Greensfelder - Catalytic and Thermal Cracking of Pure Hydrocarbons (1949)

November 1949 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 2573

(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 Co.) , 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

(28) Thomas, C. L., Hoekstra, J., and Pinkston, J. T., J . Am. Chem. Go.), U. 8. Patent 2,287,917 (June 30, 1942).

Soc., 66, 1694 (1944).

(29) Voge, H. H., Good, G. N., and Greensfelder, B. S., IND. ENQ

(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).

CHEM., 38, 1033 (1946).

RECEIVED Novomber 15, 1948. This paper is taken from a 1945 report which was part of a technioal information exchange ordered by the Petro- ieum Administrator for War in Recommendation 41. The work was per- formed at the Riverside, Ill., laboratories of Universal Oil Products Company.

Catalytic and Thermal Cracking of Pure Hvdrocarbons

J

MECHANISMS OF REACTION

B. S. GREENSFELDER, H. H. VOGE, AND G. M. GOOD Shell Development Company, Emeryd le , 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 disinte- gration. 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

RIOR work on the catalytic cracking of pure hydrocarbons P has led to a general characterization of the rates of cracking and product distributions of the principal classes of petroleum hydrocarbons (10-13). In 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 trans- formed 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-

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.

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.
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2514 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 41, No. 11

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 to include gas, liquid boiling below the original, and coke-redefined (34) to include carbon and hydrogen-all summed on a no-loss basis (9).

The apparatus and procedure used in this work have heen described (9, 15) with certain modifications for thermal craclc- ing experiments (33); analytical methods have been amplified to 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

The most successful present explanation of thermal cracking of hydrocarbons is the Rice free radical theory (2b-27) as modified by Kossiakoff and Rice (19). This will be called the "RK- theory" and is summarized from another paper (33) as follows to explain the cracking of a normal paraffin:

The normal paraffin molecule loses a hydrogen atom by colli- sion and reaction with a small free hydrocarbon radical or a free hydrogen atom, thereby becoming a free radical itself.

H HC This radical may immediately crack or may undergo

erization is a change of the position of a hydrogen atom, usually to yield a more stable radical. Cracking of either the original or isomerized radical then takes place a t a carbon-carbon bond located in the beta position to the carbon atom lacking one hydrogen atom. Cracking at the beta position gives directly an alpha olefin and a primary radical (lacking one hydrogen atom on a primary carbon atom); in this step there is no change of position of any hydrogen atom with respect to the carbon skeleton.

The primary radical derived from this step may immediately recrack a t the beta bond to give ethylene and another primary radical, or i t may first isomerize. In the absence of radical isomn- erization, only primary radicals are derived from the cracking reactions of normal paraffins; primary radicals thus give only ethylene as the olefinic product. Radical isomerization rrduces

radical isomerization prior to cracking. Radical isom- I

TABLE A Hydrocarbon Thermal Cracking Catalytic Cracking

n-Hexadecane (cetane)

Alkyl aromatics Normal olefins

Olefins

Iiaphthenos '

Alkyl aromatics (with propyl or larger sub- stituents)

Aliphatics

Major product is CP with much Ciand 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 at about same rate as corresponding pariaf- fins

Major product is Ca to Cs; few n-a-olefins above C4; aliphatics mostly branched

Cracked next to ring Double bond shifts

rapidly: extensive skele- ta 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 lower rate then Crack a t about same rate paraffins as paraffins with equiv-

alent etructural groups (9 )

Crack a t lower rate than Crack at higher rate than paraffins paraffins

Small amounts of aromatics Large amounts of aro; formed at 500 C. matica formed a t 500

C.

the amount of ethylene, but i t still reniains the major product. By successive recracking, the radicals ultimately are reduced to methyl or ethyl fragments. These radicals then react with feed stock molecules t o produce new fIee radicals and are themselves converted to methane or ethane. Thus, cracking is propagated ass a chain reaction. To start the chain and to compensate for the loss of chain carriers by side reactions, it may be assumed that 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. Radi- cal isomerization presumably occurs through a coiled configura- tion of a single radical, in which the hydrogen donor and acceptor carbon atoms must closely approach each other. This last re- striction affects the calculations for cetane vcry littlc (33).

A schematic representation of cetane cracking IS as follows:

1. Small radical, such as CH8, from prior cycle or from initial hydrocarbon rupture, combines with an I1 atom in cetane to give a cetyl radical and methane:

H I1 H H H H I H H H I H H H H -C-C-C-C-C-C-C-C-C-C-C-C-C-C-CH + CHa

H H . H H H H H H H H H H H H

2. Cetyl radical cracks beta to free valence to give, say, n pentene-1 and undecyl-1 radical:

H H H H H H H H H H H H H H H H ~~ ~~ ~~ ~~ ~~ HC-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

3. Undecyl-1 radical cracks beta to free valence to give ethylene and nonyl-1 radical; repeat process to give ethylene and heptyl-1 radical; repeat process to give ethylene and amyl-l radical; repeat process to give ethylene and propyl-1 radical; repeat process to give ethylene and methyl radical, which then reacts as in step 1 to continue the chain reaction.

4. Alternatively, some of the radicals in step 3 may isomerize to secondary forms, for example,

H H H H HC-C-C--C--R

H . H H

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 1 to continue the chain reaction.

CRACKING OF CETANE, CETENE, PARAFFIN WAX, AND ISODO- UECAXE. The authors' work on the thermal cracking of cetane a t 500 O 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.
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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).

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.

CRACKING OF NAPHTHENES AND 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.

Data 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 that otherwise there is no departure from the general prin-- ciples of the R K theory implied in the observed results. In- formation 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 tha 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 to 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 to crack within themselves in accordance with the rules of the R-K theory. In thermal cracking, there is considerable reluctance to crack a t the bond next to the aromatic ring. Thus, n-propyl and isopropyl benzenes give chiefly toluene and styrene, respectively ( 5 ) .

+3 As may be seen in another work (33) the correspondence of the

h

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 acceler- ating those cracking and secondary reactions which lead to a product distribution of considerable economic value to the petroleum refiner. The majority of the authors' published experiments have been made with a synthetic silica-zirconia- alumina 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. Cata- lysts prepared from natural clays, such as acid-treated Cali- fornian bentonitic montmorillonite, give a fairly similar product distribution and are also in commercial use.

In general, the statements regarding acidity of porous solids made herein rest on the findings of Tamele (31) which are sup- ported by the work of Thomas (32) and refer particularly to the type of acidity denoted by the term "proton availability"; this means that 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.

TABLE I. CALCULATED AND OBSERVED PRODUCTS IN THE 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 . . . 1 0 . 0 .. . 1 0 . 0 Conversion, wt. '% ... 3 1 . 5 . . . 2 6 . 6 . . . 2 4 . 2 Moles product/100

moles cracked c1 CZ Ca C4 CS Ce C7 C8 CS c 1 0 CII c 1 2 CIS C,A

Calcd. Obsvd. 61 53

139 130 50 60 27 23 15 9 17 24 14 16 12 13 11 10 10 11 9 9 8 7 7 8 7 5

Calcd. Obsvd. 4" 11

13 22 21 23 17 17 13 20 13 21 13 15 13 9 I3 18 13 15 13 14 17 13 21 7 12

Calcd. Obsvd. 0 5 0 12

95 97 97 102 72 64 41 50

7 8 6 8 5 3 4 3 4 2 4 2 4 "1 2 -.. _ _

CIS 4 ..T 4 1 4 - - 0' . . . Total hydrocarbon 378 ' - 200 - 223 339 358 Hydrogen 0 17 0 26 0 12 a Liquid hourly space velocity.

