Ind. Eng. Chem. Res. 1997, 36, 3491-3497
3491
Role of Ester Intermediates in Isobutane Alkylation and Its Consequence for the Choice of Catalyst System Sven I. Hommeltoft,* Ole Ekelund, and John Zavilla Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark
Alkylation of isobutane by olefins catalyzed by Brønsted acids involves initial reaction of the olefin with the acid to form an ester intermediate. The esters formed by the reaction of olefins with triflic acid are very short-lived under typical alkylation reaction conditions but have been made by reaction of the olefin with frozen triflic acid and characterized by NMR spectroscopy. The ester intermediates are formed in an olefin-rich environment and reacts predominantly in an environment where the acid activity is high and the olefin concentration is low. This protects against oligomerization reactions, and the formation and stability of these ester intermediates in the absence of free acid, therefore, play an important role in the selectivity to the desired high-octane components typical of alkylate gasoline. The formation of relatively stable ester type intermediates represents a route to passivation of solid acid catalysts and thus favors a liquid acid catalyst for isobutane alkylation. A new fixed-bed alkylation technology using an SLP type catalyst has been developed for isobutane alkylation. It is a compromise between the wish for improved control of the acid catalyst and the chemistry of isobutane alkylation as it provides an effective control of an otherwise liquid acid catalyst without compromising the liquid phase chemistry. Results from bench-scale tests show product qualities comparable to the qualities obtained using the established technologies. 1. Introduction and Background Alkylate gasoline formed by the alkylation of isobutane with propene, butenes, and pentenes is produced in quantities of around 1 000 000 barrels/day in North America alone and is as such an important component in the North American gasoline pool. The alkylate is a highly valued blending component for today’s motor gasoline and for tomorrow’s reformulated gasoline. It has a high content of highly branched alkanes, such as trimethylpentanes, that gives it a high octane number. It has a low vapor pressure and a narrow distillation range and contains neither olefins nor aromatics. Furthermore, the alkylation unit plays an important role in a modern fluid catalytic cracking (FCC)-based refinery as it converts LPG-range byproducts from the production of FCC naphtha into alkylate. The reaction is acid catalyzed, and today’s processes are based on the use of liquid Brønsted acidsssulfuric acid or anhydrous HFsas catalysts. These catalysts have been used with success for more than 50 years (Lafferty and Stoled, 1971; Iverson and Smerling, 1958), but concerns have been raised regarding safety and health issues in connection with the handling of the very large quantities of liquid acids used in these processes. Much of the concern regarding safety would be eliminated if a solid catalyst in a fixed bed could be used for isobutane alkylation. For decades, a quest for such a solid catalyst to replace the liquid acids has been ongoing all over the world. In spite of this, no solid catalyst has been able to challenge the dominant role of the existing liquid-catalyzed processes based on HF or sulfuric acid in the field of isobutane alkylation. The development in isobutane alkylation research has been reviewed (Corma and Martı´nez, 1993). At least part of the explanation for the lack of commercial success of the true solid acid catalysts is the * Author to whom correspondence is addressed. e-mail:
[email protected]. S0888-5885(97)00028-6 CCC: $14.00
passivation of these catalysts. Catalyst passivation is a well-known phenomenon also for the liquid catalysts. Isobutane alkylation is accompanied by formation of a highly unsaturated acid-soluble oil (ASO) byproduct occasionally referred to as conjunct polymers or “red oil” (Miron and Lee, 1963; Albright et al., 1988a), which passivates the catalyst. One of the main advantages of the existing processes is that the passivated catalyst, a liquid, can be pumped out of the alkylation reactor for acid recovery or acid regeneration without interrupting the alkylation process. This operation is less simple for a solid catalyst submerged in liquid LPG under pressure, and restoration of catalyst activation by incineration of the solid catalyst will add to the process costs (Rostrup-Nielsen, 1995; Rao and Vatcha, 1996). Passivation of solid acid catalysts is often very fast even when very pure feedstocks are used under conditions that cause only little passivation of the liquid Brønsted acid catalyst. This might indicate that there are fundamental aspects of the chemistry of isobutane alkylation that are difficult to achieve on a solid catalyst with fixed acid sites. 