C4 97 102 15 9 20 13 CS 72 64 17 24 Ce 13 21 50 41 14 16 13 15 7 8 C7 12 13 13 9 C8 6 8 11 10 I3 18 5 3 CS 10 11 13 15 c10 4 3 9 9 13 14 4 2 CII 8 7 17 13 4 2 c12 8 4 7 CIS 21 7 2 7 5 C,A -.. _12_ CIS 4 4 1 4 0' ... Total hydrocarbon 378 ' - 200 223 339 358 26 0 Hydrogen 0 17 0 12
.. .
. .. .
...
..T
a
. . ..
"1
-
- -
Liquid hourly space velocity.
Extensive work has been carried out under the direction of Tamele in t h e colloid chemistry department of these laboratories on the relation of acidity to the cracking activity of porous solids ( 3 1 ) . This work has been of essential importance to the development of the present theories of reaction mechanisms over such catalysts in terms of carbonium ions, which require available protons for their creation. It was demonstrated t h a t pure porous silica, although derived from silicic acid, had no cracking activity and had no available acidity. On the other hand, small amounts of alumina properly added to pure silica endowed the latter with considerable activity. This result was traced t o the presence of available protons in the combined silica-alumina structure, which structure so distributes the valence charges of aluminum, silicon, and oxygen atoms that additional cations are required t o obtain electrostatic neutrality. Thus, protons are incorporated into the structure when such materials are prepared in an acid environment, and those protons which are available a t the surface are readily exchanged for other cations such as sodium. The latter render the catalyst inactive since no carbonium ions can then be formed. This behavior places these solids in t h e class of baseexchange agents, a property common to conventional cracking catalysts, both synthetic and natural. Definite experimental evidence has been obtained by Tamele (SI) for the strongly acidic character of silica-alumina, silicazirconia-alumina (UOP Type B cracking catalyst), and acidtreated clays. Puve silica has been shown to be nonacidic. The activated carbon used in t h e present experiments has no indicated proton availability and shows an alkaline reaction in water; i t is also classified here as a nonacidic catalyst. The pure alumina used by the authors has been determined to be aweaklyacidic catalyst, a classification which fits well with many circumstantial observations on its properties, manner of preparation, and catalytic activity. To extend our knowledge of cracking reactions, experiments have now been made by the authors with pure silica and pure alumina of high surface areas, both of which accelerate the rate of cracking of most pure hydrocarbons with respect to the thermal rates observed over quartz chips. I n addition, the authors have made comparable experiments in some extensiveness with activated carbon of very high indicated B-E-T (Brunauer-EmmettTeller) ( 4 ) surface area, because of the unique product distribution' obtained with this material. The several catalytic agents will be discussed in approximate order of their transition from thermal to acid type of cracking. Throughout the text the relative activities of the catalysts for the cracking of pure hydrocarbons are computed by either of two methods, the preferred being the ratio of molal flow rates required for equal extents of cracking,
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OF EXTENTS OF CRACKING ovan PVRE TABLE 11. COXPARISON SILICAGEL, QUARTZCHIPS, A N D UOP-B C A T A L Y ~ T
(Process period, 1 h o w ; preqsure, atmospheric) Flow R a t e Amt. Cracked, K t . %, Temp., Moles/ Pure Hydrocarbon O C. L.h.s.v.a l./hr, Silica Gel Quartz UOP-Rb Cetane 500 0.5 1.7 20.9 oa.6 80 550 4.2c 27.2 Decalin 0.6 0.3 40 450 4.2C 27.2 n-Octenes 0.8 ... 45 Cumene 500 ' 3.8 27.2 0.3 1.1 80 hourly space velocity. * Liquid Estimated from d a t a a t other flow rates. 4pproxi.natelp.
a C
TABLE 111. CRACKING O F CETANE OVER QUARTZ CHIPS
PURE
SILICA GEL
.\VU
(Temperature, 500'
C.; pressure, atmospheric) Pnre Qnartz Silica Gel Chips C-930 C-590 Run Conversion, mt. % 20.9 81.6 0 5 0 06" Liquid hourly spare velocity Process period, min. 60 270 Moles product/100 moles cracked 35 53 C1 104 130 C? 57 60 CS 23 30 C4 Cracking Surface ,
19
C6
18 14 15
C8
c7 C8
10
CP Cro
7 10
c 1 1
7
C1e C..,.? Cl4
5
9 24
18 13
10 11
9
k
ClS
Total hydrocarbon Hydrogen Olefin content of fractions, wt. To
c, GS
42-74O C. 196-217'C. Aromatic content of fractions. mt. % 42-99' C. 99-125" C. Iso/normal utylenes ratio
b
70 92 95 97
0.7 0.08
78
nrc
90 , .
