Olefin Polymerization with Acid Catalysts G. E. LANGLOIS California Research Corp., Richmond, Culif.
The polymerization of olefin hydrocarbons with acidic catalysts is reviewed, and the more important commercial applications of acid-catalyzed polymerization are described. Catalysis by the Friedel-Crafts type catalysts as well as the conventional acid catalysts such as phosphoric acid is covered, and a reaction mechanism embracing both types of catalysts is presented. Three types of polymeriza-
tion are distinguished: true polymerization; heteropolymerization, which results in olefins which are nonintegral multiples of the monomer; and conjunct polymerization, which results in paraffins, polyolefins, and cyclic products. Mechanisms b y which these reactions occur and the reaction conditions which favor the various types are discussed.
T
apparently owes its activity to a partial conversion to free phosphoric acid. Phosphoric acid has been used principally for the polynierization of CS and Cd olefins. Ethene polymerizes only with difficulty. Pentenes and hexenes xvill polymerize readily in the presence of phosphoric acid, but the higher olefins react much lrss readily. The phosphoric acid catalysts are used in fixed-bed continuous flow-type reactors a t temperatures in the range from 100" to 250' C. and pressures from 200 to 1200 pounds per square inch gage. The catalyst activity is strongly dependent on the nater content, and the effect of acid concentration on activity is given by Langlois and Walkey ( 2 1 ) . Aviation gasoline may be prepared by the hydrogenation of butene dimer obtained by polymerization under very mild conditions of temperature and olefin conversion. Under these conditions, the optimum ratio of iso- to n-butene reacting is secured, and the structural rearrangements which result in a product of low octane number are avoided. Olefin polymer is used without hydrogenation in motor gasoline. The octane number of the olefinic material is relatively insensitive to the structure of the polymer: hence, the inclusion of propene in the feed, higher n-butene conversions, and operation at higher temperatures are possible without serious octane number degradation. A detailed description of the phosphoric acid-on-quartz process is given by Langlois and Walkey ( 2 1 ) ; the solid phosphoric acid process is described by Egloff and Weinert (6); and the copper pyrophosphate process is described by Steffens and coworkers ( 2 7 ) . Sulfuric acid of about 7070 concentration has been used commercially for the polymerization of isobutene to diisobutene a t about 20" t o 35' C. (the so-called cold acid process) and for the copolymerization of iso- and n-butenes a t about 80" t o 90' C. (the hot acid process). Operating details of these processes are described by McAllister (22). Sulfuric acid is not a good catalyst for the polymerization of ethene, propene, or n-butene alone. Silica-alumina has been employed commercially for the polymerization of butenes a t reaction temperatures from 100" to 250"C. andpressuresupto 1500 poundspersquareinchgage. Polymerization with a catalyst of this type is described by Thomas
HE field of acid-catalyzed polymerization is old and has been extensively investigated. 4 great many monomers and catalysts have been employed and a wide variety of products produced. The literature of the field is especially extensive because of the widespread commercial application of reactions of this type. Treatment of the entire field would be outside the scope of this paper, and this discussion is limited to the acid-catalyzed polymerization of olefin hydrocarbons. Thermal and free-radical catalyzed reactions are not discussed, and monomers other than mono-olefinic hydrocarbons such as conjugated diolefins and various vinyl compounds such as vinyl esters are not included, although some of the statements may be applicable to these materials. This paper describes briefly the more important applications of acid-catalyzed polymerization, shows how the product depends on reaction conditions, type of catalyst, and monomer, and presents the most widely accepted reaction mechanisms.
General Survey Olefin polymerization has been catalyzed by a wide variety of acidic materials, which may be divided roughly into three groups: conventional acids such as phosphoric acid, the metal halide or Friedel-Crafts type catalysts such as aluminum chloride, and certain oxides which are acidic in character, such as silica-alumina. Olefins ranging from ethene to hexadecene have been used as monomers, although the great bull; of the work has been done on the normally gaseous olefins. Acid-catalyzed polymerization may be broken down roughly into two classifications: high temperature and low temperature polymerization. High temperature polyinerization refers to reactions carried out at temperatures above about 0' C., and it is in this range t h a t the conventional acid catalysts and the metal oxide catalysts are effective. Many acidic materials have been investigated, including phosphoric acid, sulfuric acid, hydrogen fluoride, allrane-sulfonic acid, dihydrosyfluoboric acid, borophosphoric acid, inetal acid phosphates, boron trifluoride-sulfuric acid, hydrofluoric-hydrocyanic acid, silica-alumina, bauxite, etc. All have been more or less effective. A number of these materials have been utilized commercially, including phosphoric acid, sulfuric acid, and silica-alumina. Phosphoric acid is by far the most widely used of the conventional acid catalysts for olefin polymerization. It has been employed commercially in three different forms: ( I ) a catalyst consisting of a thin film of phosphoric acid supported on fine quartz sand; ( 2 ) a calcined composite of phosphoric acid and kieselguhr; and (3) a copper pyrophosphate composition which
1470
(28).
