CHEMISTRY CRACKING CATALYSTS CHARLES L. THOMAS' Universal Oil P r o d u c t s C o m p a n y , Chicago, I l l .
T h e chemistry of both silica and alumina in the solid state has been applied to the silica-alumina cracking catalyst. This resulted in the hypothesis t h a t the active part of the catalyst is formed when one aluminum atom shares four oxygen atoms which in turn are shared by four silicon atoms. An acldic hydrogen ion is thought to be associated with the four oxygen atoms surrounding the aluminum atom. The catalytic activity is ascribed to this acidic hydrogen. The formula for the active part of the catalyst has been written (HAlSiOJ, where x indicates
t h a t it is a part of a solid which may have the same or a different composition. The nature of (HAlSiO& is such that it cannot exist as the monomer, HAlSi04. The logic built LIP in the hypothesis leads to the conclusion that, for maximum activity, the catalyst should have a composition in which the atomic ratio of silica to alumina is one. To obtain maximum activity these catalysts should be made in special ways. Silica-alumina catalysts of maximum activity probably cannot be prepared by forming a silica hydrogel and depositing alumina on it.
T
This arrangement for silicon and oxygen is characteristic of all known crystalline forms of silica and for all solid inorganic silicates. This unit is then a monomer or building unit from which solids are built. It seems entirely reasonable that it is present in the cracking catalyst. Aluminum, in some compounds, shares its valences with four oxygen atoms equally (tetrahedrally) spaced around it. Aluminum also shares its valences with six oxygen atoms equall\ (octahedrally) spaced around it:
HE following known facts regarding synthetic silica-alumina
catalysts have been examined in the light of the chemistry of solids. The results of the examination, reported here, lead to the hypothesis that the catalytic activity is due t o hydrogen ions present in the catalysts. It is hoped that the material presented will prove to be a step toward answering the question of what makes a cracking catalyst crack. Silica alone is either inactive or only slightly active as a cracking catalyst, Alumina alone is better than silica but is an inferior cracking catalyst. The proper combination of silica-alumina is very much more active than either of its components. Silica-alumina catalysts are prepared from the hydrogels or hydrous oxides; mixtures of the anhydrous oxide with one hydrous oxide does not produce an active catalyst. The silica-alumina catalyst apparently has certain acidic p r o p erties. The catalysts are solids.
AI2
o3
40
I
o/ i
SILICA AND ALUMINA IN SOLIDS
0
/ ?
Solids have structures in much the same sense that organic niolecules have structure. I n addition, they bear a resemblance to the organic polymers in which a characteristic group is repeated indefinitely in two or three dimensions. It, is important to realize that there is no such thing as O= &=O in the solid state. Rather, each silicon is attached to four oxygen atoms and these oxygen atoms are equidistant from the silicon. The centers of the oxygen atoms are located at the corners of a tetrahedron with the silicon in the center:
TETRAHEDRAL
OCTAHEDRAL
These two types of aluminum-oxygen combimtions m a y be regarded as monomers for building more complex polymers or copolymers. If silica and alumina are combined, which type of aluminunioxygen combination should be used? The silica-alumina cat'algst gives no x-ray diffraction pattern that would permit a decision. Therefore, the following assumptions were made: 1. The aluminum in the silica-alumina cracking catalyst that contributes ta catalytic activitv is tetrahedrally linked to four oxygen atome. 2. A positive hydrogen ion is associated with tetrahedral aluminum in the cracking catalyst.
I
0
- o-.si I
-0-
1 0
3. The catalytic activity of properly prepared silica-alumina masses is due to the acidic hydrogen ion associated with it.
I TETRAHEDRAL
1
Present address, Great Lakes Carbon Corporation, Chicago, I11
Using assumption 1 the building blocks for the catalj-st become tetrahedral silica and tetrahedral alumina. The following illustrates, in two dimensions, a thrpe-dimensional network of silicon and oxygen :
2564
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1949
*
Each line represents one valence unit. Each silicon atom has four such lines to oatisfy its valence of four. Each oxygen atom has two such lines and its valence of two is satisfied. IF the central silicon atom is removed and replaced with a tetrahedral aluminum atom, the result is:
*
I
I
-Si-
01
-Si-
r-- 7,1 ,
0
I
-Si- pl
I
lo
It is known that the valence of aluminum is three. Each line attached to the aluminum atom represents three fourths of a valence unit. With four such lines the valence of aluminum ts satisfied. The oxygen atoms share one valence with a silicon atom and three fourths of a valence with aluminum. Each oxygen atom has available to it only one and three fourths valence units when its valence demands two. Each such oxygen atom is unsatisfied by one fourth of a valence unit, and there are fonr such oxygens for each aluminurn. It follows that the A101 part of the molecule is unsatisficd by a whole valence unit and, since oxygen is a negative element, this is a negative valence. The hydrogen ion (assumption 2) has been drawn to show its relation to the tetrahedral aluminum. Because of its association with four oxygen atoms it seems logical that this hydrogen atom should be ionized and possess acidic properties. On the basis of assumption 3, the silica-alumina cracking catalyst is an acid that is stable a t high temperatures. Since this conclusion assigns a definite function to the source of activity, it is desirable to rerapitulate at this point and compare the picture that has been built up with the facts outlined. It is immediately apparent that neither silica nor alumina should be active according to this picture. A definite function is assigned to the silica-alumina combination that is not present in either one alone.