2. Role of Esters in Isobutane Alkylation An important difference between a liquid phase catalyst and a true solid catalyst is the mobility of the acid sites. The importance of this is related to the mechanism of isobutane alkylation or, more precisely, the role of ester intermediates in the alkylation chemistry. It is well-known that esters can be formed and are likely intermediates in the sulfuric acid catalyzed alkylation of n-butenes (Shlegeris et al., 1969; Albright, 1977; Albright et al., 1977a,b; 1988b,c). We find this to be a common feature of a wide range of liquid acids capable of forming good quality alkylates including HF. Esters are also formed in triflic acid catalyzed alkylation by addition of the acid to the olefins. 2.1. Formation and Detection of Esters. Esters of triflic acid can be formed separately by reaction of © 1997 American Chemical Society
3492 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 1. 1H-NMR of Various Alkyl Triflates in CDCl3 (Spectras Are Recorded on Bru 1 ker AC200 and AC250 NMR Spectrometers) olefin
TfO-R
ethene
ethyl
propene
2-propyl
2-butene
2-butyl
1-butene
2-butyl
1-pentene
2-pentyl
isomer ratio
group
chemical shift, ppm
multiplicity
2.77
CH3 CH2 CH3 CH CH3 CH CH2 CH3 CH3 CH CH2 CH3 CH3 CH CH2 CH2 CH3 CH3 CH2 CH CH3 CH CH2 CH2 CH3 CH3 CH2 CH CH3
1.5 4.6 1.5 5.3 1.5 5.0 ∼1.8 1.0 1.5 5.0 ∼1.8 1.0 1.5 5.1 1.7 1.5 1.0 1.0 1.8 4.9 1.5 5.1 1.7 1.5 1.0 1.0 1.8 4.9 1.8
triplet quartet doublet septet doublet sextet multiplet triplet doublet sextet multiplet triplet doublet sextet multiplet multiplet triplet triplet doub.quart. quintet doublet sextet multiplet multiplet triplet triplet doub.quart. quintet singlet
3-pentyl 1.00 2-pentene
2-pentyl 1.05 3-pentyl 1.00
isobutene
tert-butyla
J, Hz 7.1 7.1 6.3 ∼6.3 6.3 ∼6.3 7.4 6.3 ∼6.3 7.4 6.3 6.3 7.3 7.5 7.5, 5.9 5.9 6.3 6.3 7.3 7.5 7.5, 5.9 5.9
a The 1H-NMR spectrum of tert-butyl triflate was acquired at -60 °C. The tert-butyl ester is thermally unstable and decomposes rapidly at temperatures above -30 °C. Even at -60 °C the preparation of the tert-butyl ester was accompanied by significant oligomerization of the isobutene.
Scheme 1. Reaction of Triflic Acid with Olefin
triflic acid with normal olefins (Scheme 1) at low temperatures in the presence of excess olefin. Because of the high reactivity of these esters in the presence of acid, this is conveniently done by adding a solution of the olefin to frozen triflic acid (mp: -40 °C) at -78 °C. The esters are formed selectively in accordance with Markovnikov’s rule (addition of the triflate group to the most substituted carbon atom). As long as the acid is frozen, the alkylation reaction is inhibited, and it is therefore possible to form the ester even in the presence of isoalkanes. Since the esters are much more soluble than the acid in nonpolar solvents such as alkanes or chloroform, the formation of the ester can be observed visually because of the disappearance of the separate acid phase. A list of 1H-NMR data for various triflate esters is shown in Table 1. The esters were prepared in CDCl3 at -62 °C by the method described above. Because of the high reactivity and consequently short lifetime of the esters under alkylation conditions, it is useful to be able to trap the ester intermediates that are present during the alkylation reaction. This is conveniently done by reacting the esters with a suitable reagent that reacts with the esters to form derivatives, which can be identified after the reaction. Alkylamines are suitable for this purpose. It is known that carboxylic acid esters react with amines, forming amides (March, 1992a). However, alkyl halides that can be perceived as esters of the hydrohalo acids form salts of alkylamines (March, 1992b). Triflic acid esters behave like the halides and alkylate the amine. In Scheme 2, the mentioned reactions are illustrated. n-Hexylamine is used in the experiments discussed in this paper, but other amines behave similarly. The
Scheme 2. Reactions of Amines with Esters of Carboxylic Acids, Alkyl Halides, and Esters of Triflic Acid
Table 2. MS Data of Alkylhexylamines compound mol wt, Hex-NH-R g/mol methyl ethyl
115 129
isopropyl
143
isobutyl
157
