Alkylation of lsobutane with light Olefins Using Sulfuric Acid Reaction Mechanism and Comparison with HF Alkylation Lyle F. Albright and K. W. Li' Purdue University, Lafayette, Ind. 47907
A reaction mechanism i s proposed for the alkylation of isobutane with light olefins such as Cq olefins and propylene, which i s considerably different from the one widely accepted for many years. Several previously unexplained features of sulfuric acid alkylations such as startup phenomena, secondary reactions, role of red oil (or conjunct polymers) in the acid, and the similarity of alkylates produced from all Cq olefins are explained. Cations in the red oil dissolved in the acid play important roles in alkylations. The character and perhaps the mechanism of alkylation depend to a significant extent on whether sulfuric acid or HF is used as the catalyst.
MECHANISM for the alkylation of isobutane (2-methylpropane! with light olefins in the presence of a strong acid catalyst has generally been assumed to follow the basic reaction steps listed below (Schmerling, 1945, 1955, 1964). 2-Butene is the olefin used to show the basic steps for the production of the main product by this mechanism. Initiation Steps to Produce tert-Butyl Cation. T H E
CHXH=CHCH3 + H- (from acid)CH3CH*+CHCH3 CH3CH2- CHCH, + (CH,),CH + CH3CH2CHzCH3 + (CH3)3C + Schmerling (1946) suggested that isobutane is produced rather than n-butane in the last reaction. Apparently he believed that the sec-butyl cation isomerized directly to a tert-butyl cation before hydride transfer. There is no known experimental evidence to support such a simple isomerization, however. Chain Mechanism to Form Trimethylpentanes.
(CH,), C-
+ CH?CH=CHCH,+ CH, CH,
I
I
CHY-C-CH-CH-CH3
+
l
I
+
+ (CH3)3CH
CHr-C-CH-CH-CH3
I
-+
CH3 CH3 CH, CH
,-
I I
I
C-CH-CHz-CH,
+ (CH3)3 Cs
CH3
The 2,2,3-trimethylpentyl ions can isomerize to form other trimethylpentyl ions, which by hydride transfer from isobutane can produce other trimethylpentanes and regenerate the tert-butyl cation to continue the chain. More details on this mechanism were presented by Schmerling (1945, 1955, 1964) to explain the formation of light ends (C, to C7isoparaffins), dimethylhexanes (even when 2-butene and isobutylene are used as olefins), and heavy ends (C, and heavier isoparaffins). Although this mechanism gives considerable information about the reactions, several features of alkylation are not adequately explained-for example, the recent results of Shlegeris and Albright (1969) and Li et al. (1970a,b) relative to secondary reactions and the importance of red oil (or conjunct polymers) in the acid phase. Inadequacies of Past Alkylation Mechanisms
CH3 'Present address, Continental Oil Co., Ponca City, Okla.
CH3 CH,
74601
The reaction mechanisms for alkylation depend to a considerable extent on the acid catalyst used. The mechanism for alkylations with HF is considered below, but the Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 3, 1970 447
present discussion is limited primarily to alkylations using sulfuric acid as the catalyst. Production of Trimethylpentanes. The main method for production of trimethylpentanes has been predicted to be reactions involving one mole of isobutane and one of a C4 olefin; tert-butyl and trimethylpentyl cations are intermediates. Extensive information obtained by Shlegeris and Albright (1969) and Li et al. (1970b) indicate that considerable amounts of trimethylpentanes are often not formed by this method. Instead, they are often produced in significant amounts by secondary reactions which occur after all of the olefin has reacted. Organic materials dissolved in the acid are obviously reacting in some manner with isobutane. More than 5070 of the trimethylpentanes were produced by such secondary reactions in some experimental runs. In addition, trimethylpentanes can be formed in a t least small amounts by contact of just the olefin and sulfuric acid (Li et al., 1970b; Shlegeris and Albright, 1969). Den0 et al. (1964) made a similar observation, but did not report an analysis of their hydrocarbon product. Considerable amounts of acid-soluble hydrocarbons are also formed by reactions between the olefin and sulfuric acid. The results of Hofmann and Schriesheim (1962b), who used tagged C14 olefins in their alkylation study, further indicate that the formation of trimethylpentanes is often more complicated than indicated by any previously suggested mechanism. When isobutylene was used as the olefin, fewer C14atoms were found in the trimethylpentane fractions than would be predicted if isobutane and isobutylene reacted on an equal molar basis. The C14 atoms accumulated instead more in the dimethylhexanes, light ends, and heavy ends. Dimethylhexane Production. Schmerling (1945, 1955, 1964) has suggested that the major method for the production of dimethylhexanes includes the following reaction step:
(CH,) C+ + CH,=CHCH,CH,-
This dimethylhexyl ion can isomerize to other dimethylhexyl ions, which then form dimethylhexanes, after hydride transfer with isobutane. The similarity of alkylates produced from either 1-butene or 2-butene is postulated, since these olefins quickly isomerize in strong acids to form the same equilibrium mixture of 1-butene and 2-butene. As indicated earlier, reactions between the tert-butyl cation and 2-butene lead to the formation of trimethylpentanes. If the above method for forming dimethylhexanes is of major importance, the relative ratios of trimethylpentanes to dimethylhexanes should often be essentially constant. Li et al. (1970a,b) found, however, that these ratios in alkylates vary widely from about 1 to 1 a t low octane numbers to a t least 15 to 1 at high octane numbers. Additional evidence that dimethylhexanes and trimethylpentanes are produced by very different methods was obtained as a result of a series of runs to investigate secondary reactions (Li et al., 1970b; Shlegeris and Albright, 1969). Dimethylhexanes were formed to a greater 448