\
/
/
\
The -Si-O-Al-
bonds could be formed readily by
dehydration of intimately mixed hydrous oxides-that
is,
2565
combined water is present and a picture is given of the way in which it is combined. The proportion of combined water is one water molecule for each Ale08 molecule properly combined with silica. If the acidity is thc source of catalyst activity, then maximum activity should result when maximum acidity is obtained. Maximum acidity should result from the maximum numher of Si-0A1 bonds. Maximum acidity should result from catalysts in which the atomic ratio of silica to alumina is one. In many methods of making cracking catalysts (1,ZO,I S , ,94, W7), a silica hydrogel is first formed. It is not possible to form a silica hydrogel without forming Si-0-Si bonds. Si--0-Si bonds reduce the number of Si-0-A1 bonds potentially available. From thisl it is concluded that such methods of making silicaalumina oracking catalysts cannot yield catalysts of maximum acidity. If the catalyst could be prepared so that every oxygen atom present is shared by one ailison and one aluminum, then the composition of the catalyst would be HAISi04. Since this exists as a solid it should be written (HAISiOa), just as solid NaCl should be written (NaCl),. (HAISiOd), is then the catalytic part of the catalyst. Any silica not directly associated with the acid forming part of the catalyst does not contribute to the activity. The commercial silica-alumina catalyAts ordinarily contain 10 to 15% Alp03 and 85 to 90% SiOe-that is, they contain too much silica for all of it to be in the (HAlSi04),. I n one sense, the commercial catalysts might be regarded as HAISi04 supported on inactive silica-gel. This is an incomplete picture, however, as it fails to show that the support is chemically combined with the catalyst and that one is inscparable from the others. It seems entirely reasonable that, for a hydrogen ion to be effective in a catalytic reaction, the hydrogen ion and the substance to be catalyzed must come together. I n catalytic cracking only the hydrogen ions on the surface of the catalyst should be effective. I n other words, hydrogen ions that are buried in the solid are not available to the hydrocarbons and, therefore, are not effective. The catalyst activity then should increase as the surface area of the catalyst increases and should increase as the concentration of hydrogen ions on the surface increases. These implications suggest a vast amount of experimentation, much more, in fact, than has been carried out so far. PERTINENT EXPERIMENTS
Much of this concept depends on the acidic nature of the catalyst. To determine that the acidity was due only t o a silicaalumina-water combination, a catalyst was prepared without any other inorganic materials. Ethyl orthosilicate and aluminum Isopropoxide or aluminum ethoxide were used as sources of silica and alumina, respectively. They were redistilled carefully for purifioation from possible inorganic impurities. The two substances were mixed so that the aluminum isopropoxide dissolved in the ethyl orthosilicate. This mixture was hydrolyzed by a mixture of distilled water and alcohol. The catalyst prepared in this way was acidic and was catalytically active. This method of catalyst preparation seemed to offer the possibility of preparing a series of catalysts with varying ratios of silica to alumina in such a way that the silica and alumina were more intimately related than has heretofore been possible. It offered also the opportunity to determine whether acidity is reIated t o alumina content and whether activity is related to either or both. Table I gives a summary of the results obtained. RELATION BETWEEN COMPOSITION, ACIDITY, AND WEIGHT ACTIVITY
Such bonds could not be formed readily when both or either component was anhydrous. Finally the source of the acidic nature of the catalyst is explained, and a definite function is ascribed to it. The picture fits the facts reasonably well. It is now possible to proceed with the implications of this picture: The most obvious implication is that the catalyst is not just silica and alumina. Actually, it contains both hydrogen and oxygen in excess o f AI~Os.sSi0~.The hydrogen and oxygen are present in the =me ratio that they occur in water. I n emence,
Completely independent of any hypothesis, it is possible to plot the acidity (as me. of potassium hydroxide per gram of catalyst) against the composition of the catalyst mass. The atomic ratio of alumina to silica was selected as a measure of the catalyst composition. The plot is given in Figure 1. The maximum acidity is found a t Al/Si = 1. The catalytic activity is plotted on the same figure. The maximum activity is in the region of AI/Si = 1to 2. The following procedure for KOH measurements was used:
'
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 11
A I / S i Atomic Ratio in Catalyst
Figure 1. Acidity and Weight Activity us. Al/Si Ratio Silica-alumina catalysts from (Et0)aSi
Five grams of the catalyst were weighed to the nearest 0.01 gram and added to 200 ml. of 0.1 S potassium hydroxide solution (standardized). The mixture was agitated for 0.5 minute, and a t the end of 1 minute a 10-ml. sample withdrawn with a pipet. Agitation was continued for the first half of the sccond minute, and the catalyst was allowed to settle again bcfore the second 10ml. sample was withdrawn. Samples were n-ithdrawn a t the end of 1, 2, 3, 4, 5 , 6, 10, 15, and 30 minutes; a 0.5-minute settling period was allowed before each sample was withdrawn. Each sample was dropped in one of a series of funnels fitted with filter paper. Five nil. of each filtered sample were removed from the test tube, in which it was collected, and titrated with standardized 0.1 N hydrochloric acid using phenolphthalein as an indicator. It was decided to use the value obtained after 30 minutes, which, in virtually all c a s q was very close to the final equilibrium value as indicated by close agreement b e t w x n the 15- and 30-minute values. The catalytic activity was determined by passing Pennsj-lvania gas oil over the catalyst at 500' C. and a t atmospheric pressure: Twenty-five ml. of catalyst were mcasurcd by allowing the powdered catalyst to flow into a graduate cut off to contain exactly 25 ml. The sample was wcighed and the apparent hulk density mas calculated. The sample was put into a stainless steel reactor tubr mounted vertically in a thermostatted metal block furnace. Pennsylvania gas oil was passed downward through the catalyst at tho rate of 100 ml. a n hour for 2 hours. The weight of gas formed wa5 measured. The liquid product was collected and the gasoline %-as separated by distillation. The gas plus gasoline cxprcssed as weight per cent of the gas oil charged was used as a measure of the catalytic activity. The activity was calculated using tho method described by Shankland and Schmitkons (18). The standard catalyst was a silica-alumina-zirconia catalyst (bulk density 0.74 gram per cc.) to which an :Lctivity of 100 was assigned.