1
3 0.07
The flow rate for t h e thermal run is based on t h e total volume a t 500' * bo C. occupied by quartz chips. Tho hot free space was found to be roughly 45% of this volume. The flow rate for silica gel, as with other catalysts, is based on t h e total catalyst volume, all of which was a t 500' i. 5' C .
and the other the ratio of exten's of cracking a t equal flow rates. The activity of a cracking catalyst, such as the silica-alumina type, may change rapidly with time; the values cited here are those obtained for process periods of 15 minutes t o 1hour. PURE SILICAGEL. Pure silica gel has little catalytic cracking activity, but the addition of very small amounts of alumina, even a few hundredths of one per cent, will raise the activity of pure silica gel t o a high value (7, 31). The addition of more alumina (about 10 t o 15%) makes a highly active and stable cracking catalyst, as represented by present commercial production of synthetic silica-alumina catalysts in this country. T o test t h e cracking properties of pure silica gel containing less than 0.01% by weight of alumina and with a specific surface of 531 square meters per gram, the authors used cetane. The results are shown in Table I1 together with data of Tainele (3'1) for t h e cracking of three other pure hydrocarbons, all compared over pure silica gel, quartz chips, and UOP-B catalyst. I n comparison with UOP-B catalyst, the relative activity of pure silica gel was very low. However, this latter material catalyzed the cracking of cetane by increasing t h e rate several times relative t o the rate over quartz chips. Table I11 shows a sixfold increase. Therefore, product distribution is of paramount interest for ascertaining the mode of cracking. Virtually t h e same product distribution is obtained in both cases (Table 111). The lower amounts of C1 and CZ over pure
Vol. 41, No. 11
silica gel represent a difference of less than 5$Z0 of the cetane cracked. It is concluded t h a t cetane cracks over pure silica gel by the same mechanism as over quarts chips-namely, via free radicals according t o the R-K theory. The increased rate is attributed to the high surface area of the silica gel, which suggests that free radical formation may be assisted a t a suitable solid surface. ACTIVATEDCARBOX. Steam-activated carbon from coconut charcoal, of 1600 square meters per gram specific surface, has given the authors results with cetane remarkably different both in cracking rate and product distribution from those observed thermally and over silica-alumina type catalysts. Cetane cracked from one to ten times as fast over activated carbon as over UOP-B catalyst, depending on the extent of cracking' this is equivalent t o a t least fifty times the thermal rate. The product distribution is shown in Table I in comparison with t h a t obtained thermally over quartz chips, a t a comparable extent of cracking. The cracked products from C1 to CIS are seen to be rather evenly distributed over the entire range. I n comparison with thermal cracking, 155 moles less hydrocarbon were obtained per 100 moles cetane cracked; this difference can be assigned principally to the smaller production of CI, Cf, and CB,inclusive, which is less by 187 moles. Indeed, little more than 2 moles of product were obtained per mole of cetane cracked over activated carbon in this experimenb. Very little chain-branching was noted, and the product contained more paraffins than olefins throughout the entire range. Over half of the normal olefins examined (C8, Cp, and CIZ)were alpha isomers. From these observations the authors have concluded that a carbonium ion mechanism could not explain the results. T h e high normal alpha olefin content and the lack of skeletal isomerization of olefins correspond to the thermal or free radical (R-K) mode of cracking, but the relatively high saturation of the product (over 60'%) throughout the whole range and the lack of preferential formation of CI, Cp, and CBdo not. T o solve this dilemma, the authors have proposed that t h e cracking may start via free radicals and therefore should show the effect of different ty-pes of carbon-hydrogen bonds on the rate of cracking as postulated by the R-K theory. This was tested with the five hexane isomers, as reported later in the text; the authors have obtained reasonably good agreement with experimental data by using the R-K values for the relative reactivities of primary and secondary hydrogen atoms. A slightly lower value for tertiary hydrogen is derived from the hexane tests but is not needed for the cetane calculation. Accordingly, the cracking of cetane over activated carbon may' be explained as follows:
A free radical is formed a t the catalyst surface by the removal of a hydrogen atom anywhere in the carbon chain, as in the R-K theory, and this radical cracks at the beta position, also according t o R-K theory, t o yield a normal alpha olefin and a primary free radical. This primary radical is rap dly saturated or quenched by the addition of a hydrogen atom a t the surface of the catalyst to form the corresponding normal paraffin which cracks no further unless it has high molecular weight (see discussion of n-CdT64). On the basis of these simple assumptions, t h e authors calculated the product distribution shown in Table I, wherein encouraging agreement with t h e experimental results is found. Cracking over activated carbon may thus be characterized as a radjcal mechanism at an active surface, which later enables hydrogen atoms t o combine with radicals from the first cracking step and thereby prevents their further cracking to small fragments. T h e great acceleration of cracking observed with cetane may be ascribed t o the characteristics of the high surface area of the activated carbon, nrhich acts (a)t o remove hydrogen from the hydrocarbon to generate reactive free radicalq; and conversely ( b ) to return hydrogen t o the radical derived from the cracking reaction t o convert it t o a normal paraffin. T h e saturation
November 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
2577
activated carbon proceeds by a free radical, rather than a carCRACKING OF PURE HYDROCARBONS OVER ACTIVATED bonium ion mechanism. CARBON An important postulate in the R K theory is t h a t of the (Temperature, 500° C.; pressure, atmospheric) different reactivities of the three types of hydrogen-carbon bonds Compound Chief Products Run (primary, secondary, and tertiary) in aliphatic hydrocarbons. Fairly even distribution of CI t o Cia C-708 Cetane At 500" C., these are given as 1.0,3.66, and 13.4, respectively (19). products, with high liquid/gas" C-721 Cetene If the first (and rate-controlling) step in catalytic cracking over ratio Methane a n d ethylbenzene C-686 Cumene activated carbon is the removal of a hydrogen atom, then these Hydrogen a n d naphthalene C-722 Decalin reactivities should be reflected in the rates of cracking of struca CI t o C4, inclusive. tural isomers. The authors therefore cracked the five hexane isomers over activated carbon, with the result t h a t the secondary/ TABLEV. CATALYTIC CRACKING OF HEXANEISOMERS OVER ACTIVATED CARBON primary reactivity ratio of 3.66 was confirmed t o within 4% by (Temperature, 500" C.; pressure, atmospheric; process period, 1 hour; the data for normal and neohexane, which contain only these flow rate, 7.1 moles/l./hr.) two kinds of bonds. For tertiary hydrogen, a value of 11.0 Ratios of Cracking Rates was computed from data on 3-methylpentane with respect to Hexane Isomer Calculated Observed n-hexane. With this value, fairly good agreement was obtained NeohexnnR 0.56 0.54 0.98 1.08 2,3iDimethylbutana for the rate of cracking of 2,3-dimethylbutane, as shown in 1.00a 1.00b 3-Methylpentane Table V; 2-methylpentane fell out of line. Considering the good 1.00 1 27 2-Methyfpentane 1:02b 1.02 n-Hexane agreement in four out of five cases, i t seems quite believable a Assigned value of unity. that removal of a hydrogen atom is the initial step in catalytic Experiments used t o determine *he rate ratio of tertiary to primary hydrogen removal. cracking over activated carbon. Another test of the proposed mechanism is the prediction of cracked products from the isomeric hexanes, which represent of total aliphatic products above the theoretical 50% may be many different carbon-carbon groupings within small paraffin correlated with the observed activity of this catalyst for hydrogen molecules. Applying the relative rates of removal of 1, 3.66, transfer, a property which falls in line with the reactions noted and 11.0 for primary, secondary, and tertiary hydrogen atoms, above. Normal p a r f f i s , C,, larger than cetane should yield the beta fission rule, and the postulate for activated carbon that some amount of normal paraffins and olefins of 16 t o n - 1 carbon radicals from the first cracking step are resaturated t o paraffins atoms under the conditions just given. These products should and not recracked, fair agreement with the experimental rethen extensively recrack. The authors found this t o be exactly sults is obtained here, as depicted in Table VI. These calcuthe case for a Borneo wax of approximate average formula nlations offer further confirmation of the suggested mode of CzsH54, which yielded 363 moles of hydrocarbon product