Ind. Eng. Chern. Process Des. Develop., Vol. 9,No. 3, 1970
extent by primary reactions than trimethylpentanes in these experiments. Since primary reactions tend to involve olefins to a greater extent and isobutane to a lesser extent than is true for secondary reactions, dimethylhexane production is a t least sometimes affected to a greater extent by olefin reactions. When isobutylene is used as the olefin, production of dimethylhexanes has never been well explained by mechanisms involving the reactions of isobutylene and a tertbutyl cation. Zimmerman et al. (1962) suggested that trimethylpentyl cations isomerize to some extent to dimethylhexyl cations, but did not present direct experimental evidence to support this hypothesis. The finding of Li et al. (1970a) that the ratios of trimethylpentanes to dimethylhexanes vary widely implies, however, that the postulate is not correct. Hofmann and Schriesheim (1962a) postulated that when isobutylene is used as the olefin, dimethylhexanes are formed by a mechanism involving the allylic cation formed from isobutylene. They presumably believe that when n-butenes are used, dimethylhexanes are produced by a mechanism involving the reaction between tert-butyl cations and 1-butene. The results of Li et al. (1970a) now strongly imply that the mechanism and intermediates for producing dimethylhexanes are similar if not essentially identical, regardless of the C4 olefin used. A similar conclusion seems justified for production of trimethylpentanes. Such conclusions differ significantly from earlier postulates. Production of Light and Heavy Ends. The experimental results of Li et al. (1970a,b), Shlegeris and Albright (19691, and Hofmann and Schriesheim (1962a,b) indicate that light ends and especially heavy ends are sometimes produced to a greater extent from C4 olefins than from isobutane. In addition, Hofmann and Schriesheim concluded, on the basis of experiments with C14-tagged olefins, that the light ends are formed from similar intermediate groups. Li et al. made a similar conclusion, since they found that the compositions of light ends in the alkylate were almost identical, regardless of the quality of the alkylate or the specific C4 olefin used. Schmerling (1945, 1955, 1964) had indicated that overall reaction for the production of light ends by destructive alkylation involved 2 moles of isobutane and 1 mole of olefin. Hofmann and Schriesheim (1962a,b) confirmed the over-all stoichiometry of the reaction when sulfuric acid is used. It is not clear how common intermediate(s1 would be produced from the three C4 olefins by destructive alkylation. Roles of Acid in Alkylation. Schmerling (1964) indicated that the acid serves as a source of protons and that the negative ion produced from the acid plays “an essential, if lesser, role in the reaction.” This later role was, however, not specified. Hofmann and Schriesheim (1962a1, however, suggested that the acid and the red oil in the acid may also be involved in the hydride steps which are occurring. Although their arguments seem to be logical, there is no evidence that their postulate was widely accepted. Based on available information, several features still need to be explained, such as the ability of the acid to promote hydrogenation steps. The reactions between olefins and strong acids (Li et al., 1970b) clearly indicate that hydrogenation and dehydrogenation steps are occurring, presumably by the transfer of protons and hydride ions. The formation of the saturated hydrocarbons from
olefins means, of course, that hydrogen at.oms were obtained from some source. There is no evidence that the acid was permanently decomposed to serve as such a source. Rather, the acid-soluble hydrocarbons produced were apparently highly unsaturated. The role of these acid-soluble hydrocarbons in secondary reactions also needs to be explained. Such hydrocarbons not only affect the physical properties of the acid (Albright, 1966a,b) but also react in some cases to form alkylate (Li et al., 1970b; Hofmann and Schriesheim, 1962a). Although red oil concentrations in the acid have been known for some time (Albright, 1966a; Hofmann and Schriesheim, 1962b; Mosby and Albright, 1966) to affect the quality of alkylate produced, explanations of this effect have, a t best, been partial. Some investigators have, in fact, ignored the role of the red oil altogether in proposing reaction mechanisms. The results of Li et al. (1970b) now indicate that the red oil affects both the physical (character of emulsion, solubility, viscosity, etc.) and the chemical phenomena of the over-all process. The fact that the compositions of various families, such as the trimethylpentane family, vary significantly with changes in acid strength strongly suggests that chemical factors are affected by the red oil. The red oil constituents in the acid are known to increase as the acid is used for alkylation. An induction or startup phenomenon has been reported on several occasions until some red oil was present. During these initial periods, alkylates of poor quality are produced. Only relatively limited information is available on the chemical compositions of these acid-soluble hydrocarbons. These hydrocarbons can be separated from alkylation acids by water-dilution techniques, and the separated hydrocarbons were analyzed by Miron and Lee (1963). Perhaps the hydrocarbons change to some extent, however, when they are removed from the acid. I t has not been reported if the red oil is the same as the acid-soluble hydrocarbons involved in hydrogenation-dehydrogenation steps or in the secondary reactions. Hofmann and Schriesheim (1962a) report that cyclization of the 2,5-dimethylhexenyl cation is the first step in the formation of red oil. Den0 (1964) also has shown steps for the formation of various cyclopentenyl ions which seem to be likely precursors for red oil. Red oil is known to be ionized to a considerable extent. The sulfuric acid acts to a t least some extent as an oxidizing agent to form allylic ions by reactions such as the following (Leftin and Hobson, 1963):