TABLE I.
There is thus ample evidence that catalyst activity, catalyst acidity, and composition are ielated in silica-alumina catalysts prepared in a way to exclude mineral acids. The maximum acitivity and maximum acidity occur in almost the same region of composition. These are observed facts and are indeprndcnt of any hypothesis. It is pointed out that these observed facts are in qualitative agreement with the postulate that the active constituent is ( HAISi04),. This qualitative agreement can be shown in anothei way. In catalysts containing an excess of silica (AI/Si < 1)It was assumed that all the alumina is in form of (HAlSiOd), and from thp composition, the pcrcentagc of (HBlSiOd), can be calculated. For catalvsts containing an exress of alumina (Al/Si > 1) it was &Esumed that all the silica was in the form of (HhlSiOd), and the weight % of (HAISiO4)= rnlculated from the composition. The
COMPOSITION, hCTIVITY, AND ACIDITY O F SILICA-~%LLXtlINAC.4TALYSTS
Calcd. Catalytic Wt. % (HAISi04)s Activity rimiiming all S! is present as (HdlSl04)s 0.024 4.7 13 0,047 9.0 36 9 . 0 22 0 . 0 4 7 (Et03)41 0.095 17.4 100 17.4 82 0.095 39 31 27..04 0 . 0 9 5 (Et0s)Al 128 0.19 32.0 114 0 . 1 9 (Et0a)Al 55.1 160 0.38 181 73.2 0.575 73.2 188 0.578 193 Q11.8 0.92 95.8 161 0 . 9 2 (EtOdAl ~ 1 5 p i Ratio
1.85 1.85 3.66 7.3 14.9 z8.6 08.8 0
+ Al(0R)s
Assuming all Si is present as (HAlBi04)~ 73.4 202 73.4 194 47.1 142 27.1 127 14.6 80 7.9 69 76 3.9
u
0
.hcidity, h , ~ C~ a. l c u l a t e d Catalytic Composition ?& ICOH/G. % ?& SiOzc Catalyst (H.41SiOi)~b . & 1 2 0 a c
... ...
0.60 0.90 1.09 1.22
7.2 10.8 13.1 14.6
1:4
95.3 01.0 91.0 84.0
0.87 1.42 1.40 2.09
10.4 17.1 16.8 25.1
2.9 6.6 6.9 16.1
86.7 76.3 19.3 08.8
2.26 2.43 2.26
27.1 29. I 27.1
hi:7 30.6
52.2
2.17
26.0
...
...
, . .
1.46 1.10 0,70 0.56 0.39 1.03 0.31
...
...
, . .
17.5 15.7 8.4 6.7 4.7
... ...
...
. .
I
.
.
,
Calcd. Catalvtio Activityd
47 71 86 97
... 60
I15 113 172
...
40.3
31 . 3
41.4
187 205 192
'14.4
:i 1, 6
189
65.1 78.5
17.4 5.8 5.1 0.6 0.0 100.0 0 .0
144
...
88.11 92.7 95.3 0 100. 0
...
138 9 : 86 74 0
47 0 45 Made from (iPr0)aAl unless indicated otherwise. Calculated from observed acidit c Calculated from Al/Si ratio andVbbservod acidity except where there is insufficient alumina, theii c a l r u l a t r ~ i assuming all alumina present as (HAlSiOds. Calculated taking 655 a s activity of (HAlSiOdr, 4 5 a s activity of A1203. and zero as activity of SiOz. m
a
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1949
Difference 2 is probably due t o the incomplete reaction between Bilica and alumina. The formation of a solid from a solution is comparable to polymerization; the formation of (IXAISi04), compares with copolymerization. By this analogy silica may polymerize with itself or with alumina. The yield of (HAlSi04), depends on the proportion of copolymerization. As the proportion of copolymerization increases, the acidity should approach 8.33 for the Al/Si = I catalyst and the activity should increase. Difference 3 can be explained if it is assumed that alumina has a small cracking activity that is not accompanied by an acidity. It is possible to use t,he explanation of differences 2 and 3 to make a few pertinent calculations: Assume that any silicaalumina catalyst that has not been overheated is made up of SiOn, AlzOa, and (HAlSiO&; that SiOl has zero catalytic activity and that ALOa and (HAISi04), are catalytically active. Furthei assume that the acidity (me. of potassium hydroxide per gram of catalyst) is a measure of the (HAlSiO,), content and that any remaining SiOzand A1203 are present as such. From the acidity and the composition of the catalyst, i t is possible to calculate the percentage of SiOz, &Os, and (HAlSiOd), for most of the catalysts. For some of the catalysts very rich in Si02 or i11203the observed acidity is greater than the potential (HAISiOl),. In these cases it has been assumed that all of the minor constituent is present as ( HA1SiO4),. The activity of the catalyst may be expressed:
i
(HAiSi0i)z in Catalyst, Cslcd. Wt. %
Calculated (HAlSiO& Content us. Activity of Silica-Alumina Catalysts
Figure 2.