Low temperature polymerization refers to reactions carried out between about 0' and -100' 6.with metal halide or FriedelCrafts type catalysts. Many catalysts have been investigated, including boron trifluoride, aluminum chloride, aluminum bromide, titanium tetrachloride, stannic chloride, boron trichloride, ferric chloride, etc., of which boron trifluoride and aluminum
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 45, No. 7
. chloride are the most important. A variety of olefins has been successfully polymerized, but the reaction of most interest is the polymerization of isobutene for the production of polymers of high molecular weight ranging from viscous oils to rubbery-type polymers. A large number of excellent papers on this subject have been written (8-10,iW-i5, dS-d5,69).
Reaction Mechanisms
x
-
The proposed mechanisms for acid-catalyzed polymerization have been reviewed in detail by Schmerling and Ipatieff (16),and t h e mechanisms proposed for polymerization with Friedel-Crafts catalysts have been reviewed by Heiligmann (15); hence, a complete treatment of t h e subject is not presented here. However, a brief review is given t o serve as a basis for subsequent discussion. The most widely accepted mechanism is the carbonium ion mechanism postulated by Whitmore (SO). According to this mechanism, the reaction proceeds through an intermediate carbonium ion formed in the presence of a hydrogen acid by the addition of a proton to the pi electrons of the double bond:
CH3 H3C:e: :CH2
Q
*
CHI
+ H X --+H3C:C:CH3 + X+
(1)
The existence of a free alkyl carbonium ion in the 3ame sense as an ion in aqueous solution has been seriously questioned, and many investigators prefer to consider t h a t the carbonium ion is never separated any great distance from the anion of the catalyst, but the two exist as an ion pair. Some experimental evidence of the existence of such ion pairs has been obtained ( i - 3 ) . Others prefer to consider an even lesser degree of charge separation, thinking of the intermediate in terms of a polarized molecule. For the purpose of explaining the structure of the products formed, the degree of charge separation is of no importance; i t is necessary only to postulate the existence of a positive charge of greater or lesser magnitude localized on a particular carbon atom. I n subsequent discussion the term “carbonium ion” is used, and the customary representation, as in Equation 1, is employed without showing the anion, although its presence somewhere near the positive charge is t o be understood. Silica-alumina type catalysts also contain acidic hydrogen, and KazanskiI and Rozengart (19j have postulated t h a t they are capable of forming carbonium ions, as indicated above, by the addition of a proton. The mechanism of the formation of the ionic intermediate in the case of the Friedel-Crafts type catalysts is less clear-cut. These materials in themselves are acidic only in the more general sense of being electron acceptors. It is not possible for these substances in the absence of a promoter t o form a carbonium ion by means of the addition of a proton, and it has been suggested by Hunter and Yohe ( 1 6 ) that the active complex is formed in this case by the direct addition of the catalyst t o the olefin, with the pi electrons of the double bond entering and completing the octet shell around the metal atom and leaving a carbon atom with only six electrons, as, for example: :Cl:
H R
+ BF3 + HzO
R-CH=CHz
R-CH-CHI
-+
+ BFIOH-
(3)
Because boron fluoride and aluminum bromide are the most active of all the Friedel-Crafts catalysts for the polymerization of isobutene, and yet in the absence of a promoter are incapable of catalyzing the polymerization, it now appears unlikely that Reaction 2 occurs t o any appreciable extent, or if it does occur the resulting complex cannot have any appreciable activity for initiating polymerization. It is probable, therefore, that in all cases the effective catalysts for acid-catalyzed polymerization of olefins are acids in the more restricted sense that they must be able t o supply a proton to initiate the reaction. Of the olefins of low molecular weight, isobutene is polymerized the most readily with acid catalysts, n-butene and propene are polymerized less readily, while ethene is polymerized only with difficulty. From this, and inferences drawn from the composition of the reaction products, Whitmore and others of the early workers in this field reached the empirical conclusion that a tertiary carbonium ion is the most stable, a secondary ion less stable, and a primary ion very unstable. Evans and Polanyi (IO)calculated proton affinities of the doubly bonded carbons in isobutene, propene, and ethene, Their calculations showed the proton affinity of the primary carbon of the double bond t o be in the order isobutene > propene > ethene; and furthermore, that the proton affinity of the primary carbon of the double bond is greater than t h a t of a secondary or tertiary carbon of the double bond for the cases of propene and isobutene. These calculations confirm the early empirical observations. I n accordance with this concept, we should expect the following carbonium ions from isobutene and propene: CH3
9H3 CH3-dH=CH2