-CH =CH--CHZ-
+ 4H2S04 -+ --C
-+ --- C Z C -
I
H 2H30-
l
H
l
+
H
+ SO? + 3HS04-
Kramer (1967a,b) has shown that saturated hydrocarbons produced during alkylation are sometimes oxidized with sulfuric acid to produce carbonium ions. Oxidation reactions obviously occur in the dissolved red oil, and the cations formed are presumably involved in the alkylation reactions. Sulfur dioxide remains t o a considerable extent dissolved in used alkylation acids. Initiation Phenomena. Light paraffins such as n-butane and propane would be expected to be formed by the
initiation reactions according to previous mechanisms when 2-butene (or 1-butene) and propylene, respectively, are used as olefins. Industrial units do not, however, produce measurable quantities, a t least in most cases, when sulfuric acid is employed as the catalyst. Furthermore, the results of Li et al. (1970a,b) did not indicate any such production. I t can be argued that the amount of these light paraffins produced is small, since the chain lengths for the main reaction are very long. When H F is used, however, as a catalyst for alkylations using propylene, up to 10 to 15% of the propylene is often converted to propane. Obviously, H F and sulfuric acid have some rather different behavior patterns which have never been really clarified. Although n-butane is definitely not produced in large quantities when 1-butene or 2-butene is used, Hofmann and Schriesheim (1962b) found that isobutane was formed in experiments with C14-tagged 1-butene. Whether this production is related to the initiation steps is not reported, but the results indicate that the reactions are more complex than had previously been postulated. Experimental information to support the initiation mechanism proposed earlier is hence lacking for alkylations when sulfuric acid is the catalyst. The startup results of Li et al. (1970b) and Hofmann and Schriesheim (1962a) further indicate that the initiation steps may involve red oil hydrocarbons. Importance of Physical Phenomena. Previous investigators who have postulated reaction mechanisms have underestimated the importance of mass transfer steps and the effect of the operating variables on physical phenomena associated with alkylation. Such features have been discussed thoroughly by Albright (1966a) and Li et al. (1970a,b). Consequently, previous investigators have overemphasized the chemical features in trying to explain the alkylation reaction. I t is now clear that the quality and the composition of the alkylate are controlled largely by the dissolved ratios of isobutane to olefin in the acid phase. Mass transfer considerations have a major effect on such a ratio. Changes of the chemical mechanism directly caused by temperature and the specific C, olefin used are of lesser importance-for example, the large changes in alkylate quality and composition which result for a given reaction system as 2-butene is substituted for isobutylene can be explained to a considerable extent at least by differences in the rates of solution of these two olefins in the acid phase. Such a change in rates affects the dissolved ratio of reactants in the acid. Proposed Alkylation Mechanism with Sulfuric Acid Catalyst
The mechanism proposed for alkylation uses sulfuric acid as the catalyst. Before outlining the numerous steps of the mechanism, the types of ions present in the alkylation acid are discussed. Such an acid contains red oil (or conjunct polymers). Tertiary (or secondary) cations are present in the red oil fraction. Nonconjugated double bonds occur, and a proton would add to each such bond to form a cation. The red oil is known to be highly branched. If a secondary ion should form first, isomerization would probably occur quickly to form a tertiary ion. Such a cation is designated as
R,+. Allylic cations are also present in the red oil fraction. Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 3, 1970 449
+
-CHZCICH-C-
I I
Conjugated double bonds are common in the red oil hydrocarbons which have been separated from the acid, but no two bonds are present in the same ring (Miron and Lee, 1963). When the red oil is dissolved in the acid, allylic cations such as the one shown above would occur, designated as R2'. Protons would be present throughout the acid phase. These cations are involved in the reaction sequences which occur in the acid phase or probably to a considerable extent primarily at the interface between the acid and organic phases. The number of reactions which occur are probably far greater than previously postulated because of the several types of ions present. At least three types of reactions occur with olefins. The first involves the addition of a proton to the olefin to form an alkyl cation. Such a step has been proposed in previous mechanisms. Olefins also react with the red oil cations (Rlf and possibly R 2 ' ) to form larger red oil cations. I n the case of isobutylene, the following reaction occurs:
CH3 RI'
+ CH,=C
I
CH3 2 R1-CH2-C
CH,
I
+
I
CH,
Such a tertiary ion, designated as RQ: is probably very stable, but the above reaction is reversible to a t least some extent. When 2-butene is used as the olefin, the addition reaction with red oil cations is
CH3 R1-
+ CHiCH =CH-CH3