sec-butyl
157
tert-butyl
157
fragment ions and intensities, m/e (%) 115 (4), 44 (100), 30 (5) 129 (6), 128 (1), 114 (2), 58 (100), 44 (16), 30 (33) 143 (6), 142 (3), 128 (52), 72 (100), 44 (52), 30 (83) 157 (6), 156 (1), 114 (65), 86 (36), 44 (100), 30 (60) 157 (1), 156 (1), 142 (14), 128 (100), 86 (39), 44 (58), 30 (60) 157 (2), 142 (100), 86 (26), 58 (63), 30 (78)
alkylated amines were liberated from the salts by treatment with aqueous NaOH and identified by GCMS. GC reference samples of the alkylated amines were prepared separately by reaction of the corresponding alkyl iodides with n-hexylamine. MS data for various n-hexylalkylamines are shown in Table 2. Addition of the amine to an alkylation mixture results in an immediate quench of the isobutane alkylation reaction by fast reaction with the free acid removing the strong acid activity. This stabilizes the present esters sufficiently to allow them time to react with the amine. For the esters of the type TfOR where R ) Me, Et, i-Pr, sec-Bu, the alkyl group is transferred to the amine without rearrangement. Therefore, the formed alkylamine can be used as a probe for the presence of
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3493 Scheme 3. Mechanism of Butene Alkylation of Isobutane Catalyzed by Triflic Acid
ester intermediate in the alkylation reaction using n-butene or propene as olefin. 2.2. Reactivity of the Alkyl Triflates. The alkylation reaction in which the esters are converted into alkylate typically requires not only the presence of a suitable hydrocarbon substrate (an isoalkane) but also a high acid activity. In the absence of free acid the esters are relatively stable in an isoalkane solution. However, the esters slowly decompose thermally, thus liberating acid which catalyzes further reaction. Therefore, the reaction of the esters with isoalkanes is autocatalytic. Because of this, esters in an isoalkane solution must be stabilized by a reagent that can trap liberated acid. Excess n-olefin is suitable for this purpose, and solutions of sec-butyl triflate in isobutane are stable for hours when either 1-butene or 2-butene is present. The reactivity of the different triflate esters in the alkylation of isobutane varies considerably. Methyl triflate and ethyl triflate remain unreacted for hours in the presence of triflic acid and isobutane at 20 °C. sec-Butyl triflate reacts within seconds in a similar mixture at -40 °C in the presence of liquid acid. tertButyl ester differs from the sec-butyl ester in that it is considerably more reactive. In accordance with Olah and co-workers (Olah et al., 1996), we have found that even at temperatures below the freezing point of triflic acid the chemistry proceeds to C5+ products when isobutene is reacted in the presence of isobutane. Conceivably, this may proceed either through unstable tert-butyl esters or directly through traditional carbenium ion chemistry without any ester intermediates. Using the amine quench technique, it has been possible indirectly to show that, in spite of its low stability, tert-butyl triflate is present during alkylation of isobutane by isobutene containing olefin feeds. Quenching of the reaction mixture by an amine solution resulted in the formation of tert-butyl-substituted amines. Since the tert-butyl-substituted amines are not observed in the quench of an alkylation reaction when isobutene
is not present in the alkylation feed, their formation is taken as an indication of the presence of tert-butyl triflate in the reaction of isobutene. The triflic esters are soluble in isoalkanes, but they are also soluble in triflic acid. At equilibrium, when both phases are present, much of the esters will be found in the acid phase. By using the amine quench method, the distribution of methyl triflate in a mixture of triflic acid and isopentane in equilibrium at 25 °C was found to be 6.5:1 (1.43% (w/w) in the acid phase and 0.22% (w/w) in the isopentane phase). In the case of ethyl triflate, the distribution in a similar mixture of acid and isopentane was 4.5:1 (2.36% (w/w) in the acid phase and 0.52% (w/w) in the isopentane phase). The high reactivity of the isopropyl and sec-butyl esters prevented direct measurement of the equilibrium distribution for these esters at ambient conditions, but it is assumed that also these esters have a substantial solubility in the acid. The formation of the ester intermediates is not dependent on the presence of a separate liquid acid phase. However, considering the ionic nature of the carbenium ion chemistry