Extensive work has been carried out under the direction of Tamele in the colloid chemistry department of these laboratories on the relation of acidity to the cracking activity of porous solids (31) . 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 that 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 to 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 en- vironment, 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 the class of base- exchange 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, silica- zirconia-alumina (UOP Type B cracking catalyst), and acid- treated clays. Puve silica has been shown to be nonacidic. The activated carbon used in the 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-Emmett- Teller) ( 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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TABLE 11. COXPARISON OF EXTENTS OF CRACKING ovan PVRE SILICA GEL, QUARTZ CHIPS, AND UOP-B CATALY~T

(Process period, 1 h o w ; preqsure, atmospheric) Flow Rate

Temp., Moles/ Pure Amt. Cracked, Kt . %,

Hydrocarbon O C. L.h.s.v.a l./hr, Silica Gel Quartz UOP-Rb Cetane 500 0 . 5 1.7 20.9 o a . 6 80 Decalin 550 4 .2c 27.2 0 . 6 0 . 3 40 n-Octenes 450 4.2C 27.2 0 . 8 . . . 45 Cumene 500 ' 3 . 8 27.2 0 . 3 1 . 1 80

a Liquid hourly space velocity. * Estimated from data a t other flow rates. C 4pproxi.natelp.

TABLE 111. CRACKING O F CETANE OVER PURE SILICA GEL . \VU QUARTZ CHIPS

(Temperature, 500' C.; pressure, atmospheric) Pnre Qnartz

Cracking Surface , Silica Gel Chips Run C-930 C-590 Conversion, mt. % 20 .9 8 1 . 6 Liquid hourly spare velocity 0 5 0 06" Process period, min. 60 270 Moles

C1 C? CS C4

C8 c7 C8 CP Cro c11 C1e C , ?

C6

product/100 moles cracked

. .. Cl4 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. b Iso/normal utylenes ratio

35 104

57 30 19 18 14 15 10 7 10 7 5

53 130 60 23

9 24 18 13 10 11

9 k

70 78 92 nrc 95 90 97 , .

1 3 0.7

0.08 0.07

The flow rate for the thermal run is based on the total volume a t 500' * Tho hot free space was found to be roughly

The flow rate for silica gel, as with other catalysts, bo C. occupied by quartz chips. 45% of this volume. is based on the 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 to 1 hour.

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 to 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.

To test the 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 the 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 the 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 the same product distribution is obtained in both cases (Table 111). The lower amounts of C1 and CZ over pure

PURE SILICA GEL.

silica gel represent a difference of less than 5$Z0 of the cetane cracked. It is concluded that cetane cracks over pure silica gel by the same mechanism as over quarts chips-namely, via free radicals according to 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.

ACTIVATED CARBOX. Steam-activated carbon from coco- nut 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 to a t least fifty times the thermal rate. The product distribution is shown in Table I in comparison with that 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. In 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. The 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 CB do not. T o solve this di- lemma, the authors have proposed that the 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 to R-K theory, to 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 at 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, the authors calculated the product distribution shown in Table I, wherein en- couraging agreement with the 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 to combine with radicals from the first cracking step and thereby prevents their further cracking to small fragments.

The great acceleration of cracking observed with cetane may be ascribed to the characteristics of the high surface area of the activated carbon, nrhich acts (a) to remove hydrogen from the hydrocarbon to generate reactive free radicalq; and conversely ( b ) to return hydrogen to the radical derived from the cracking reaction to convert it to a normal paraffin. The saturation
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TABLE IV. CRACKING OF PURE HYDROCARBONS OVER ACTIVATED CARBON

(Temperature, 500 C.; pressure, atmospheric) Compound Chief Products Run

Fairly even distribution of CI t o Cia C-708 products, with high liquid/gas" C-721 ratio

Methane and ethylbenzene C-686 Hydrogen and naphthalene C-722

1 Cetane Cetene Cumene Decalin

a CI to C4, inclusive.

TABLE V. CATALYTIC CRACKING OF HEXANE ISOMERS OVER ACTIVATED CARBON

(Temperature, 500" C.; pressure, atmospheric; process period, 1 hour; flow rate, 7.1 moles/l./hr.)

Ratios of Cracking Rates Hexane Isomer Calculated Observed

NeohexnnR 2,3iDimethylbutana 3-Methylpentane 2-Methyfpentane n-Hexane

0.56 0 . 5 4 0.98 1 .08

1.00 1 27 1.00a 1.00b

1 .02 1 : 0 2 b a Assigned value of unity.

Experiments used to determine *he rate ratio of tertiary to primary hydrogen removal.

of total aliphatic products above the theoretical 50% may be correlated with the observed activity of this catalyst for hydrogen transfer, a property which falls in line with the reactions noted above. Normal p a r f f i s , C,, larger than cetane should yield some amount of normal paraffins and olefins of 16 to n - 1 carbon atoms under the conditions just given. These products should then extensively recrack. The authors found this to be exactly the case for a Borneo wax of approximate average formula n- CzsH54, which yielded 363 moles of hydrocarbon product per 100 moles wax cracked at 44y0 conversion (run C-715) in these experiments. Recracking of most of the material above Cla is indicated. The 2 to 1 mole ratio of hydrocarbon product to cracked feed stock which was approached by cetane represents the lowest possible value of this ratio.

To characterize further the catalysis over activated carbon, the authors tested four other hydrocarbons. The chief products from cetane, cetene, cumene, and Decalin are shown in Table IV. Cetene cracked a t about the same rate as cetane. In contrast, cetene cracks far faster than cetane over acidio catalysts. The cracked products were similar to those from cetane, although somewhat less saturated (about 55% unsaturation of the aliphatics). This considerable saturation of product from an olefinic feed stock may be correlated with the simultaneous production of aromatics. (The release of hydrogen by aromatic formation may also enter into the excess saturation-60yo instead of theoretical 50y0'o--of the products from cetane cracking, but does not alter the concept of radical saturation a t the carbon surface, whatever the source of the hydrogen may be.) Cumene gave methane and ethylbenzene, the same bond division as in thermal cracking but accompanied by saturation of the vinyl side chain, Decalin was dehydrogenated to naphthalene and showed a 200- fold acceleration of this important thermal reaction, which demonstrates the high dehydrogenation activity of activated carbon. These observations support the view that cracking over

activated carbon proceeds by a free radical, rather than a carbonium ion mechanism.

An important postulate in the R K theory is that of the different reactivities of the three types of hydrogen-carbon bonds (primary, secondary, and tertiary) in aliphatic hydrocarbons. At 500" C., these are given as 1.0,3.66, and 13.4, respectively (19). If the first (and rate-controlling) step in catalytic cracking over activated carbon is the removal of a hydrogen atom, then these reactivities should be reflected in the rates of cracking of structural isomers. The authors therefore cracked the five hexane isomers over activated carbon, with the result that the secondary/ primary reactivity ratio of 3.66 was confirmed to within 4% by the data for normal and neohexane, which contain only these two kinds of bonds. For tertiary hydrogen, a value of 11.0 was computed from data on 3-methylpentane with respect to n-hexane. With this value, fairly good agreement was obtained for the rate of cracking of 2,3-dimethylbutane, as shown in Table V; 2-methylpentane fell out of line. Considering the good agreement in four out of five cases, i t seems quite believable that removal of a hydrogen atom is the initial step in catalytic cracking over activated carbon.