I
+
R1-CH-CH-CH3
The above secondary ion can isomerize by first a methide shift to produce another secondary ion and then by a hydride shift to form R?:
This isomerization is probably almost irreversible, since tertiary ions are more stable than secondary ions. If 1-butene is used, it probably first isomerizes rapidly to form 2-butene, which then reacts as shown above to form Rs-. The addition reaction of 1-butene directly with red oil ions results in relatively unstable secondary ions, which probably cannot easily isomerize to a stable tertiary ion. Formation of RQ- ions explains why Hofmann and Schriesheim (196213) noted that C14-tagged isobutane was formed when C14-tagged 1-butene was used. The RS- ion formed from 1-butene releases isobutylene, and part of this isobutylene is converted to isobutane by the addition of first a proton and then a hydride ion. Production of Trimethylpentanes. Trimethylpentanes are produced by several relatively different sequences, or series of reactions. 450
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 3, 1970
1. The first sequence for production of trimethylpentanes is probably of major importance when C4 olefins are used, regardless of the quality of the alkylate produced. A considerable portion of the Cq olefins reacts first to form Rs' ions. Such an intermediate ion is formed in considerable amounts because of the high rate of solution of the olefins in the acid phase. When isobutane enters the acid phase, it reacts there with red oil ions, such as R l + , to form tert-butyl ions:
R1-
+ (CHB)3CH 2 RiH + (CH3)sC-
Such a reaction is, of course, reversible. In the meantime, the R3+ ion slowly decomposes to free some isobutylene. This isobutylene then quickly reacts with the available tert-butyl ions to form trimethylpentyl cations, which can isomerize. Trimethylpentanes are then formed by hydride ion transfer. It is thought that the hydride ions are transferred from suitable red oil groups (RH):
Such transfer seems most logical, since it is known that red oil contains considerable branching and hence must contain some tertiary C-H bonds. Although the above reaction is reversible, the isooctane is very insoluble in the acid phase and hence will quickly transfer to the hydrocarbon phase. Hydride transfer from isobutane to the C8H17+ ions probably is in most cases of secondary importance. 2. The second sequence has been postulated on many occasions in the past, but is of lesser importance than the first. The tert-butyl cation in the acid phase reacts with isobutylene or 2-butene as the latter transfers from the hydrocarbon phase to the acid phase. A trimethylpentyl cation is formed by such a reaction and can then isomerize; hydride transfer then produces the trimethylpentanes. In this sequence, 1-butene is first isomerized to 2-butene. In the over-all reaction scheme, trimethylpentanes tend to be produced by this sequence a t earlier stages of the reaction than by the first sequence. The trimethylpentanes produced from 2-butene by the second sequence are thought to contain a smaller fraction of 2,2,4-trimethylpentane than are produced from isobutylene. A relatively stable 2,2,4-trimethylpentyl cation is formed when isobutylene reacts with the tert-butyl cation, whereas a 2,2,3-trirnethylpentyl cation is initially formed when 2-butene reacts. A significant fraction of this latter ion isomerizes to 2,3,3- and 2,3,4-trimethylpentyl cations (Mosby and Albright, 1966). 3. The third sequence is of importance when the dissolved ratio of isobutane to C 4 olefins in the acid phase is low. In this case, the trimethylpentanes are formed primarily, if not completely, from olefins and red oil. The red oil can be produced from olefins if necessary. Isobutylene is the common intermediate involved in this sequence, regardless of the olefin used. When n-butenes are used as the olefins, isobutylene is released from R3+ cations. A portion of the isobutylene combines with protons to form tert-butyl ions, which then react with released isobutylene to form trimethylpentyl ions, which eventually form trimethylpentanes. This sequence is thought to be of somewhat more importance when isobutylene is used as the olefin as compared to the other two C4 olefins. Such a postulate seems obvious first because the isobutylene feed can react immediately by this sequence. When
n-butenes are used as olefins, several reaction steps are required, however, before isobutylene forms. Second, isobutylene has a much more rapid rate of solution than the other two olefins (Davis, 1928; Davis and Schuler, 1930), and, as a result, the dissolved ratios of isobutane to olefin tend to be lower when isobutylene is used. 4. The fourth sequence (self-alkylation of isobutane) is always of some importance when isobutane is alkylated. The acid and red oil ions dehydrogenate part of the isobutane to produce some isobutylene in situ. This isobutylene then reacts with tert-butyl ions, as outlined above, t o form trimethylpentanes. Hofmann (1964a) demonstrated that self-alkylation becomes more important with increasing molecular weight and branching of the olefin used during alkylation. Self-alkylation is probably always the major sequence for production of trimethylpentanes when propylene and amylenes are the olefins used. Secondary reactions, which occur later in the reaction sequence and have been noted by Shlegeris and Albright (1969) and Li et al. (1970b), are explained by the first sequence for producing trimethylpentanes. The first sequence also explains why the composition of the trimethylpentane family is similar, regardless of the olefin used. Common red oil cations, such as RQ: are formed by all three C4 olefins, and reactions between isobutylene and tert-butyl ions then