associated with isobutane alkylation and the high solubility of the esters in the acid phase, it is reasonable to assume that the carbenium ion alkylation chemistry initiated by the reaction of the ester takes place predominantly in the acid phase and is inhibited by the absence of such a liquid phase. This is consistent with the fact that ester intermediates can be synthesized from n-butenes in isoalkane solvent in the presence of a separate triflic acid phase without alkylation of the isoalkane only under conditions where the acid is frozen. The important role of the esters in isobutane alkylation is illustrated by the mechanism proposed in Scheme 3, in which the key steps of the alkylation reaction mechanism involving the initial formation of esters are illustrated for the alkylation of isobutane by n-butenes using triflic acid as catalyst. Please note that the presented mechanism includes only the most important steps of relevance to understanding the importance of
3494 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
the ester intermediates. In particular, the carbenium ion part of the chemistry is highly complex, and a detailed discussion of this would be highly speculative and is beyond the scope of this paper. The reaction of triflic acid with n-butenes results in the rapid formation of sec-butyl triflate, presumably through initial protonation of the olefin to form a secondary carbenium ion (eq 1), which rapidly combines with the triflate anion to yield the sec-butyl ester (eq 2). The formation of sec-butyl esters shows that the rate of ester formation (eq 2) is much faster than the rate of structural isomerization of the sec-butyl carbenium ion to a tert-butylcarbenium ion. In the reaction of triflic acid with 1-pentene and 2-pentene, Table 1 shows that mixtures of 2- and 3-pentyl triflate are obtained in both cases but that the ratio between the esters depends on the olefin. When the reaction was performed at -78 °C in n-hexane instead of CDCl3, the reaction of triflic acid with 1-pentene resulted in predominantly 2-pentyl triflate as the ratio between 2- and 3-pentyl triflate was found to be 16:1 identified through the amine quench reaction. At similar conditions, 2-pentene reacts with triflic acid, resulting in a 1:1 mixture of 2-pentyl triflate and 3-pentyl triflate. This shows that the rate of ester formation from the carbenium ion and the triflate ion is faster than even the isomerization between the 2-pentylcarbenium ion and the 3-pentylcarbenium ion. The overall ester formation (eq 1 + eq 2) is exothermic and appears to have a low activation energy. Once formed, the ester is relatively stable in the absence of high acid activity. A plausible explanation for this is that in the subsequent step the ester reacts reversibly with acid to liberate the butylcarbenium ion, which can then enter into the actual alkylation chemistry (eq 3). When the acid activity is low, this equilibrium is shifted toward the ester, thus limiting further reaction. Once the butylcarbenium ion is liberated, it can react with another molecule of ester, forming a heavier carbenium ion while liberating the acid (eq 4). Alternatively, it is conceivable that the carbenium ion can react with the olefin present in low concentrations in equilibrium with the carbenium ions (eq 4a) as suggested by other authors (Albright et al., 1977b; Albright, 1977; Benazzi et al., 1996). However, there seems to be no direct evidence for a reversibility of the ester formation reaction that could give rise to a low olefin concentration. (Protonation of isobutene is reported to be irreversible in magic acid (Olah et al., 1993), but the reversibility of this reaction could be dependent on the acid strength and triflic acid is not as strong an acid as magic acid.) In either case, the product of this reaction (eq 4 or 4a) is an octylcarbenium ion, and the further reaction of this ion is decisive for the determination of the product quality. The octylcarbenium ion can abstract hydride from isobutane to yield high-quality alkylate which, when formed this way, consists of mostly trimethylpentanes (Albright et al., 1988c) (eq 5). (Conceivably, hydride sources such as other isoalkanes or acid soluble oil may replace isobutane in this reaction, but isobutane is the most important hydride donor.) Alternatively, the octylcarbenium ion can react with more olefin to yield heavier carbenium ions (eq 6), which is the first step toward a low-quality product. The addition of olefin appears to have a lower activation energy than the hydride transfer from isobutane, so the production of alkylate of the highest quality requires that the reaction