+ Hf
1
CH3-C-CH3
--L
\f CHI-
+H
CH1-CH=CH2
:Cl:H R
However. i t has been reported recently (9, 93,25) that boron trifluoride, titanium tetrachloride, and stannic chloride cannot catalyze the polymerization of isobutene in the absence of promoters or cocatalysts. Effective cocatalysts are materials such as water, tertbutyl alcohol. acetic acid, etc., all of which contain active hydrogen. Fontana and Kidder (13) also reported that aluminum bromide will not catalyze the polymerization of propene in the absence of a promoter. They found that hydrobromic acid greatly accelerated the reaction and that the rate of
July 1953
reaction was approximately proportional to the concentration of the promoter. Norrish and Russell ($3) found t h a t in the polymerization of isobutene with stannic chloride a cocatalyst such as water was necessary, that the rate of reaction was proportional t o the concentration of the cocatalyst, and that the maximum rate was achieved when the water and stannic chloride were present in equimolal proportions. Experimental evidence of the production of carbonium ions by the interaction of boron trifluoride monohydrate with 1,I-diphenylethylene was obtained by Evans and Hamann ( 7 ) . It is probable, therefore, that the reaction proceeds through an intermediate formed from the reaction of the olefin, metal halide, and promoter, with the latter serving as the source of the proton necessary for the production of the carbonium ion:
+
\
(Probable)
(4)
-CH?+
I
H
CH3-CH-CH3 CHa-CH2-CH2
(Probable) ( 5 ) +
The possibility of the formation of minor quantities of the two less favored isomers cannot be excluded, particularly a t high temperatures, but the species indicated should be the predominant ones. Whitmore has postulated that a carbonium ion once formed may undergo a variety of reactions:
1. Addition of a negative ion, X, with the net result being the addition of H X to the double bond. R-CH-CHs
+
+ X- S R-CH-CHa
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
(6)
X 1471
2. Addition to the double bond of a n olefin to form a new carbonium ion. This is the polymerization step. The reverse reaction which may also occur results in depolymerization. RC
+ R’-CH=CH,
R’-CH-CH>R +
4. bIigration of a proton with its bonding electrons from another carbon to the atom deficient in electrons, leaving a new carbonium ion. CH3 I
e CH3-C-CH2-CH3
CH3-&%-CH-CH3
(9)
5 . Migration of a methyl group with its two electrons to the positive carbon atom, generating a nen- carbonium ion with a new skeletal arrangement. CHsCH3 I I CH3-C-CH-CH-CH3
e CH3-C-CH-CH-CH3 -
I
(10)
CHI 6. Extraction of hydrogen from another molecule t o form a saturated molecule and a new carbonium ion.
RH
+ Ri+
R+
+ RiH
l o w Temperature Polymerization
(7)
3. Elimination of a proton t o form the original olefin or a different one.
CH3
olefins is not covered, however, b e c a u e this has recently been rcviewed in detail by Schmerling and Ipatieff (Z6).
(11)
8 1 1 the above reactions are indicated to be reversible, but the forward and reverse reactions may not occur a t similar rates. Usually one will be favored clearly over the other.
The polymer produced in acid-catalyzed reactions can vai y in molecular weight over wide extremes, depending on the conditions employed. With isobutene, for example, it is possible to produce a polymer which is predominantly the dimer or to produce solid polymers with molecular weights in excess of 100,000, The principal factors affecting molecular weight of the polymer appear to be catalyst activity, reaction temperature, and structure of the monomer. Plesch and coworkers ( 2 5 ) reported data obtained i,v Fairbrother and Seymour, who polymerized isobutene a t -80” C. with a variety of catalysts and found that the molecular weight of the polymer decreased as the activity of the catalyst decreased. No quantitative scale for determining relative catalyst activity was described, but the catalysts can be arranged qualitatively in order of activity in terms of concentration of catalyst required, t h e 1 eaction time, and the monomer conversion obtained. These data are shown belov,-: Catalyst B FI AlBrt
TiClr
TiBra BCIB BBrs SnClr
One of the most pronounced characteristics of acid-catalyzed reactions involving carbonium ions is the lack of specificity. Almost never is a single product formed and normally even under the most mild conditions a large number of compounds are formed. Ipatieff and Pines ( 1 7 ) have distinguished two classes of polymerization. The “true” polymerization in which the reaction products are mono-olefins with molecular weights integral multiples of the monomer molecular weight, and “conjunct” polymerization in which the reaction product is a complex mixture of olefins, diolefins, paraffins, naphthenes, cyclo-olefins, and aromatics. I t is here proposed t h a t a third designation, “heteropolymerization,” be applied t o those cases in which the product is composed only of mono-olefins but in which the molecular weights of the polymer molecules are no longer integral multiples of the monomer molecular weight. Even in the case of so-called true polymerization, the numbei and complexity of reactions occurling may be very great. For example, in the polymerization