2567
values so calculated are plotted against the activity in Figure 2. The correlation is remarkably good considering the wide variations in composition and the differences in preparation. Close examination of Figures 1 and 2 reveals quantitative discrepancies: 1. Acidities occur in the zero and low alumina content and in the zero SiOzcontent masses that are higher than can be explained by (HAISiO& for the sim le reason that there is not enough alumina (or silica) to form tge necessary (HAlSiOa),. 2. The maximum acidity observed is 2.43 me. per gram whereas that calculated for (HAlSi04), is 8.33. 3. The maximum activity and acidlty should come at Al/Si = 1. Actually the acidity does, but the activity comes a t about 2.
No attempt will be made here to reconcile difference 1. Silicon dioxide prepared according to this procedure has an acidity of 1.03 and aluminum oxide has a n acidity of 0.31 me. per gram.
A = aB -I bC where A = activity of the total catalyst; B = activity of AI2O3; C = activity of (HAlSiO&; a = weight % A1203; and b = weight % (HAISiO&. The data for these calculations are given in Table I. From some of the known activities and compositions it was found that B = 45 and C = 655. Using these values, all the activities of the catalysts were calculated from the acidity and the composition. These results are given in Figure 3 along with the observed values, plotted as a function of the atomic ratio of aluminum to silicon. The agreement between calculated and observed values is apparent. To summarize the observations and the deductions made from them: catalysts of maximum activity were obtained a t Al/Si atomic ratios of about 2; catalysts of maximum acidity were obtained a t Al/Si atomic ratios of I; to a first approximation, the catalyst mass behaves as if it were made up of an active part having an AI/Si ratio of 1plus an inert support. The last two observations are consistent with the hypothesis that the active component is (HAlSiO&. Using the acidity as a measure of (HA1Si04), content and the composition of the catalyst mass, i t is possible to calculate the activities of these catalysts by assuming that the activity of (HAlSiOl), as determined by the acidity is 655, the activity of excess alumina is 45, and the activity of silica is zero. OTHER CATALYSTS
AI/Si Atomic Ratio in Catalyst
Figure 3.
Comparison of Activity Calculated from Acidity w i t h Observed Activity and Composition
Wt. yo Activity (HAISi04). = 655 AlnOc = 45 Si02
= o
x
= Obaerved
Calod. from acidity plus excess A1208 = Calcd. from SiOa or AlzOa content
0 =
A
Silica Plus Octahedral Alumina. So far, only tetrahedral silica and tetrahedral alumina have been considered. I n octahedral alumina,
INDUSTRIAL AND ENGINEERING CHEMISTRY
2568
d -0-Alk-0-
I/ O
Vol. 41, No. 11
the catalyst and to the conclusion that the catalyst of mavimum activity will have Si/Zr = 2.
/
0
I
5’
each A1-0 line represents one half a valence unit. act with such a system
0
0-si-o
Silica can re0
-0
0
’ /
0-A1--0
0’
/
\
0
I -0
0-9 U
b
‘0
so that each oxygen shares one tetrahedral silicon atom and two octahedral aluminum atoms. This produces a saturated aluminum silicate that should have no acidity and, therefore, no activity. There are several aluminum silicates known in which the aluminum is octahedral. So far, none of these has been found that is a cracking catalyst. This tends, at least in part, t o substantiate assumption 1. Silica-Magnesia. Xagnesium occurs in both tetrahedral and octahedral types with oxygen. The tetrahedral magnesia gives rise t o the following type of structure:
If the catalyst actually has four hydrogen ions per zirconium, there is a reasonable doubt that more than one, or a t most, two, is active enough to be calaytic. Alumina-Boria ( 2 ) . Boron usually shares three oxygen atoms ( l e f t ) . Active alumina-boria 0‘ catalysis can be made from gamma alumina. \ B-0Gamma alumina is thought to contain two octahedral aluminum atomfi to each tetrahcdral / aluminum atom: O \
1
-0
-0
x! 6-B
*---I
0
Each Si-0-Mg oxygen is unsatisfied by one half a valence unit. There are four of these so there are two full negative valences to be satisfied. Two hydrogen ions are postulated to satisfy this deficiency. The catalytic part of the catalyst is then (HtMgSiO& This means that catalysts of maximum activity should be produced when the ratio of magnesia to silica is one, provided they are properly prepared. It will be recalled that most of the better silica-magnesia catalysts that have been prepared had a magnesia to silica ratio of about one ( 2 2 ) . Is (HnMgSi04), a stronger acid than (HA1Si04),? This question relates t,o the activity to be expected. It seems likely that the (H&IgSiO& may well be a weaker acid because i t is a dibasic acid and because of the basic nature of magnesium that forms part of the anion. Silica-Zirconia (6, 15). Zirconium is associatcd with eight oxygen atoms.
