Isomerization of 1-Butene to Isobutene at Low Temperature - Industrial

This peculiar oligomerization behavior of 1-butene has been noted previously in the literature and the present study deals with its explanation. Evide...
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Ind. Eng. Chem. Res. 2004, 43, 6325-6330

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Isomerization of 1-Butene to Isobutene at Low Temperature Arno de Klerk* Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and Development, P.O. Box 1, Sasolburg 1947, South Africa

Solid phosphoric acid catalyzed oligomerization of 1-butene rich material yielded highly branched products that were not expected from a standard carbocation mechanism. This peculiar oligomerization behavior of 1-butene has been noted previously in the literature and the present study deals with its explanation. Evidence for a low-temperature skeletal isomerization pathway of 1-butene to isobutene is presented. Although double bond isomerization caused equilibration of the n-butenes, there was a significant period during which 1-butene determined the product properties and oligomerization rate. During this period double bond isomerization was stereospecific, favoring cis-2-butene, and the ratio of trimethylpentenes to total C8 olefins did not change. Macroscopic evidence indicated that a monomolecular, or pseudo-monomolecular mechanism was active for isomerization and an explanation that did not involve the formation of a primary carbocation could be presented. Introduction Acid-catalyzed reactions of olefins are commonly believed to take place via a carbocation intermediate. The relative stability of the carbocation is used to explain differences in reactivity, migration of substituents, and product selectivity. In many instances it provides an adequate explanation of olefin oligomerization, but in cracking and skeletal isomerization some shortcomings have been noted. For C5 and heavier hydrocarbons the mechanism has been modified to recognize the role of a protonated cyclopropane (PCP) intermediate.1 In this respect butene occupies a special place. Butene skeletal rearrangement by an acyclic carbocation mechanism would result in a primary carbocation intermediate, a suggestion that has been considered and rejected early in the debate on its mechanism.2 If the mechanism involving a PCP intermediate is used, butene skeletal rearrangement still requires a primary carbocation intermediate during the reaction sequence. This can be prevented if one accepts a bimolecular mechanism, which requires dimerization to precede skeletal rearrangement and cracking, but the debate on the mechanism is far from settled.3-6 This is in contrast to the mechanism of butene oligomerization, which seems to be adequately explained by the standard carbocation mechanism. One would therefore expect similar products from the acid-catalyzed oligomerization of 1-butene, cis-2-butene, and trans-2-butene, since they have a common acyclic secondary carbocation intermediate. Yet, this assumption was not borne out by experimental evidence and literature is speckled with accounts of 1-butene and 2-butene showing different behavior. For example, it was found that 1-butene and 2-butene oligomerization * Tel: +27 16 960-2549. Fax: +27 16 960-2826. E-mail: [email protected].

yielded different products,7 that 1-butene resulted in stereospecific double bond isomerization,8 and that 1-butene and 2-butene reacted differently during skeletal isomerization.9 From an industrial point of view and specifically from a Fischer-Tropsch product refining perspective, the peculiar oligomerization behavior of 1-butene is of interest. The butene fraction derived from a hightemperature Fischer-Tropsch process contains more than 70% 1-butene.10 Surprisingly, it was reported that oligomerization of 1-butene yielded better motor gasoline than 2-butene7 and it was noted that 1-butene oligomerization behavior was closer to that of isobutene (2-methylpropene) than to that of the 2-butenes. This deviant behavior of 1-butene was not noted with all acid catalysts,11,12 but on some catalysts it seemed that the mechanism of butene oligomerization departed from the standard carbocation mechanism. Oligomerization studies with high-temperature Fischer-Tropsch derived feed material confirmed that oligomerization of 1-butene over a solid phosphoric acid catalyst deviated from the standard carbocation mechanism. This implied that on solid phosphoric acid the adsorbed state of 1-butene and 2-butene had to be different, since a common intermediate would make it impossible to explain the differences. The product from 1-butene oligomerization showed a high degree of branching, similar to isobutene dimerization. Data will be presented to show that 1-butene was involved in some form of skeletal rearrangement at low temperature and macroscopic evidence will be provided for a probable mechanism. Experimental Section Three different reactors were used in the experiments: a batch reactor and two packed-bed reactors of slightly different configuration. The stirred batch reac-