per 100 cracking over activated carbon, especially in view of the possible moles wax cracked at 44y0 conversion (run C-715) in these experidisturbing effects of different carbon atom groupings arranged ments. Recracking of most of the material above Cla is indiin such close proximity t o one another in the hexane isomers. cated. The 2 t o 1 mole ratio of hydrocarbon product to cracked PUREALUMINA. Porous alumina in various forms, such as feed stock which was approached by cetane represents the lowest bauxite or precipitated alumina, is an important type of catalyst possible value of this ratio. either alone or in combination with other substances. When To characterize further the catalysis over activated carbon, the mixed with small amounts of silica, many aluminas acquire the authors tested four other hydrocarbons. The chief products cracking properties of the commercial silica-alumina catalyst from cetane, cetene, cumene, and Decalin are shown in Table to some degree (20). T o observe the behavior of a pure alumina, IV. Cetene cracked a t about the same rate as cetane. I n cona sample prepared by the authors containing below 0.01 yo by trast, cetene cracks far faster than cetane over acidio catalysts. weight silica with specific surface 180 square meters per gram The cracked products were similar to those from cetane, although was teated with cetane and cumene. somewhat less saturated (about 55% unsaturation of the aliThe authors found that cetane cracked a t about half the rate phatics). This considerable saturation of product from an oleover pure alumina as over UOP-B, with t h e comparative product finic feed stock may be correlated with the simultaneous producdistribution shown in Table VII. The amount of each comtion of aromatics. (The release of hydrogen by aromatic formaponent is intermediate t o t h a t obtained over UOP-B and actition may also enter into the excess saturation-60yo instead of vated carbon, with the exception of CI, Cz, CS,and C6. I n each theoretical 50y0'o--of the products from cetane cracking, but does of the latter cases the value is closer t o that for activated carbon not alter the concept of radical saturation a t the carbon surface, than to the value for thermal cracking. whatever the source of the hydrogen may be.) Cumene gave Cumene was cracked about 10% at 500" C. and 1.9 liquid hourly methane and ethylbenzene, the same bond division as in thermal Bpace velocity, compared with about 1% over quartz chips or silica cracking but accompanied by saturation of the vinyl side chain, gel, 8% over activated carbon, and 84% over UOP-B. The Decalin was dehydrogenated t o naphthalene and showed a 200gas composition indicated a much higher ratio of Cs to methane fold acceleration of this important thermal reaction, which than for thermal or activated carbon cracking, but considerably demonstrates the high dehydrogenation activity of activated lower than for UOP-B. Since removal of the entire alkyl group carbon. These observations support the view t h a t cracking over as propylene is characteristic of acid type cracking, and the production of methane and CS aromatics is found thermally (6) and over activated carbon, an interTABLE VI. CATALYTIC CRACKING OF HEXANE ISOMERS OVY~RACTIVATED mediate type of cracking is evident, both with reCARBON spect t o rate and product distribution. (Temperature, 500n C.; pressure, atmospheric. process period, 1 hour; flow rate, 7.1 The authors have concluded from the foregoing moles/l./Lr.) t h a t this weakly acidic pure alumina displays 3-Methyl2-Methyl2,a-DimethylCracked a mixed type of cracking, intermediate t o t h a t products, %-Hexane Neohexane pentane pentane butane .\%ole% Obsvd. Calcd. Obsvd. Calcd. Obsvd. Calod. Obsvd. Calcd. Obsvd. Calcd. over strongly acidic oxides and activated carbon, CI 15 5 27 28 25 21 13 13 36 37 with perhaps some accelerated thermal cracking cz 28 24 33 22 29 29 22 20 3 0 cs 35 42 7 0 10 0 39 34 20 26 entering to a small degree. The butylenes and c 4 17 24 28 22 24 29 21 20 12 0 amylenes were found t o be skeletally isomerized C E 5 5 5 28 12 21 6 13 19 37 to equilibrium; this demonstrated the existence of
TABLE IV.
1
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
2578
TABLE VII. CRACKING OF CETANEOVER PCREh . v > I I X . 4
AND
OTHER CATlLYsTs
(Temperature, 500" C.; pressure, atmospheric) Catalyst or surface Alumina UOP-B Carbon C-587 C-710 C-931 Run Liquid hourly space velocity 0.5 1.0 3.9 6 8 . 0 68.4a Conversion. wt. 70 61.7 moles cracked
29
44
58 53 22 11
15
11 12 11
A 3
a
50
32 7 5 5
4 3 2 1
1
13
26
28
26 28 16
24 13 20 16 10 14 7 4 4
53 130
60 23 9 24 16 13
10
11 9 7 8 5
206
,..