0
\
I/
/o/l‘o0
I
0
I n the absence of any other basis for selection, one would be inclined to favor the octahedral alumina as the one that gives a cracking catalyst with boria. On this basis the catalyst is (€I~AlB20~)r and the catalyst of maximum activity has BjAl = 2. Titania-Boria (7). Titanium shares six oxygen atoms in t h same way that octahedral aluminum does. Excopt for the difference in valence, titania-boria looks very much like aluminaboria: 0 €3
\O
0
/ 0 0
I
-0-Zr-0-
/’
\_.___’---*’
I
-0
I
‘0
0
/
0-
0-SI
P
/
0
0-9
0
0’
\
Since zirconium has a valence of four, each Zr-0 line represents one half a valence unit, the same value t h a t was encountered in tetrahedral magnesia. This leads to (H4ZrSi10s)z as the formula of the catalytic part of
From this, the catalyst is (HzTiHzOs)zand the catalyst of maximum activity should contain B/Ti = 2.
November 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY CONCLUSIONS
The following generalizations are offered with maximum tentativeness. The term coordination number means the number of atoms surrounding a given atom-for example, the tetrahedral oxides all have a coordination number of four. 1. When a positive element, having a given valence and coordination number, replaces a second positive element having a higher valence and the same coordination number, a catalyst can be formed if the valence deficiency is made u by hydrogen ions. 2. Two positive elements, one having dougle the coordination number of the other when combined with a negative element, tend to form an acid that can act as a cracking catalyst,
. I
I
All the catalysts discussed here come within the scope of generalizations 1 and 2 above. Even so, five catalysts are hardly enough to place much emphasis on the generalizations. However, these generalizations are quite as important for the oxide mixtures they exclude as for the ones they include. I n the absence of any hypothesis it is difficult to understand why SiOa-BzOs does not form a cracking catalyst. Boron is in the same Periodic Table group with aluminum and forms many similar compounds. The author knows that many attempts have been made to prepare such catalysts, but he knows of no one who obtained products that were active for catalytic cracking. The silica-boria mixture does not obey either generalization 1 or 2:
0 0-Si
I
I
l
o
‘
B
0 In silica the line from silicon to oxygen represents one valence unit. The same is true for boron and oxygen. If they share an oxygen, the oxygen is completely satisfied. There is no place for any hydrogen ion and, therefore, no acidity. A combination of octahedral titania and tetrahedral silica that might have acidic properties can be written. It is [H4Tir Si,0,2]z. This combination does not fit generalizations 1 and 2. So far as the writer knows, active cracking catalysts have not been prepared from silica-titania masses. MECHANISM OF CATALYTIC CRACKING
From the work reported it is clear that the cracking catalysts are acids and the catalyst activity is related to the acidity of the catalyst. Carbonium ion reactions are known to occur in the presence of acids. It is proposed that the hydrocarbon reactions catalyzed by the cracking catalysts are carbonium ion reactions. Hansford (Id) has suggested that both carbonium ions and carbanions are involved. In the following section, the established types of carbonium ion reactions will be reviewed. It will then be shown that the reaction products resulting from the catalytic cracking of hydrocarbons can be explained adequately by carbonium ion mechanisms, Reactions of Carbonium Ions (30,31). A carbonium ion carbon has only six electrons associated with it. The various reactions it undergoes are the result of the ion taking an additional pair of electrons from some other atom, either in the same molecule or in a different molecule. Reactions which are thought to involve carbonium ions have been studied enough to permit useful generalizations to be made about the reactions of these ions. Before going to the hydrocarbon reactions, it is desirable to review
.. .. ..
t h e types of reaction briefly. For purposes of illustration, :A :B: will be used to represent a carbonium ion; B represents the cmbonium ion carbon atom. REACTIONTYPE1. Carbonium ion B may have a greater affinity for electrons than A. When this is the case the following change occurs:
.. .. ..
:A:B:
.. ..
:A:B:
2569
so that A becomes the carbonium ion carbon atom. The electron pair that migrates from A to B carries whatever atom or group, that shares these electrons along with them, to B. Reaction 1 was proposed by Whitmore (31). REACTION TYPE2. A carbonium ion may react with a neutral molecule to form a new carbonium ion and a new molecule.
:A:B .. .. + :R:H-
:A:B:H
.. ..
+ :R
This reaction has a t least two distinct forms: ( a ) R H is a paraffin hydrocarbon-for example, isobutane (3, 27); ( b ) R H is an olefin molecule so that an unsaturated carbonium ion is formed (a). :A:B
*. ..
H H + R:C:C-C=CH, I I
+ :A:B: .. ..