10.1021/ie049585m CCC: $27.50 © 2004 American Chemical Society Published on Web 09/01/2004

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tor had a useful internal volume of about 150 mL and temperature was controlled by an external heater. The reactor was fitted with an ampule-breaking device, allowing the catalyst to be loaded in a glass ampule that could be broken in situ. The two packed bed reactors had internal diameters of 27.9 mm and 26.6 mm respectively, with a 6-mm central thermowell, and the temperature was controlled by a series of external heaters. The oligomerization catalyst used was a commercial C84-3 solid phosphoric acid from Su¨d-Chemie-Sasol, manufactured in Sasolburg, South Africa. The catalyst was crushed and sealed in glass ampules for the batch reactor work. In the packed bed reactors the catalyst was diluted with inert material for hydrodynamic reasons, as well as to aid isothermal operation. The calculations of catalyst hydration levels were based on the Brown and Whitt13 vapor pressure data. The feed material used in runs A-M and Q-R was taken from the cold condensate of the commercial hightemperature iron-based Fischer-Tropsch synthesis at the Sasol refineries in Secunda, South Africa. Different cuts were prepared by distillation under pressure at the pilot plant facilities of Sasol Technology in Sasolburg, South Africa. Feed was kept in the liquid state in cylinders to prevent degradation and evaporative loss. For runs N-P pure commercially obtained 1-butene, cis2-butene, and trans-2-butene were used. The isobuteneenriched feed used for runs F and G were prepared by mixing the Fischer-Tropsch feed with pure isobutene. The composition of the feed material used has been reported with the results. For runs where the feed compositions have not been listed with the results, a standard high-temperature Fischer-Tropsch C4 mixture was used: 4% isobutene, 44% 1-butene, 4% cis-2butene, and 5% trans-2-butene, with the remainder being mostly C4 paraffins. The combined water and oxygenate content of the commercial feed mixtures was about 75-100 µg‚g-1. Analyses were mostly done by gas chromatography with flame ionization detection (GC-FID). To help with identification of the species produced during oligomerization, products were hydrogenated and compared with commercially obtained pure compounds, as well as the analyses from gas chromatography with mass spectrometry detection (GC-MS). Mass balances were 95-98% for run A, and 97-100% for runs B-G. The data for runs H-M were expressed relative to an internal standard and no mass balance calculations were done. Results and Discussion Skeletal Isomerization of Butene over Solid Phosphoric Acid. The conditions commercially used for butene oligomerization over solid phosphoric acid catalysts are low temperature (800 kPa).14 This is different from the conditions used for the skeletal isomerization of butene, which is high temperature (>300 °C) and low pressure (close to atmospheric). The formation of isobutene from 1-butene is consequently not expected at oligomerization conditions. However, the thermodynamics for such a conversion is very favorable.15 It has long been known that supported phosphoric acid catalysts are effective for butene skeletal isomerization.16 Supported phosphoric acid has been described in process context and was used in many of the earlier studies to determine equilibrium data.15 However, cur-

Table 1. Skeletal Isomerization of Butene over Solid Phosphoric Acid Catalyst (Run A) days on stream