Another test of the proposed mechanism is the prediction of cracked products from the isomeric hexanes, which represent many different carbon-carbon groupings within small paraffin molecules. Applying the relative rates of removal of 1, 3.66, and 11.0 for primary, secondary, and tertiary hydrogen atoms, the beta fission rule, and the postulate for activated carbon that radicals from the first cracking step are resaturated to paraffins and not recracked, fair agreement with the experimental results is obtained here, as depicted in Table VI. These calculations offer further confirmation of the suggested mode of cracking over activated carbon, especially in view of the possible disturbing effects of different carbon atom groupings arranged in such close proximity to one another in the hexane isomers.

PURE ALUMINA. Porous alumina in various forms, such as bauxite or precipitated alumina, is an important type of catalyst either alone or in combination with other substances. When mixed with small amounts of silica, many aluminas acquire the cracking properties of the commercial silica-alumina catalyst to some degree (20). To observe the behavior of a pure alumina, a sample prepared by the authors containing below 0.01 yo by weight silica with specific surface 180 square meters per gram was teated with cetane and cumene.

The authors found that cetane cracked a t about half the rate over pure alumina as over UOP-B, with the comparative product distribution shown in Table VII. The amount of each com- ponent is intermediate to that obtained over UOP-B and activated carbon, with the exception of CI, Cz, CS, and C6. In each of the latter cases the value is closer to that for activated carbon than to the value for thermal cracking.

Cumene was cracked about 10% at 500" C. and 1.9 liquid hourly Bpace velocity, compared with about 1 % over quartz chips or silica gel, 8% over activated carbon, and 84% over UOP-B. The gas composition indicated a much higher ratio of Cs to methane than for thermal or activated carbon cracking, but considerably lower than for UOP-B. Since removal of the entire alkyl group as propylene is characteristic of acid type cracking, and the pro-

duction of methane and CS aromatics is found thermally (6) and over activated carbon, an inter-

TABLE VI. CATALYTIC CRACKING OF HEXANE ISOMERS OVY~R ACTIVATED mediate type of cracking is evident, both with re- CARBON spect to 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.) that this weakly acidic pure alumina displays a mixed type of cracking, intermediate to that 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

amylenes were found to be skeletally isomerized c4 17 24 28 22 24 29 21 20 12 0 C E 5 5 5 28 12 21 6 13 19 37 to equilibrium; this demonstrated the existence of

3-Methyl- 2-Methyl- 2,a-Dimethyl-

.\%ole % Obsvd. Calcd. Obsvd. Calcd. Obsvd. Calod. Obsvd. Calcd. Obsvd. Calcd.

Cracked products, %-Hexane Neohexane pentane pentane butane


TABLE VII. CRACKING OF CETANE OVER PCRE h.v>IIX.4 AND OTHER CATlLYsTs

(Temperature, 500" C.; pressure, atmospheric) Catalyst or surface Alumina UOP-B Carbon Quartz Run C-931 C-587 C-710 C-590 Liquid hourly space velocity 0 .5 1 . 0 3 . 9 0.05 Conversion. wt. 70 61.7 68 .0 68.4a 31.5

moles cracked

Total hydrocarbon Hydrogen

a Does not include coke.

29 44 58 53 22 11 15 11 12 11

A 3 4

206 199

-

1 1 18

116 113 50 32

7 5 5 4 3 2 1 1

367 12

, . . -

13 26 28 26 28 16 24 13 20 16 10 14

7 4 4

247 16

-

53 130 60 23

9 24 16 13 10 11 9 7 8 5

378 17

. . . -

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 to the rat,her low hydrogen transfer activity, as suggested by the high butenes to butanes mtio, coupled with considerable dehydrogenation activit,y to form aromatics (as observed) and to release molecular hydrogen. Anot,her contributing factor to 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 an adsorbed hydrogen atom a t the surface of the catalyst, in contrast t o the addition of a hydrogen atom to form a paraffin, as in the case of activated carbon. Then the weak acidic character of the pure alumina Tvould come into play and encour- sge the cracking of the resultant large olefins according to the carbonium ion mechanism to be proposed for acidic oxide catalysts. These latter catalyst's have much greater activity for cracking olefins than for cracking paraffins. In 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'alysts- for example, synthetic silica-alumina gel with 10 to 15% by weight alumina and specific surface area ranging from 250 to 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 4.3y0 alumina, and which had specific surface about 330 square meters per gram.

The behavior of over sixty pure hydrocarbons in the 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 that cracking over the nonacidic cat'alysta, pure silica gel, and activated carbon can be explained as the simple acceleration of thermal free radical-type cracking for the 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 to 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 solu- tions of the foregoing, liquid organic complexes with aluminum

chloride, porous solids impregnated with acidic substances, acid- treated clays, and acidic mixtures of refractory oxides, such as the present commercial cracking catalysts. These substances all regisler acidity intrinsically or in contact with water and can be viewed as the source of the protons required to convert hydrocarbons into reactive carbonium ions.

The 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 to specific rules (36). Somo of these rules are reviewed here for the sake of exposition of the concepts to be applied to catalytic cracking. The formation of a carbonium ion from a hydroca,rbon may occur in several different, ways. In general, unsaturates add a proton to form a carbonium ion, and saturates lose a hydride ion to form a carbonium ion (36). The authors believe that protons (H+j, hydrideions (H-j, and carbonium ions (R+) in the catalytic systems under discussion are always associated with, and are t,ransferred to and from, com- plementary 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 to many ot,hcr acid-catalyzed hydrocarbon reactions. To give subst,aiice and example to these concept,si, the simplest and best-known set of reactions, involving the addition of a proton to an 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 ~ 017- tained :

CARBONIUM ION REACTIO~S.

IT-+ + >C=C< + >c-c< :111d >c-

2579 November 1949 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

fore, the initial structure of a polymer ion can be directly p r e dicted, as first proposed by Whitmore (37). For example, isobutene dimer ion will be:

C C c c C-Cf I . 3- c=c-c I --f c-c-c-c-c I I

ei d tert-Butyl Ion Isobutene Iso-Octyl Carbonium Ion

with the Carbonium ion carbon atom in the beta position to the added positive group.

Carbonium ions undergo several types of rearrangement (56, 38, 99) which will determine the structure of the final product. These rearrangements are in general accord with the relative energies of the initial and rearranged ions as specified directly by, or by analogy with, the Evans and Polanyi values, but there are not yet adequate data to cover all cases.

Two of the most important rearrangements comprise a shift of hydrogen or a methyl group, which are best explained as movement of a proton or methyl carbonium ion, respectively. This provides a pattern more consistent with the energetics of the system than does the H- or CHa- shift proposed by Whitmore (36). These reactions are shown as follows:

H H S H H , + - - c H , + Hzc=LCH, --f NoC-+--CH8 A (1)

I H+ H

Primary Ion Olefin + Proton Secondary Ion in which n-propyl ion becomes sec-propyl ion via movement of a proton through ah intermediate resonance position (36).

H~c-cH-cH~-~-cH, -+ HaC-CH=CH$ -ic i.

CHs

H I + as in

I Eauation 1 I H~C-C-CH, - H~C---~~--CH~ (2)

Here the sec-butyl ion becomes the more stable tertbutyl ion by skeletal isomerization via movement of a methyl carbonium ion. It is the key step in the important secondary reaction of olefin skeletal isomerization ($4). These rearrangements will be seen later to have much influence upon the minimum size of the fragments produced in catalytic cracking.