result. The small differences noted for the trimethylpentane composition can be explained by differences in the relative importance of the several reaction sequences. Furthermore, the trimethylpentanes formed are subject to degradation reactions. Such reactions obviously have a more pronounced effect on trimethylpentanes formed during the initial stages of the reaction. Trimethylpentanes in this category include those produced by the second sequence and those produced from isobutylene by the third sequence. Composition changes of the trimethylpentane family for n-butene alkylates as the quality increases above 96 octane number indicate the change in the relative importance of the first and second sequences for the production of trimethylpentanes. The degree of agitation is high whenever the octane number is large, and consequently, the transfer rate of reactants, and especially isobutane, to the acid phase is increased. As a result, the concentrations of the tert-butyl cations are higher at such conditions. Consequently, reactions of 2-butene via the second sequence are of increased importance. The composition changes of the trimethylpentane family support such a conclusion, except for the somewhat decreased importance of 2,2,3-trimethylpentane. The probable reason for this decrease is the instability of 2,2,3-trimethylpentyl cations andfor 2,2,3-trimethylpentane. Such an explanation seems reasonable, since the second sequence for production of trimethylpentanes occurs relatively early in the over-all reaction scheme; hence, considerable time is available for the isomerization steps to occur. Additional information is still obviously needed relative to the decomposition or degradation of all trimethylpentanes a t alkylation conditions. Such information would help clarify the mechanism. Other sequences of steps may also be of some importance for production of trimethylpentanes. One such possibility may involve the reaction between a R3+ cation and a C4 olefin to form a cation that has a Cg side group attached to the red oil. If this side group should split off, it might eventually form a trimethylpentane molecule. Still another possibility
is that some trimethylpentanes may be produced via thc light ends mechanisms discussed below. The importance of red oil in the sulfuric acid is explaineci by the above reaction schemes. Red oil promotes thr rapid production of tert-butyl ions when isobutane dissolves in the acid. In addition, increased amounts of red oi allow faster rates of hydride ion transfer to form the final alkylate product. Furthermore, higher amounts o red oil produce more red oil ions (RI' or R,') whicb can react to form R 3 + . More red oil is desired in the acid when low quality alkylate is produced (Li et al 1970b). In such a case, the dissolved ratio of isobutanc to olefin in the acid phase is low. More red oil is henct needed, so that an increased amount of the olefin initiallj reacts to form R3' rather than form undesired heav! ends. Production of Dimethylhexanes. Dimethylhexanes arc formed mainly by reactions involving isobutylene ever when n-butenes are used as feed olefins. Much of the isobutylene is released from R3+. The basic mechanislr for dimethylhexane formation is similar to the one outlinec by Hofmann and Schriesheim (1962a). In the first step an allylic ion is produced from isobutylene. They postu lated that an alkyl cation, such as a trimethylpentyl cation reacts with isobutylene to form the allylic cation:
I
CH, The allylic cation is probably formed, however, to i greater extent by the reaction between a red oil catior (Ri+ or R2-) and isobutylene. The allylic ion formec then reacts with another isobutylene molecule as follows
+
C HY
C H F - - C - C H 2+ CHz=A
I
-+
I
CH3 CH~= C-CH,-CH2-C-CH+
I
CH, This latter ion, which can isomerize, can then be saturate by hydride ion and proton transfer to form dimethylhexan molecules. By the revised mechanism proposed here, mor than one mole of dimethylhexane could be produced pe mole of trimethylpentane. The proposed reaction scheme for production c dimethylhexanes explains all known experimental resulti including those of Li et al. (1970a,b). When the dissolved ratio of isobutane to olefin in t h acid phase is high, most of the isobutylene (formed i: situ in the acid phase) reacts with readily available t e n butyl cations to form trimethylpentanes instead ( dimethylhexanes. At low dissolved ratios, however, higher fraction of the isobutylene reacts to forr dimethylhexanes, since fewer tert-butyl ions are availablc When the red oil concentration of acid is increasec lower amounts of dimethylhexanes are produced, sinc Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 3, 1970 45
higher concentrations of tert-butyl ions would be present, because of the reactions between red oil ions and isobutane. Consequently, the isobutylene that is released tends to react to an increased extent to form trimethylpentanes. When isobutylene is used as the olefin instead of n-butenes, a somewhat lower dissolved ratio of tert-butyl ions to isobutylene would be expected and hence more dimethylhexanes would be formed, as was experimentally observed. If 1-butene reacted in significant amounts with tertbutyl cations, as has been widely accepted in the past, to form dimethylhexyl cations which then yield dimethylhexanes, such an occurrence would be most probable for n-butene alkylations at high rates of agitation. At such conditions, both the concentration of tert-butyl cations and the amounts of 1-butene present in the acid phase or at the acid interface would be high. The dimethylhexane content, however, actually decreased at high rates of agitation. Of interest, the amounts of trimethylpentanes produced from 2-butene and tert-butyl cations (by the second reaction sequence) increased with increased degrees of agitation. I t seems safe to conclude that most dimethylhexanes are not produced by the reaction of 1-butene and fert-butyl cations. Production of Light and Heavy Ends. The production scheme for light ends may be more complicated than previously suggested, and an additional method is proposed. Two, or more likely three, C4olefins such as indicated in the following equation add to a given red oil cation such as RI:
Ri+ + 3C4Hs-+ Ri-(C4HS)Z-C4Hs
+
Isomerization reactions occur which involve the ions produced or which convert 1-butene to 2-butene. As a result, the resulting red oil ions are essentially identical for all C4 olefins. The positive charge is located on a branch group, and this branch then enters into a complicated series of steps including at least: Reaction of the positive ion with a hydride ion to form an isoalkyl branch. Loss of proton to form an unsaturated side group. Subsequent gain or loss of hydride, methide, ethide, etc., ions or of protons from the side group. Such transfers sometimes involve adjacent red oil groups. As a result of the various transfer steps, the side group can change significantly in regard to the position of the positive charge on it and the skeleton arrangement. The large red oil cation eventually fragments a t a position p to the positive charge to form an isoalkyl ion or an olefin with the skeleton of a light end component. Transfer of hydride ions and protons results in the formation of the various isoparaffins in the light end fraction. Some isobutane is involved in the formation of light ends, and some tert-butyl cations formed from isobutane lose a hydride ion to produce isobutylene. The resulting isobutylene then enters the above reaction scheme. The small change in the composition of the light end fraction as the acid strength is varied, is probably caused by changes in the amount or rate of hydride, methide, .ethide, etc., ion transferred from adjacent red oil groups. Light ends probably are also produced by mechanisms proposed earlier by Schmerling (1945, 1955, 1964) and Hofmann and Schriesheim (1962a,b). The relative importance of those mechanisms and the one just proposed would likely depend primarily on the relative concentra452
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 3, 1970
tions of the tert-butyl cations and the red oil ions such as R1+and R2'. The tert-butyl cations are present in higher concentrations when high quality alkylate is being produced. Based on that reasoning, the mechanism of Hofmann and Schriesheim is of relatively greater importance for alkylates of high octane number. The various methods for production of light ends help explain why maximum amounts of light ends are formed in intermediate quality (88 to 90 O.N.) alkylates. Most heavy ends are produced primarily by reactions involving only olefins, at least when the dissolved ratio of isobutane to olefin in the acid phase is low. The same basic mechanism for formation of heavy ends is probably applicable when the dissolved ratio is high (and when high quality alkylate is produced). Polymerization reactions, some fragmentation of the polymeric ions formed, and transfer of hydride, methide, ethide, etc., ions and protons probably complete the reaction scheme for heavy end formation. The techniques for production of heavy ends and of light ends are sometimes competing for the olefins. When the alkylation system is operated so that very high local concentrations of olefins are in contact with the acid, production of heavy ends is high. Better dispersion of the olefins in the acid, such as by improved agitation, results in increased production of light ends. Such conclusions are based on the results of Li et al. (1970a), especially those in which an olefin was in direct contact with sulfuric acid. The results of Li et al. indicated also that at least one compound (2,2,5-trimethylhexane) in the CSand heavier products is formed to a considerable extent by the alkylation of isopentane with C4 olefins. Possibly other side alkylation reactions are occurring to form some CIO, CI1,etc., isoparaffins. Such reactions are probably of lesser importance, however, since isohexanes, isoheptanes, etc., probably have limited solubility in the acid phase. Future research is, however, recommended in regard to the chemical mechanisms involved in the production of both light and heavy ends. Analytical techniques to separate and identify the major compounds in the heavy ends need to be developed. Hydride Ion and Proton Transfer. Transfer steps involving hydride ions and protons are most important during alkylation. Numerous types of such steps are possible, and sources of hydride ions during the alkylation reaction include isobutane, olefin, red oil, and alkylate. Kramer (1965) has postulated that hydride transfer reactions sometimes occur across the acid-hydrocarbon interface. Such reactions with methylcyclopentane and methylcyclohexane were relatively slow, even though these compounds are known to be good hydride transfer agents. Higher rates of hydride transfer would be expected, however, when transfer occurs within the acid phase such as with red oil hydrocarbons. Some hydride ions are transferred from alkylate hydrocarbons, probably mainly across the acid-hydrocarbon interface to produce alkyl cations, and such a transfer is the first step in degradation reactions such as have been reported for the degradation of trimethylpentanes (Hofmann, 1962h; Kramer, 1967a). The resulting alkyl cations in contact with the acid isomerize, fragment, etc., to produce all major constituents found in alkylate. Some degradation reactions during secondary reactions were also noted by Li et al. (1970b).