**
HA
H H + R:C:&--&==CI&
A
H
I
REACTIONTYPE3. A carbonium ion carbon may take an electron pair from an adjacent carbon atom without taking the group which shared that electron pair ($9). This is the reverse of adding a carbonium ion to an olefin.
.. .. .. .. ., ..--f :R.. + A: .. :B ..
:R:A:B
THE BETARULE. When a carbonium ion carbon takes a pair of electrons intramolecularly, that pair of electrons always comprises one of the bonds beta to the original carbonium ion carbon. REACTION TYPE4. A carbonium ion may react directly with an olefin, converting the carbonium ion into an olefin and forming a new carbonium ion.
:A:B:
..
.. .. + :c::c:* :A::B:f :c:c:
Not all carbonium ions form with equal readiness and all are not equally stable. Many carbonium ion reactions are best explained if it is assumed that the tertiary carbonium ions form most readily and are most stable, secondary next, and primary next. Of the primary carbonium ions, ethyl and methyl seem to occupy special places so that a series of decreasing stability can be written: tertiary > secondary > primary > ethyl > methyl. This might well be termed the carbonium ion rule. It cannot be emphasized too strongly that the above-given reactions of carbonium ions are not independent of the catalyst. The carbonium ion is always intimately associated with the acid catalyst; whether or not a given reaction will occur is a function of the catalyst. Formation of Carbonium Ions. In hydrocarbon reactions, carbonium ions are most readily formed from olefins. The reaction can be represented:
:C: :C:
+ H +H:C:C: ..
APPLICATION OF CARBONIUM ION THEORY TO HYDROCARBON REACTIONS IN PRESENCE OF CRACKING CATALYSTS
I n hydrocarbon reactions the catalyst will be designated HA where H represents the hydrogen ion that makes the catalyst acidic and A represents the anionic part of the catalyst. Cracking of Olefinic Hydrocarbons ( 8 , I l ) . Step 1is reaction (A). Using diisobutylene as an example of the olefin the reaction can be written: C-
XA I
-4- =C
+ HA +C-
s g
-4- -C
+ A-
This is followed by step 2 which is reaction type 3 according to “the Beta rule”:
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 11
c:
I
C Step 3 is reaction 4:
c
c
C
C
c
C
C
c
c
Steps 2 and 3 continue indefinitely. There is an alternate to step 3:
I
c:
c
C C-k+
Steps 2 and 3 continue indefinitely. Skeletal Isomerization: step 1is reaction (A) as indicated above for the double bond shift. Step 2 is reaction type 1 according to the Bcta rule:
+ -+
I
C=C-C+HA
c-c-c-c-c
C This breaks the ieaction chain. The regenerated catalyst, H 4, then is re-used in step 1 to repeat the sequence. Throughout this discussion this type of alternate will exist. Although the alternates mill not be written down, they are implied. Dealkylation of Aromatic Hydrocarbons (13, 28). The dealkylation of aromatic hydrocarbons is anothrr type of cracking reaction. Step 1 is again reaction (A) in ahich the double bo,id involved is one of the double bonds in the aromatic ring:
Step 2 is again reaction type 3 according to the Beta rulo:
Step 3 is reaction type 4 using one of the double bonds in the aromatic ring:
Steps 2 and 3 are continued indefinitely. The Beta rule calls for the severance of the side chain at thc. aromatic ring. The experimental evidence shows that this is the predominant if not’the exclusive reaction. The relative ease of remova,l of the side chains calls for some comment, A teyt-butyl or tert-amyl side chain comes off very easily. An isopropyl group comes off easily but, vr-it,h greater difficulty than the tertiary groups. An n-butyl group comes oft’ with still greater difficulty and t,he et,hyl group is difficult indeed. The methyl group is substantially unattacked. This scale of refractoriness is in perfect agreement with the carbonium ion rule. T o crack tert-butylbenzene a tertiary carbonium ion nlust be formed. Since this is readily formed, the cracking is easy. K i t h isopropylbenzene a secondary ion must be formed, and this is more difficult. With n-butylbenzene a primary carbonium ion n u s t be formed, and this is still more difficult. The observat,ions regarding methyl and ethyl groups indicate that there are differences among the primary carbonium ions. Isomerization of Ole& Hydrocarbons (8, 11, 21, 25, 29). Ilouhle bond shift: step 1 is reaction (-4) :
C-(%C-&=C
+ H A +C---C--(2--C---C t
-
$-
A
t
I C I --+ fC-- c-c
c
I i --t c--c--c C f
Step 3 i8 reaction type 4:
C I
Steps 2 and 3 continue indefinitely. The same types of reaction occur in both double bond shift and skeletal isomerization. In I)ot,h cases an electron pair moves to the carbonium ion. When the nioving electron pair takes a hydrogen atom with it, a double bond shift results. When the moving electron pair takes a11 alkyl group with it, skeletal isomerization results. The same type of change occurs uhon the olefinic double bond is present in a ring. The isomerization of cyclohexene can be written ad follows: Ht,ep I is reaction A:
Step 2 is reaction type 1 aocorcling to the Beta rulc:
Step 3 is ieaction type 4. Relation between Isomerization and Cracking. There is a simple relation between cracking of olefins and isomerization. This relation should be made clear before proceeding further Both involve the Beta yule. It will bp recalled that a carbonium ion contains a mrbon atom associated with only six electrons and that the reactions of the carbonium ion involve this carbon atom’s attempts to obtain two more electrons. The behavior of the carbonium ion that makes the beta bond weak suggests that the carbonium ion carhon trici hardest to obtain electron? Frorn the beta bond I t can obtain these cllectrons in two ways: the, entire group from the beta bond to the end of the molecule, including it>s electron pair, can migrate, or the electron pa11 done nlay migrate When the first occurs, t h r result is I earrangement (isomeriznonrl owuri the rriult is clacking.