3

5

6

7

8

temperature (°C) WHSV (h-1) isobutene/n-butenes

300 0.52 0.11

350 0.28 0.17

350 0.28 0.14

350 0.28 0.12

350 0.28 0.11

rent commercial butene skeletal isomerization processes favor ferrierite17 mainly due to its near equilibrium conversion and slower deactivation rate. The reason for the high isobutene selectivity of ferrierite compared to other materials is still being actively researched and debated.18 It was necessary to confirm the ability of the C84-3 solid phosphoric acid to catalyze the skeletal isomerization of n-butenes. The smaller of the two packed-bed reactors was loaded with 30 g of catalyst diluted five times with carborundum and was operated at atmospheric pressure. The temperature and weight hourly space velocity (WHSV) were changed during the run. The feed was mixed with 2-propanol in a 100:3.5 ratio to maintain a low level of catalyst hydration, about 110115% H3PO4. The isobutene content in the product was well below the equilibrium value (Table 1). The results indicated rapid catalyst deactivation, which was expected from previous studies.19 Furthermore, the level of catalyst hydration and high temperature favored coke formation14 and the catalyst deactivation rate was consequently high. The test nevertheless confirmed that the catalyst had the inherent ability to catalyze the skeletal isomerization of butenes. Oligomerization of Butene over Solid Phosphoric Acid. It has already been noted that some catalysts oligomerize 1-butene and 2-butene differently, but most of the work had been done on silica-alumina.7,11,12 The solid phosphoric acid oligomerization of various C2-C4 olefinic feed mixtures14,20-23 seemed to show that the product was insensitive to the type of linear olefin in the feed. Even the commercial design of the solid phosphoric acid oligomerization units to process Fischer-Tropsch derived feed made no provision for the fact that the feed was 1-butene rich.24 The only indication that solid phosphoric acid showed deviant behavior was provided by Ipatieff and Schaad,25 who reported a slightly higher than expected octane number for the hydrogenated product from the oligomerization of a 1-butene rich C4 feed. The tert-butyl content in the product was determined by depolymerization26 and indicated that the degree of branching of the product derived from the 1-butene rich mixture was higher than that from the 2-butene rich mixture. This needed to be confirmed, because it could be argued that carbocation addition to an R-olefin would potentially yield a product with a lower degree of branching. The larger of the two packed-bed reactors was loaded with 250 mL of catalyst and the void spaces were filled with inert material. It was operated at typical commercial conditions as follows: 160 °C, 3.8 MPa, and liquid hourly space velocity (LHSV) of 0.7 h-1. The hydration level of the catalyst was equivalent to about 110% H3PO4. Samples were taken at steady state and the results are based on a composite of at least 3 days (Table 2). Special attention was paid to the trimethyl and tetramethyl branched species. The maximum amount of isobutene that could be present in the liquid product, if all the isobutene in the feed reacted, was calculated.

Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6327 Table 2. Oligomerization of 1-Butene Rich Fischer-Tropsch Material at 160 °C and 3.8 MPa

Table 4. Oligomerization of Fischer-Tropsch Butenes at 130 °C and Atmospheric Pressure

run

B

C

D

run

H

I

J

1-butene in feed (%) isobutene in feed (%) 2-butenes in feed (%) olefin conversion (%) gasoline range product (%) maximum isobutene content possible (%) TMP and TMB content in motor gasoline (%) isobutene content in iso-derived C8’s (%)

29 3 4 75 82 8 7 7

43 5 9 82 83 12 33 12

54 6 7 78 84 12 38 13

isobutene conversion (%) 1-butene conversion (%) cis-2-butene conversion (%) trans-2-butene conversion (%) overall butene conversion (%) cis-2-butene/trans-2-butene in product

-0.3 4.3 -0.4 0.2 3.8 0.84

0.1 8.3 -2.4 -1.8 4.1 0.87

0.9 22.4 -7.4 -6.6 9.3 0.93

Table 3. Increase in TMP Content with Isobutene Enrichment at 160 °C and 3.8 MPa

Table 5. Oligomerization of Fischer-Tropsch Butenes at 160 °C and Atmospheric Pressure run

run

E

F

G

1-butene in feed (%) isobutene in feed (%) 2-butenes in feed (%) olefin conversion (%) gasoline range product (%) maximum isobutene content possible (%) TMP and TMB content in motor gasoline (%) isobutene content in iso-derived C8’s (%)