of nbutene over a phosphoric acid catalyst under relatively mild conditions, a total of 15 isomers n ere identified in the hydrogenated dimer fraction (4). I t is indicated, therefore, that not only ale the many reactions of carbonium ions listed above possible, but the rates of the various reactions are of similar orders of magnitude, so that in a n y given polymerization reaction all may contribute more or less significantly. At the present time i t is possible, with the aid of the carbonium ion theory, t o explain satisfactorily the product distribution observed, but it is largely impossible t o make specific predictions for a new case. Although the possible reactions which may occur are known, their relative rates and the factors affecting their relative rates are not well understood. The subsequent discussion reviews some of the available data on the effect of reaction variables on the polymer product. The extensive work of Whitmore and others on the structures of the various isomeis formed during the true polymerization of low molecular n eight 1472
Reaction Time Seconds 1-5 min. 20-70 min. 12-18 hours 12-18 hours 12-18 hours 17-50 hours
0.05 0.05 0.25 1.5 1.5 1.0 2.5
AIonomer Conversion,
Molecular
io-90
100
I?O-l50 120-150
35-50
100-130
%
30-50 0.5-1.5 0.5-1.5 10-18
Weight 10-1
x
70-90 30-50
20-30 12-25
The mechanism proposed for the reaction using titanium tctmchloride as a typical example is:
-
+ HzO TiCI40H2+ M --+ TiCI4
Factors Affecting Character of Polymer Product
Catalyst Concentration
MnH+ M,H+
+ TiClaOH+ TiChOH-
4
+
TiC140H*
MH’
+ TiCl4OH-]
+
Initiation
(13)
3f7A TiC140H2 M,,HOH
+ Ticla
(12)
Termination
(16)
(17)
M represents a monomer molecule. Equation 16 is the moit likely termination reaction. Equation 17 is unlikely in view of the findings by Evans and Meadows (a), that in the polymerization of isobutene with boron fluoride as many as 2000 polymer molecules are formed per molecule of water present. Plesch and coworkers noted that the order of catalyst activity appears t o be the same as the order of relative acidity of the catalysts. This is consistent with the above mechanism and t h e observed rates of reaction. The over-all rate of reaction ip higher for the more active catalysts, indicating either that the total number of active molecules existing a t any time is greater or that the activated molecule formed is more reactive. This latter possibility can be interpreted in ternis of the assumed carbonium ion intermediate as an increase in the degree of charge separation with a consequent increase in the reactivity, A more acidic catalyst would be favorable t o both possibilities, so that a grcater over-all rate of reaction would be expected from the more acidic catalyst. The molecular weight of the polymer is fixed by the relative rates of the propagation and termination reactions. As the most acidic catalysts would form the weakest conjugate base? after giving up a proton, the termination reaction (Equation 16) might be slower for the more active catalysts, and this, togethei with the higher rates of the propagation reaction, would account for the high molecular weight polymer obtained with the more actlve catalysts. D a t a obtained by Pepper ( 2 4 ) on t h r polymerization of styiene with stannic chloride showed that the 1ate of reaction increased
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 7
as the dielectric constant of the solvent was increased, indicating an ionic mechanism. Furthermore, the molecular weight of the polymer increased as the dielectric constant of the solvent increased, which is consistent with the concept t h a t the molecular weight of the polymer increases with the stability of the charged intermediate.
.
Figure 1. Effect of Reaction Temperature on Molecular Weight of Polymer Data of Thomas and coworkers ( 2 9 )
a
I n the low temperature polymerization it appears that once an ionic intermediate is formed it remains as an entity until the polymer is completely formed-that is, the carbonium ion does not dissociate and reform repeatedly. Evidence for this is the fact that the molecular weight of the polymer formed is very high, even a t monomer conversions of only 1%. Also, the fact reported by Thomas and coworkers (69) that the presence of di- and triisobutene in the reaction mix slows the reaction and reduces the molecular weight indicates that these olefins as such are not intermediates in the reaction. The effect of temperature on molecular weight of the polymer may be illustrated by thb data of Thomas and coworkers (gg), which are shown in Figure 1, where the molecular weights of polymers prepared from isobutene are plotted against reaction temperature. These data indicate that the stability of the ionic intermediate decreases with increasing temperature. As the temperature rises, the life of the ionic intermediate is shortened, with the result that the molecular weight decreases. This dependence of molecular weight on reaction temperature is usual for low temperature acid-catalyzed polymerization. The structure of the monomer may influence the molecular weight of the polymer through steric hindrance. Even in the case of isobutene if a head-to-tail polymer of the type
8 '
c c- -c-c-c-c
x
c I
h d t !