Finally, an ole& nil1 accept a proton In a definite position, yielding a specific carbonium ion. This ion, either directly or after rearrangement, may dissociate to a smaller olefin and a carbonium ion. The latter can then dissociate to a proton and an olefin. This is the essence of the catalytic cracking of olefins, and is the mechanism of depolymerization of olefins (by acid catalysts) for the special case of the cracked product being identical with the monomer.

In conclusion, the high reactivity of olefins in catalytic cracking and other acid-catalyzed systems may be ascribed t o the high attraction of the ethylenic double bond for a proton, which results in the rapid formation of reactive carbonium ions.

Saturates (Paraffins and Naphthenes): The catalytic cracking of petroleum saturates-namely, paraffins and naphthenes- poses a special problem. There are no Pi, or double bond, electrons available for direct proton attack as in olefins, nor are the hydrogen atoms rapidly exchanged for protons (or their isotopes) in dilute acids, as are those in aromatics (17) . Since cetane

cracks in a manner similiar to cetene but much more slowly, it is logical to postulate the dow formation of a carbonium ion from cetane as the first step to establish a common point of departure for their subsequent reactions.

Therefore, a mode of initial formation of carbonium ions from saturates is required To avoid the step of removing a hydride ion therefrom directly by the catalyst to give a carbonium ion, Thomas (8.2) proposed that saturates first undergo some thermal cracking to give olefins, which then become carbonium ions by simple proton addition. After cracking, the small carbonium ions produced continue the reaction chainwise via the hydride ion exchange mechanism proposed by Bartlett (1 ) for the alkylation of olefins with paraffins.

This reaction may be written, for example, as tert-CaHp+ + iso-C6HIZ --+ iso-CaHlo + tert-CaH1,+

in which a hydride ion (H- or H:) is transferred from isopentane to the tertbutyl ion, yielding tert-amyl ion and isobutane.

The same problem of the initiatory mechanism arises in the carbonium ion reaction of butane isomerization, which has been reported to occur in the absence of olefins or other promoters using HBr 3. AIBrs catalyst ($1, page 825, Table IV). Other evidence (3.4) indicates need of a promoting agent; even 0.01% butenes suffice. There are many additional data bearing on this quastion, including the effects of hydrogen, oxygen, and mater @I), which lead to the conclusion that it is difficult to decide which of the several following possibilities initiates the attack of the acid catalyst upon butane: ( a ) direct attack of catalyst proton to remove hydride ion from paraffin, yielding molecular hydrogen (5); ( b ) addition of catalyst proton to traces of olefin to start paraffi-carbonium ion exchange; ( c ) dehydrogenation of pa ra f i by oxygen to yield olefin to react as in ( b ) ; ( d ) addition of catalyst proton to water to provide hydronium ion (H30+) which may attack paraffin as in ( a ) ; (e) formation of catalyst-promoter complex which provides a sufficiently activated proton to react as in (a) .

All these possibilities enter into the catalytic cracking of saturates, except that ( b ) may corraspond to olefins derived from some thermal cracking and (e) becomes even more indefinite.

Aromatics: The catalytic cracking of aromatics differs greatly from that of paraffins, olefins, and naphthenes. It is discussed separately at the end of the text.

Cracking qf Cetane over an Acidic Oxide Type Catalyst (UOP-B). Using the principles set forth, the results of cracking cetane will be discussed in detail. Secondary reactions of olefins (34) play an important part in determining the composition of the final products of catalytic cracking, among which reactions those of doublebond isomerization, skeletal isomerization, and hydrogen transfer are greatly influenced by experimental conditions. Since the purpose of this work is to study the primary cracking reactions, all products have been grouped according to carbon number; thereby the effects of these secondary reactions are largely removed from view. Table VI11 presents a series of five new experiments the authors have made with cetane up to a twenty-five-fold increase of flow rate; other conditions were constant, and the percentage cracked varied from 11 to 68%. Excellent consistency of product distribution by carbon number is seen to hold, with only 5% deviation from the average total 361 moles of hydrocarbon product per 100 moles cetane cracked. These data indicate that a uniform mechanism of primary cracking prevails in this system, and that it should be possible to apply definite rules thereto with the aim of predicting product dis- tr?oution.

Reaction Mechanism for the Cracking of Cetane (or Any Kormal Paraffin): From the data in Table VI11 a mechanism based on the properties of carbonium ions has been devised to explain the cracking of cetane, the same in many respects as that which Thomas (32) proposed, without specific calculation, to woctme. The steprvise process in a certain formalistic sense

2580 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 41, No, 11

TABLE VIII. CATALYTIC CRACKING OF CETANE (Temperature, 500 C.; catalyst UOP-B. ressure, atmospheric; process

pd iod , 1 hoh7 Run C-614 C-679 C-578 C-613 C-587 Flow rate moles/l./hr. 85.2 3 4 . 0 13 6 8 . 8 3.4 Conversidn, wt. % (through Cir) 11.0 24.2 40.0 53.5 68.0

Total hydrocarbon Hydrogen

S 17 87

103 53 35 17 9

3 3 2 1 1

343 12

7

I

5 12 97

102 64 60 8 8 3 3 2 2 2 1

12 E

4 16

112 116 43 38 7 8 7 4 3 1 1 1

361 14

I

12 18

113 116 60 29

9 5 4 3 3 2 1 1

9 3%

11 18

115 113

SO 32 7 5 5 4 3 2 1 1

367 12

- . -

a Cia was not determined in these tcsts. I n some similar experiments one mole of CIS product was obtained per 100 moles of cetane cracked.

shows parallelism to the R-X mode of thermal cracking. Thus, hydrogen is first removed from the molecule, leaving a hydrogen- deficient entity, which then cracks at a carbon-carbon bond beta to the hydrogen-deficient carbon atom, producing an alpha ole& and a new hydrogen-deficient entity. The latter repeats the process until a small uncrackable group is left, which then becomes saturated by acquisition of hydrogen. However, applied to this generalized process are the definite, special properties of carbonium ions. These govern the primary cracking reactions by the preferential formation of certain initial carbonium ions, the rearrangement of most primary to secondary ions, and some secondary to tertiary ions, and the cracking of these ions into fragments not smaller than three carbon atoms each. These properties also govern many of the secondary reactions by the almost complete equilibrat,ion of olefin double bonds, the extensive skeletal isonierization of olefins, and the saturation of olefins by hydrogen transfer. The product distribution d t h respect to carbon number, paraffin to olefin ratio, and paraffin and olefin isomers is thereby greatly altered from that of thermal cracking, and in a very characteristic and definable manner.

The differences in energy of the isomeric forms of a given carbonium ion are determinative features of cracking over acid oata- lysts, because most of the carbonium ion reactions dealt with here, with the exception of hydrogen transfer, include the isomerization of carbonium ions (both hydrogen shift and skeletal rearrangement) as an essential step. Their energies decrease and their stabilities increase in the order primary, secondary, tertiary. Accordingly the following mechanism of cetane cracking is set forth:

The cetane molecule first reacts with a proton or small carbonium ion at the surface of the catalyst to form cetyl carbonium ion by the loss of a hydride ion, H-. Secondary cetyl carbonium ions will immediately predominate, since there are twenty-eight secondary and only six primary hydrogen atoms, and the latter are twice as slow to be removed, as will be shown later. Furthermore, any primary ions formed may rearrange to secondary ions before cracking, in accordance with the observed properties of carbonium ion systems.

STAGE I.