of the octylcarbenium ions takes place under conditions where the olefin concentration is as low as possible. Since a rapid ester formation keeps the acid activity low when the n-olefin concentration is high and since the alkylation chemistry forming the octylcarbenium ion requires a high acid activity, the alkylation chemistry takes place predominantly under conditions with low olefin concentration, favoring a trimethylpentane-rich product. These favorable conditions are found in the liquid acid phase, and as mentioned above the esters are highly soluble in the acid phase, making this phase the most likely environment for the carbenium ion chemistry. Once the heavy carbenium ion is formed (eq 6), it can react in a number of ways, all ultimately resulting in the formation of less desirable products. The heavy carbenium ions can abstract hydride from isobutane, just like the octylcarbenium ion, but hydride transfer to the heavy carbenium ions results in the formation of heavy alkylates (eq 7). However, the heavy carbenium ions can also undergo cracking reactions, yielding lighter products. One such cracking reaction is eq 8, which, in principle, is the reverse of eq 6 except that the bond breaking may take place at a different position in the hydrocarbon backbone. Reaction (8) first yields an olefin, which reenters the alkylation chemistry (eq 9), and a lighter carbenium ion, which through hydride transfer can form lighter isoalkanes including isopentane, isohexanes, and isoheptane (eq 10). Also isooctanes and heavier isoalkanes are formed this way, but the isooctane fraction formed through cracking reactions has much lower contents of the preferred trimethylpentanes than the products of eq 5. We find that formation of light-end alkanes is almost always accompanied by catalyst passivation. This can be explained by a different cracking reaction of the heavy carbenium ion which results in the direct formation of light-end alkanes, as shown in reaction (11). In this case, the cationic product from the cracking is an unsaturated carbenium ion which under the acidic conditions rearranges rapidly to an allylcarbenium ion. Once the proton is tied up in an allylcarbenium ion, the acidity is lowered considerably and the acid is passivated with respect to alkylation chemistry. This step initiates the formation of protonated acid-soluble oil (ASO) and illustrates that, though many measures can be taken to limit it, catalyst passivation cannot be completely avoided in isobutane alkylation even with the most ideal feed and at the most ideal operating conditions. All the products from the cracking reactions under typical alkylation conditions contain four or more carbon atoms in the carbon chain. There is no evidence for the formation of C3 or smaller fragments. 3. Consequence of Ester-Type Intermediates for Solid Acid Catalysts The formation of intermediates between the acid sites and the olefin feed offers a plausible explanation to the rapid passivation often observed when solid acids are used in isobutane alkylation. When a true solid acid catalyst with fixed acid sites is used as a catalyst, it is conceivable that the reaction between the olefin and the acid sites at first forms intermediates which like the ester intermediates known from the liquid Brønsted acids are relatively stable in the absence of excess acid. Quantum chemical studies
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3495
do indeed suggest that the surface alkoxides conceivably formed by reaction of olefins with Brønsted acid sites in solid acids such as zeolites are thermodynamically relatively stable (Kazansky et al., 1996; Engelhardt and Hall, 1995). The formation of ester-type intermediates requiring extra acidity for further reaction would make fixed acid sites prone to passivation by the olefins. Thus, when an acid site is converted into an “intermediate” by reaction with the olefin, an acid site nearby would be required for further reaction unless the intermediate could be decomposed thermally. Conversion of all acid sites in a given region of the catalyst would, therefore, passivate this region. Assuming that this is the case, the optimal conditions for minimizing the passivation would be to use a catalyst with high acid site density and to perform the alkylation under conditions where the olefin concentration is low everywhere around the catalyst. Observations from the application of zeolites