Protons transfer from the acid to various unsaturated groups, including those of the olefin or the red oil hydrocarbons, to produce various types of hydrocarbon cations. These cations, including the tert-butyl cation, can also eject a proton to form an olefin. Such an ejection step is more probable as the reaction temperature increases. The increased amounts of dimethylhexanes produced with increased temperature (Li et al., 1970b) were probably caused by increased decomposition of the tert-butyl cation and of R3' to form more isobutylene in situ. In many alkylations, the olefins because of solubility considerations tend to ''dissolve'' in the acid from a mixture of isobutane and olefins at a faster rate than the isobutane. In such a case, the net flow of hydride ions may be considered to be from the red oil to the alkyl cations. As the isobutane enters the acid (or reaction medium) at a somewhat later stage of the reaction, the net flow is now from the isobutane to the red oil. The red oil, in a sense, acts as a reservoir for hydride ions with flows initially out and then later in as the alkylation progresses. There is also obviously some flow of hydride ions from one portion of the red oil to another portion during and after alkylation. Secondary Reactions and Formation of Red Oil. Additional information was obtained by Li et al. (1970b) relative to the formation of red oil. Under some conditions, secondary reactions not only minimized the permanent drop of acid strength but also formed trimethylpentanes. As indicated, R3+cations reacted in such cases with isobutane. If the acid phase was separated from the hydrocarbon phase, some information is available regarding the fate of R3+. At least a part of the loss of acid strength becomes permanent, indicating the buildup of red oil. It is postulated that R3+ slowly decomposes to release isobutylene. In this case, the isobutylene reacts to produce red oil and alkylate of rather low quality. Such an alkylate often slowly forms and separates from used alkylation acid, as noted in numerous cases. Clearly, there are advantages at least in some cases in maintaining intimate contact between the acid and hydrocarbon phases after the main reaction sequence is completed. Lower acid consumption and increased production of trimethylpentanes are sometimes realized. Hofmann and Schriesheim (1962a) found, for example, that a small amount of alkylate was obtained when isobutane was in contact with a used acid. Some reactions occur in the decanters for separating the hydrocarbon and acid phases. Recycling the used alkylation acid back to the main alkylation reactor may also be advantageous in promoting these desired reactions, if it is done quickly after the separation step (Li et al., 1970b). Long-term contact between alkylate and acid is, however, detrimental (Hofmann, 196413; Kramer, 1967a), and there is obviously an optimum time of contact for production of the best quality alkylate. The proposed mechanism explains the induction periods and startup phenomena which have been noted on frequent occasion in both commercial and laboratory alkylation units. Both red oil and red oil cations such as R3+ are necessary in order to obtain high quality alkylate. Apparently, the formation of each is relatively slow. In this regard, Kramer (1967a,b) was probably producing some red oil ions during the induction period noted when he investigated the oxidation of various hydrocarbons with sulfuric acid.