41 5 8 82 83 12 38 11

40 14 8 84 87 28 43 13

36 21 6 84 89 41 52 14

This constituted the “maximum isobutene content possible”. It was also noted that the product contained a significant amount of trimethylpentene (TMP) and tetramethylbutene (TMB). Not all of the highly branched species could be explained by isobutene oligomerization and a fair degree of skeletal rearrangement could be seen. The isomers in the C8 fraction that could be directly related to isobutene plus isobutene dimerization and isobutene plus n-butene dimerization were expressed in terms of their isobutene content as a percentage of the liquid product. This is the “isobutene content in the isoderived C8’s”. The results of run B (Table 2) showed that the isobutene content in the feed could explain the highly branched products. However, run C and D (Table 2) contained much more TMP and TMB than could be explained by the isobutene in the feed. Even if skeletal rearrangement was disregarded, despite it clearly being active, the isobutene content in the feed could not even explain the C8 isomers characteristic of isobutene dimerization in run D. When the product of runs C and D was hydrogenated, it was found that the motor gasoline fraction had a research octane number (RON) and motor octane number (MON) in the order of 86-88. This was high, considering that the main product that was expected from n-butene dimerization was 3,4-dimethylhexene, which has a hydrogenated RON of 76 and MON of 82.27 It was also higher than the octane values reported by Ipatieff and Schaad.25 Similar test runs were done with isobutene enriched feed (runs E-G, Table 3) to observe the effect of isobutene on a 1-butene rich mixture. The data made it quite clear that isobutene enrichment increased the TMP and TMB content, but it was equally clear that the baseline TMP and TMB content was already quite high. There appeared to be a correlation between the combined isobutene and 1-butene content in the feed and the content of highly branched oligomers in the product. Since the isobutene in the feed could not explain all of the highly branched products, these products had to be formed by the skeletal isomerization of n-butenes and most likely by 1-butene. However, since the skeletal isomerization of n-butenes seemed

K

L

M

isobutene conversion (%) -0.2 -0.1 0.3 1-butene conversion (%) 16.9 25.4 46.8 cis-2-butene conversion (%) -6.1 -8.9 -14.5 trans-2-butene conversion (%) -6.2 -10.2 -21.1 overall butene conversion (%) 4.5 6.2 11.5 cis-2-butene/trans-2-butene in product 0.86 0.83 0.71

unlikely at the operating conditions used, the possibility of product skeletal rearrangement as source of the highly branched material had to be evaluated. Skeletal Rearrangement during Oligomerization. The liquid product of run B (Table 2) contained 4% 2,3,4-trimethylpentene, while that of runs C and D (Table 2) and E-G (Table 3) contained between 20% and 30% 2,3,4-trimethylpentene. In studies by Whitmore and co-workers28 it was found that the reaction of equal amounts of isobutene and 2-butene resulted in a product containing 35% of the rearrangement isomer 2,3,4trimethylpentene. This can be explained in terms of steric effects due to the close proximity of the methyl groups in isobutene-derived dimers, like 2,2,4-trimethylpentene and 2,2,3-trimethylpentene, which cause skeletal rearrangement to a less crowded structure, as in 2,3,4-trimethylpentene. Thermodynamically there is little driving force to increase the degree of branching, and skeletal rearrangement of isobutene-derived material would tend to result in a decrease in the degree of branching.29 It is consequently not possible to explain the formation of highly branched products in terms of the skeletal rearrangement propensity of solid phosphoric acid. Interaction of Butene with Solid Phosphoric Acid. Phosphoric acid forms stable esters with ethene and propene at low temperature.30 These esters involve a formal bond between the R-carbon of the olefin and the phosphoric acid. The esters of butenes are more labile, but analogous. The species formed by 1-butene involve either the R- or β-carbon, while the species formed by 2-butene involve only a β-carbon. Solid phosphoric acid therefore has the ability to interact differently with 1-butene and 2-butene. Low-Temperature 1-Butene to Isobutene Conversion. The high TMP and TMB content of runs C-E (Tables 2 and 3) still had to be explained, and although it seemed unlikely that isobutene could be formed during the low-temperature oligomerization of 1-butene rich feed, this possibility had to be investigated. Runs H-J at 130 °C (Table 4) and runs K-M at 160 °C (Table 5) were done at atmospheric pressure to increase the possibility of detecting the formation of isobutene. The hydration level of the catalyst was 108110% H3PO4. All tests were done in the smaller packedbed reactor. The WHSV was varied to manipulate the conversion. The system was allowed to reach steady state and three or more samples were analyzed during each run, with samples being taken at least 6 h apart.