November 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
This is illustrated by the following equations: H H H H H
H H H Rt.
%
L-A & =
H H H +H:C::(!d%H
+LA
I!€L
(Cracking),
In the isomerization, a n-propyl group and its electron pair moved to the carbonium ion carbon producing a rearranged carbonium ion. In the cracking reaction only the electron pair moved, forming an olefin from the original carbonium ion carbon, and n-propyl carbonium ion. Hydrogen Transfer (21, 19, 29). I n the hydrogen transfer or self-saturation reaction, olefinic hydrocarbons are converted into paraffins without any external source of hydrogen. The hydrogen is supplied by converting a part of the olefinic hydrocarbons into hydrocarbons containing less hydrogen. Step 1 is reaction (A) to form the carbonium ion. Step 2 is reaction type 2. This may be illustrated by the equation:
H R:C:CHa -t
+ C-C-C=C-IE’
+ R-
r
+
-CHa
IH
C-C-C=C-R‘
+
in which a carbonium ion reacts with an olefin to give a paraffin and an olefinic carbonium ion. Step 3 is reaction type 4. H H R-C=CH2 C-C-C=C-R’ R-C-CHa
+
+
+
C=C-C=C-R’ As a result of steps 2 and 3 one can start with two olefin molecules and convert one to a paraffin and the other to a diolefin. By repeating the same type of process, the diolefin may be converted to a triolefin and on to an aromatic hydrocarbon. The formation of aromatic hydrocarbons has been clearly demonstrated. The diolefins and triolefins are presumed to remain associated with the catalyst as a carbonaceous catalyst deposit (19). It seems likely that a major part of the catalyst deposit in commercial catalytic cracking arises from the hydrogen transfer reaction. Certain naphthenes such as decalin can supply hydrogen for hydrogen transfer. The most likely reaction seems to be: H2 H Hz Hs+ H2
+
Catalytic Cracking of Paraffins and Naphthenes (4, 8, 9, 10, 12,221. An olefin hydrocarbon can add to the catalyst t o produce a carbonium ion (reaction A). A paraffin hydrocarbon is not able t o do this. This difference seems pertinent to the fact that olefins are much easier than paraffins to crack catalytically, in contrast to thermal cracking where olefins and paraffins crack with approximately the same difficulty. It is well known that paraffins form olefins on thermal cracking. It is postulated that a certain amount of thermal cracking occurs in the catalytic cracking of paraffins. The thermal cracking forms olefins that in turn are able to form carbonium ions and thus start the catalytic part of the process. The fact that the thermal cracking part of the process requires more severe conditions (higher temperatures) is thought to account for the fact that the paraffins are more difficult to crack catalytically than olefins. There are other postulates for starting the paraffin reactions. BIoch, Pines, and Schmerling (6) proposed a reaction for starting the paraffin isomerization reaction :
RH
k + ;i + H~
+ HA-
The occurrence of this type of reaction on silica-alumina catalysts may serve to start the cracking of paraffins. Parravano, Hammel, and Taylor (16) studied the behavior of a mixture containing CD,, CD,H, and CH4 over a silica-alumina catalyst. They showed that the catalyst caused the CD4 to CDsH ratio to decrease and then come to equilibrium. The first stage of this reaction can be expressed:
+A +
+
C D ~ HAcb, HD This represents one form of the initial dehydrogenation proposed by Taylor et al. (16). When written this way, the close relation between the initial dehydrogenation step of paraffins for isomerization and for catalytic cracking becomes apparent. The same general type of reaction should occur with cycloparaffins. The following equations can be written t o illustrate the types of reactions that occur in the catalytic cracking of n-octane: Step 1 is thermal cracking:
C”
n-C,His Step 2 is reaction type (A): CTHu
+ C7H14
+ HA +C-C-C-C-C-C-C
+ ;i
Step 3 is reaction type 3 according t o the Beta rule:
c-c-c-c-c-c-c c
-----f
c-c=c + +c-c-c-c
Step 4 may be reaction type 1:
Hz()sl
R + +HZH a C H2 U 4 RH Hz H P H2 €IsH HP The alkyl carbonium ion reacts with the cycloparaffin to form a cycloalkyl carbonium ion and a paraffin. The cycloalkyl carbonium ion may react. with an olefin to continue the process:
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+c-c-c-c
-
c-c-c-c
C
+ ++
c-b-c
C
c-b-c
Step 5 is reaction type 2:
C-
8
-C
C +
n-CsHls
I
+ C-C-C
+ c-c-c-c-c-c-c-c +
Step 6 may be one or more of a large number of type 1reactions:
c-c-c-c-c-c-c-c-c(b)
C
This cycle of events continues until the decalin is converted into tetralin and naphthalene.