H I

Example: Cl& + GHi+ + C6HlI-F-ClOHPI + C3Ha For simplicity, the important rearrangement of Becondsry to

tertiary carbonium ions has been omitted in this description. The significance of carbonium ion rearrangement, with respect to product distribution is this: Secondary ions can give no olefins smaller than Cs, since a secondary carbonium ion carbon atom by definition must be at least in the 2 position with respect to the end of the chain. Beta fission ail1 then yield propylene as the smallest product olefin. Since a tertiary carbonium ion is more stable than a secondary carbonium ion, a number of secondary ions will rearmnge to tertiary, still leaving the carbonium ion

carbon atom in at least the 2 position with respect to the end of the chain. Beta fission in this case will yield isobutylene as the smallest product olefin. Therefore, the smallest fragments in important yield from catalytic cracking are Ca and C, hydrocarbons, Any large olefins initially formed will tend to recrack because of their great reactivity over acidic oxide catalysts. Finally, the theory .and data indicate that carbonium ions that cannot crack by this mechanism into two fragments, eachCaor larger, will crack but very little, which corresponds to the large amounts of Cg and iso-C6 in the product. Of course, some methane and CZ are formed, which need not be ascribed to thermal cracking, but to adequate activation of much less favored types of cracking.

Convincing evidence of this rearrangement is found in cetane cracking, which gives large amounts of small isoparaffins. Thw, at 11% conversion, 99 moles of total paraffins came from 100 moles cetane cracked (in nearly complete hydrogen balance), of which 53 moles were isoparaffins. Since 30 moles were CHI, CzHe, and CsHs, the significant is0 to normal ratio is then 53 to 16.

STAGE 11. The carbonium ion will split at a carbon-carbon bond in a beta position to the carbonium ion carbon atom. The two electrons from this bond will move to the original carbonium ion carbon atom and neutralize the single positive charge, simul- taneously forming an ethylenic double bond. Thus, an alpha olefin is produced which rearranges, and the other fragment becomes a primary carbonium ion, Example:

STAGE 111. The primary carbonium ion derived from cracking will in turn rearzange to a secondary carbonium ion a3 in stage I and crack beta to the carbonium ion carbon atom as before.

H Example: HJ -C~HI~ + HaC--C-GHls -r --+

H HIC-C=CHZ -i- H&-CSHii +

This process will continue until a carbonium ion which cannot yield fragments of three or more carbon atoms is produced- for example, a normal secondary CS carbonium ion. It is ar experimental fact that methyl and ethyl carbonium ions are harc to produce; this corresponds to their high electron impac appearance potentials (16) from their parent paraffins. There fore, cracking to such small ions occurs to but a small extent.

STAGE I f . The final carbonium ion from stage 111 reacts wit a new cetane molecule by hydride ion exchange to produce a sma paraffin and a new cetyl carbonium ion, thereby propagating tE reaction chain. The example is the same as in stage I.

The beta fission rule is not a purely arbitrary law, but is description of that mode of molecular division which will yield neutral olefin and ti smaller carbonium ion without the rearrang ment of any carbon or hydrogen atoms within the ion during t cracking process. Only electrons are shifted, and then only frc one side to the other of the prospective ethylenic carbon atom the point of division. This mode of behavior corresponds to I principle of least motion or the principle of least rearrangeme It has been shown in the analogous case of free radicals that SI a process should have a lower work of activation than any ot possible process (2, 87).

To illustrate the applicability of these rules to cetane, experimental and calculated product distributions by car number are shown in Table I and Figure 1. Calculations v made on the following basis:

Cetane forms cetyl carbonium ion by random loss of dride ion from any secondary position.

The secondary cetyl carbonium ion cracks at a bond to the carbonium ion carbon, forming an olefin, CnH2, a primary carbonium ion, CI~--nH+33-z,1. (Whenever altern bonds are available ior cracking, all are considered equally, viding that fragments Ca or larger are formed.) The pri carbonium ion then isomerizes by proton shift to an equa tribution of all possible secondary ions. Continued crackin isomerization proceeds w described until all ions are reduc c6 or smaller. These become amall parafins by acquir hydride ion.

3. The olefins formed in step 2 should react about j extent to be expected from the authors cracking tests wit olefin feeds. Accordingly, half of these olefins are assur form carbonium ions which then crack according to thr outlined above. The extent of recracking of olefins wi! with the experimental conditions.

1.

2.
ThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlight


of Hansford (14) which show easier exchange of tertiary hydrogen (compared with secondary or primary) with deuterium incorporated in a silica-alumina cracking catalyst are in agreement with this view.

Cracking of Other Hydrocarbon Classes. Naphthenes: The primary cracking reactions of naphthenes are expected to follow a pattern similar to that of paraffins (IS, 32). Naphthenes have been thought to be more reactive than paraffinsof a similar molecular weight ( I S ) . However, certain naphthenes and parafKns with the same number of carbon atoms and about the same number and types of carbon-hydrogen groupings more recently have been found by the authors to crack at similar rates (9). Thus, Decalin and 2,7-dimethyloctane (CIO hydrocarbons with two

I ' E lo I ' l2 l3 l4 l5 tertiary carbon atoms) showed similar conversions (runs C-641 and 642 in Table X). These results indicate that the naphthene ring structure by itself may be of secondary importance in determining the rate of cracking over this acidic oxide catalyst.

--- ---- -_---- CARBON No. OF PRODUCT

Figure 1. Catalytic Cracking of Cetane at 500' C. . Experimental, 24% conversion over UOP-B catalyst

Cracking of the Five Hexane Isomers over UOP-B Catalyst. These experiments on the cracking of the five hexane isomers over activated carbon provided data for the calculation of the relative reactivities of primary ( P ) , secondary (S), and tertiary ( T ) hydrogen. A similar calculation may be made for UOP-B catalyst a t 550" C. using the authors' data (9), as will be shown, leading to the coefficients P = 0.62, S = 1.29, and T = 12.3% of charge. These coefficients, when multiplied by the number of C-H bonds of corresponding type, add up to the extent of cracking of a given hexane isomer.

Number and Type of Hexane Isomer C-H Bonds % Cracked

2 3-Dimethylbutane ;-Hexane 2,2-Dimethylbutitne

12P + 2T O P + 8s 12P + 2s

32 14 10

The relative values for the three types of C-H bonds are then close to 1, 2, and 20 for P, S, and T a t 550" C. For comparison, the R K thermal values a t 500" C. were 1.0,3.66, and 13.4, and those calculated for activated carbon were 1.0, 3.66, and 11.0, respectively.

These data lead to the conclusion that the relative rates of removal of hydride ions fall in the same order as those for the removal of hydrogen atoms, but the values are different. Of particular significance is the high value for tertiary hydrogen for cracking with an acidic oxide catalyst.

Applying these numbers to the cracking of the two other hexane isomers, good agreement is obtained, as shown in Table IX. In addition, the cracking of two octanes and two dodecanes is shown, with good agreement of relative rates. The effect of molecular weight on extent of cracking is not simple (9, 10, 1.9) and has been left out of consideration.