as catalysts for isobutane alkylation do indeed show that “high acid site density is crucial for prolonging catalyst lifetime” (de Jong, et al., 1996) and that a stirred reactor drastically increases the lifetime of the solid alkylation catalyst relative to a fixed-bed reactor by keeping the olefin concentration low (Nowak, et al., 1996). The problem of acid site passivation by the olefin can conceivably be dealt with by choosing the catalyst formulation or the operating conditions in such a way that intermediates do either not form or are thermally labile. However, this would represent a change in the role of the intermediate and thus a potential change in the kinetics of the alkylation reaction. In this context, it is important to realize that though isobutane alkylation gives a mixture of products the product distribution, including the high yield of trimethylpentanes, is determined by the kinetics of the reaction and not by the thermodynamics. This is illustrated by thermodynamic considerations which show that trimethylpentanes constitute only a minor fraction of the isooctanes at equilibrium (Prosen et al., 1945). It is also illustrated by the effect of prolonged treatment of high-octane alkylate with a strong acid resulting in a drop in the concentration of trimethylpentanes and the formation of a wide range of mostly inferior products (Doshi and Albright, 1976). To be more specific, destabilization of the reaction intermediate would deemphasize its role in the selectivity to high-octane alkylate via reaction (5) and thus favor less desired products and increased catalyst passivation via reaction (6). A chemically more appealing solution to the passivation of the acid sites by the olefin is the addition of a mobile acid which can catalyze the further reaction of these intermediates with isobutane. The addition of strong Lewis acids such as BF3, SbF5, or AlCl3 to the solid acid catalyst is an approach that has been applied in several cases (Collins et al., 1996; Crossland, 1993; Corma and Martı´nez, 1993), and there is little doubt that this does improve the lifetime of the solid catalyst. However, it also introduces handling of a typically unpleasant volatile Lewis acid. 4. Alkylation in a Fixed Bed with an SLP Type Catalyst The above discussion indicates that in spite of the obvious safety advantages related to the use of a solid acid for isobutane alkylation there are important factors
Figure 1. Solid-supported liquid catalyst in a moving catalyst zone. Table 3. RON Data for Isobutane Alkylation (Hommeltoft, 1996) olefin
I/O (w/w)a
bath temp (°C)
RON (ASTM)
propylene 2-butene 1-butene 30% isobutene, 70% 2-butene 1-pentene 2-methyl-2-butene MTBE raffinateb C3-C4 mixed feedb C5 mixed feed, C6+ productb
12.3 9 9 9 7 7 9 13 14
0 0 0 -10 0 -10 -10 0 0
91.6 97.6 97.1 95.1 88.1 93.0 97c 94c 87
a I/O ) isobutane/olefin ratio (wt/wt). b Real industrial feed acquired from a refinery. c Estimated from GC data.
favoring the use of a liquid-phase catalyst for isobutane alkylation, and the liquid catalyst may be the optimal solution. However, this requires that the safety and health issues can be dealt with satisfactorily. The addition of a volatile Lewis acid improves the solid catalysts’ ability to cope with the ester formation, but the problem of passivation of the fixed acid sites by acid-soluble oil still remains and the handling of a volatile Lewis acid such as BF3 adds to the complexity of the process. Mobility of the acid sites can be achieved and passivation of the solid bed can be minimized by choosing an SLP-type catalyst of a kind that allows for the withdrawal of passivated catalyst. This is the case for a new fixed-bed alkylation (FBA) process based on the use of a supported liquid acid catalyst (Hommeltoft, 1996; Hommeltoft et al., 1996; Søgaard-Andersen et al., 1996). The new technology represents a compromise between the wish for a solid catalyst and the reality of isobutane alkylation chemistry that favors a liquid catalysts. In this process the catalyst is a liquid acid supported on a fixed bed of suitable support material in a well-defined catalyst zone, which takes up a fraction of the packed bed. The hydrocarbons move through the bed in plug flow, and the catalyst zone moves through the bed in the same
3496 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 2. Effect of isobutene in the alkylation of isobutane with butenes. Bench-scale data for alkylation with 1-butene in a mixture with isobutene containing a total of 10% (w/w) butenes in isobutane at 0 °C.