(18)
is formed, there is some steric interaction between the methyl side chains. When a more bulky monomer such as diisobutene is used, the steric hindrance apparently becomes very great, as evidenced by the data of Evans and-Polanyi (11), who found that the polymerization of diisobutene with boron fluoride resulted only in the dimerization to tetraisobutene. The molecular weight of the polymer formed from isobutene with a boron fluoride catalyst is greater than that from n-butene. This is illustrated by the data of Thomas and coworkers (69) in Figure 2, where the molecular weight of the polymer is shown as a function of the n-butene content of the feed. The observed deJuly 1953
crease in molecular weight might be explained on the basis of the concept that high molecular weight polymer results from the most stable ionic intermediate, because Evans and Polyani's proton affinity calculations indicate that the tertiary carbonium ion formed from isobutene would be considerably more stable than the secondary ion produced from n-butene. No such simple picture as that given above is adequate to explain all the data, however. Fontana, Kidder, and Herold ( 1 4 ) showed that with aluminum bromide catalyst at high promoter levels the molecular weight of 1-butene polymer went through a maximum with decreasing reaction temperature, in contrast t o the steady increase in molecular weight obtained with isobutene. These results are explained on the basis of an intermolecular hydride ion transfer of a tertiary hydrogen to the carbonium ion, resulting in the formation of multiply-charged carbonium ions with two or more sites upon which polymer growth may occur. The maximum in the molecular weight-temperature relationship would result from the hydride transfer reaction becoming less important a t lower temperatures. Fontana and coworkers ( 1 4 ) showed t h a t with the aluminum bromide catalyst at high promoter levels, polymer of 1-butene had a higher molecular weight than the corresponding isobutene polymer, while at low promoter levels the converse was true. They also showed that while high promoter levels result in high molecular weight propene polymer with aluminum bromide catalyst, the opposite is the case with aluminum chloride catalyst. No unified theory correlating polymer molecular weight with all the reaction variables has as yet been advanced.
40,0001 0
I
I
10
5
I I5
I
20
X r(-BUTENES IN ISOBUTENE Figure 2.
Effect of n-Butenes on Molecular Weight of Polyisobutene
BFa catalyst, - 9 5 ' C .
Data o f Thomas and coworkers ( 2 9 )
Although the evidence is not conclusive, an analysis by Dainton and Sutherland ( 5 )of the infrared spectra of low temperature isobutene polymer indicated that the structure was that shown above for the simple head-to-tail reaction, from which it may be inferred that changes in carbon skeleton, which would result from the proton and methyl group shifts postulated for carbonium ions, do not occur to any appreciable extent. I n general, therefore, the low temperature polymerization reactions are characterized by relative simplicity. Conventional Acid Catalysis
I n the case of polymerization carried out with the conventional acid catalysts, such as phosphoric acid and sulfuric acid, and also for polymerization with the Friedel-Crafts catalysts a t elevated temperatures, the reactions become much more complex. I n the temperature range in which these reactions are normally carried
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1473
out, 0 " to 300" C., the rates of the various possible reactions of carbonium ions indicated in Equations 6 to 11become more nearly equal, with the result that a much wider variety of products is formed. These relative rates vary considerably with reaction conditions, so that the type of product can frequently be changed greatly with moderate changes in reaction conditions.
VOLUME % OVERHEAD
Figure 3. Effect of Reaction Temperature on Composition of Propene Polymers Made with Phosphoric Acid Catalyst
As indicated earlier, Ipatieff and Pines have distinguished beh c e n "tiue" polymerization and the "conjunct" polymerization which occurs under drastic conditions and which results in the formation of paraffins, cycloparaffins, diolefins, aiomatics, etc. It is frequently not realized, homevrr, that under relatively mild conditions where the polymer product is completely olefinic in character the reaction may still be very complex, and the product contain much material other than true polymer. Reactions leading to olefin products other than true polymer will be referred to as heteropolymerization. The evtent to which heteropolymerization occurs and some of the factors affccting it ale illustrated bel0W.