As in the case of activated carbon, the determinative influence of the numbers and types of carbon-hydrogen bonds on the extent of cracking provides strong indication that the carbon-hydrogen bond is the critical point of attack in paraffins. The experiments

TABLE IX. CATALYTIC CRACKING OF HYDROCARBONS AT 550' C. OVER UOP-B CATALYST

Conversion, Wt. % of Total Products Numbers and Types of C-H

Hydrocarbon Experimental Calculateda Bond Present (a) 2-Methylpentans 25 23 9 P 4 5 1T (b) 3-Methylpentane, 25 23 9P: 4s: 1T

ratio a/b 1 .0 1.0 ( a ) Isooctane (b) n-Ootan~,

ratio a /b (a) n-Dodecane (b) Isododecane,

ratio a /b

49 24 16P, 2S, IT 42 19 0 P , 12s

35 30 6P 20s 1.2 1 .3

32 31 21P:4S, 1T 1.1 1.0

a Feed space rates were not the same for the digerent groups of isomers and other factors enter into the differences between the ex erimental and calculated absolute conversions of Cs and CIZ such as the e&ct of molecular weight, so that only the ratios a /b are ekpected to agree.

Olefins: In accordance with the prior discussions of olefin and paraffi reactions, the formation of a carbonium ion is postulated to be the first step in their cracking. The very rapid addition reactions of proton to olefin (or of a small carbonium ion to an olefin to give a larger carbonium ion) account for the high re- sponse of olefins to acid catalysts. The Carbonium ion is the common intermediate in both paraffin and olefin cracking; according to the mechanism of para& cracking set forth earlier in the text, the cracked products should in general be similar for paraffins and olefins of similar skeletal structure, differing mainly in degree of saturation and extent of recracking of product olefins (11).

Several secondary reactions of olefins, which contribute to the ultimate product distribution in catalytic cracking, have already been discussed. One other important secondary reaction, that of hydrogen transfer, as exemplified by the saturation of olefins, proceeds by two steps, the addition of a proton to olefin to form a carbonium ion, and the transfer of hydride ion from a neutral donor molecule to give a saturate and a new carbonium ion derived from the donor. Thus,

This is shown in run C-46, Table XI.

(CHa)&=CH2 + H + + (CHa)aC*

or, (CH3)sC+ + (CH&C=CH1 --+ (CH&CH + C&+ The donor may then lose a proton to become a 185s saturated molecule. This reaction is another example of hydride ion exchange, as proposed by Bartlett (1, page 1536). An identical mechanism waa proposed by Thomas (Sf?).

Tertiary carbonium ions are formed more readily (6) than secondary ions, and this difference is believed to be related to the preferential saturation of tertiary olefins in catalytic cracking (34).

Aromatics: Alkyl-substituted aromatic hydrocarbons are highly reactive in catalytic cracking systems when the alkyl groups are Ca or larger. Therefore, the characteristics of both the aromatic ring and the alkyl side chain are responsible for the ease of cracking of such compounds, although the aromatic nucleus remains essentially intact. The unique properties of the benzene ring with respect to protons are now reviewed in the light of well-known concepts. First, there is much unsaturation in the benzene ring, but there are no ethylenic double bonds, since the six surplus (Pi) electrons are resonating among all the carbon-to-carbon bonds. Therefore, n6 simple ethylenic double bond type of carbonium ion can be formed without altering the resonance energy of the benzene ring. Secondly, the benzene ring has much affinity for protons in the sense of attracting them and exchanging the hydrogen (or deuterium) originally present
ThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlightThomasHighlight


3 109 . . . . . . . . . g 2 c o - m . . . . . . . . . . . . . . . . . . B 6%"

. . . . . . . . . . s ".N.??R . . . . . . . . . . m m - o c y . . . . . . . . . u 0,

a:u?*'Ludi . . . . . . . . . . . . . . . . . . ' * Ncld

. . . . . . . . . . . . . . c o t - i c y . . . . . . . . . . . . . . 0

d 1

.C

R L

for protons (or deuterons) in the vicinity, ati demonstrated by Ingold and co-workers (17 ) with sulfuric acid a s cat,alyst. Thus,

+ 1.'-

H where an intermediate form is depicted to indicate the hypothetical transition state.

The authors believe that the same mechanism can be applied to the alkylation of aromatics, by assuming that a n alkyl carbonium ion plays the same part as a proton, as follows:

H H

where R+ is an alkyl carbonium ion derived from the combination of an olefin and a proton. The cracking of aromatics over acidic catalysts is the exact reverse of this reaction, followed by the dissociation of R+ to a n olefin and a proton. Thus

H H

ii B H

H The distinctive characteristic of catalytic

compared with thermal cracking of alkyl aromatics is the severance of the entire alkyl group from the ring, instead of cracking of a carbon-to-carbon bond within the side chain. This has been extensively reported (IS), and i t is plain that the rates of cracking of isomeric alkyl aromatics correspond t o the relative ease of formation of the several carbonium ions. Suitable data are not available for computing the strength of attachment of a n alkyl carbonium ion to an aromatic ring for comparison with the same covalent bond strength. However, the energies of combination of methyl, ethyl, isopropyl, and tertiary butyl positive ions with hydride ion (H-) are 319, 282, 264, and 247 kg.-cal., respectively (26, 69). The energy differences in this series are similar when an alkyl group is substituted for hydrogen and may be assumed to be correspondingly similar with respect to an aromatic nucleus. These differences may be correlated with the relative extents of cracking (dealkylation) of methyl-, ethyl-, and iso- propylbenzenes, which are 1, 11, and 8470, respectively, at 500" C. under the same ex-
ThomasHighlight

November 1949 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R , Y 2583

TABLE XI. DATA ON THE CRACKING OF PURE HYDROCARBONS Hydrocarbon c Cetane -. 7-Cetene- Wax 7--Cumene- Catalyst - UOP-B . -Carbon- Silica Quartz" Alumina UOP-B -Carbon- UOP-B Carbon Alumina Temperature, C: 500 - 400 500 550 --500- Process period, min. , 60 - 270 -60-- 90 r 60 Liquid. hourly space

velocity 24.5 1 0 . 0 4 . 0 2 . 0 1 . 0 10.0 3 . 9 0 .5 0.05 0 . 5 2 . 0 7.9 31.7 -1.9- Conversion wt. '% 11.0 24.2 40 .0 53.5 68.0 26.6 68.4 20.9 31.5 61.7 62.0 37 .8 44.0 83.6 7 . 5 11.1 Moles product/100

moles cracked HZ CHI Cz H4 CzHa CUHS Ca He C4Ha

n-C& iso-C4Hio n-CIHio tert-CsHio sec-CsHlo i80-CsHit n-CsHiz Ce (74O)d c7 (990) Ce (152O) Cio (174')

Cia (2389 ci4 (255') Cia 273')

dSO-CIH8

C8 (125')

2; t%,' ClS t2900)

Total Material balance, wt.

12 12 5 5

11 7 6 5

68 76 19 21 0 0

29 26 41 39 22 25 11 12 23 25 14 17 11 16 5 6

35 50 17 8 9 8 7 3 3 3 3 2 2 2

2 . . . . . . . . . . . .

355370

14 4 9 7

85 27 1

2 8 40 32 15 14 10 13 6

38 7 8 7 4 3 1 1 1 . . . ...

375

9 12 10 8

82 31 0

25 37 37 17 21 13 20 6

29 9 5 4 3 3 2 1 1 . . . . . . -

385

12 11 9 9

81 34 1

20 31 44 17 16 10 19 5

32 7 5 5 4 3 2 1 1 ... . . . -

379

26 11 6

16 9

14 0 0 6 0

11 1 7 1

11 21 15 9

18 15 14 13 7 4

14

249 ... -

16 13 6

19 10 18 0 1 8 1

15 2 8 1

17 16 24 13 20 16 10 14 7 4 4

263 . . . -

33 35 58 46 40 17 1 1

23 1 4 3

14 0 2

18 14 15 10 7

10 7 5 4 5

373 ... -

17 53 77 53 47 13 2 2

16 1 2 3 6 0 0

24 16 13 10 11 9 7 8 5 ... ...