direction as the hydrocarbon flow but at a much lower velocity (see Figure 1). This ensures an efficient control of the contact between the hydrocarbon stream and the catalyst, including control of the contact time which is important particularly when using a liquid super acid such as triflic acid as catalyst. The mobility of the acid catalyst zone enables the withdrawal of passivated catalyst from the reactor without interruption of the alkylation process. The concept is applicable to a range of acid catalysts and support materials. Bench-scale data have been published for the alkylation of isobutane with various olefin feeds using triflic acid supported on silica gel support as catalyst (Hommeltoft, 1996). RON for typical feeds alkylated in the bench-scale reactor at lowtemperature conditions are shown in Table 3. Generally speaking, the reactor concept is capable of processing any olefin feed that can be processed with the existing commercial technologies based on HF or H2SO4, and the product qualities obtained are similar to what can be obtained in the industry today. This is consistent with the mechanistic similarities in the essential chemistry. Some evidence for the effect of the ester intermediate in triflic acid catalyzed alkylation can be seen by comparing the product from alkylation of isobutane using pure n-butenes with the product from a mixture of n-butenes using isobutene. As mentioned above, the stability of the tert-butyl esters formed by reaction of isobutene with triflic acid is lowsan observation which is also made in sulfuric acid catalyzed alkylation (Albright et al., 1988b). Since isobutene does not form stable esters, a decrease in C8 selectivity is expected when isobutene is mixed into the feed. As illustrated in Figure 2, which compares alkylation with pure 1-butene with alkylation using a mixture of 30% isobutene and 70% 1-butene, this is indeed what is observed. Incidentally, since tert-butyl fluorides can be formed in the reaction of isobutene with HF and are sufficiently stable to measure the vapor pressure at 12 °C (Grosse et al., 1940; Grosse and Linn, 1938), HF may differ from triflic acid and sulfuric acid when used as catalyst for alkylation with isobutene containing feeds. HF has
indeed been noted to yield better alkylates than sulfuric acid in alkylation with isobutene (Chapin et al., 1985; Corma and Martı´nez, 1993), and the higher stability of tert-butyl fluoride could be the explanation for this. 5. Conclusion The mechanism of isobutane alkylation catalyzed by liquid Brønsted acids involves the formation of ester intermediates that play an important role in the selectivity to high-octane alkylate. The formation of the esters and their relative stability in the absence of high acid activity ensure that the alkylation chemistry takes place in a region of the catalyst where the acid activity is high and the olefin concentration is low. There is evidence to suggest that when liquid Brønsted acids are used as catalyst, the desired alkylation chemistry to a large extent takes place in the polar acidic environment of the acid phase. Whereas the ester intermediates play a positive role in alkylation when it is catalyzed by liquid acids, formation of such intermediates represents a potent route to fast catalyst passivation for a solid catalyst with fixed acid sites. It represents a serious challenge for the use of solid acids for isobutane alkylation. Also, the formation of acid-soluble oil byproducts which to some extent are formed as part of the mechanism of alkylation represents a serious complication for the use of a solid acid as catalyst for isobutane alkylation. Therefore, while safety and environmental considerations may favor a true, solid catalyst, the chemistry strongly favors a liquid catalyst. A new process based on a liquid catalyst supported in a mobile catalyst zone on a fixed bed of solid support represents a compromise between the wish for a true, solid catalyst and the chemistry of the isobutane alkylation. Since the reaction takes place on a solid support in a fixed bed, the operation resembles that of a solid catalyst in a fixed bed regarding safety and engineering. However, the mechanism of the isobutane alkylation in the process is very similar to the chemistry of the existing processes based on liquid acids, and, conse-