The true boiling point distillation curves of polymers prepared by the polymerization of propene a t 150" and 205" C. over a phosphoric acid-on-quartz catalyst are shown in Figure 3. The low molecular weight of the polymer is consistent n i t h the generalization made in connection with the Friedel-Crafts type catalysts, that higher temperatures and less active catalysts result in low molecular weight polymer. The effect of increasing temperature in decreasing the molecular weight of the polymer is still directionally the same, but the magnitude of the effect is small. Of particular interest is the difference in homogeneity of the two polymers. That prepared a t 150' C. contains principally true polymer, although significant quantities of heteropolymer also exist. I n the case of the polymer prepared a t 205" C., the material is predominantly heteropolymer with only minor quantities of true polymer. Despite the large quantity of heteropolynirr produced, conjunct polymerization in the normal sense is not occurring, since the material is still essentially completely olefinic. The formation of this heteropolymer probably occurs through
CII,
CHd CH-C-CH,-CH,-CH-CH,
1474
CHd
a reaction analogous to depolymerization. I t was indicated in Equation 7 t h a t the addition of a monomer molecule to a carbonium ion is reversible. If a hydrogen or methyl shift occurs prior to the reverse reaction, however, the original monomer molecule will no longer be produced. This may be illustrated by Reactions 19, 20, and 21. Rupture occurs a t the carbon-carbon bond beta t o the positively charged carbon and has been termed beta-scission by Whitmore (SI). The molecules with four and five carbon atoms resulting from Reaction 21 can then react' furt'her with the propene nionomer t o form the heteropolymer products. As can be seen in Figure 3, the relative import,ance of Reaction 21 increases rapidly with increasing reaction temperature. This is even more strikingly illustrated in the case of t'he polymerization of diisobutene with 7 i % sulfuric acid reported by Ipatieff and Pines (28). They carried out this reaction a t 0"and 55" C., and distillation curves of the polymer products are shown in Figure 4. At the low temperature only true polymerization occurred, whereas a t 55 O C. the product was almost completely heteropolymer. 4 rather surprising fact, is the critical dependence of the degree of heteropolynierization on reaction pressure in polymerization over a phosphoric acid catalyst. I n Figure 5 are shown distillation curves of propene polymers prepared a t 205" C. and a t 250, 500, and 7800 pounds per square inch gage. The extent of heteropolymerization is reduced rapidly as the pressure is iricreased and is largely eliminated a t 1800 pounds per square iiicbh gage. This phenomenon may be explained if it is assumed that the beta-scission reaction indicated by Equation 21 will not occur to any appreciable extent unless another catalyst site is available t o accept the other fragment formed. The postulated reaction scheme is: >In c e1LC (22)
+
M,bc+ M 31nC
+c
+
;\I,+lC
(23)
M'C f M"C
(24)
--f
11 is the monomer, M, is a polymer of n units, XI' and 11" are olefin components whose molecular weights are not int'cgral multiples of the monomer molecular weight, C is a vacant catalyst site, and MnC is the ionic catalyst-olefin complex-i.e., the carbonium ion intermediate. Reaction 22 is assumed to be very rapid arid in equilibrium, 7%-hileReactions 23 and 24 are the slon- rate-controlling reactions. Bn increase in reaction pressure would increase the concentration of the reactants and decrease the concentration of unoccupied catalyst sites. This would have the effect of increasing the rate of Reaction 23, which results in true polymerization and of decreasing t'he rate of Reaction 24, n-hic-h results in heteropolymerization. This mechanism also offers a plausible explanation for the increase of heteropolymerizat,ion with increasing temperature. Thc extent of activated adsorption represented by Equation 22 would be expected t o decrease with increasing temperature, so that other factors being equal, the concentration of vacant catalyet sites would increase with increasing temperature with a resu1t:tnt increase in the rate of het'eropolymerizat'ion. The almost complet,e absence of heteropolymerization observed a t 1800 pounds per square inch gage would indicate that a t t h f . pressure the available catalj,st sites are almost completely occupied, This does not mean that every molecule of acid or even
4 CHa + &H=CH--CH2-CH~
(21
INDUSTRIAL AND ENGINEERING CHEMISTRY
but that several are required in order t o stabilize the carbonium ion through solvation.
Vol. 45, No. 7
Further increase in reaction temperature with the phosphoric acid catalyst t o about 300" C. results in conjunct polymerization in which saturates, polyolefins, and cyclic compounds are formed. The conjunct-type polymerization occurs with all the acid catalysts if sufficiently drastic conditions are used. Ipatieff and Pines (18) have shown that with a sulfuric acid catalyst the extent of conjunct polymerization increases with increasing temperature, acid concentration, and ratio of acid t o olefin. A typical mechanism postulated for the formation of paraffins and diolefins is: R+
+ R'-CHZ-CHZ-CH=CH~ R'-CHz-CH-CH=CHz +
+ RH
J . +
the extreme sensitiveness of acid-catalyzed polymerization to reaction conditions. Failure t o explain this diversity in the behavior of different catalysts is one of the most serious flaws in carbonium ion theory. The customary representation of a carbonium ion carries with it the implication of a n independent existence and of properties independent of the medium in which it exists and of the acidic material providing the proton. Obviously this independence does not exist. The charged intermediate or carbonium ion formed
(25)
+ H+
R'-CH=CH-CH=CHz
=
O
r
r
?
-
T
-
n
and for the formation of cyclic structures:
*
5OOPSIG
+&
2
R-CHZ-CHZ-CHZ-CHZCH=CHZ -P RIH R-CH-CH~-CHZ-CH~-CH=CHZ
+
(26)
I50
I
.1 R-CH
/CH2-CHz \
CHz
>H,-cH
/
30 0
20
40
60
80
100
VOLUME % OVERHEAD
These mechanisms do not postulate the necessity of a second active catalyst site, as proposed above for the beta-scission reaction, but it is very likely that one is involved in some fashion. Supporting this idea are the data of Ipatieff and Pines (18), which show that the extent of conjunct polymerization with a sulfuric acid catalyst increases rapidly as the ratio of acid to olefin is increased.