247
... -
199
12
16
17
4
Total hydrocarbon Hydrogen
11 18 116 113
Quartz C-590 0.05 31.5
367
378
Does not include coke.
some carbonium ion or acid type activity. The observed data may be correlated with the relatively weak acidity arid high surface area of the pure porous alumina. The large amount of hydrogen produced in cetane cracking can be partially attributed t o the rat,her low hydrogen transfer activity, as suggested by the high butenes to butanes mtio, coupled with considerable dehydrogenation activit,y t o form aromatics (as observed) and t o release molecular hydrogen. Anot,her contributing factor t o high hydrogen production might be the stabilization of some of the free radicals from the first, craclring step by the removal of a second hydrogen atom to form an olefin and a n adsorbed hydrogen atom a t the surface of t h e catalyst, in contrast t o the addition of a hydrogen atom to form a paraffin, as in t h e case of activated carbon. Then the weak acidic character of the pure alumina Tvould come into play and encoursge the cracking of the resultant large olefins according t o the carbonium ion mechanism t o be proposed for acidic oxide catalysts. These latter catalyst's have much greater activity for cracking olefins than for cracking paraffins. I n proper balance, such a mechanism can account for a product distribut'ion for cetane over alumina intermediate to those observed over activated carbon and UOP-B catalysts. ACIDIC OXIDE CATALYSTS.Industrial cataly-tic cracking of petroleum fractions utilizes porous solid acidic oxide cat'alystsfor example, synthetic silica-alumina gel with 10 t o 15% by weight alumina and specific surface area ranging from 250 t o 600 square meters per gram. Most of our experiments have been made with a synthetic gel catalyst of virtually the same cracking characteristics-namely, UOP Type R, which analyzed 86.2% silica, 9.470 zirconia, and 4.3y0alumina, and which had specific surface about 330 square meters per gram. The behavior of over sixty pure hydrocarbons in t h e presence of this catalyst has beon reported (9-13,34.). A comparison n-ith thermal (free radical-type) cracking \-,-as made earlier in the paper, and i t has been proposed here t h a t cracking over the nonacidic cat'alysta, pure silica gel, and activated carbon can be explained as t h e simple acceleration of thermal free radical-type cracking for t h e former and as an accelerated but modified, quenched free radical-type cracking for the latter. Cracking over porous solid acidic catalysts appears to comprise a process of a very different kind, closely allied t o those hydrocarbon reactions which always require the presence of an acidic catalyst but which can occur a t lower temperatures. Acidic catalysts exist in many forms, including solid heteropoly acids, solid aluminum chloride with various promoters and supports, liquid sulfuric, phosphoric, borofluohydrjc, and hydrofluoric acids, aqueous solutions of the foregoing, liquid organic complexes with aluminum
Vol. 41, No. 11
chloride, porous solids impregnated with acidic substances, acidtreated clays, and acidic mixtures of refractory oxides, such as t h e present commercial cracking catalysts. These substances all regisler acidity intrinsically or in contact with water and can be viewed a s the source of t h e protons required to convert hydrocarbons into reactive carbonium ions. CARBONIUM IONR E A C T I O ~ ST. h e common features of these: acid-catalyzed hydrocarbon reactions are the attack on the hydrocarbon, the production of a carboniuni ion, and the behavior of carbonium ions according t o specific rules (36). Somo of these rules are reviewed here for the sake of exposition of the concepts t o be applied t o catalytic cracking. The formation of a carbonium ion from a hydroca,rbon may occur in several different, ways. I n general, unsaturates add a proton t o form a carbonium ion, and saturates lose a hydride ion t o form a carbonium ion (36). The authors believe t h a t protons ( H + j , hydrideions (H-j, and carbonium ions ( R + )in the catalytic systems under discussion are always associated with, and are t,ransferred t o and from, complementary electronegative or electropositive atoms, groups, molecules, or catalyst surface regions. The carbonium ion can be regarded as a simplified concept of a polarized state, but a concept which usefully predicts reactions according to a definite set of rules. These rules serve to emphasize the close relation of catalytic cracking t o many ot,hcr acid-catalyzed hydrocarbon reactions. T o give subst,aiice and example t o these concept,si, t h e simplest and best-known set of reactions, involving the addition of a proton t o a n olefin, will be discussed next. Olefins: A proton will combine with an olefin by taking the two Pi electrons from the ethylenic double bond to form an ordinary hydrogen-carbon bond comprising one pair of electrons (36). This bond will be made with one of the h o carbon atoms sharing the original double bond; the other carbon atom will now carry a positive charge and may be designated the "carbonium ion carbon atom," as will be shown. For a symmetrical olefin, two equivalent struct,ures a r ~017tained :
+ >C=C
c-c
c-