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3497
quently, the high product qualities known from the liquid-phase processes are readily obtained. Acknowledgment We thank Susanna Røen for her efforts in the lab and Anne Marie Bauer and Ole Stahl for their operation of the bench-scale units, and we express our gratitude to Dr. Jens Rostrup-Nielsen for his support throughout this project. We also thank Professor Chr. Pedersen for his assistance in acquiring NMR data. Literature Cited Albright, L. F. Mechanism for Alkylation of of Isobutane with Light Olefins. ACS Symp. Ser. 1977, 55, 128. Albright, L. F.; Doshi, B.; Ferman, M. A.; Ewo, A. Two-Step Alkylation of Isobutane with C4 Olefins: Reactions of C4 Olefins with Sulfuric Acid. ACS Symp. Ser. 1977a, 55, 96. Albright, L. F.; Doshi, B. M.; Ferman, M. A.; Ewo, A. Two-Step Alkylation of Isobutane with C4 Olefins: Reaction of Isobutane with Initial Reaction Products. ASC Symp. Ser. 1977b, 55, 109. Albright, L. F.; Spalding, M. A.; Kopser, C. G.; Eckert, R. E. Alkylation of Isobutane with C4 Olefins. 2. Production and Characterization of Conjunct Polymers. Ind. Eng. Chem. Res. 1988a, 27, 386. Albright, L. F.; Spalding, M. A.; Nowinski, J. A.; Ybarra, R. M.; Eckert, R. E. Alkylation of Isobutane with C4 Olefins. 1. First Step Using Sulfuric Acid Catalyst. Ind. Eng. Chem. Res. 1988b, 27, 381. Albright, L. F.; Spalding, M. A.; Faunce, J.; Eckert, R. E. Alkylation of Isobutane with C4 Olefins. 3. Two-Step Process Using Sulfuric Acid as Catalyst. Ind. Eng. Chem. Res. 1988c, 27, 391. Benazzi, E.; Joly, J. J.; Latieule, S.; Marcilly, C. Study of Sulfuric Alkylation of Isobutane with Butenes at Low Temperature Using Model Supported Acid-Liquid Phases. ACS Div. Petr. Chem. Prep. 1996, 41 (4), 711. Chapin, L. E.; Liolios, G. C.; Robertson, T. M. Which AlkylationHF or H2SO4. Hydrocarbon Processing, 1985, September, 67. Collins, N. A.; Child, J. E.; Huss, A. BF3 Promoted IsobutaneOlefin Alkylation at Low I/O. ACS Div. Petr. Chem. Prep. 1996, 41 (4), 706. Corma, A.; Martı´nez, A. Chemistry, Catalysts, and Processes for Isoparaffin-Olefin Alkylation: Actual Situation and Future Trends. Catal. Rev.-Sci. Eng. 1993, 35 (4), 483. Crossland, C. S. Advances in Solid Acid Alkylation. Proceedings Worldwide Solid Acid Process Conference, Houston, TX, 1993. de Jong, K. P.; Mesters, C. M. A. M.; Peferoen, D. G. R.; van Brugge, P. T. M.; de Groot, C. Paraffin Alkylation using Zeolite Catalysts in a Slurry Reactor: Chemical Engineering Principles to Extend Catalyst Lifetime. Chem. Eng. Sci. 1996, 51 (10), 2053. Doshi, B.; Albright, L. F. Degradation and Isomerisation Reactions Occurring during Alkylation of Isobutane with Light Olefins. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 53. Engelhardt, J.; Hall, W. K. Peculiarities Observed in H-D Exchange between Perdeuteroisobutane and H-Zeolites. J. Catal. 1995, 151, 1. Grosse, A. V.; Linn, C. B. The Addition of Hydrogen Fluoride to the Double Bond. J. Org. Chem. 1938, 3, 26.
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Received for review January 6, 1997 Revised manuscript received April 8, 1997 Accepted May 6, 1997X IE970028S
X Abstract published in Advance ACS Abstracts, July, 1; 1997.