Figure 5. Effect of Reaction Pressure on Composition of Propene Polymers Made with Phosphoric Acid Catalyst
with one acidic material may have properties and characteristics much different from that formed with another acidic substance. It was stated earlier that in order to explain the structure of the reaction products an exact description of the carbonium ion was not necessary and that only the existence of a localized positive charge need be postulated. However, the differences in the character of the polymerization reaction resulting from changes in catalyst and reaction conditions cannot be explained with such a simple concept. A more thorough understanding of the nature of the ionic intermediate and of the role of the catalyst in these reactions will be necessary before a complete elucidation of the field of acid-catalyzed polymerization will be possible.
Experimental I 0
20
I
I
I
40
60
BO
I 100
WEIGHT % OVERHEAD
Figure 4. Effect of Reaction Temperature on Composition of Diisobutene Polymer Made with 77% Sulfuric Acid Data of lpatieff and Pines ( 1 8 )
The relative importance of the many possible reactions of the carbonium ion intermediate and the relative response to changes in reaction conditions vary with the catalyst. With phosphoric acid, for example, there is a rather gradual transition from true polymerization occurring a t temperatures around 100 O C. to conjunct polymerization at temperatures above about 300' C. With sulfuric acid the temperature region separating these extremes is smaller, and with hydrofluoric acid no case of true polymerization has been reported, and only conjunct-type polymerization occurs. Kuhn (20) reports that with anhydrous hydrogen fluoride, the effect of temperature on the nature of the propene polymerization reaction is especially sharp. I n raising the temperature from -5' t o +loo C . the reaction changes sharply from one of predominantly isopropyl fluoride formation to one of almost entirely conjunct polymerization, again illustrating July 1953
The propene polymerization experiments with the phosphoric acid catalyst were carried out as follows: The catalyst was phosphoric acid on uartz (21) prepared by soaking crushed quartz with 75% phos8oric acid and draining off excess acid. A charge of catalyst was placed in a stainless steel tube, which was surrounded with a boiling liquid bath for temperature control. The acid was concentrated t o polymerizing strength by e uilibrating with 16-mm. water vapor pressure a t 205' C. A Ca?raction containing 35 mole % propene and 65 mole %propane was preheated to reaction temperature and passed continuously downflow over the catalyst. The reaction mixture was fed to a distillation column which was operatedcontinuously, where the propane and unreacted pro ene were taken overhead and the liquid polymer product was witEdrawn as bottoms. The distillations shown in Figures 3 and 5 were distillations of this whole polymer product. As the reaction pressure was increased from experiment t o experiment, the feed rate was also increased proportionally, so that the contact time in all the experiments was approximately constant a t about 3 minutes. Under these conditions the conversion of propene t o polymer was about 80% in each case. Dew point calculations indicated that the reaction mixture was completely vaporized in all the experiments. The duration of the runs varied from a few t o several hundred hours. Only minor changes in composition of the polymer as a function of time on stream were observed.
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literature Cited Bentley, A , and Evans, A. G., J . Chem. SOC.,1952,3468. (2) Bentley, A,, and Evans, A. G., Research, 5,575 (1952). (3) Bentley, A., Evans, A. G., and Halpern, J., T r a n s . Faraday Soc., 47,711 (1951). (4) California Research Corp., unpublished data. (5) Dainton, F. S., and Sutherland, G. B. B. M., J . Polymer Sei., 4, 37 (1949). (6) Egloff, G., and Weinert, P. C., W o r l d Petroleum Congr., PTOC. 3rd Congr., Hague, 1951, Section IV, 201. (7) Evans, A. G., and Hamann, S.D., J . A p p l . Chem., 1 , 2 4 0 (1951). (8) Evans, A. G., and hleadows, G. R., J . Polymer Sei., 4, 359 (1949). (9) Evans, A. G., and Meadows, G. W., T r a n s . Faraday Soc., 46, 327 (1950). (10) Evans, A. G., and Polanyi, M., J . Chem. SOC.,1947, 252. (11) Evans, A. G., and Polanyi, M., A-ature, 152, 738 (1943). (12) Fontana, C. M., Herold, R. J., Kinney, E. J., and RIiller, R. C., IND. ENQ.CHEM.,44, 2955 (1952). (13) Fontana, C. M., and Kidder, G. A , , J . Am. Chem. Soc., 70, 3745 (1948). (14) Fontana, C. M., Kidder, G. A., and Herold, R. J., IND.EXG. CHEM.,44, 1688 (1952). (15) Heiligmann, R. G., J . Polymer Sei., 4, 183 (1949). (16) Hunter, W. H., and Yohe, R. V., J . Am. Chem. Soc., 55, 1248 (1933). (1)