395

199 29 24 20 42 16 0

17 26 3 7 6

10 1 5

11 15 11 12 11 8

15 3 4

495 ... -

1 3 1 1

16 3 0 8

15 18

42

33

73

10

15

... - 239

13 21

7 20 17 16 0 0

10 0

13 1 8 1 7

23 10 16 17 10 12 11 10 7 9

259 ... -

15 25 31 29 27 19 1 1

14 1

12

29 24 20 19 20 15 17 14 11 12 7

378 -

2 3 1 1

78 1 0 1 1

:}

100

. . . ... . . . ... ... . . . ... . . . is-

73 62 12 11 8

52 0 0 0 6 0 0 0 0

13 20 25 . . . . . . . .. . . . . . . . . . . . . . . . ~

282

% of charge Gas below Ca 4 . 9 11.1 20.8 28.4 36.5 3.3 10.8 7.3 11.9 20.5 8 . 4 6 . 2 7 .5 23.3 Liquid boiling below

feed 5 . 8 12.8 17.7 23.4 29.2 22.7 5 7 . 1 12.7 19.2 37 .1 49.4 28.7 35.8 50.7 Remainingliquid 89.4 75.3 59 .4 45.9 31 .9 72 .6 31.3 78.9 69 .3 38.1 36 2 61.2 55.6 15 3 Coke on catalyst 0 . 3 0 . 1 1.1 1.1 2 . 3 0 . 3 0.0 0.9 0 . 7 3.9 1 : 2 b 2 . 2 0 . 4 3 :9* Loss Q -0 .4 0 . 7 1 0 1 . 2 0 .1 1 . 1 0.8 0 . 2 -1.1 0 . 4 4 . 8 1 . 7 0 . 7 6.8

Run C-614 C-579 C-578 C-613 C-587 '2-708 (2-710 C-930 C-590 C-931 '2-46 C-721 C-715 C-131 Thermal run. Carbon only.

C Negative values si nify gain. 1 Upper cut points for Cn fractions from aliphatic feedstocks in ' C. The CS, CT, and Cs fractions from cumene were cut a t 99O, 125'

epec tively.

2 .9 3 .9

3.3 6.7 90.2 88.3 1.1 0 .4 2 . 5 0 .7

'2-686 C-1015

and 150' C., re-

perimental conditions (13). Normal, secondary, and tertiary butyibenzenes cracked 14, 52, and 84%, respectively, a t only 400" C. (IS). The relation of the ease of cracking to the differences in energy of the carbonium ions is clearly apparent and provides support for the belief that the reaction corresponds to the carbonium ion mechanism.

Further information can be obtained by changing the electron distribution in the benzene ring. Thus, in an enlightening study by Roberts (88) of these laboratories on the cracking of cumenes substituted in particular manners, their cracking rates decreased in the order: 1,3-dimethy1-Pisopropylbenzene>p- cymene > 1,3-dimethy1-5-isopropylbenzene > cumene > p chlorocumene > trichlorocumene, which corresponds to the order of decreasing electron density of the carbon atom bonded to the isopropyl group (89). This is added evidence for the proposed cracking mechanism-namely, proton attack a t the ring carbon atom which carries the vulnerable isopropyl group.

ACKNOWLEDGMENT

The pioneer work of F. C. Whitmore on carbonium ion systems has provided basic concepts for the mechanism of reactions of hydrocarbons in the presence of acidic catalysts. The work of P. D. Bartlett ( I ) on the role of hydride ion in alkylation has supplied an essential step to explain the cracking of saturates and the transfer of hydrogen to olefins.

The authors acknowledge appreciatively the earlier unpublished work (38) of Charles L. Thomas, then of Universal Oil Products Company, upon an independent approach to the problem in terms of carbonium ion theory and the intrinsic acidity of the catalysts employed. The work of F. 0. Rice and A. Kossiakoff on free radical cracking mechanisms has been applied with success to the thermal cracking of relatively large paraffins and has proved to be of much use in comparing different cracking systems.

The authors particularly thank A. E. LacomblO, chairman of the board of directors and former director of research, for his

interest and encouragement, The data and information supplied by the work of their colleagues, 0. Beeck, M. W. Tamele, R. C. Archibald, R . M. Roberts, D. P. Stevenson, C. W. Bittner, C. P. Brewer, J. W. Gibson, N. C . May, and D. H. Rowe, as well as their helpful suggestions, are most appreciatively acknowl- edged.

CATALYSTS

ALUMINA (low silica). From aluminum chloride by precipita- tion with ammonium hydroxide followed by washing. Dried at 150" C., calcined a t 565' C. Fe 0.001-0.01%, Si 0.03-0.010/0, and traces of Cu, Mg, Mo, Sn, Ag, Ni, Ca, and Cr by spectro- graphic analysis. Loss on ignition a t 1000" C., 33.3% by weight. Specific surface 180 m.a/g.

ACTIVATED CARBON. Columbia Activated Carbon, Grade S, size 4-14, A2, mesh, Carbide and Carbon Chemicals Corpora- tion. Calcined at 565" C. Specific surface 1600 m.a/g. Ash content, 2.1%.

SILICA (low alumina). Wilmington, Calif. AI less than O.OOl%, Ca 0.02%. Spec& surface 531 m.e/g. Bulk density 0.38.

SILICA-ZIRCONIA-ALUMINA. Universal Oil Products Com- pany, Type B, l/a-inch pills. Three lots were used:

1. This lot waa used for all runs except for those listed later. Specific surface 346 m.e/g.

2. 3. .UOP-B (R-51) used in runs C-139 and 140. Contained

86.2% silica, 9.4% zirconia, and 4.3% alumina.

Made by Shell Development Compan

UOP-B.

UOP-B-2 used in run C-46.

FEEDSTOCKS Feedstocks are arranged in order of carbon number. TL-HEXANE.

Stock A. Shell Oil Company Inc., Houston, Tex. Boiling range wss 0.4" C., nv 1.3759, d:O 0.6613, bromine No. 0.2. Used in run C-623.

Stock B. Phillips Petroleum Company. n? 1.3753, d:O 0.6611, bromine No. 0.2.

Stock A. From hydrogenation of 2-methylpentenes. Boil- ing range 59.6-66.0" C., nv 1.3730, d i0 0.6603, bromine No. 0.3. Used in run C-529.

Stock B. Phillips Petroleum Company. Technical grade, designated greater than 95% pure. Used in run (2-996.

Used in run C-822. Z-METHYLPENTANE.
ThomasHighlight


3-METHYLPENTANE. Stock A. Distilled from hydrogenated catalytically cracked

gasoline from Standard Oil Company of New Jersey (Louisiana division). After silica gel treatment, boiling range was 0.4" C., a? 1.3755, dzo 0.6619, bromine No. 0.2. Used in runs '2-530 and 553.

Stock B. Phillips Petroleum Company. nso 1.3772, dqo 0.6638, bromine No. 0.5.

2,3-DIhlETHYLBUT.4KE. From alkylation of isobutane with ethylene. Boiling range 56.5-57.3' C., ny 1.3732, d:" 0.6603, bromine KO. 0.3.

NEOHEXANE. Stock A. From isomerization of methylpentanes with AlCla

catalyst. Boiling range 48.5-48.9" C., nZ,O 1.3710, d i0 0.6542, bromine No. 0.3.