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An internal standard was used to ensure consistency. Runs were also done at 95 °C, but conditions were too favorable for isobutene oligomerization in relation to the other reactions to learn much. Reaction at 130 °C (Table 4) and 160 °C (Table 5) not only showed some isobutene production at short contact time, but also showed little apparent isobutene conversion at overall butene conversion levels in the order of 10%. This suggested the existence of a low-temperature isomerization pathway and a monomolecular mechanism being active. The temperature was too low for significant cracking of oligomers and no propene was detected. This did not rule out a bimolecular mechanism, but with no odd-numbered cracking products such as propene being coproduced, there was no evidence supporting a bimolecular mechanism. The small increase in isobutene at low conversion should not be cited as proof of isobutene formation, since it was within the error margin of the analytical technique. What could be said with certainty, however, was that isobutene conversion was very low compared to that of 1-butene. This is notable, since isobutene is much more reactive than the n-butenes. As a matter of fact, the reactivity of isobutene has been commercially exploited to selectively convert isobutene in an n-butene matrix over a solid phosphoric acid catalyst for the production of high-octane fuel.14,23 The fact that isobutene is seldom detected in the off-gas from low-temperature oligomerization, the study by Ipatieff and Schaad25 being a notable exception, is therefore hardly surprising. It can be argued that despite the lack of conclusive evidence to show an increase in isobutene content, that the low conversion of isobutene provided indirect evidence of the formation of isobutene. The stereospecificity of the double bond isomerization confirmed that 1-butene did not form a classical secondary carbocation. The cis-isomer was favored (Tables 4 and 5) contrary to what was expected from thermodynamic considerations. This could be explained by a 1-butene adsorbed state that limited the free movement of the R- and β-carbon atoms.8 The role of neighboring hydrogen participation in the formation of a π-complex during the formation of the carbocation intermediate has also been suggested31 and it was further noted that the nature of the ionic intermediate played a role in determining whether elimination would be E2 (bimolecular) or E1 (unimolecular).32 In all instances stereospecificity has been explained as a consequence of an intermediate state that does not resemble a classic secondary carbocation, but involves a carbocation bonded to the catalyst. Stereospecificity would consequently provide indirect evidence for the interaction of solid phosphoric acid and R-carbon of the 1-butene. This is very important, because it implies that the R-carbon of the intermediate species is no longer a primary carbon. Any rearrangement that requires the R-carbon to have a positive charge therefore does not involve the formation of a primary carbocation. The PCP mechanism can then be used to explain skeletal isomerization without the implications of a primary carbocation intermediate. This contention is supported by the observations of Skell and Starer,33 who found cyclopropane formation only during the dehydration of 1-propanol and not with 2-propanol. There is also an obvious analogy with the suggestion that high-temperature skeletal isomerization takes place on the coke layer, to justify a pseudo-

Table 6. Reaction Rate of Pure n-Butenes at 140 °C, 3.8 MPa, and 1.6 h-1 WHSV Equivalent run

N

O

P

feed material conversion (%) relative reaction rate

1-butene 22 1

trans-2-butene 10 0.45

cis-2-butene 13 0.57

Table 7. Oligomerization Rate and 1-Butene Conversion at 130 °C and 3.8 MPa (run Q) reaction period (µmol‚s-1)

oligomerization rate 1-butene conversion (%) Σ butenes conversion (%) Σ TMP/Σ C8 olefins cis/trans-2-butene in product

1

2

3

4.2 1.9 1.7 0.9 0.8 9 25 38 70 87 2.6 3.4 4.6 13.6 15.9 0.7 0.7 0.7 0.7 0.7 0.67 0.70 0.69 0.56 0.53

Table 8. Oligomerization Rate and 1-Butene Conversion at 140 °C and 3.8 MPa (run R) reaction period oligomerization rate (µmol‚s-1) 1-butene conversion (%) Σ butenes conversion (%) Σ TMP/Σ C8 olefins cis/trans-2-butene in product