(17) Ipatieff, V. N., and Pines, H., IND. EXG.CHEM.,28, 684 (1936). (18) Ipatieff, V. N., and Pines, H., J . Org. Chem., 1, 465 (1936). (19) Kazanskii, B. A,, and Rozengart, SI. I., J . Gen. Chem. U.S.S.R., 13, 304 (1943); tr. by J. G. Tolpin, .VatZ. Petroleum News. 36, R-643 (1944). (20) Kuhn, C. S., Jr., U. S.Patent 2,400,520 (1946). (21) Langlois, G. E., and Walkey, J. E., W o r l d Petroleunz Congr., Proc. 3rd Congr., Hague, 1951, Section IV, 191. (22) McAllister, 5.H., Oil a n d Gas J., 36, KO.26, 139 (1937). (23) Sorrish, R. G. W., and Russell, K. E., T r a n s . F a r a d a y Soc , 48, 91 (1952). (24) Pepper, D. C., Ibid., 45, 397 (1949). (25) Plesch, P. H.. Polanyi, M.,and Skinner, H. A.. J . Chem. Soc.. 1947,257. (26) Schmerling, L., and Ipatieff, V. N., “Advances in Catalysis,” Vol. 11,pp. 21-80, S e w Pork Academic Press, Inc., 1950. (27) Steffens, J. H., Zimmerman, M. U., and Laituri, M, J., Chem. Eng. Progr., 45, 269 (1949). ( 2 8 ) Thomas, C. L., IND. EXQ.CHEY.,37, 543 (1945). (29) Thomas, R. M., Sparks, W. J., Frolich, P. K., Otto, AI,, and Mueller-Cunradi, &I., J . Am. Chem. Soc., 62,276 (1940). (30) Whitmore, F. C., IND. ESQ. CHEM., 26, 94 (1934). (31) Whitmore, F. C., and Llosher, W. A . , J . Am. Chem. Soc., 68,281 (1946). RECEIVED for review December 4, 19.52.
ACCEPTED May 14, lY5S
Oxides of the Transition Metals as Catalysts ALFRED CLARK Research Division, Phillips Pefroleum
Co., Barflesville, Okla.
The purpose of this paper i s to characterize in a general way the behavior of transition metal oxides in catalytic reactions in the light of current theories of solid catalysts. The catalytic properties of these oxides span those of the transition metals and of the silica-alumina type. Evidence from hydrogen-deuterium exchange measurements indi-
cotes that hydrogenation-dehydrogenation reactions over these oxides are associated with the presence of excess metal ion in the oxide structure. In many instances, the same catalysts will also promote “acid” type reactions such as isomerization and polymerization, indicating a bifunctional catalyst surface.
HIS paper has as its object a general characterization of the
genation and dehydrogenation reactions, another group of solid catalysts, broadly referred to as insulators, tend to catalyze positive ion or “acid” type reactions, such as polymerization, alkylation, cracking, and isomerization. These are the oxides and halides of the lower elements of groups 3, 4, and 5 of the peiiodic table. Of the oxides, silica-alumina is the most important. According to recent work, the catalytic activity of silica-alumina is associated with protons located on the c a t a l p t surface. Dowden (6) postulates that the active centers in these catalysts are protons originating from the residual water which theye catalysts always contain and located a t cation vacancies in the oxide lattice. Electron transfer again is considered to be an important step in the catalytic action, but the mechanism differs considerably in detail from that of transition metal catalysis. Thus, there are two types of solid catalysts-the transition metals and the solid oxide insulators-which can be differentiated a t least qualitatively with reasonable clarity on the basis of electronic mechanism and types of reactions vhich they catalyze. Figure 1, in which rate of reaction is plotted against R-eight per cent silica in silica-alumina catalysts for various reactionshydrogen transfer, propylene polymerization, ethylene hydrogenation, and ,hydrogen-deuterium exchange-readily illustrates the differences. Hydrogen transfer and propylene polymerization are definitely acid-type reactions, usually associated with a positive ion mechanism whose reaction velocity constants are
T
behavior of txansition metal oxides in catalytic reactions in the light of current theories of solid catalysts. The catalytic properties of these oxides appear t o span those of the transition metals and the “irreducible” oxides of the lower elements of groups 3 , 4 , and 5 of the periodic table ( 1 5 ) . The transition metals, as far as they have been studied, appear to have a special significance with respect to catalysis. According t o recent theories, the catalytic activity of these metals is associated with their partially empty d-bands. Couper and Eley ( 6 ) ,for example, working with palladium-gold alloys, have shown that as the d-band holes in palladium are filled by the svalence electron of gold, significant changes take place in activity for para-hydrogen conversion and electrical resistance. Dowden and Reynolds ( 7 ) present comparable information, using nickeliron and nickel-copper alloys, on the rate of hydrogenation of styrene and decomposition of hydrogen peroxide, formic acid, and methanol. With active catalysts, a transfer of electrons is believed t o take place from the adsorbed species to the partially filled d-band of the metal atoms on the surface. The bond between the metal and adsorbate may be largely covalent. These and other current investigations are doing much toward focusing attention on “electronic” interpretations of catalytic mechanisms a t the expense of purely geometric interpretations. While the transition metals predominantly catalyze hydro1476
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