I c-c-c-c-c-c-c + (0
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
2572
c+ I
c--c-e-c-c-e-e-c +
e--+ C-C-C-C-C--C-C
-4
Any of these various carbonium ions may react with octane by a type 2 reaction or crack by reaction type 3 according to the Beta rule. It is known that a major part of n-octane cracks t o Ca and Cg hydrocarbons. There are several routes by which this could occur. I n fact, a, b, f, g, and h can crack (reaction type 3) according to the Beta rule to give Ca and c6 hydrocarbons. The rracking reactions may be illustrated as follows:
c-c=c + L
c-c-c--cc-c-c--c---+ +
Vol. 41, No. 11
ing activated (forming carbonium ions)-for example, by reaction type 2, and cracking catalytically. Catalytic Cracking of Naphthenes (I,12, 26). Naphthenes (cycloparaffins) resemble paraffins in that there is no simple way in which they can unite directly with the catalyst to form a carbonium ion. A small amount of thermal cracking can supply olefins that start the carbonium ion chain. Cyclohexane is less susceptible than deralin to catalytic cracking. It will be recalled that cyclohexane contains only secondary carbon a t o m whereas decalin contains two tertiary ones. It will also be recalled that tertiary carbonium ions are more readily formed than secondary carbonium ions. This difference would cause a difference in the susceptibility to catalytic cracking in the observed direction. I t has been clearly established that decalin is isomerized by the cracking catalyst t o produce bicycloparaffins having lower boiling points than decalin (4, 26). Cracking also occurs t o produce lower boiling hydrocarbons, especially C3H8, C3Hs, i- and n-C4Hg, C ~ H ~methylcyclopentane, O, and probably cyclohexane. Hydrogen transfer also takes place. All the reactions are, broadly, the same types that have been discussed above for other hydrocarbons. There is little doubt that the same types of Carbonium ion reactions take place. More of the products will have to be identified and determined quantitatively before it will become profitable to try to write n mechanism for the reactions. ACKNOWLEDGMENT
- - C
~
c: -c
The author is grateful to Universal Oil Products Company for permission to publish this work. The author also wishes t o acknowledge with sincere appreciation the aid of James Hoekstra who prepared the catalysts for this study.
LITERATURE CITED
Ahlberg, J. E., and Tlionias, C. L. (to Universal Oil Ploducts Co.), U. S. Patent 2,282,922 (May 12, 1942). Bailey, W. A., Jr. (to Shell Development Co.), U. S. Patent 2,777,744 (June 5, 1945). Bartlett, P. D., J . Am. Chem. Soc., 66, 1531 (1944). Bloch, H. S., and Thomas, C. L., Ibid., pp. 1589-94 (1944). Bloch, H. S., Pines, Herman, and Schmerling, Louis, I b i d . , 68, 153 (1946).
Connolly, G. C . (to Standard 011 Development Co.), U. S Patent 2,364,949 (Dec. 12, 1944). I b i d . , 2,424,152 (June 15, 1947). Egloff, G., Morrell, J. C., Thomas, C. L., and Bloch, H. S , J . Carbonium ions that contain four or more carbon atoms may rearrange. Either before or after rearranging they may exchange with a n-octane molecule to give an octyl carbonium ion and a paraffin. It is of considerable importance t o be sure that such an eschange reaction does or does not occur. If it does not occur, each molecule of paraffin must crack thermally before the catalyst can work on the olefinic part. If it does occur, then a small proportion of thermal cracking can furnish olefins (and therefrom carbonium ions) for the conversion of a number of paraffin molecules. The facts available are from early studies on the cracking of n-octane (8). Approximately 70% of the catalytic reaction may be written: CS
c 6
4CO
Thermally, about 15y0 of the reaction proceeds this way. It, is not possible t o crack thermally a n-octane molecule in any way and let the cracking catalyst work on the products and obtain 70% C g CB. Therefore, i t must be concluded that a major part of the reaction occurs by the entire octane molecule becom-
+
Am. Chem. Xoc., 61, 3571 (1939).
Good, G. M.,Voge, €I. H., and Greensfelder, B. S., IND. EXG. CHEM.,39, 1032 (1947). Gieensfelder, B S., and Voge, II. I-I., Ibid.,37,514 (1945). Ibid.,p. 983. Ibid., p. 1038.
Greensfelder, B. S., Voge, H. H., and Good, G. M.. Ibul., 37 1168 (1945).
Hansford, R. C., Ibid., 39, 849 (1947). Lee, E. C . (to Universal Oil Products Company), U. S. Patent 2,382,239 (August 14, 1945). Parravano, G., Hammel, E. F., and Taylor, H. S., J . Am. Chem SOC.,70,2269 (1948).
Schmerling, L., Ibid , 66, 1422 (1944). Shankland, R. U., and Sohmitkons, G. E., Proc. Am. Petroleum Inst., 27, [111]57 (1947).
Thomas, C. L , J . Am. Chem. Soc., 66, 1586 (1944). Thomas, C. L. (to Universal Oil Products Co.), U. S. Patent 2,270,090 (Jan. 13,1942). Ibid., 2,328,753 (Sept. 7, 1943). Ibid.,2,432,634 (Dee. 16. 1947).
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).
2573
(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 t o 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 to 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-