Alkylations with Propylene. The results obtained by Li
et al. (1970a) for alkylations with propylene further clarify the alkylation mechanism. In this case, it is obvious that the following reaction is of importance:
(CH3)BCA+ CH2=CH-CH3-+
CH, CH3--C-CHz-
I
+ CH--CHs
CH, These dimethylpentyl cations then isomerize and finally combine with hydride ions to produce dimethylpentanes. Two sequences (or series) of reaction steps undoubtedly occur for the production of dimethylpentanes. These sequences are similar to the first two discussed earlier for the production of trimethylpentanes when Cq olefins are used. The first technlque (related to the first discussed for trimethylpentanes) involves addition of propylene to a cation in the acid phase. Several types of cations probably are involved in this sequence. Propylene combines to at least some extent with protons to form propyl sulfates (Goldsby, 1966), and this technique is now being used commercially. Propylene may also add to red oil cations such as R1+ or R2+to form larger red oil cations. The propyl sulfate and the larger red oil cations then slowly decompose to release the propylene, which reacts with tert-butyl cations. Higher temperatures, such as 20" to 40°C,are required to obtain fairly fast kinetics for these decomposition reactions. The second sequence of reactions involves the reaction between the tert-butyl cation and propylene as the latter transfers from the hydrocarbon to the acid phase. This sequence is the one generally accepted for many years for the production of dimethylpentanes. I t may he relatively more important in propylene alkylations than the comparable sequence for butene alkylations, because of the considerably slower rates of absorption of propylene in sulfuric acid as compared to the rates for the C4 olefins (Davis, 1928; Davis and Schuler, 1930). The production of trimethylpentanes in propylene alkylation involves self-alkylation of isobutane, as explained above. Isobutylene obtained by dehydrogenation of isobutane is involved in the production of both dimethylhexanes and methylhexanes. The allylic ion mechanism discussed earlier explains the production of dimethylhexanes. The same allylic ion reacts with propylene to form the skeleton of methylhexanes; hydride and proton transfer produces the final methylhexanes. Production of the other light ends (C, and CS isoparaffins) and of heavy ends in propylene alkylation is probably by similar mechanisms as compared to butene alkylations. The above mechanisms explain the several families of compounds reported by Li et al. (1970a). Mechanism of Alkylation When HF Is Catalyst
An effort was made to determine if the alkylation mechanism varied significantly depending on the acid catalyst used. Data for HF alkylation are very limited, but available data were compared for HF and sulfuric acid alkylations. Some composition data for alkylates produced using HF and 1-butene, 2-butene, or isobutylene (Albright, 1966b) were compared to those of alkylates of the same Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 3; 1970
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quality produced using sulfuric acid. These composition data for H F alkylates differed significantly in all cases with the extensive graphs or tables prepared by Li et al. (1970a) for sulfuric acid alkylates. Specific differences were noted. When isobutylene is used as the olefin, H F alkylates contain considerably more trimethylpentanes, especially 2,2,4-trimethylpentane, but less light ends than sulfuric acid alkylates. When 1-butene is used, H F alkylates contain extremely high amounts of dimethylhexanes, especially 2,3-dimethylhexane, and lower amounts of trimethylpentanes than sulfuric acid alkylates. Alkylates produced using H F and 2-butene show some similarity to those alkylates produced using sulfuric acid, but the compositions of the trimethylpentane family are significantly different. The composition and quality of alkylates produced from 1-butene and 2-butene are almost identical for comparable runs when sulfuric acid is used, but are very different when H F is used. H F alkylates produced from 1-butene have considerably higher amounts of dimethylhexanes, less trimethylpentanes, and a much lower octane number than H F alkylates produced from 2-butene. The compositions of the Cs fraction in alkylates produced using H F as the catalyst are relatively identical regardless of the Cq olefin used (Kennedy, 1958). Although similar compositions are noted in alkylates produced using sulfuric acid, the compositions differ drastically when the alkylates of the two acids are compared. Differences in the alkylates produced using either sulfuric acid or H F are undoubtedly caused in part a t least by significantly different physical properties of the two acids. Physical properties known to have important effects on the character of the emulsion include viscosity, interfacial tensions between acid and hydrocarbon phases, and density. For sulfuric acid alkylations, the dissolved ratio of isobutane to olefin in the acid phase is extremely important relative to the quality to the alkylate produced (Albright, 1966a; Li et al., 1970a). Presumably, a similar conclusion is valid for H F alkylations, but the factors affecting such a dissolved ratio may vary rather significantly depending on the acid used. Since red oil (or conjunct polymers) is also produced in HF, reaction steps involving the red oil ions, hydride transfer from the red oil, etc., likely occur to at least some extent when H F is used. Extensive data such as were obtained by Li et al. (1970a,b) for sulfuric acid alkylation are, however, needed for H F alkylations before a firm conclusion can be made relative to the mechanism when H F is used as the catalyst. Conclusions
The mechanism proposed for alkylation using sulfuric acid as the catalyst should be most helpful in explaining alkylation involving other olefins such as pentenes (or
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amylenes) and even heavier olefins. Insufficient data are available to propose a detailed mechanism when these olefins are used, but the importance of the red oil and red oil ions in alkylation seems obvious, regardless of the olefin used for alkylation. Acknowledgment
The financial assistance of the M. W. Kellogg Co. is gratefully acknowledged. The authors also appreciate helpful suggestions made by several personnel of that company. literature Cited
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RECEIVED for review June 9, 1969 ACCEPTED January 8, 1970 Division of Petroleum Chemistry, 158th Meeting, ACS, New Yo& N. Y.,September 1969.