2 5.6 32 5.1 0.7 0.74

3 4.0 42 7.3 0.7 0.68

2.6 68 9.5 0.7 0.60

2.6 87 12.1 0.7 0.55

monomolecular mechanism that does not require the formation of a primary carbocation.34 Rate Regimes and Double Bond Isomerization. Although evidence was provided for the formation of highly branched products characteristic of isobutene oligomerization, the role of 1-butene has not been proven per se. It could be argued that double bond isomerization of 1-butene rapidly equilibrated the n-butenes, since it is known to be a facile reaction35 and that n-butenes in general exhibited such behavior. Some earlier work by Ipatieff and co-workers indicated that the rate of double bond isomerization is suppressed by pressure,36 but this was not confirmed. Since 1-butene was the only reagent being significantly depleted in runs H-M (Tables 4 and 5), it was likely that 1-butene played an important role even at commercially relevant conditions, like those used for runs B-G (Tables 2 and 3). A batch reactor study (runs N-P, Table 6) was used to compare the reaction rate of different pure butene species. Crushed solid phosphoric acid catalyst was used at 140 °C, 3.8 MPa, and residence time equivalent of 1.6 h-1 WHSV. This gave the apparent rate of the pure species under conditions where equilibration would take place during the reaction. It is not a measure of the true ratio of reaction rates, since that would require differential conversion. However, it showed that the time period before equilibration of the butenes took place had a significant effect on the apparent rate. It also established the n-butene reactivity sequence: 1-butene > cis2-butene > trans-2-butene. Batch reactors were also used to study the rate of oligomerization in butene mixtures. The residence time was used to control feed conversion. Run Q was done at 130 °C (Table 7) and run R was done at 140 °C (Table 8), both at 3.8 MPa pressure. The rates reported are integral rates and not derivative rates. The cis-2-butene to trans-2-butene ratio in the feed was 0.62. Three reaction periods could be identified (Tables 7 and 8). During the first very short period, the oligomerization rate was very high and resembled isobutene oligomerization. In the second period the reaction rate was still high, but seemed to be determined by 1-butene. Two productive reactions and one unproductive reaction occurred. Some 1-butene was isomerized to isobutene,

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which rapidly oligomerized, some 1-butene oligomerized directly, and some 1-butene was isomerized to 2-butene. As the supply of 1-butene was depleted, the reaction moved into the third period. This was the oligomerization of an unequilibrated n-butene mixture, rich in 2-butenes. It should be noted that the stereospecificity of the 1-butene double bond isomerization was still apparent in the third period, emphasizing the rate difference between 1-butene and the 2-butenes. Once the n-butene mixture equilibrated, it entered the last period (not shown by these runs), which is the reaction of an equilibrated n-butene mixture. The oligomerization rate decreased as the reaction progressed and the cis-2-butene to trans-2-butene ratio moved closer to equilibrium. During the first three periods, before the n-butenes equilibrated, the ratio of TMP to total C8 olefins did not change. The rate of TMP formation was somehow linked to the oligomerization rate and it was a relationship that extended into the fourth period. This again pointed to the role of isobutene and 1-butene and hinted that they had a common intermediate for TMP formation, rather than 1-butene and 2-butene having a common intermediate. It is therefore likely that TMP formation by an equilibrated n-butene mixture takes place via the 1-butene present in such a mixture. Mechanism of Low-Temperature Skeletal Isomerization. The high activity of a solid phosphoric acid catalyst, the rapid oligomerization of isobutene, and the propensity of TMP for skeletal rearrangement made mechanistic studies difficult, and the discussion on the mechanism was based on macroscopic observations only. The observations pointed to a monomolecular (or pseudomonomolecular) mechanism for the low-temperature skeletal isomerization of 1-butene. However, in the absence of 13C-experiments, like those used to study high-temperature butene skeletal isomerization,37 this could not be proven. Conclusion The ability of solid phosphoric acid to catalyze the skeletal isomerization of n-butenes to isobutene at high temperature was confirmed. It was subsequently shown that, at lower temperatures, a 1-butene rich feed yielded a product that contained more highly branched species (TMP and TMB) than could be explained by the isobutene content of the feed. This could not be explained by skeletal rearrangement of the product and evidence was presented for a low temperature isomerization pathway from 1-butene to isobutene. Although double bond isomerization caused 1-butene to equilibrate, there was a significant period during which 1-butene determined the product properties and oligomerization rate. During this period double bond isomerization was stereospecific, favoring cis-2-butene and the ratio of TMP to total C8 olefins did not change. The catalyst interacted differently with 1-butene and 2-butene and the stereospecificity of double bond isomerization provided indirect evidence for the formation of a carbocation intermediate that bonded the R-carbon of the 1-butene. This implied that the R-carbon of the carbocation intermediate was no longer a primary carbon and an explanation for the skeletal isomerization of 1-butene that did not involve a primary carbocation could be presented. Macroscopic observations pointed to a monomolecular (or pseudomonomolecular) mechanism being operative, but this was not proven.

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Received for review May 17, 2004 Revised manuscript received July 12, 2004 Accepted July 22, 2004 IE049585M