Stock B. Phillips Petroleum Company. n2$ 1.3690, d?" 0.6492. bromine h'o. 0.01.

Used in runs C-821 and 998.

Used in run C-527.

TJsed in run '2-823. OCTANE. Eastman 1

126.2" C., nag 1.3978, d i0

99.4" C., n%? 1.3925, d ? O 0.6935; bromine KO. 0.04. ISOOCTANE. Shell - -

Codal< Company. Roiling range 124- 3.7025, bromine No. 0.16.

Oil Company, Inc. Boiling range 98.6-

CUMENE. Stock A. Dow Chemical Company. Boiling range 152-

153" C., n y 1.4912, dZo 0.8621. Stock B. Eastman Kodak Company. n9 1.4908, d:"

0.8629. Used in run C-1015 (97.7y0 cumene by infrared analysis).

Stock C. From alkylation of benzene with propylene. Boiling range, 95% from 150-152.7" C., n2,0 1.4886, 0.8579, bromine KO. 3.3.

DECALIN. Eastman Kodak Company. Purified over silica gel.

2,7-DIMETHYLOCTANE. Eastman Kodak Company. Boiling range 158-160" C., ny 1.4089, di0 0.7248, bromine No. 2.4.

~-DODECANE. From lauryl alcohol from Eastman Kodak Company. Boiling point 217" C., melting point -9.8" C., nSo 1.4216, d:O 0.7486.

ISODODECANE. Hydrogenated triisobutenes from Shell Oil Company, Inc. from cold acid polymerization of isobutene. A.S.T.M. 5% and 95y0 distillation temperatures were 175.8' and 178.7"C., n2$ 1.4201, d:O 0.7474, bromine No. 3.9.

N. V. de Bataafsche Pet,roleum Mij., Delft, Holland. Stock A. Melting point 4.05" C., n'g 1.4409, d io 0.7813,

bromine KO. 69.3. Stock B. Melting point 3.6" C., ny 1.4398, dqo 0.7822,

bromine No. 68.5. E. I. du Pont de Nemours & Company, Inc. Several

lots of similar properties were used. Range of properties: melting point _16.7-17.0" C., ng 1.4345-1.4348, di0 0.7726-0.7742, bromine no. 0.2-0.3.

Kestern Waxed Paper Company, Emeryville, Calif. nY 1.4356, d:" 0.7786, melting point 53.4" C., molecular weight 368.

Used in run C-131.

Used in run C-686.

n? 1.4755, d:O 0.8837, bromine No. 0.4.

CETENE.

Used in run C-46.

Used in run C-721. CETANE.

BORNEO Wax.

LITERATURE CITED

(1) Bartlett, P. D., Condon, F. E., and SchneideI, A, J. Am. Chem.

( 2 ) Bawn, C . E. H., Trans. Faraday Soc., 34, 598 (1938). (3) Bloch, H. S., Pines, H., and Schmerling, L., J . Am. Chem. Soc.,

Soc., 66, 1531 (1944).

68, 153 (1946).

Brunauer, S., Emmett, P. H., and Teller, E., Zbid . , 60, 309

Dobryanskii, A. F., Kanep, E. K., and Katsman, S. V., Trans.

Evans, A. G., and Polanyi, M., J . Chem. Soc., 1947, p. 252. Frost, A. V., J . Phgs. Chem. (U.S.S.R.) , 14, N o . 9/10, 1313-18

Gault, H., and Altchidjian, Y., Ann. chim., (10) 2, 209 (1924). Good, G. M . , Voge, H. H., and Greensfelder, B. S., IND. ENG.

Greensfelder, B. S., and Voge, H. H., Ib id . , 37, 514 (1945). Ib id . , p. 983. Ib id . , p. 1038. Greensfelder, B. S., Voge, H . H., and Good, G. M . , I h i d . ,

p. 1168. Hansford, R. C., Ibid. , 39, 849 (1947). Henriques, H. J., Ibid. , p, 1564. Hipple, J. A . , and Stevenson, D. P., J . Am. Chem. SOC., 64,

Ingold, C. K., Raisin, C. G., and Wilson, C. L., Nature, 134,

Kasanskii. B. A., and Plate, A. F., Ber., 67, 1023 (1934). Kossiakoff, A, and Rice, F. O., J . Am. Chem. SOC., 65, 590

Marisic, M. M. (to Socony-Vacuum Oil Co. ) , U. S. Patent 2,394,-

Oblad, A. G., and Gorin, hf. H., ISD. ESG. CHEM., 38, 822

Pauling, Linus, "The Nat,ure of the Chemical Bond," 2nd ed.,

Peski, A. 3. van (to Shell Development Co.) , U. S. Patent 2,172,-

Pines, H., and Wackher, R. C., J. Am. Chem. SOC., 68, 595

Rice, F. O., Ibid. , 55, 3035 (1933). Rice, F. O., and Rice, K. K., "The Aliphatic Free Radicals,"

Rice, F. O., and Teller, E., J. Chem. Phys., 6 , 489 (1938); 7,

Roberts, R. M,, Shell Development Company, Emeryville,

Shell Development Company, Emeryville, unpublished work. Sundgren, A,, Ann. ofice natl. combustibles liquides, 5, 35 (1930). Tamele, M. W., and co-workers, Shell Development Company,

Thomas, C. L., IND. ENQ. CHEM., 41, 2564 (1949). Voge, H. H., and Good, G. M., J. Am. Chem. Soc., 71, 593

Voge, H. H., Good, G. AM., and Greensfelder, B. S., IND. ENG.

Wheland, G. W., "The Theory of Resonance," pp. 233--4,

(1938).

Reaearch Plant Khimgaz, 3, 1 (1936).

(1940).

CHEM., 39, 1032 (1947).

1590, 2766, 2769 (1942).

847 (1934) ; J . Chem. Soc., 1936, pp. 915, 1643.

(1943).

796 (1946).

(1946).

Ithaca, N. Y., Cornell University Press, 1940.

228 (1938).

(1946).

Baltimore, Johns Hopkins Press, 1938.

199 (1939).

Calif., unpublished work.

Emeryville, unpublished work.

(1949).

CHEM., 38, 1033 (1946) I

New York. John Wilev & Sons. 1944. (36) Whitmore, F. C., Chem. knp. News, 26, 668 (1948). (37) Whitmore, F. C., IND. ENG. CHEM., 26, 94 (1934). (38) Whitmore, F. C., J . Am. Chem. Soc., 54, 3274 (1932). (39) Whitmore, F. C., and Stahly, E. E., Ibid., 55, 4153 (1933).

RECEIYED August 2 , 1948.

LEON 0. WINSTROM AND LAURENCE KULP' Vapor pressure Allied Chemical and Dye Corporation, New York, N . y . ~

of Maleic

TEMPERATURE RANGE FROM 35" TO 77" C.

The vapor pressure of maleic anhydride has been measured in the temperature range from 35" to 77" C. A new static method for the measurement of the vapor pressure of sublimable substances is described.

URIKG the course of process development in expanding the production of maleic anhydride (Toxilic anhydride),

it became necessary t o know the vapor pressure of this substance a t temperatures near its melting point. The only data available in the literature are those tabulated in Landolt-Bornstein (4) which refer to unpublished data obtained from I. G. Farben. Weiss and Down8 (6) determined this property a t higher teni- peratures but they give only one value in the range of interest,.

1 Present address, Coliimbia University, New York 27, S. Y .

Documents

Greensfelder - Catalytic and Thermal Cracking of Pure Hydrocarbons (1949)